Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session NP10: Poster Session V (NSTX-U, Pegasus, STs, HBT-EP and General Tokamak; General Spherical Torus Configuration; General Tokamak Configuration; LPI/Short Pulse; HED and Pinches; Diagnostic Techniques, FuZE, ZaPD-HD, Mirrors and other Magnetic Confinement)Poster
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Room: Exhibit Hall 1 |
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NP10.00001: MFE: NSTX-U, PEGASUS, STs, HBT-EP AND GENERAL TOKAMAKs |
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NP10.00002: Commissioning and Plans for the NSTX-U Facility Masayuki Ono The National Spherical Torus Experiment - Upgrade (NSTX-U) has started its first year of plasma operations after the successful completion of the CD-4 milestones. The unique operating regimes of NSTX-U can contribute to several important issues in the physics of burning plasmas to optimize the performance of ITER. The major mission of NSTX-U is also to develop the physics and technology basis for an ST-based Fusion Nuclear Science Facility (FNSF). The new center stack will provide toroidal field of 1 Tesla at a major radius of 0.93 m which should enable a plasma current of up to 2 mega-Amp for 5 sec. A much more tangential 2nd NBI system, with 2-3 times higher current drive efficiency compared to the 1st NBI system, is installed. NSTX-U is designed to attain the 100{\%} non-inductive operation needed for a compact FNSF design. With higher fields and heating powers of 14 MW, the NSTX-U plasma collisionality will be reduced by a factor of 3-6 to help explore the trend in transport towards the low collisionality FNSF regime. If the favorable trends observed on NSTX holds at low collisionality, high fusion neutron fluences could be achievable in very compact ST devices. [Preview Abstract] |
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NP10.00003: Operational characteristics and non-inductive plasmas on NSTX-U Dennis Mueller Start-up and ramp-up of NSTX-U plasmas with nearly full solenoid pre-charge required higher loop voltage than on NSTX in agreement with modeling. NSTX-U has operated with a plasma current (I$_{\mathrm{p}})$ of 0.65 MA at a toroidal field (B$_{\mathrm{T}})$ of 0.63 T for 2s in L-Mode. These plasmas allowed for the initial investigation of error field correction which found a difference between correction during Ip flattop and during ramp-up. Plasma control using feedback on the X-Point locations or the X-point height and outer strike point locations is routinely used. Because an ST does not have a single coil set that controls the inner gap, it was challenging to control the time at which the plasma diverted. A novel approach was used to trade off accuracy on the outer plasma shape to achieve a reproducible inner gap. These control tools allowed study of ELMy H-mode operation at I$_{\mathrm{p\thinspace }}=$ 1 MA with boronized walls. A major long-term goal for NSTX-U is totally non-inductive operation. The plan calls for initiating the plasma with coaxial helicity injection (CHI) heatied by ECH, then current drive and heating by HHFW and NBI. CHI will be used in the new geometry to demonstrate results comparable to NSTX and provide information to inform plans to upgrade the available voltage from 1.65 to 2 kV next year. Low I$_{\mathrm{p}}$ plasmas will be used to study the dependence of current drive on neutral beam voltage and injection angle. [Preview Abstract] |
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NP10.00004: Progress with MGI and CHI Research on NSTX-U R Raman, W-S Lay, T.R Jarboe, B.A Nelson, D Mueller, S.P Gerhardt, F Ebrahimi, S.C Jardin, G Taylor NSTX-U experiments on Massive Gas Injection (MGI) will offer new insight to the MGI database by studying gas assimilation efficiencies for MGI gas injection from different poloidal locations. In support of this research, two ITER-type MGI valves have been successfully commissioned on NSTX-U. Results from the planned experiment `Comparison of Private Flux Region with Conventional Mid-plane MGI on NSTX-U', will be reported. In support of planned Coaxial Helicity Injection (CHI) research on NSTX-U, a new high-resolution grid has been generated for TSC simulations of CHI. This improves the resolution of the CHI injector region, and better models the closely-spaced divertor coils on NSTX-U. These new simulations support previous analysis that suggests a solenoid-free plasma current initiation capability of more than 400kA on NSTX-U. This work is supported by U.S. DOE Contracts: DE-AC02-09CH11466, DE-FG02-99ER54519 AM08, and DE-SC0006757. [Preview Abstract] |
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NP10.00005: Impact of the Digital Coil Protection System and Plasma Shutdown Handler on NSTX-U Operations Stefan Gerhardt, D. Battaglia, M. Boyer, K. Erickson, D. Mueller, C Myers, D. Mueller, S.A. Sabbagh In order to prevent excessive forces on the NSTX-U vessel and coils, a digital coil protection system (DCPS) has been implemented. This system computes approximately 400 different forces/torques/stresses, and terminates the discharge if limits on those quantities are exceeded. It is desirable, however, to prevent these coil system trips from ever happening. Given that many of these limits would be reached during transients associated with disruptions, as ``discharge shutdown handler'' was coded in the plasma control system to automatically control the plasma shutdown. This is a state machine with five states, and a set of rules for transitioning between states. The first use of these systems during plasma operations on NSTX-U will be described, with a focus on operational experiences and directions for future improvements. Work Supported by U.S.D.O.E. Contract No. DE-AC02-09CH11466. [Preview Abstract] |
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NP10.00006: Model Predictive Control with Integral Action for Current Density Profile Tracking in NSTX-U Z.O. Ilhan, W.P. Wehner, E. Schuster, M.D. Boyer Active control of the toroidal current density profile may play a critical role in non-inductively sustained long-pulse, high-beta scenarios in a spherical torus (ST) configuration, which is among the missions of the NSTX-U facility. In this work, a previously developed physics-based control-oriented model is embedded in a feedback control scheme based on a model predictive control (MPC) strategy to track a desired current density profile evolution specified indirectly by a desired rotational transform profile. An integrator is embedded into the standard MPC formulation to reject various modeling uncertainties and external disturbances. Neutral beam powers, electron density, and total plasma current are used as actuators. The proposed MPC strategy incorporates various state and actuator constraints directly into the control design process by solving a constrained optimization problem in real-time to determine the optimal actuator requests. The effectiveness of the proposed controller in regulating the current density profile in NSTX-U is demonstrated in closed-loop nonlinear simulations. [Preview Abstract] |
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NP10.00007: TRANSP-based Trajectory Optimization of the Current Profile Evolution to Facilitate Robust Non-inductive Ramp-up in NSTX-U William Wehner, Eugenio Schuster, Francesca Poli Initial progress towards the design of non-inductive current ramp-up scenarios in the National Spherical Torus Experiment Upgrade (NSTX-U) has been made through the use of TRANSP predictive simulations [Nucl. Fusion \textbf{55 }(2015) 123011 (12pp)]. The strategy involves, first, ramping the plasma current with high harmonic fast waves (HHFW) to about 400 kA, and then further ramping to 900 kA with neutral beam injection (NBI). However, the early ramping of neutral beams and application of HHFW leads to an undesirably peaked current profile making the plasma unstable to ballooning modes. We present an optimization-based control approach to improve on the non-inductive ramp-up strategy. We combine the TRANSP code with an optimization algorithm based on sequential quadratic programming to search for time evolutions of the NBI powers, the HHFW powers, and the line averaged density that define an open-loop actuator strategy that maximizes the non-inductive current while satisfying constraints associated with the current profile evolution for MHD stable plasmas. This technique has the potential of playing a critical role in achieving robustly stable non-inductive ramp-up, which will ultimately be necessary to demonstrate applicability of the spherical torus concept to larger devices without sufficient room for a central coil. [Preview Abstract] |
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NP10.00008: Development of the NSTX-U Advanced Divertor Control Patrick Vail, Egemen Kolemen Advanced magnetic divertor configurations such as the snowflake (SF) divertor are being investigated at NSTX-U for reducing the peak heat flux onto plasma-facing components. Initial efforts include development of plasma scenarios incorporating SF configurations using an upgraded set of divertor coils as well as implementation of a feedback control system for real-time detection and manipulation of two closely-spaced magnetic null points. Closed-loop plasma simulations are performed to demonstrate precise control of various SF configurations. The simulations are then used to demonstrate that the controller can be enhanced to regulate additional parameters such as strike point location and divertor flux expansion. The advanced divertor control will be used in the coming years to enable experiments investigating the physics of advanced divertors at NSTX-U. [Preview Abstract] |
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NP10.00009: Initial operation of the NSTX-U Real-Time Velocity diagnostic M. Podesta, R. E. Bell A Real-Time Velocity (RTV) diagnostic based on active charge-exchange recombination spectroscopy is now operational on the National Spherical Torus Experiment-Upgrade (NSTX-U) spherical torus. The system has been designed to supply plasma velocity data in real time to the NSTX-U Plasma Control System, as required for the implementation of toroidal rotation control. Measurements are available from four radii, spanning from the core to the plasma edge, at a maximum sampling frequency of 5 kHz. Post-discharge analysis of RTV data provides additional information on ion temperature, toroidal velocity and density of carbon impurities. Initial results from RTV measurements are presented and compared with those from the main NSTX-U charge-exchange recombination system. Examples of physics studies enabled by RTV measurements from initial operations of NSTX-U are then discussed, with emphasis on the effects of plasma disturbances such as sawteeth and MHD instabilities on toroidal velocity and its temporal evolution. [Preview Abstract] |
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NP10.00010: Transport properties of NSTX-U L- and H-mode plasmas Stanley Kaye, Walter Guttenfelder, Ron Bell, Ahmed Diallo, Ben LeBlanc, Mario Podesta The confinement and transport properties of L- and H-mode plasmas in NSTX-U has been studied using the TRANSP code. A dedicated series of L-mode discharges was obtained to study the dependence of confinement and transport on power level and beam aiming angle. The latter is made possible by having two beamlines with 3 sources each, capable of injecting with tangency radii from R$_{\mathrm{tan}}=$50 to 130 cm (R$_{\mathrm{geo}}=$92 cm). L-mode plasmas typically have confinement enhancement factors with H98y,2$=$0.6 to 0.65, exhibiting a 25{\%} decrease in confinement time as the beam power is raised from 1 to 3 MW. Associated with this is an increase in the electron thermal diffusivity in the core of the plasma from 3.5 to 10 m$^{\mathrm{2}}$/s. Electron thermal transport is the dominant energy loss channel in these plasmas. H-mode plasmas exhibit improved confinement, with H98y,2$=$1 or above, and core electron thermal diffusivity values \textless 1 m$^{\mathrm{2}}$/s. Details of these studies will be presented, along with the results of the beam tangency radius scan in L-mode plasmas. [Preview Abstract] |
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NP10.00011: Study of ion and electron scale turbulence in an NSTX H-mode plasma using a synthetic high-k diagnostic and gyrokinetic simuation. J. Ruiz Ruiz, A. White, W. Guttenfelder, Y. Ren, F. Poli, S. Kaye, B. P. Leblanc, E. Mazzucato, K. C. Lee, C. W. Domier, D. R. Smith, H. Yuh Electron scale turbulence is studied on NSTX with a combination of experimental measurements from a high-k scattering system and gyrokinetic simulations. Recent work has shown that electron scale turbulence can be stabilized by the equilibrium electron density gradient after a controlled current ramp down experiment in an NSTX H-mode [1]. Nonlinear electron scale gyrokinetic simulation has shown to underpredict the experimental level of electron heat flux, both before and after the current ramp down. These results suggest ion scale turbulence might not be completely suppressed by ExB shear. Recent nonlinear gyrokinetic simulation results of ion-scale turbulence and its contributions to electron thermal transport for this NSTX plasma will be presented. In addition, a novel synthetic diagnostic for the high-k scattering system is under development to provide quantitative comparisons and constraints between experiments and simulations of electron-scale turbulence. Progress on the development of the synthetic diagnostic along with recent results applied to NSTX and NSTX-U plasmas will be presented. [1] Ruiz Ruiz et al., PoP 22, 122501 (2015). [Preview Abstract] |
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NP10.00012: Recent progress in understanding electron thermal transport in NSTX and NSTX-U Y. Ren, E. Belova, W. Guttenfelder, N. Gorelenkov, S.M. Kaye, E. Mazzucato, W.X. Wang, K. Tritz, R.E. Bell, B.P. LeBlanc, C.W. Domier, D.R. Smith, H. Yuh, K.C. Lee The National Spherical Torus eXperiment (NSTX) provides a unique laboratory for studying plasma instabilities and their relation to electron thermal transport due to its low toroidal field, high plasma beta, low aspect ratio and large ExB flow shear. Recently commissioned NSTX-Upgrade (NSTX-U) has doubled toroidal field, plasma current and NBI heating power, which allows it to reach new parameter regimes more relevant to future devices and to have new capabilities, e.g. modifying current and flow profiles. This upgrade makes it possible to make new turbulence measurements and to isolate/determine the regime of validity of a variety of instabilities in driving electron thermal transport. In addition, enhanced turbulence diagnostics measuring both large and small wavenumbers on NSTX-U will allow more detailed comparisons with nonlinear gyrokinetic simulations. We will present recent progress in understanding the roles of a variety of instabilities in driving electron thermal transport in NSTX and NSTX-U, and associated future plans for NSTX-U will also be presented. [Preview Abstract] |
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NP10.00013: Gyrokinetic linear stability analysis of NSTX L-mode plasmas Ke Han, Yang Ren NSTX offered unique opportunities in studying transport and turbulence with low aspect ratio, strong plasma shaping and strong E$\times$B shear. NSTX L-mode plasmas have some favorable properties to facilitate the study of the relation between microturbulence and thermal transport: easier to obtain stationary profiles; easier to maintain MHD quiescence; no complications from edge transport barrier. Studies of NSTX RF/NBI-heated L-mode plasmas have provided new insight into the role of ion and electron-scale turbulence in driving anomalous transport [1,2]. Here we present linear stability analysis of some NSTX L-mode plasmas with GS2 gyrokinetic code. GS2 is an initial value gyrokinetic code which, in its linear mode, finds the fastest growing mode for a given pair of poloidal and radial wavenumbers. The linear simulations used local Miller equilibria and plasma parameters derived from measured experimental profiles with electromagnetic effects, electron and ion collisions and carbon impurity.\\ 1. Y. Ren et al, Phys. Plasmas 22, (2015) 110701 \\ 2. W.X Wang et al., Phys. Plasmas 22, (2015) 102509 [Preview Abstract] |
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NP10.00014: Initial Characterization of L-mode and H-mode Edge Turbulence in NSTX-U using Beam Emission Spectroscopy David Kriete, Raymond Fonck, George McKee, David Smith Turbulence in L-mode and H-mode plasmas in NSTX-U has been measured using the upgraded 2D BES system. Plasma discharges exhibit a broadband turbulence spectrum up to $\sim $150 kHz. In addition, a broadband feature centered at 100 kHz is observed in the early, low density L-mode phase of a discharge with high neutral beam heating, but not in the late, high density L-mode phase of a discharge with low neutral beam heating. Normalized density fluctuation power reduces after the L-H transition by a factor of $\sim $10 in the outer edge region, and $\sim $5 in the inner edge region. More detailed characterization results, including correlation lengths, decorrelation times, and flow dynamics across the L-H transition, will be presented. Due to discontinuation of the photodiode currently used in BES detectors, the capacitances of several modern, PIN photodiodes have been measured to assess their suitability for BES measurements. The BES preamplifier board layout has been redesigned to test each of the potential replacement diodes. The redesign also enables automated fabrication and assembly of the preamplifiers, simplifying future expansions to the BES system. Finally, the design specifications of a survey spectrometer for impurity measurements with BES detectors will be discussed. [Preview Abstract] |
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NP10.00015: Identification of characteristic ELM evolution patterns with Alfven-scale measurements and unsupervised machine learning analysis* David R. Smith, R.J. Fonck, G.R. McKee, A. Diallo, S.M. Kaye, B.P. LeBlanc, S.A. Sabbagh Edge localized mode (ELM) saturation mechanisms, filament dynamics, and multi-mode interactions require nonlinear models, and validation of nonlinear ELM models requires fast, localized measurements on Alfven timescales. Recently, we investigated characteristic ELM evolution patterns with Alfven-scale measurements from the NSTX/NSTX-U beam emission spectroscopy (BES) system [1].~We~applied clustering algorithms from the machine learning domain to ELM time-series data. ~The algorithms identified two or three groups of ELM events with distinct evolution patterns. In addition, we found that the identified ELM groups correspond to distinct parameter regimes for plasma current, shape, magnetic balance, and density pedestal profile [1].~The observed characteristic evolution patterns and corresponding parameter regimes suggest genuine variation in the underlying physical mechanisms that influence the evolution of ELM events and motivate nonlinear MHD simulations. Here, we review the previous results for characteristic ELM evolution patterns and parameter regimes, and we report on a new effort to explore the identified ELM groups with 2D BES measurements and nonlinear MHD simulations. [1] D. R. Smith et al, PPCF 58, 045003 (2016). *Supported by U.S. Department of Energy Award Numbers DE-SC0001288 and DE-AC02-09CH11466. [Preview Abstract] |
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NP10.00016: Measurements of fast ion confinement and transport using dual-view Fast-Ion D-Alpha Diagnostics on NSTX-U. G.Z. Hao, W.W. Heidbrink, D. Liu, L. Stagner, M. Podesta, E. Fredrickson, A. Bortolon, D. Darrow On spherical tokamak NSTX-U, it is found that the fast-ion transport induced by sawtooth is larger for passing particles than for trapped particles. The result is identified by the fact that the sawtooth induces the slight decrease of both vertical and tangential FIDA signal at the inner channels, whilst strongly enhances t-FIDA signal at outer channels. FIDA results are consistent with the SSNPA measurements which identify the enhanced passing particle losses caused by sawtooth. Here, the vertical/radial and tangential views of FIDA and SSNPA are sensitive to the trapped and passing particles, respectively. In addition, low frequency mode (\textless 60 kHz) decreases both trapped and passing fast-ion population in the core region, and increases the passing particle population in the edge area. Furthermore, for FIDA data analysis, background subtraction technique works well for v-FIDA, however, requires the improvement for t-FIDA at edge channels which are more sensitive to the plasma condition. Work supported by U.S. DOE DE-AC0209CH11466, DE-FG02-06ER54867, and DE-FG03-02ER54681 [Preview Abstract] |
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NP10.00017: Beam ion susceptibility to loss in NSTX-U plasmas Douglass Darrow, Eric Fredrickson, Mario Podesta, Deyong Liu, Roscoe White NSTX-U has operated with three additional neutral beam sources whose tangency radii of 1.1, 1.2, and 1.3 m are significantly larger than the 0.5, 0.6, and 0.7 m tangency radii of the neutral beams previously used in NSTX. These latter beams have also be retained for NSTX-U. Here, we present an estimate of the susceptibility of the beam ions from all the various sources to loss under a range of NSTX-U plasma conditions. This estimation is based upon TRANSP calculations of beam ion deposition in phase space, and the location of the FLR-corrected loss boundary in that phase space. Since losses are often observed at the injection energy, a simple measure of loss susceptibility is the change in canonical toroidal momentum required to move beam ions from their deposition point to the loss boundary, as a function of magnetic moment. To augment this simple estimate, we intend to report some associated transport coefficients of beam ions due to AE activity. [Preview Abstract] |
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NP10.00018: Nonlinear Simulation of Beam-driven Multiple TAEs in NSTX Guoyong Fu, Feng Wang Energetic particle modes and Alfv\'{e}nic modes driven by super-Alfv\'{e}nic beam ions were routinely observed in neutral beam heated plasmas on the National Spherical Torus Experiment (NSTX). These modes can significantly impact beam-ion transport, thus causing beam-ion redistribution and losses. NSTX experimental results show that multiple low-amplitude beam-driven TAEs with weak frequency chirping can transit to mode avalanche with much larger amplitudes and stronger frequency chirping. In order to explore mechanisms of the TAE avalanche, M3D-K nonlinear simulations of multiple beam-driven TAEs and the n$=$1 fishbone have been carried out. The results show strong interaction between TAEs and fishbone that either enhances or reduces saturation level of individual modes. The simulated saturation levels are found to be very sensitive to the minimum value of safety factor q$_{\mathrm{\mathbf{min.}}$ when q$_{\mathrm{\mathbf{min}}}$ drops below a critical value $\sim$1.19, the saturated mode amplitudes increase sharply to large values with stronger chirping. This result is similar to the observed TAE avalanche in the later phase of NSTX discharges as q$_{\mathrm{\mathbf{min}}}$ drops in time. [Preview Abstract] |
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NP10.00019: Parametric investigation of compressional and global Alfv\'{e}n eigenmode instability and effect on thermal confinement in NSTX-U S. Tang, N.A. Crocker, T.A. Carter, E.D. Fredrickson, N.N. Gorelenkov, W. Guttenfelder The leading candidates for anomalous electron transport in NSTX with increasing beam power are high-frequency Alfv\'{e}n eigenmodes excited through Doppler-shifted cyclotron resonance with beam ions. However, there exists no current model for predicting the spectra, structure, and amplitude of these modes, which consist of compressional (CAE) and global (GAE) Alfv\'{e}n eigenmodes. An existing database of neutral beam heated NSTX shots spanning a broad range of plasma parameters is extended to include measurements of CAE/GAE activity in order to statistically investigate the physics parameters controlling the characteristics of these modes and how they contribute to anomalous electron transport. Mode power is found to scale with beam power as \textbar dB\textbar ~$\sim $~P$^{\mathrm{2.6}}$. Average frequency is shown to correlate strongly with average toroidal mode number (\textbar n\textbar ~decreases as f increases) across a wide range of beam powers. This correlation might be explained by the parallel resonance condition expected to govern the instability of these modes. Central electron temperature is also found to correlate with mode frequency. A possible explanation is that the higher frequency, lower \textbar n\textbar ~modes are more effective at electron thermal transport. The physical causes of these correlations require further investigation. [Preview Abstract] |
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NP10.00020: Comparison of simulated heat transport in NSTX via high frequency Alfv\'{e}n eigenmode-induced electron orbit modification with~TRANSP power balance modeling. N. A. Crocker, K. Tritz, R. B. White, E. D. Fredrickson, N. N. Gorelenkov Compressional (CAE) and global (GAE) AEs have been hypothesized to cause an anomalously high electron thermal diffusivity ($\chi_{\mathrm{e}})$ routinely inferred via TRANSP power balance modeling in the core ($r/a$ \textless $\sim $ 0.3) of NSTX beam heated plasmas. New simulations with the guiding-center code ORBIT test a leading proposed transport mechanism: electron orbit stochastization by multiple modes. Simulations with a set of modes identified as GAEs in a high performance, beam heated plasma---using experimentally determined amplitudes, frequencies and wave numbers---yield a $\chi_{\mathrm{e}}$ insufficient to match TRANSP. To produce a comparable $\chi_{\mathrm{e}}$, the amplitudes must be increased by a factor of $\sim $ 10, which is outside the bounds of measurement uncertainty. Many observed modes, identified as CAEs, could not be included without modifications to ORBIT. These are in progress. However, given the uncertainties in identification, it is informative to calculate $\chi _{\mathrm{e}}$ assuming all the observed modes are GAEs. This leads to substantially higher $\chi_{\mathrm{e}}$, although an amplitude increase by a factor \textgreater $\sim $ 3 is still necessary to match TRANSP. [Preview Abstract] |
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NP10.00021: Microwave diagnostics to measure internal magnetic fluctuations, intermediate-k density fluctuations, and flows on NSTX-U T. L. Rhodes, N. A. Crocker, S. Kubota, W. A. Peebles New microwave diagnostics are being installed on the NSTX-U spherical tokamak that will address a range of important physics topics including multi-scale turbulence and transport, energetic particles, and pedestal turbulence and flows. These multi-radial point diagnostics include (a) Doppler backscattering for intermediate-k density fluctuations (\~{n}) and flows and (b) cross-polarization scattering for internal, localized magnetic fluctuations. Doppler backscattering has significant measurement capabilities and is able to measure \~{n} levels, mean and fluctuating flow, sheared flows, GAMs, ELM and EHO activity (with wavenumber range k$_{\mathrm{\theta }}\rho_{\mathrm{s}}=$0.5--10, spatial and temporal resolutions $\Delta $r$\le $1cm and $\Delta $t$\le $1$\mu $s). Cross-polarization scattering measurements of internal magnetic fluctuations cover an even broader wavenumber range (k$_{\mathrm{\theta }}\rho _{\mathrm{s}}\sim $0.2--17) with high time and space resolutions ($\Delta $r$\sim $1cm, $\Delta $t$=$1$\mu $s). Important and interesting instabilities addressed include microtearing, ITG, TEM, KBM, lower-k ETG, and kinetic Alfv\'{e}n waves. [Preview Abstract] |
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NP10.00022: Nonlinear simulations of Neutral-beam-driven Compressional Alfv\'{e}n Eigenmodes / Kinetic Alfv\'{e}n Waves in NSTX Elena Belova, N.N. Gorelenkov, N.A. Crocker, J.B. Lestz, E.D. Fredrickson, S. Tang Results of 3D nonlinear simulations of neutral-beam-driven compressional Alfv\'{e}n eigenmodes (CAEs) in the National Spherical Torus Experiment (NSTX) are presented. Hybrid MHD-particle simulations for the H-mode NSTX discharge (shot 141398) using the HYM code show unstable CAE modes for a range of toroidal mode numbers, n$=$4-9, and frequencies below the ion cyclotron frequency. It is found that the essential feature of CAEs is their coupling to kinetic Alfven wave (KAW) that occurs on the high-field side at the Alfven resonance location. Nonlinear simulations demonstrate that CAEs can channel the energy of the beam ions from the injection region near the magnetic axis to the location of the resonant mode conversion at the edge of the beam density profile. This mechanism provides an alternative explanation to the observed reduced heating of the plasma core in the NSTX. A set of nonlinear simulations show that the CAE instability saturates due to nonlinear particle trapping, and a large fraction of beam energy can be transferred to several unstable CAEs of relatively large amplitudes and absorbed at the resonant location. Absorption rate shows a strong scaling with the beam power. [Preview Abstract] |
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NP10.00023: Detailed comparison of simulations, experiments, and theory of sub-cyclotron Alfven eigenmodes in NSTX Jeff Lestz, Elena Belova, Nikolai Gorelenkov, Shawn Tang, Neal Crocker High frequency compressional (CAE) and global (GAE) Alfv\'{e}n eigenmodes are often driven unstable by super-Alfv\'{e}nic beam ions in NSTX, and have been linked to anomalous electron temperature profile flattening at high beam power [D. Stutman, PRL 2009]. A large set of 3D MHD-$\delta f$ hybrid simulations show that GAE are ubiquitous at beam energies $V_b/V_A > 2.5$, while CAE are not excited until $V_b/V_A > 4$. The frequency of the most unstable GAE changes significantly with the normalized beam energy, consistent with trends described by its dispersion and resonance condition. These simulation results are analyzed and compared with a new, extensive experimental survey of NSTX discharges, as well as analytic studies. Interestingly, simulations find no case where counter-propagating CAE are more unstable than co-CAE, whereas experiments routinely observe both co- and counter-CAE. Moreover, simulations find co-GAE to be very unstable for beams peaked around $\lambda \leq 0.5$, yet these modes have not yet been thoroughly investigated experimentally. Preliminary predictions are also made for the CAE/GAE instability in ITER-like plasmas, which are expected to operate near similar values of $V_b/V_A$ as those studied for NSTX. [Preview Abstract] |
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NP10.00024: Parallel Electron Force Balance and the L-H Transition T. Stoltzfus-Dueck, A. Diallo, S. Zweben, S. Banerjee In a popular description of the L-H transition, energy transfer to the mean flows directly depletes turbulence fluctuation energy, resulting in suppression of the turbulence and a corresponding transport bifurcation. However, electron parallel force balance couples nonzonal velocity fluctuations with electron pressure fluctuations on rapid timescales, comparable with the electron transit time. For this reason, energy in the nonzonal velocity stays in a fairly fixed ratio to electron thermal free energy, at least for frequency scales much slower than electron transit. In order for direct depletion of the energy in turbulent fluctuations to cause the L-H transition, energy transfer via Reynolds stress must therefore drain enough energy to significantly reduce the sum of the free energy in nonzonal velocities and electron pressure fluctuations. At low $k_{\perp}$, the electron thermal free energy is much larger than the energy in nonzonal velocities, posing a stark challenge for this model of the L-H transition. [Preview Abstract] |
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NP10.00025: Carbon microgranule injection into NSTX-U discharges for edge diagnostic research Robert Lunsford, A. Lane Roquemore, Filippo Scotti, Dennis Mansfield, Alessandro Bortolon, Robert Kaita, Rajesh Maingi Microgranule injection is a versatile means for investigating edge plasmas in fusion devices. Employing a dual bladed rotary turbine, carbon microgranules ranging in diameter from 300 - 700 microns are radially injected into NSTX-U discharges at approximately 50 m/sec. Utilizing multiple high speed camera views, a 3D reconstruction of the injection geometry is created which characterizes the ablation rate and granule trajectory. By coupling this with a neutral gas shielding (NGS) ablation model, the granule mass deposition profile can be determined. Simulation projects a depositional barycenter near the pedestal shoulder for H-mode discharges, and \textasciitilde 20 cm inboard of the LCFS for L-mode discharges. Spectroscopic measurements of this localized particle source can be used to characterize impurity transport within the discharge, and potentially allows for direct measurement of the safety factor profile (q). In addition, the transient pressure peaking resultant from injection into H-mode plasmas can also result in the prompt triggering of an edge localized mode (ELM). [Preview Abstract] |
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NP10.00026: First results of NSTX-U SOL reflectometer Cornwall Lau, John Caughman, Joel Hosea, Rory Perkins, Gary Taylor, John Wilgen The goal of the Oak Ridge National Laboratory (ORNL) scrape-off-layer (SOL) reflectometer is to measure the density profiles and fluctuations in front of the HHFW antenna on NSTX-U to help understand plasma-antenna coupling and RF-edge interactions, such as density profile modifications due to field-aligned power losses and/or parametric decay instabilities. Originally designed for NSTX parameters, the reflectometer has been upgraded to operate at the increased magnetic fields of NSTX-U by using a combination of O-mode cutoffs, and X-mode L and R cutoffs instead of only X-mode R-cutoff. The use of the X-mode L-cutoff, in particular, is necessary to achieve density profile measurements at the expected full magnetic field capability of NSTX-U. Reflectometer electronics and digitization systems were also upgraded to take measurements with a 20$\mu $s time resolution, so as to reduce the effects of turbulence on the density profile measurement. The first results of these reflectometry measurements on NSTX-U will be shown for a range of plasma conditions. Demonstration that the reflectometer can measure the different cutoffs will also be shown. [Preview Abstract] |
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NP10.00027: Gas puff imaging diagnostic on NSTX-U$ S.J. Zweben, F. Scotti, D.L. Stotler, A. Diallo, N. Mandell, J.L. Terry, W. Han The first results and plans for the gas puff imaging (GPI) diagnostic on NSTX-U will be described. The GPI optical efficiency has been improved by about x10 using a new fiber bundle and interference filter, and the new optics has a zoom lens which can potentially resolve turbulence below the ion gyroradius scale. Experiments are planned to study high-k edge turbulence, correlations of edge turbulence with the SOL heat flux width, and the trigger mechanism of the L-H transition. A second fast camera is planned to view the GPI gas cloud from across the machine, which can potentially measure the field line pitch by simultaneously viewing individual field-aligned blob filaments in the radial vs. poloidal (GPI) and toroidal vs. poloidal (second camera) directions. An incoming collaboration from MIT will bring a 9x10 pixel APD-based detector array from Alcator C-Mod to NSTX-U, initially for faster and more sensitive imaging of the existing GPI gas puff. New results and further diagnostic plans will be described. [Preview Abstract] |
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NP10.00028: Characterization of intermittent divertor filaments in L-mode discharges in NSTX and NSTX-U F. Scotti, R.J. Maqueda, V.A. Soukhanovskii, S. Zweben Divertor filaments due to intermittent fluctuations are studied in L-mode discharges in NSTX and NSTX-U to understand transport due to edge blobs and their role in the divertor particle fluxes. In diverted Ohmic L-mode NSTX discharges, intermittent filaments on the divertor target were imaged via neutral lithium emission with frame rates up to 200 kHz and $\leq 1$ cm resolution. Broadband fluctuations up to 20-50\% in RMS/mean are observed between $\Psi_N \sim$ 1.02 and 1.3. Spiral-shaped divertor correlation regions are observed up to $\Psi_N \sim$ 1.02 and extend for over a toroidal turn. The spiral motion of the filaments at the target is consistent with a radial and poloidal downward motion upstream as previously observed in NSTX H-mode discharges. Divertor filaments are correlated with midplane blobs measured by the gas puff imaging diagnostic. The cross-correlation with midplane blobs is observed to peak at zero delay at every radius, with values up to 0.8 in the far SOL and decreasing to 0.4 at $\Psi_N \sim$ 1.05. In NSTX-U, a more sensitive camera with optimized throughput allowed divertor turbulence imaging using C III emission at up to f= 100 kHz, enabling the study of filament dynamics along the inner and outer divertor legs in NBI-heated L-mode discharges. [Preview Abstract] |
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NP10.00029: Numerical Study of the Impact of Resonant Magnetic Perturbations on Recycling Sources In Advanced Divertor Configurations of NSTX-U Ian Waters, Kurt Flesch, Heinke Frerichs, Oliver Schmitz, Joon-Wook Ahn, Gustavo Canal, Todd Evans, Vsevolod Soukhanovskii Explorations are under way to optimize the magnetic topology in the plasma edge of NSTX-U with the goal of improving neutral and impurity fueling and exhaust. Advanced divertor configurations combined with resonant magnetic perturbation (RMP) fields are being considered to improve peak heat and particle loads, stabilize edge instabilities, adjust plasma refueling, and control impurity transport. In this study, the EMC3-EIRENE fluid plasma and kinetic neutral transport code is used to investigate snowflake divertor configurations with and without RMP fields. Analysis of the edge recycling sources show that RMP fields induce a transition from a linear recycling regime into a high recycling regime at densities that are lower than in non-perturbed cases. This transition is also accompanied by a shift in the spatial distribution of these recycling sources and neutral atoms, and is impacted by the strength of the perturbations. An overview of results from different standard and snowflake divertor configurations will be presented. This work was funded by the Department of Energy under grant DE-SC0012315. [Preview Abstract] |
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NP10.00030: M3D-C$^{\mathrm{1}}$ simulations of the plasma response to $n=$ 3 magnetic perturbations applied to the NSTX-U snowflake divertor G.P. Canal, N.M. Ferraro, T.E. Evans, T.H. Osborne, J.E. Menard, J-W. Ahn, R. Maingi, A. Wingen, D. Ciro, H. Frerichs, O. Schmitz, V. Soukhanoviskii, I. Waters Single- and two-fluid resistive magnetohydrodynamic simulations, performed with the code M3D-C$^{\mathrm{1}}$, are used to investigate the effect of $n=$ 3 magnetic perturbations on the SF divertor configuration. The calculations are based on simulated NSTX-U plasmas and the results show that additional and longer magnetic lobes are created in the null-point region of the SF configuration, compared to those in the conventional single-null. The intersection of these additional and longer lobes with the divertor plates are expected to cause more striations in the particle and heat flux target profiles. In addition, the results indicate that the size of the magnetic lobes, in both single-null and SF configurations, are more sensitive to resonant than to non-resonant magnetic perturbations. The results also suggest that lower values of current in non-axisymmetric control coils close enough to the primary x-point would be required to suppress edge localized modes in plasmas with the SF configuration. [Preview Abstract] |
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NP10.00031: UEDGE modeling of snowflake divertors in NSTX-U O. Izacard, F. Scotti, V.A. Soukhanovskii, M.E. Rensink, T.D. Rognlien, M.V. Umansky New UEDGE code capabilities have been developed for simulations of snowflake (SF) divertors (SF-plus/SF-minus) with NSTX-U simulated equilibria. A robust grid generator for SF magnetic configurations including nonorthogonal plate geometries has been developed. UEDGE convergence is achieved on SF grids with a secondary X-point. Different SF-minus and SF-plus geometries are compared with each other permitting various physics studies such as investigating the effects of leg lengths or plate geometries using different impurity and transport models. The results include: (i) Leg lengths of a SF-minus affect the distribution of the parallel velocity, core performance (temperature) and detachment for a fixed fraction impurity model. (ii) Heat flux at the additional strike point in a SF-minus geometry is observed without enhancing the transport. (iii) Radiation front moves further away from divertor plates for shorter legs. Post-processing analyses are performed via a UEDGE module under OMFIT. [Preview Abstract] |
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NP10.00032: RF power deposition effects observed for the scrape off layer in NSTX/NSTX\textunderscore U and EAST and the accompanying RF effects on divertor Langmuir probes* J. Hosea, R.J. Perkins, M. Jaworski, N. Bertelli, G. Taylor, C. Qin, L. Wang, J. Yang, X.J. Zhang Strong RF power deposition effects in the divertor regions have been observed in NSTX for the HHFW regime [1] and in EAST for the minority ICRF regime [2]. On NSTX the RF power deposition in the scrape off layer (SOL) follows the magnetic field lines from in front of the antenna to an RF heat deposition spiral on the divertor regions. The strong SOL deposition and the spiral formation occur for edge densities above the cutoff density in front of the antenna. On EAST the RF heat deposition appears to be less intense as predicted with AORSA simulations. At coupled powers on EAST up to \textasciitilde 700 kW here, bands of deposition are observed on the lower divertor. RF deposition is also indicated on Langmuir probes on the lower outer divertors. For divertor probes in NSTX located to intercept field lines passing in the SOL away from the antenna, the floating potential is pushed negatively as expected for RF rectification. Similarly, on EAST the floating potential is pushed negatively for the field lines out in front of the antenna, but more positively for field lines that intercept the antenna/wall. To understand this latter behavior, probe IV characteristics will be investigated on NSTX-U to establish the electron energy distribution and space potential at a set of probes covering the entire SOL field strike point range. [1] R.J. Perkins et al., PoP \textbf{22} (2015) 042506. [2] J. Hosea et al., US-PRC MFCW (2016) poster 21. *This work is supported by USDOE Contract No. DE-AC02-09CH11466. [Preview Abstract] |
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NP10.00033: Possible phase coherence of annulus resonant modes in a cylindrical cold plasma: a perspective on SOL losses of fast-wave power on NSTX Rory Perkins, Joel Hosea, Nicola Bertelli, Gary Taylor, James Wilson Efficient high-harmonic fast-wave (HHFW) heating on the National Spherical Torus eXperiment (NSTX) would enable new experiments in turbulence, energetic particles, and impurity transport. However, scrape-off layer (SOL) losses of HHFW power can severely limit the heating efficiency down to \textasciitilde 40{\%}. The power is lost along scrape-off layer field lines, creating bright spirals of heat deposition on the divertor [1]. A cylindrical cold-plasma model finds modes, named ``annulus resonances,'' that conduct a large fraction of the wave power in the outer low-density region [2], making such modes a potential candidate to explain the SOL losses on NSTX. Here, we present result for full three-dimensional reconstructions of the wave fields. There is typically one such mode for each azimuthal mode number and a near linear relationship between azimuthal and axial wavenumbers, suggesting the existence of helices of constant phase. The potential role of these helices in relation to the field-aligned SOL losses will be discussed. This work was supported in part by DOE Contract No. DE-AC02-09CH11466. [1] J. C. Hosea et al., AIP Conf. Proc. 1187 (2009) 105. [2] R. J. Perkins et al., accepted by Phys. Plasmas. [Preview Abstract] |
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NP10.00034: Interaction between high harmonic fast waves and fast ions in NSTX/NSTX-U plasmas N. Bertelli, E.J. Valeo, M. Gorelenkova, D.L. Green Fast wave (FW) heating in the ion cyclotron range of frequency (ICRF) has been successfully used to sustain and control the fusion plasma performance, and it will likely play an important role in the ITER experiment. As demonstrated in the NSTX and DIII-D experiments the interactions between fast waves and fast ions can be so strong to significantly modify the fast ion population from neutral beam injection. In fact, it has been recently found in NSTX that FWs can modify and, under certain conditions, even suppress the energetic particle driven instabilities, such as toroidal Alfv\'en eigenmodes and global Alfv\'en eigenmodes and fishbones. This paper examines such interactions in NSTX/NSTX-U plasmas by using the recent extension of the RF full-wave code TORIC to include non-Maxwellian ions distribution functions. Particular attention is given to the evolution of the fast ions distribution function w/ and w/o RF. Tests on the RF kick-operator implemented in the Monte-Carlo particle code NUBEAM is also discussed in order to move towards a self consistent evaluation of the RF wave-field and the ion distribution functions in the TRANSP code. [Preview Abstract] |
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NP10.00035: Generation of Non-Inductive H-Mode Plasmas with 30 MHz Fast Wave Heating in NSTX-U G. Taylor, N. Bertelli, S. P. Gerhardt, J. C. Hosea, D. Mueller, R. J. Perkins, F. M. Poli, J. R. Wilson, R. Raman A Fusion Nuclear Science Facility based on a spherical tokamak must generate the plasma current (I$_{\mathrm{p}})$ with little or no central solenoid field. The NSTX-U non-inductive (NI) plasma research program is addressing this goal by developing NI start-up, ramp-up and sustainment scenarios separately. 4 MW of 30 MHz fast wave power is predicted to ramp I$_{\mathrm{p}}$ to 400 kA, a level sufficient to avoid significant shine-through of 90 keV ions from neutral beam injection. In 2010, experiments in NSTX demonstrated that 1.4 MW of 30 MHz high-harmonic fast wave (HHFW) power could generate an I$_{\mathrm{p}}=$ 300 kA H-mode discharge with a NI I$_{\mathrm{p}}$ fraction, f$_{\mathrm{NI}}$, around 0.7 at the maximum axial toroidal field (B$_{\mathrm{T}}$(0)) in NSTX of 0.55 T. NSTX-U is a major upgrade of NSTX that will eventually allow the generation of plasmas with B$_{\mathrm{T}}$(0) up to 1 T. Full wave simulations of 30 MHz HHFW heating in NSTX-U predict reduced FW power loss in the plasma edge as B$_{\mathrm{T}}$(0) is increased. HHFW experiments this year aim to couple 3 -- 4 MW of 30 MHz HHFW power into an I$_{\mathrm{p}}=$ 250 -- 350 kA plasma with B$_{\mathrm{T}}$(0) up to 0.75 T to generate a f$_{\mathrm{NI}}=$ 1 H-mode plasma. These experiments should benefit from the improved fast wave coupling predicted at higher B$_{\mathrm{T}}$(0) in NSTX-U. [Preview Abstract] |
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NP10.00036: Synthetic Aperture Microwave Imaging (SAMI) of the plasma edge on NSTX-U Roddy Vann, Gary Taylor, Jakob Brunner, Bob Ellis, David Thomas The Synthetic Aperture Microwave Imaging (SAMI) system is a unique phased-array microwave camera with a $\pm 40^{\circ}$field of view in both directions. It can image cut-off surfaces corresponding to frequencies in the range 10-34.5GHz; these surfaces are typically in the plasma edge. SAMI operates in two modes: either imaging thermal emission from the plasma (often modified by its interaction with the plasma edge e.g. via BXO mode conversion) or ``active probing'' i.e. injecting a broad beam at the plasma surface and imaging the reflected/back-scattered signal. SAMI was successfully pioneered on the Mega-Amp Spherical Tokamak (MAST) at Culham Centre for Fusion Energy [Shevchenko \textit{et al.}, \textit{JINST} \textbf{7} P10016 (2012); Thomas \textit{et al.}, \textit{Nucl. Fusion} \textbf{56} 026013 (2016)]. SAMI has now been installed and commissioned on the National Spherical Torus Experiment Upgrade (NSTX-U) at Princeton Plasma Physics Laboratory. The firmware has been upgraded to include real-time digital filtering, which enables continuous acquisition of the Doppler back-scattered active probing data. In this poster we shall present SAMI's analysis of the plasma edge on NSTX-U including measurements of the edge pitch angle on NSTX-U using SAMI's unique 2-D Doppler-backscattering capability. [Preview Abstract] |
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NP10.00037: Feasibility study of ECRH in NSTX-U startup plasma N. A. Lopez, F. Poli, G. Taylor, R. Harvey, Yu. Petrov A key mission goal of the National Spherical Torus eXperiment Upgrade (NSTX-U) is the demonstration of fully non-inductive startup and operation. In part to accomplish this, a 1MW, 28 GHz ECRH system is presently being developed for implementation on NSTX-U in 2018. Like most spherical tokamaks, NSTX-U operates in the overdense regime (f$_{\mathrm{pe\thinspace }}$\textgreater f$_{\mathrm{ce}})$, which limits traditional ECRH to the early startup phase. An extensive modelling effort of the propagation and absorption of EC waves in the evolving plasma is thus required to define the most effective window of operation, and to optimize the launcher geometry for maximal heating and for current drive during this window. In fact, the ECRH system will play an important role in preparing a target plasma for subsequent injection of IC waves and NBI. Here we assess the feasibility of O1-mode ECRH in NSTX-U startup plasma at full field of 1T through time-dependent simulations performed with the transport solver TRANSP. Linear ray-tracing calculations conducted by GENRAY are coupled into the TRANSP framework, allowing the plasma equilibrium and the temperature profiles to evolve self-consistently in response to the injected microwave power. Furthermore, we investigate additional possibilities of heating and current drive made available through coupling the injected O-mode power to the electrostatic EBW via the slow X-mode as an intermediary. [Preview Abstract] |
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NP10.00038: Modeling of surface temperature effects on mixed material migration in NSTX-U J.H. Nichols, M.A. Jaworski, K. Schmid NSTX-U will initially operate with graphite walls, periodically coated with thin lithium films to improve plasma performance. However, the spatial and temporal evolution of these films during and after plasma exposure is poorly understood. The WallDYN global mixed-material surface evolution model [K. Schmid et al., J. Nucl. Mater. 415, S284-S288 (2011)] has recently been applied to the NSTX-U geometry to simulate the evolution of poloidally inhomogenous mixed C/Li/O plasma-facing surfaces. The WallDYN model couples local erosion and deposition processes with plasma impurity transport in a non-iterative, self-consistent manner that maintains overall material balance. Temperature-dependent sputtering of lithium has been added to WallDYN, utilizing an adatom sputtering model developed from test stand experimental data. Additionally, a simplified temperature-dependent diffusion model has been added to WallDYN so as to capture the intercalation of lithium into a graphite bulk matrix. The sensitivity of global lithium migration patterns to changes in surface temperature magnitude and distribution will be examined. The effect of intra-discharge increases in surface temperature due to plasma heating, such as those observed during NSTX Liquid Lithium Divertor experiments, will also be examined. [Preview Abstract] |
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NP10.00039: Upstream Density for Plasma Detachment with Conventional and Lithium Vapor-Box Divertors RJ Goldston, JA Schwartz Fusion power plants are likely to require detachment of the divertor plasma from material targets. The lithium vapor box divertor is designed to achieve this, while limiting the flux of lithium vapor to the main plasma. We develop a simple model of near-detachment to evaluate the required upstream plasma density, for both conventional and lithium vapor-box divertors, based on particle and dynamic pressure balance between up- and down-stream, at near-detachment conditions. A remarkable general result is found, not just for lithium-induced detachment, that the upstream density divided by the Greenwald-limit density scales as $(P^{5/8}/B^{3/8})T_{det}^{1/2}/(\epsilon_{cool} +\gamma T_{det})$, with no explicit size scaling. $T_{det}$ is the temperature just before strong pressure loss, $\sim$ 1/2 of the ionization potential of the dominant recycling species, $\epsilon_{cool}$ is the average plasma energy lost per injected hydrogenic and impurity atom, and $\gamma$ is the sheath heat transmission factor. A recent 1-D calculation (A. Kallenbach et al. PPCF $\bf{58}$ (2016) 04501) agrees well with this scaling. The implication is that the plasma exhaust problem cannot be solved by increasing $R$. Instead significant innovation, such as the lithium vapor box divertor, will be required. [Preview Abstract] |
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NP10.00040: A Lithium Vapor Box similarity experiment employing water vapor JA Schwartz, C Jagoe, RJ Goldston, MA Jaworski Handling high power loads and heat flux in the divertor is a major challenge for fusion power plants. A detached plasma will likely be required. However, hydrogenic and impurity puffing experiments show that detached operation leads easily to X-point MARFEs, impure plasmas, degradation in confinement, and lower helium pressure at the exhaust. The concept of the Lithium Vapor Box Divertor is to use local evaporation and strong differential pumping through condensation to localize the gas-phase material that absorbs the plasma heat flux, and so avoid those difficulties. In order to design such a box first the vapor without plasma must be simulated. The density of vapor required can be estimated using the SOL power, major radius, poloidal box length, and cooling energy per lithium atom. For an NSTX-U-sized machine, the Knudsen number Kn spans $\sim$ 0.01 to 1, the transitional flow regime. This regime cannot handled by fluid codes or collisionless Monte Carlo codes, but can be handled by Direct Simulation Monte Carlo (DSMC) codes. To validate a DSMC model, we plan to build a vapor box test stand employing more-convenient water vapor instead of lithium vapor as the working fluid. Transport of vapor between the chambers at $\sim$-50C will be measured and compared to the model. [Preview Abstract] |
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NP10.00041: Analysis of surface chemistry of boronized TZM samples in NSTX-U between plasma exposures Hanna Schamis, Felipe Bedoya, Jean Paul Allain, Robert Kaita, Bruce Koel In the National Spherical Torus Experiment Upgrade (NSTX-U) a new plasma facing component diagnostic, the Material Analysis and Particle Probe (MAPP), was installed. MAPP has the capability of conducting XPS studies on materials without exposing them to atmospheric conditions. MAPP was used to conduct XPS studies of TZM (99{\%} Mo, 0.5{\%} Ti, 0.08{\%} Zr) samples. XPS gives information about the chemical composition of up to about 5 nm of the surface, and can be conducted on a day-to-day basis or at higher temporal resolutions e.g. close to in-between plasma shots. MAPP characterization gives insight on boron deposition and fuel retention by following the evolution of atomic concentrations and oxidation states. The data shows that the boron deposited layer was thicker than 5 nm. Additionally, the data shows evidence of sputtering of the boron layers following tens of plasma shots. The data also shows an increase in the oxygen concentration with plasma exposure. The next NSTX-U experimental campaign will feature TZM tiles in the lower divertor region, while the rest of the first wall will continue to be ATJ graphite. Our data provides the basis to analyze how the surface chemistry of the new set of tiles will be influenced by plasma operations, boron conditioning and carbon migration. [Preview Abstract] |
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NP10.00042: Improved measurement of {T}$_{\mathbf{e}}$ and {Z}$_{\mathrm{\mathbf{eff}}}$ on NSTX-U using Integrated Data Analysis L. M. Reusch, D. J. Den Hartog, A. Diallo We have begun to develop IDA to improve the precision and temporal resolution of electron temperature ($T_{e})$ profiles and increase the reliability of effective ionic charge ($Z_{\mathrm{eff}})$ measurements on NSTX-Upgrade. Experimental measurements of a physical system are always limited in scope, scale, and resolution. In addition, uncertainties are always present, and while statistical uncertainties can often be estimated, systematic uncertainties are usually more difficult to quantify. Integrated Data Analysis (IDA) provides methods to overcome these measurement limitations and maximize the value of experimental measurements. The goal of IDA is to combine data from heterogeneous and complementary diagnostics, considering all dependencies within and between diagnostics, in order to obtain the most reliable results in a transparent and standardized way. Bayesian probability theory provides a natural framework for this type of analysis, and will be applied to this project. Initially, work will concentrate on combining data from the multi-energy soft x-ray (ME-SXR) diagnostic with charge exchange recombination spectroscopy measurements to improve $Z_{\mathrm{eff}} $estimation. As appropriate and available, data from other diagnostics will be incorporated. The second goal of this project, to improve $T_{e\thinspace }$measurement, will combine ME-SXR and Thomson scattering data. This work is supported by the U.S. DOE. [Preview Abstract] |
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NP10.00043: Advanced Plasma Diagnostic Analysis using Neural Networks Kevin Tritz, Matt Reinke Machine learning techniques, specifically neural networks (NN), are used with sufficient internal complexity to develop an empirically weighted relationship between a set of filtered X-ray emission measurements and the electron temperature (T$_{\mathrm{e}})$ profile for a specific class of discharges on NSTX. The NN response matrix is used to calculate the T$_{\mathrm{e}}$ profile directly from the filtered X-ray diode measurements which extends the electron temperature time response from the 60Hz Thomson Scattering profile measurements to fast timescales (\textgreater 10kHz) and greatly expands the applicability of T$_{\mathrm{e}}$ profile information to fast plasma phenomena, such as ELM dynamics. This process can be improved by providing additional information which helps the neural network refine the relationship between T$_{\mathrm{e}}$ and the corresponding X-ray emission. NN supplement limited measurements of a particular quantity using related measurements with higher time or spatial resolution. For example, the radiated power (P$_{\mathrm{rad}})$ determined using resistive foil bolometers is related to similar measurements using AXUV diode arrays through a complex and slowly time-evolving quantum efficiency curve in the VUV spectral region. Results from a NN trained using Alcator C-Mod resistive foil bolometry and AXUV diodes are presented, working towards hybrid P$_{\mathrm{rad}}$ measurements with the quantitative accuracy of resistive foil bolometers and with the enhanced temporal and spatial resolution of the unfiltered AXUV diode arrays. [Preview Abstract] |
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NP10.00044: TRANSP: status and planning R Andre, J Carlsson, M Gorelenkova, S Jardin, S Kaye, F Poli, X Yuan TRANSP is an integrated interpretive and predictive transport analysis tool that incorporates state of the art heating/current drive sources and transport models. The treatments and transport solvers are becoming increasingly sophisticated and comprehensive. For instance, the ISOLVER component provides a free boundary equilibrium solution, while the PT- SOLVER transport solver is especially suited for stiff transport models such as TGLF. TRANSP incorporates high fidelity heating and current drive source models, such as NUBEAM for neutral beam injection, the beam tracing code TORBEAM for EC, TORIC for ICRF, the ray tracing TORAY and GENRAY for EC. The implementation of selected components makes efficient use of MPI for speed up of code calculations. Recently the GENRAY-CQL3D solver for modeling of LH heating and current drive has been implemented and currently being extended to multiple antennas, to allow modeling of EAST discharges. Also, GENRAY+CQL3D is being extended to the use of EC/EBW and of HHFW for NSTX-U. This poster will describe present uses of the code worldwide, as well as plans for upgrading the physics modules and code framework. [Preview Abstract] |
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NP10.00045: Progress on the FIReTIP Diagnostic on NSTX-U Evan Scott, Robert Barchfeld, Paul Riemenschneider, Chris Muscatello, Mohammad Sohrabi, Calvin Domier, Yang Ren, Robert Kaita, Neville Luhmann, Jr. The Far-infrared Tangential Interferometer/Polarimeter (FIReTIP) system on NSTX-U at the PPPL aims to provide robust, line-averaged electron density measurements. The system consists of three optically-pumped 119 \textmu m methanol lasers, one of which can be tuned via Stark broadening, allowing for uniquely high intermediate frequencies and time resolutions. One of the major goals of FIReTIP is to incorporate it into the NSTX-U plasma control system (PCS) for real-time plasma density feedback control. The front-end optics mounted to Bay G, which shape and position the beam going into the plasma, and internal retroreflector located near Bay B, which facilitates double-pass measurements, are hard-mounted to the NSTX-U vacuum vessel. Because interferometric density measurements are sensitive to vibrational effects, FIReTIP has been upgraded to a two-color interferometer system with the inclusion of a 633 nm laser interferometer for the direct measurement of vibrations and a field programmable gate array (FPGA) for the subsequent subtraction of vibrational effects from the density measurement in real-time. [Preview Abstract] |
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NP10.00046: Design of tangential multi-energy soft x-ray camera for NSTX-U Luis F. Delgado-Aparicio, J. Maddox, N. Pablant, K. Hill, M. Bitter, B. Stratton, Phillip Efthimion For tokamaks and future facilities to operate safely in a high-pressure long-pulse discharge, it is imperative to address key issues associated with impurity sources, core transport and high-Z impurity accumulation. Multi-energy SXR imaging provides a unique opportunity for measuring, simultaneously, a variety of important plasma properties ($T_{e}$, $n_{Z}$ and $\Delta Z_{eff}$). A new tangential multi-energy soft x-ray pin-hole camera is being design to sample the continuum- and line-emission from low-, medium- and high-Z impurities. This new x-ray diagnostic will be installed on an equatorial midplane port of NSTX-U tokamak and will measure the radial structure of the photon emissivity with a radial resolution below 1 cm at a 500 Hz frame rate and a photon-energy resolution of ~500 eV. The layout and response expected of the new system will be shown for different plasma conditions and impurity concentrations. The effect of toroidal rotation driving poloidal asymmetries in the core radiation is also addressed. This effort is designed to contribute to the near- and long-term highest priority research goals for NSTX-U which will integrate a non-inductive operation at reduced collisionality, long energy-confinement-times and a transition to a divertor solution with metal walls. [Preview Abstract] |
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NP10.00047: Development of Radiated Power Diagnostics for NSTX-U Matthew Reinke, G.G. van Eden, Jack Lovell, Byron Peterson, Travis Gray, Rian Chandra, Brent Stratton, Robert Ellis New tools to measure radiated power in NSTX-U are under development to support a range of core and boundary physics research. Multiple resistive bolometer pinhole cameras are being built and calibrated to support FY17 operations, all utilizing standard Au-foil sensors from IPT-Albrecht. The radiation in the lower divertor will be measured using two, 8 channel arrays viewing both vertically and radially to enable estimates of the 2D radiation structure. The core radiation will be measured using a 24 channel array viewing tangentially near the midplane, observing the full cross-section from the inner to outer limiter. This enables characterization of the centrifugally-driven in/out radiation asymmetry expected from mid-Z and high-Z impurities in highly rotating NSTX-U plasmas. All sensors utilize novel FPGA-based BOLO8BLF analyzers from D-tAcq Solutions. Resistive bolometer measurements are complemented by an InfraRed Video Bolometer (IRVB) which measures the temperature change of radiation absorber using an IR camera. A prototype IRVB system viewing the lower divertor was installed on NSTX-U for FY16 operations. Initial results from the plasma and benchtop testing are used to demonstrate the relative advantages between IRVB and resistive bolometers. [Preview Abstract] |
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NP10.00048: Thomson Scattering on NSTX-U Benoit LeBlanc, Ahmed Diallo The MPTS Thomson scattering diagnostic has been supporting NSTX-U since its start of operation in August 2015. At the time of this writing more than 1000 plasmas have been documented. While most MPTS elements from ``NSTX'' were reutilized, significant changes were necessitated: The laser-beam path was re-aimed in order to accommodate for the larger center-stack diameter of NSTX-U. The presence of a new neutral beam box required the introduction of mirror optics in order to dump the laser beams away from the measurement region. The FY2016 run marks the start of operation of a previously installed upgrade to 42 channels. Details of the new MPTS configuration will be given and experimental results will be presented. Future plans will also be discussed. This work was funded by DOE contract DE-AC02-09CH11466. [Preview Abstract] |
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NP10.00049: Status and Plans for Infrared Thermography and Heat flux Measurements on NSTX-U Travis Gray, Joon-Wook Ahn, Kaifu Gan, Alistair McGann, Matthew Reinke, Rajesh Maingi, Brian Wirth Improvements and expansion of IR thermography tools on NSTX-U are being pursued to support a range of boundary physics research. Due to a carbon-lithium mixed material environment and upcoming use of high-Z materials, NSTX-U presents a challenge in determining the deposited power flux to plasma facing components (PFCs). The majority of the PFCs are graphite which has a high surface emissivity but extensive use of lithium wall conditioning creates a mixed material divertor environment. Furthermore, a row of low emissivity/highly reflective molybdenum tiles will be installed in the outboard divertor for the next run campaign. To overcome these challenges as well assess overall power balance in NSTX-U, infrared coverage of the PFCs has been increased. The lower divertor outer strike point (OSP) is observed by a 1.6 kHz IR camera equipped with dual-band optics to account for the changes in surface emissivity introduced with the addition of lithium [AG MCLean, RSI 2012]. A wide-angle view of the lower divertor and a tangential view of the HHFW antenna and limiters has been added for the commencement of plasma operations on the NSTX-U. Measurements of the lower divertor, inner strike point (ISP) as well as the upper diverter OSP will be implemented for the FY17 run campaign. The installation of the molybdenum tiles will also include calorimeters to further constrain the heat flux measurements on those tiles with plans to increase calorimeter coverage. [Preview Abstract] |
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NP10.00050: Impacts of Shear Flow on the Low-n Kink Instabilities Jianguo Chen, Xueqiao Xu We report the progress on studies of the effects of shear flow on the edge instabilities using the reduced 3-field two fluid MHD model under the BOUT$++$ framework. Using the equilibrium profiles in JET-like Tokamak geometry with a circular cross section, the results of simulations demonstrate that: (1) the low-n peeling modes are mainly driven by the gradient of parallel current and the large pressure gradient leads to high-n ballooning modes; (2) in low density cases, the low-n kink modes are sensitive to the Er shear; (3) using the shear flow's profiles measured from DIII-D experiment, the intermediate-n modes(n$\sim$20) are triggered firstly and the peak of it shifts to low-n mode with narrower mode spectrum when increasing the shear flow in the linear simulation; (4) the nonlinear results show the enhanced nonlinear mode-mode interaction in saturate phase and are quantitatively consistent with the transition from coherent harmonic oscillation(EHO) to the broad band turbulence state discovered in DIII-D discharge with net-zero NBI torque\footnote{K. H. Burrell, et al., \textit{Phys. Plasmas} \textbf{23}, 056103 (2016).} and the QH-mode can be achieved by NBI in both co- and counter direction. It's significant for understanding the mechanism of EHO and QH-mode. [Preview Abstract] |
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NP10.00051: GENERAL SPHERICAL TORUS CONFIGURATION; GENERAL TOKAMAK CONFIGURATION |
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NP10.00052: Overview of Non-Solenoidal Startup Studies in the Pegasus ST M.W. Bongard, J.L. Barr, G.M. Bodner, M.G. Burke, R.J. Fonck, J.L. Pachicano, J.M. Perry, J.A. Reusch, N.J. Richner, C. Rodriguez Sanchez, D.J. Schlossberg Local helicity injection (LHI) is a non-solenoidal startup method pursued on Pegasus utilizing compact, high power current sources ($A_{inj} \sim 2-4$ cm$^{\mathrm{2}}$, $I_{inj} \sim 10$ kA, $V_{inj} \sim 1$ kV) at the plasma edge. Outboard injectors ($N_{inj} =4$, $A_{inj} =8$ cm$^{\mathrm{2}})$ produce $I_{p} \sim 170$ kA plasmas compatible with Ohmic drive. A 0-D model that treats the plasma as a resistive element with time-varying inductance and enforces $I_{p} $ limits from Taylor relaxation is used to interpret experimental $I_{p} (t)$ in several scenarios. Strong inductive drive arises from the plasma shape evolution, in addition to poloidal field induction. A new injector system has recently been installed in the lower divertor region ($N_{inj} =2$, $A_{inj} =8$ cm$^{\mathrm{2}})$ to explore the implications of geometric placement of the helicity injectors on LHI startup. This geometry supports tests of reconnection dynamics seen in NIMROD simulations, high-$B_{T} $ effects expected in larger devices, and LHI electron confinement with and without inductive assist. Plasmas with $I_{p} >130$ kA, $V_{inj} \sim 0.5$ kV, $\Delta t_{pulse} \sim 8$ ms and $B_{T} /B_{T,max} \le 50\% $ are produced with the inboard system to date, consistent with performance expectations. Higher $I_{p} $ is expected with increased $B_{T} $, $V_{inj} $, and $\Delta t_{pulse} $. Thomson scattering data in both geometries indicate high $T_{e} \ge 100$ eV during LHI, suggesting the confinement is not strongly stochastic. Conceptual design work is exploring the feasibility of coaxial helicity injection and ECH heating on Pegasus in addition to LHI. [Preview Abstract] |
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NP10.00053: Non-Solenoidal Tokamak Startup via Inboard Local Helicity Injection on the Pegasus ST J.M. Perry, J.L. Barr, G.M. Bodner, M.W. Bongard, R.J. Fonck, J.L. Pachicano, J.A. Reusch, C. Rodriguez Sanchez, N.J. Richner, D.J. Schlossberg Local helicity injection (LHI) is a non-solenoidal startup technique utilizing small injectors at the plasma edge to source current along helical magnetic field lines. Unstable injected current streams relax to a tokamak-like configuration with high toroidal current multiplication. Flexible placement of injectors permits tradeoffs between helicity injection rate, poloidal field induction, and magnetic geometry requirements for initial relaxation. Experiments using a new set of large-area injectors in the lower divertor explore the efficacy of high-field-side (HFS) injection. The increased area (4 cm$^{\mathrm{2}})$ current source is functional up to full Pegasus toroidal field ($B_{T,inj} =0.23$ T). However, relaxation to a tokamak state is increasingly frustrated for $B_{T,inj} >0.15$ T with uniform vacuum vertical field. Paths to relaxation at increased field include: manipulation of vacuum poloidal field geometry; increased injector current; and plasma initiation with outboard injectors, subsequently transitioning to divertor injector drive. During initial tests of HFS injectors, achieved $V_{inj} $ was limited to $\sim 600$ V by plasma-material interactions on the divertor plate, which may be mitigated by increasing injector elevation. In experiments with helicity injection as the dominant current drive $I_{p} \sim 0.13$ MA has been attained, with $\overline T_{e} >100$ eV and $\bar{{n}}_{e} \sim 10^{19}$ m$^{\mathrm{-3}}$. Extrapolation to full $B_{T} $, longer pulse length, and $V_{inj} \sim 1$ kV suggest $I_{p} >0.25$ MA should be attainable in a plasma dominated by helicity drive. [Preview Abstract] |
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NP10.00054: Thomson Scattering Measurements During Local Helicity Injection in the Pegasus Toroidal Experiment G.M. Bodner, M.W. Bongard, R.J. Fonck, J.A. Reusch, C. Rodriguez Sanchez, D.J. Schlossberg Local helicity injection (LHI) is a non-solenoidal startup technique
currently being developed at the Pegasus Toroidal Experiment. In LHI,
helicity is injected by compact, high-power current sources located in the
plasma scrape off layer that drive bulk plasma current through magnetic
reconnection. Investigations of the electron temperature and density
evolution in LHI plasmas are being pursued using the multi-point Thomson
scattering diagnostic on Pegasus. It has been expanded to provide a total of
24 spatial channels using a set of three high-throughput transmission
gratings and intensified CCD cameras. Measurements have been made in two
separate helicity injector configurations: a low-field-side (outboard
midplane) configuration; and a high-field-side (lower divertor)
configuration. Initial observations during injection showed $50 |
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NP10.00055: MHD and Reconnection Activity During Local Helicity Injection J.L. Barr, M.W. Bongard, M.G. Burke, R.J. Fonck, J.A. Reusch, N.J. Richner Scaling local helicity injection (LHI) to larger devices requires a validated, predictive model of its current drive mechanism. NIMROD simulations predict the injected helical current streams persist in the edge and periodically reconnect to form axisymmetric current rings that travel into the bulk plasma to grow $I_{p} $ and poloidal flux. In simulation, these events result in discrete bursts of Alfv\'{e}nic-frequency MHD activity and jumps in $I_{p} $ of order $\Delta I_{p} \sim I_{inj} $, in qualitative agreement with large $n=1$ activity found in experiment. Fast imaging prior to tokamak formation supports the instability of, and apparent reconnection between, adjacent helical streams. The bursts exhibit toroidal amplitude asymmetries consistent with a kink structure singly line-tied to the injectors. Internal measurements localize this activity to the injector radial location. Pairwise correlations of poloidal Mirnov coil amplitude and phase match expectations of an edge-localized current stream carrying $I_{inj} $. Prior to tokamak formation, reconnection from both adjacent helical windings and co-injected current streams are shown to strongly heat impurity ions. After tokamak formation, strong anomalous ion heating in the plasma edge is attributed to continuous reconnection between colinear streams. The $n=1$ bursts occur less frequently as $I_{p} $ rises, likely caused by increased stream stability as $B_{v} $ rises and $q_{edge} $ drops. This evidence supports the general NIMROD model of LHI, confirms the persistence and role of the edge current streams, and motivates experiments at higher $I_{inj} $ and $B_{T} $. [Preview Abstract] |
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NP10.00056: H-mode and ELM Characteristics at Ultralow Aspect Ratio in the Pegasus Experiment R.J. Fonck, J.L. Barr, M.W. Bongard, D.M. Kriete, J.M. Perry, J.A. Reusch, K.E. Thome Operation at low $B_{T} $ and $A<1.3$ allows access to the H-mode regime in
the Pegasus experiment using only Ohmic heating.\footnote{ K.E. Thome \textit{et al.},
\textit{Phys. Rev. Lett.} \textbf{116}, 175001 (2016).} Modest plasma parameters in this regime
permit detailed probe measurements of the edge pedestal region. H-mode
plasmas have standard L-H transition phenomena: a drop in $D_{\alpha } $
radiation; formation of pressure and current pedestals; field-aligned
filament ejection during ELMs; and a doubling of $\tau_{E} $ from $H_{98}
\sim 0.5$ to $\sim 1$. The L-H power threshold $P_{LH} $ increases
monotonically with $n_{e} $, consistent with both the ITPA08 scaling,
$P_{ITPA08}_{\mathrm{,}}$ used for ITER and the theoretical
FM$^{\mathrm{3}}$ power threshold model. Unlike at high $A$, $P_{LH} $ is
comparable in limited and single-null diverted topologies at $A\sim 1.2$,
consistent with FM$^{\mathrm{3}}$ predictions. $P_{LH} /P_{ITPA08} $
increases rapidly as $A\to 1$, and is $>10$ for $A<1.3$. Multiple-$n$ modes
are observed during ELMs, consistent with excitation of multiple
peeling-ballooning modes. Small, Type-III-like ELMs occur at $P_{OH} \sim
P_{LH} $ with $n\le 4$. Large, Type-I-like ELMs occur with $P_{OH} >P_{LH} $
and intermediate $5 |
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NP10.00057: Development of a Technique for Measuring Local Electric Field Fluctuations in High Temperature Plasmas M.G. Burke, M.R. Bakken, R.J. Fonck, B.T. Lewicki, A.T. Rhodes, G.R. Winz A novel diagnostic for measuring local electric field fluctuations in high temperature plasmas is being developed. It employs high-speed measurements of the spectral separation and/or line intensities of the motional Stark effect (MSE) $H_{\alpha } $ multiplet emitted from a low divergence, 80 keV diagnostic neutral beam. A spatial heterodyne spectrometer (SHS) coupled to a 500 kHz CMOS camera provides the high resolution ($\approx $0.025 nm) and throughput ($\le $0.1 cm$^{\mathrm{2}}$str) required for the measurement. The Fizeau fringe pattern produced by the SHS provides the Fourier transform of the input spectrum. Line broadening due to the large collection lens at the tokamak can be compensated by phase correcting the resulting fringe pattern. Based on simple tokamak turbulence scalings, $\tilde{{E}}/E_{MSE} \approx 10^{-3}$ is expected for the core plasma in present experiments. To observe these low fluctuation levels, cross-correlation between adjacent spatial points and/or simultaneously measured $\tilde{{n}}$ will be employed to suppress photon noise that is comparable to the turbulent signal. The SHS Littrow wavenumber and grating constant can be chosen to reduce the number of detectors needed to resolve changes in the input spectrum. This allows multi-spatial point measurements using 4--6 discrete photodiodes each, with no loss in sensitivity to $\tilde{{E}}/E_{MSE} $. To validate this diagnostic concept, the diagnostic neutral beam will be fired into a magnetized target plasma (B$\le $0.5 T) comparable to a tokamak edge, with $\tilde{{E}}$ applied parallel or perpendicular to $E_{MSE} $ via biased electrodes. [Preview Abstract] |
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NP10.00058: Neutral Beam Source and Target Plasma for Development of a Local Electric Field Fluctuation Diagnostic M.R. Bakken, M.G. Burke, R.J. Fonck, B.T. Lewicki, A.T. Rhodes, G.R. Winz A new diagnostic measuring local $\vec{{E}}(r,t)$ fluctuations is being developed for plasma turbulence studies in tokamaks. This is accomplished by measuring fluctuations in the separation of the $\pi $ components in the $H_{\alpha } $ motional Stark spectrum. Fluctuations in this separation are expected to be ${\tilde{{E}}} \mathord{\left/ {\vphantom {{\tilde{{E}}} {E_{MSE} \sim 10^{-3}}}} \right. \kern-\nulldelimiterspace} {E_{MSE} \sim 10^{-3}}$. In addition to a high throughput, high speed spectrometer, the project requires a low divergence ($\Omega \approx 0.5^{\circ})$, 80 keV, 2.5 A $H^{0}$ beam and a target plasma test stand. The beam employs a washer-stack arc ion source to achieve a high species fraction at full energy. Laboratory tests of the ion source demonstrate repeatable plasmas with $T_{e} \sim 10$ eV and $n_{e} \approx 1.6\times 10^{17}$ m$^{\mathrm{-3}}$, sufficient for the beam ion optics requirements. $T_{e} $ and $n_{e} $ scalings of the ion source plasma are presented with respect to operational parameters. A novel three-phase resonant converter power supply will provide 6 mA/cm$^{\mathrm{2}}$ of 80 keV $H^{0}$ at the focal plane for pulse lengths up to 15 ms, with low ripple $\delta V/80$ keV $\approx 0.05\% $ at 280 kHz. Diagnostic development and validation tests will be performed on a magnetized plasma test stand with $\sim 0.5$ T field. The test chamber will utilize a washer-stack arc source to produce a target plasma comparable to edge tokamak plasmas. A bias-plate with programmable power supply will be used to impose $\tilde{{E}}$ within the target plasma. [Preview Abstract] |
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NP10.00059: Overview of the Lithium Tokamak eXperiment-Beta* R. Kaita, D.P. Boyle, T. Kozub, M. Lucia, R. Majeski, E. Merino, F. Scotti, B. E. Koel, S. Kubota, T. Rhodes, T. Biewer, M. Reinke, D. Donovan The Lithium Tokamak Experiment-Beta (LTX-Beta) seeks to extend the plasma performance of LTX with higher plasma current and toroidal magnetic field. A new neutral beam injection (NBI) system will provide significant auxiliary heating and core fueling. The NBI will enable measurements of the core ion temperature and lithium concentration using the ORNL charge-exchange spectroscopy (CHERS) system. Toroidal rotation measurements and studies of toroidal momentum transport will also be possible with the CHERS diagnostic. Upgrades to the UCLA reflectometer and interferometer systems will be implemented for core and edge electron density fluctuations. A new capability for transferring samples in vacuo after plasma exposure to an offline surface science laboratory is also planned for analysis with higher resolution instrumentation. This presentation will describe progress on these facilities, and discuss how they enable LTX-beta to build on past LTX results in the further exploration of low recycling, high confinement regimes. [Preview Abstract] |
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NP10.00060: Measurements of impurity concentrations and transport in the Lithium Tokamak Experiment D.P. Boyle, R.E. Bell, R. Kaita, M. Lucia, J.C. Schmitt, F. Scotti, S. Kubota, C. Hansen, T.M. Biewer, T.K. Gray The Lithium Tokamak Experiment (LTX) is a modest-sized spherical tokamak with all-metal plasma facing components (PFCs), uniquely capable of operating with large area solid and/or liquid lithium coatings essentially surrounding the entire plasma. This work presents measurements of core plasma impurity concentrations and transport in LTX. In discharges with solid Li coatings, volume averaged impurity concentrations were low but non-negligible, with $\sim2-4\%$ Li, $\sim0.6-2\%$ C, $\sim0.4-0.7\%$ O, and $Z_{eff} < 1.2$. Transport was assessed using the TRANSP, NCLASS, and MIST codes. Collisions with the main H ions dominated the neoclassical impurity transport, and neoclassical transport coefficients calculated with NCLASS were similar across all impurity species and differed no more than a factor of two. However, time-independent simulations with MIST indicated that neoclassical theory did not fully capture the impurity transport and anomalous transport likely played a significant role in determining impurity profiles. Progress on additional analysis, including time-dependent impurity transport simulations and impurity measurements with liquid lithium coatings, and plans for diagnostic upgrades and future experiments in LTX-$\beta$ will also be presented. [Preview Abstract] |
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NP10.00061: Microwave and Millimeter-Wave Diagnostics Upgrades for LTX-$\beta$ S. Kubota, R. Majeski, X.V. Nguyen, W.A. Peebles, T.L. Rhodes, R. Kaita Measurements of fluctuations and their relation to transport are of key interest in the LTX-$\beta$ device, which will feature higher $B_T$ and $I_P$, and neutral beam heating. Improvements are underway for the microwave and millimeter-wave diagnostics on LTX-$\beta$, with the goal of significantly enhancing the capabilities for fluctuation measurements. Currently, a 296 GHz single-chord interferometer provides radial line density measurements, while an FM-CW (frequency-modulated continuous-wave) reflectometer (13.5$-$33 GHz) provides fast density profile measurements. Additional hardware and data acquisition for these systems will provide both higher bandwidths and better noise rejection. Two new variable-frequency reflectometer channels with frequency ranges of 13$-$20 GHz and 27$-$40 GHz will provide quadrature measurements of edge and core electron density fluctuations. New data analysis techniques include using the FM-CW system as a correlation reflectometer for low-$k$ fluctuations near the cutoff layer, as well as a radial backscattering ($k_r$$\leq$16 cm$^{-1}$) system for fluctuations far from the cutoff, while the interferometer can function as a far-forward scattering system. [Preview Abstract] |
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NP10.00062: Progress on the Implementation of a Neutral Beam for the Lithium Tokamak eXperiment-Beta Enrique Merino, Thomas Kozub, Dennis Boyle, Richard Majeski, Robert Kaita, Artem Smirnov, Ryan Catalano In the Lithium Tokamak eXperiment (LTX), good performance discharges have been achieved with reduced-recycling lithium walls. Two hydrogen neutral beams (NB) have been loaned to the LTX project by Tri-Alpha Energy, Inc. To further improve plasma parameters, one of these neutral beams is being installed as part of an upgrade to LTX (LTX-Beta). Current ohmic input power in LTX is less than 100 kW. The NB will provide core plasma fueling with up to 700 kW of injected power. Requirements for accommodating the NB include the addition of injection and beam-dump ports on the vessel, and their designs have been finalized. Progress has also been made on the NB power supplies, including the preparation of a new room to accommodate them. A description of these activities and the status of other improvements to LTX for LTX-Beta will be presented. [Preview Abstract] |
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NP10.00063: Magnetism of toroidal field in two-fluid equilibrium of CHI driven spherical torus T. Kanki, M. Nagata Double-pulsing CHI (D-CHI) experiment has been conducted in the HIST device to achieve a quasi-steady sustainment and good confinement of spherical torus (ST) plasmas. The feature of CHI driven ST such as diamagnetic toroidal field in the central open flux column (OFC) region and strong poloidal flow shear around the separatrix in the high field side suggests the two-fluid effect. The relationship between the magnetism of the toroidal field and the poloidal flow velocity is investigated by modelling the D-CHI (mainly driving the poloidal electron flow along the open flux) in the two-fluid equilibrium calculations. The poloidal component of Ampere's law leads that the toroidal field is related to the difference between the stream functions of ion $\bar{{\psi }}_{i} $ and electron $\bar{{\psi }}_{e} $for the poloidal flow, indicating that the toroidal field with $\bar{{\psi }}_{e} >\bar{{\psi }}_{i} $ results in a diamagnetic profile, while that with $\bar{{\psi }}_{e} <\bar{{\psi }}_{i} $ results in a paramagnetic one. The gradient of the stream function determines the polarity and the strength of the poloidal flow velocity. It is found that the two-fluid equilibrium of CHI driven ST satisfies $\bar{{\psi }}_{e} >0$ and $\bar{{\psi }}_{i} <0$ in the OFC region, and $\bar{{\psi }}_{e} <0$ and $\bar{{\psi }}_{i} <0$ in the closed flux region. The toroidal field is a diamagnetic profile in the OFC region due to $\bar{{\psi }}_{e} >\bar{{\psi }}_{i} $ and $\left| {u_{ez} } \right|>\left| {u_{iz} } \right|$, where $u_{ez} $ and $u_{iz} $ denote the poloidal electron and ion flow velocities, respectively. It becomes from a diamagnetic to a paramagnetic profile in the closed flux region, because $\bar{{\psi }}_{e} (u_{ez} )$ approaches $\bar{{\psi }}_{i} (u_{iz} )$ around the magnetic axis. The poloidal ion flow shear is enhanced in the OFC region due to the ion inertial effect through the toroidal ion flow velocity. [Preview Abstract] |
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NP10.00064: Plasma current ramp-up by lower hybrid wave using innovative antennas on TST-2 Yuichi Takase, Akira Ejiri, Charles Moeller, Benedikt Roidl, Takahiro Shinya, Naoto Tsujii, Satoru Yajima, Hibiki Yamazaki, Akichika Kitayama, Naoki Matsumoto, Akito Sato, Masateru Sonehara, Wataru Takahashi, Yoshiyuki Tajiri, Yuki Takei, Hiro Togashi, Kazuya Toida, Yusuke Yoshida Non-inductive plasma current ($I_{\mathrm{p}})$ ramp-up by RF power in the lower hybrid frequency range is being studied on the TST-2 spherical tokamak ($R \quad =$ 0.36 m, $a \quad =$ 0.23 m, $B_{\mathrm{t}} \quad =$ 0.3 T, $I_{\mathrm{p}} \quad =$ 0.1 MA). Up to 400 kW of RF power is available at a frequency of 200 MHz. An innovative antenna called the capacitively-coupled combline (CCC) antenna was developed to excite a sharp, highly directional traveling wave with the electric field polarized in the toroidal direction. It is an array of resonant circuit elements made of capacitance and inductance, coupled to neighboring elements by mutual capacitance. Two CCC antennas are installed in TST-2, a 13-element outboard-launch antenna and a 6-element top-launch antenna. The latter was installed in March 2016 to improve accessibility to the core and to achieve single-pass damping. The suspected wave power loss in the scrape-off layer plasma should also be avoided. $I_{\mathrm{p}}$ ramp-up to 25 kA has been achieved so far. An upgrade of the $B_{\mathrm{t}}$ power supply is planned to take advantage of the observed improvement of $I_{\mathrm{p}}$ ramp-up with $B_{\mathrm{t}}$. Higher $B_{\mathrm{t}}$ for longer pulses should improve the $I_{\mathrm{p}}$ ramp-up efficiency by improving wave accessibility and by reducing prompt orbit losses of energetic electrons. [Preview Abstract] |
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NP10.00065: Shaping of the plasma column in a small aspect ratio tokamak Julio Herrera, Ismael Arroyo, Esteban Chavez, Miguel Angel Segura This is a follow-up to the work presented in a precious meeting, on the conceptual design of a small aspect ratio tokamak of variable configuration. The base parameters for this device would be similar to those in the START tokamak. The shaping of the plasma column is known to have important effects in the plasma performance, including the value of $\beta $, bootstrap currents, and intrinsic rotation. The main feature being explored here is the inclusion of independent control coils in the inboard and outboard sides; six in the first case, and up to seven in the latter. By varying the strength in their currents it is possible to achieve a wide variety of shapes: elliptical, conventional D-shape, inverse D-shape, and Bean-shape. As the control coils are activated, the strength of the toroidal magnetic field needs to he weakened, in order to keep reasonable values of the safety factor $q.$The study presented here is made by means of the 3D-MAPTOR code, which produces the Poincar\'{e} maps of the magnetic field lines, given the currents. For this purpose, a seed plasma current must be provided. All studies presented here assume equatorial symmetry, due to limitations in the code. [Preview Abstract] |
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NP10.00066: The Aneutronic Rodless Ultra Low Aspect Ratio Tokamak Celso Ribeiro The replacement of the metal centre-post in spherical tokamaks (STs) by a plasma centre-post (PCP, the TF current carrier) is the ideal scenario for a ST reactor. A simple rodless ultra low aspect-ratio tokamak (RULART) using a screw-pinch PCP ECR-assisted with an external solenoid has been proposed in the most compact RULART [Ribeiro C, SOFE-15]. There the solenoid provided the stabilizing field for the PCP and the toroidal electrical field for the tokamak start-up, which will stabilize further the PCP, acting as stabilizing closed conducting surface. Relative low TF will be required. The compactness (high ratio of plasma-spherical vessel volume) may provide passive stabilization and easier access to L-H mode transition. It is presented here: 1) stability analysis of the PCP (initially MHD stable due to the hollow $J$ profile); 2) tokamak equilibrium simulations, and 3) potential use for aneutronic reactions studies via pairs of proton $p$ and boron \textit{11B} ion beams in He plasmas. The beams' line-of-sights sufficiently miss the sources of each other, thus allowing a near maximum relative velocities and reactivity. The reactions should occur close to the PCP mid-plane. Some born alphas should cross the PCP and be dragged by the ion flow (higher momentum exchange) towards the anode but escape directly to a direct electricity converter. Others will reach evenly the vessel directly or via thermal diffusion (favourable heating by the large excursion \textasciitilde 2a), leading to the lowest power wall load possible. This might be a potential hybrid direct-steam cycle conversion reactor scheme, nearly aneutronic, and with no ash or particle retention problems, as opposed to the D-T thermal reaction proposals. [Preview Abstract] |
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NP10.00067: HBT-EP Program: MHD Dynamics and Active Control through 3D Fields and Currents G.A. Navratil, M.C. Abler, J. Bialek, J.W. Brooks, P.J. Byrne, S. DeSanto, P.E. Hughes, J.P. Levesque, M.E. Mauel, D.J. Rhodes, C.J. Hansen The HBT-EP active mode control research program aims to: (i) advance understanding of the effects of 3D shaping on advanced tokamak fusion performance, (ii) resolve important MHD issues associated with disruptions, and (iii) measure and mitigate the effects of 3D scrape-off layer (SOL) currents through active and passive control of the plasma edge and conducting boundary structures. A GPU-based low latency control system uses 96 inputs and 64 outputs to control the plasma boundary. An in-vessel adjustable ferritic wall is used to study ferritic RWMs with increased growth rates, RMP response, and disruptivity. A quasi-linear sharp-boundary model is developed to study effects of toroidal curvature and plasma shaping on beta limits with resistive plasmas and walls. Measurement of currents between vessel sections reveals currents running from the plasma to the wall during wall-touching kink modes and disruptions. Asymmetries in plasma current are observed using segmented Rogowski coils. Biased electrodes in the plasma are used to control rotation of external kinks and drive currents in the SOL. An extensive array of SOL current monitors and edge drive electrodes will be installed for pioneering studies of helical edge current control. [Preview Abstract] |
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NP10.00068: Ferritic wall studies on HBT-EP Paul E. Hughes, J.P. Levesque, M.E. Mauel, G.A. Navratil Low-activation ferritic steels are leading material candidates for use in next-generation fusion development experiments such as ITER and DEMO. Understanding the interaction of plasmas with a ferromagnetic wall will provide crucial physics for these experiments. Although the ferritic wall mode (FWM) was not observed in JFT-2M [1], it has been been studied in HBT-EP [2], while the effects of related error fields have been studied on DIII-D [3]. HBT-EP operates with a high-permeability tiled ferritic first wall, characterizing its MHD effects using high-resolution magnetic diagnostics. We report on our study of FWM dynamics comparing stainless and ferritic wall configurations, including increases in plasma response to RMPs, plasma disruptivity, and natural mode growth [2,4]; new results include differences in scrape-off layer (SOL) current dynamics and mode rotation frequency dependence of the FWM growth rate [4]. Additionally, we present the effects of toroidally asymmetric distribution of ferromagnetic material on mode rotation.\\ [1] K. Tsuzuki, et. al. Nucl Fus 46 (2006)\\ [2] J.P. Levesque, et al. Phys. Plasmas 22 (2015)\\ [3] M.J. Schaffer, et al. Nucl Fus 51 (2011)\\ [4] P.E. Hughes, Ph.D. Thesis (2016) [Preview Abstract] |
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NP10.00069: Scrape-off layer current measurements on HBT-EP M.C. Abler, J.P. Levesque, J.W. Brooks, J. Bialek, M.E. Mauel, G.A. Navratil Scrape-off layer currents flowing between the plasma and the wall are a topic of interest in the fusion community due to possible ramifications for the operation of the ITER tokamak and future magnetic confinement devices. HBT-EP, with four toroidally-distributed poloidal quartz breaks in the vacuum vessel, is uniquely positioned to study scrape-off layer currents as they flow toroidally between chamber sections. We study these currents during natural kink mode activity, applied RMPs, and disruptions using a set of copper straps between otherwise-isolated chamber segments and two toroidally-opposite segmented Rogowski coils. Both diagnostic sets detect currents which evolve dynamically with MHD activity, show an order of magnitude increase in vacuum vessel currents during disruptions, and demonstrate current flow between the vacuum vessel and the plasma scrape-off layer. We also describe new in-vessel diagnostics for high-resolution scrape-off layer current measurement and discuss planned experiments as part of our ongoing upgrade. [Preview Abstract] |
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NP10.00070: Extreme ultraviolet diagnostic upgrades for kink mode control on the HBT-EP tokamak J.P. Levesque, J.W. Brooks, S. DeSanto, M.E. Mauel, G.A. Navratil, J.W. Page, C.J. Hansen, L. Delgado-Aparicio Optical diagnostics can provide non-invasive measurements of tokamak equilibria and the internal characteristics of MHD mode activity. We present research plans and ongoing progress on upgrading extreme ultraviolet (EUV) diagnostics in the HBT-EP tokamak. Four sets of 16 poloidal views will allow tomographic reconstruction of plasma emissivity and internal kink mode structure. Emission characteristics of naturally-occurring \emph{m/n} = 2/1, 3/2, and 3/1 tearing and kink modes will be compared with expectations from a synthetic diagnostic. Coupling between internal and external modes leading up to disruptions is studied. The internal plasma response to external magnetic perturbations is investigated, and compared with magnetic response measurements. Correlation between internal emissivity and external magnetic measurements provides a global picture of long-wavelength MHD instabilities. Measurements are input to HBT-EP's GPU-based feedback system, allowing active feedback for kink modes using only optical sensors and both magnetic and edge current actuators. A separate two-color, 16-chord tangential system will be installed next year to allow reconstruction of temperature profiles and their fluctuations versus time. [Preview Abstract] |
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NP10.00071: Control of scrape-off layer currents in HBT-EP using biased electrodes J.W. Brooks, M.C. Abler, J. Bialek, J.P. Levesque, M.E. Mauel, G.A. Navratil Scrape-off layer (SOL) currents and their paths through tokamaks are not well understood, but their control may prove crucial to the success of ITER and future fusion energy devices. We extend Columbia University High Beta Tokamak-Extended Pulse's (HBT-EP) diagnostics and feedback system to study the SOL and control MHD instabilities - typically around 7 kHz - by actively controlling these currents. We conduct these experiments in two phases: first with a single probe, then with multiple, independent probes. With a single probe, we have shown that active feedback alters the rotation and magnitude of slowly growing kink instabilities. Present work focuses on multiple probes in both a net-zero current configuration and independent configurations with an active GPU feedback system to control the magnitude and phase of MHD instabilities. In addition to the biased electrodes, two triple-probes are placed in the SOL to help us better-understand SOL density and temperature. This research also provides insight into the next phase of research, a multi-element SOL control upgrade of HBT-EP currently underway. [Preview Abstract] |
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NP10.00072: Shaping Effects on Resistive-Plasma Resistive-Wall Mode Stability in a Tokamak Dov Rhodes, A.J. Cole, G.A. Navratil, J.P. Levesque, M.E. Mauel, D.P. Brennan, J.M. Finn, R. Fitzpatrick A sharp-boundary MHD model is used to explore the effects of toroidal curvature and cross-sectional shaping on resistive-plasma resistive-wall modes in a tokamak. Building on the work of Fitzpatrick [1], we investigate mode stability with fixed toroidal number n$=$1 and a broad spectrum of poloidal m-numbers, given varying aspect-ratio, elongation, triangularity and up-down asymmetry. The speed and versatility of the sharp-boundary model facilitate exploration of a large parameter space, revealing qualitative trends to be further investigated by larger codes. In addition, the study addresses the effect of geometric mode-coupling on higher beta stability limits associated with an ideal-plasma or ideal-wall. These beta limits were used by Brennan and Finn [2] to identify plasma response domains for feedback control. Present results show how geometric mode-coupling affects the stability limits and plasma response domains. The results are explained by an analytic reduced-MHD model with two coupled modes having different m-numbers. The next phase of this work will explore feedback control in different tokamak geometries. Refs: [1] R. Fitzpatrick, Phys. Plasmas 17, 112502 (2010). [2] D. P. Brennan and J. M. Finn, Phys. Plasmas 21, 102507 (2014). [Preview Abstract] |
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NP10.00073: MHD Spectroscopy of External Kink Modes in Diverted Plasmas in HBT-EP P.J. Byrne, J. Bialek, P.E. Hughes, J.P. Levesque, M.E. Mauel, G.A. Navratil, Q. Peng, C.C. Stoafer We report on studies using high-resolution sets of magnetic coils for passive detection and internal saddle coils for active excitation of external kinks in non-circular plasmas. HBT-EP has a zero-net-turns poloidal field coil for local shaping near the inboard midplane, up to and including diverted operation. The shape, amplitude, and mode rotation frequency of the dominant, marginally stable kink mode in shaped plasmas is compared to that of circular plasmas. There also exist stable, low-m, low-n kink modes that in diverted plasmas have high amplitude near the X-point and low amplitude elsewhere, with relevance for magnetic boundary shape in the divertor region of a tokamak. These modes are excited on a feed-forward basis by our control coils, and the effect on mode response of varying RMP amplitude and helicity is studied. VALEN is used to project DCON calculations of the mode structure at the surface of the plasma to both our sensors and control coils, to clarify observations and improve control coupling. [Preview Abstract] |
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NP10.00074: LPI AND SHORT PULSE INTERACTIONS |
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NP10.00075: Controlling stimulated Raman scattering by multi-color light in inertial confinement fusions Zhanjun Liu, Chunyang Zheng A method is put forward to control the stimulated Raman scattering in inertial confinement fusions. Using different frequency lights combined with $\mbox{3}\omega $ light can control the Raman scattering of $\mbox{3}\omega $ light. Numerical simulation results validate this method. The Raman scattering of $\mbox{3}\omega $light can be prevented to develop by using another $\mbox{2}\omega $ light, which ensures the $\mbox{3}\omega $ light depositing energy to the desired place. The Raman or Brillouin scattering of $\mbox{3}\omega $light can modify the electron density. And the inverse bremsstrahlung absorption of $\mbox{2}\omega $ light increase the electron temperature and then decrease the density in the laser path due to the pressure equilibrium. The increased inhomogenous of plasma density and electrons temperature and low density can decrease the scattering level of $\mbox{3}\omega $ light. [Preview Abstract] |
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NP10.00076: Reducing parametric backscattering by polarization rotation Ido Barth, Nathaniel Fisch When a laser passes through underdense plasmas, Raman and Brillouin Backscattering can reflect a substantial portion of the incident laser energy. This is a major loss mechanism, for example, in inertial confinement fusion. However, by slow rotation of the incident linear polarization, the overall reflectivity can be reduced significantly. Particle in cell simulations show that, for parameters similar to those of indirect drive fusion experiments, polarization rotation reduces the reflectivity by a factor of 5. A general, fluid-model based, analytical estimation for the reflectivity reduction agrees with simulations. [Preview Abstract] |
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NP10.00077: Development of a New Fluid Code to Study Laser-Plasma Instabilities Liang Hao, Rui Yan, Jun Li, Chuang Ren We have developed a new fluid code to study laser-plasma instabilities. Its physics model is based on the plasma two-fluid model combined with the vector potential equations of light without any envelope approximations. Incident lasers are launched from an antenna and the perfect-matched layers (PML's) technique is implemented in our boundary conditions. Landau damping of Langmiur waves and ion-acoustic waves are included by solving the damping terms in phase space based on the time-split method. Currently, a one-dimensional version of the code is complete and has been tested in study of the laser-plasma instabilities for shock ignition. [Preview Abstract] |
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NP10.00078: Density-Modulation-Induced Absolute Laser-Plasma-Instabilities in Inertial Confinement Fusion Jun Li, Rui Yan, Chuang Ren Fluid simulations show that when a static sinusoidal density modulation is superimposed on a linear density profile, convective instabilities can become absolutely unstable. This conversion can occur for two-plasmon-decay and stimulated Raman scattering instabilities under realistic direct-drive inertial confinement fusion conditions and can affect hot-electron generation and laser-energy deposition. Analysis of the three-wave model shows that a sufficiently large change of the density gradient in a linear density profile can turn convective instabilities into absolute ones. An analytical expression is given for the threshold of the gradient change, which depends only on the convective gain. This work was supported by DOE under Grant No. DE-SC0012316; by NSF under Grant No. PHY-1314734; and by Laboratory for Laser Energetics. The research used resources of the National Energy Research Scientific Computing Center. [Preview Abstract] |
(Author Not Attending)
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NP10.00079: Observation of filamentation instability of 1-D laser plasma Solitons Deepa Verma, Ratan Kumar Bera, Amita Das The 1-D solitary wave solutions for laser plasma system have been studied extensively. This includes complete characterization of the exact solutions (both stationary and propagating) in the parameter space of laser frequency and the group speed [1]. The dynamical evolution of these structures in 1-D has also been studied which identifies the solutions with single peak (of light wave field ) as stable solutions whereas those having multiple peaks as unstable to forward Raman scattering instability [2]. It is shown here with the help of 2-D fluid simulations that the single peak solutions remain intact for several plasma periods but ultimately unstable to transverse filamentation instability. The numerical growth rate obtained from simulations is seen to compare well with the analytical value. It is also shown that the multiple peak solutions in 2-D first undergo the regular 1-D forward Raman scattering instability. Subsequently, however, they too are found to be unstable to filamentation instability. The filamentation instability having a slower growth rate appears later. [1] S. Poornakala, A. Das, A. Sen and P. K. Kaw. Physics of Plasmas, 9(5), 2002. [2] V. Saxena, A. Das, S. Sengupta, P. K. Kaw and A. Sen. Physics of Plasmas, 14(072307), 2007. [Preview Abstract] |
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NP10.00080: The role of collisions in the laser filamentation and Weibel instabilities R. Bingham, R. Trines, P. Norreys, R.A. Cairns, L.O. Silva We report on two instabilities that are associated with inertial fusion, namely the laser filamentation instability and the Weibel instability that can filament the relativistic electron beam in fast ignition. We look at the role collisions have in these instabilities. First we consider the filamentation instability of lasers in high Z plasmas typical of hohlraum targets. The wavelength dependence of the two principal laser filamentation mechanisms, namely the ponderomotive force and Joule heating, are examined and deductions are made of their relative importance for current holhraum experiments. The Joule heating mechanism is important for short wavelengths and high Z materials while the ponderomotive force becomes more important for longer wavelengths and low Z materials. The high Z targets are susceptible to thermal filamentation, creating non-uniform plasmas. The effect of plasma density and laser bandwidth is also examined and reported on. In fast ignition, we show that the Weibel instability growth rate, which drives the filamentation of a relativistic electron beam, is reduced when collisions are considered. [Preview Abstract] |
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NP10.00081: Preliminary numerical investigation of bandwidth effects on CBET using the LPSE-CBET code Jason Bates, Jason Myatt, John Shaw, James Weaver, Keith Obenschain, Robert Lehmberg, Steve Obenschain Cross beam energy transfer (CBET) is a significant energy-loss mechanism for direct-drive implosions on the OMEGA laser facility. Recently, a working group that includes participants from the Laboratory for Laser Energetics (LLE) at the University of Rochester and the U.S. Naval Research Laboratory (NRL) was formed to investigate strategies for ameliorating the deleterious effects of CBET. As part of this collaboration, the wave-based code LPSE-CBET developed at LLE has been made available to researchers at NRL and is being used to study the feasibility of suppressing CBET through the enhancement of laser bandwidth by stimulated rotational Raman scattering (SRRS). In this poster, we present some preliminary results on this subject. In particular, we discuss initial efforts to evaluate mitigation levels of 4 discrete Stokes lines from SRRS in air and compare our findings with ray-based simulation results of wavelength shifted (-6{\AA} ,0, $+$6{\AA}) driver-lines on OMEGA. [Preview Abstract] |
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NP10.00082: CBET Experiments with Wavelength Shifting at the Nike Laser James Weaver, P. McKenty, J. Bates, J. Myatt, J. Shaw, K. Obenschain, J. Oh, D. Kehne, S. Obenschain, R. H. Lehmberg, F. Tsung, A. J. Schmitt, V. Serlin Studies conducted at NRL during 2015 searched for cross-beam energy transport (CBET) in small-scale plastic targets with strong gradients in planar, cylindrical, and spherical geometries. The targets were irradiated by two widely separated beam arrays in a geometry similar to polar direct drive. Data from these shots will be presented that show a lack of a clear CBET signature even with wavelength shifting of one set of beams. This poster will discuss the next campaign being planned, in part, with modelling codes developed at LLE. The next experiments will use a target configuration optimized to create stronger SBS growth. The primary path under consideration is to increase scale lengths 5-10x over the previous study by using exploding foils or low density foams. In addition to simulations, the presentation will also discuss improvements to the diagnostic suite and laser operations; for example, a new set of etalons will be available for the next campaign that should double the range of wavelength shifting between the two beam arrays. [Preview Abstract] |
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NP10.00083: How external magnetic fields alter the parameter dependence of reflectivity in stimulated Raman scattering B. J. Winjum, A. Tableman, F. S. Tsung, W. B. Mori We show the parameter dependence of stimulated Raman scattering (SRS) reflectivity over a range of electron temperatures and densities, laser intensities, and external magnetic field ($B_0$) amplitudes and orientations in particle-in-cell simulations with $k\lambda_D = 0.2-0.4$ for the backscatter plasma wave. $B_0$ can modify kinetic SRS by altering the phasespace dynamics of trapped particles. We show how $B_0$ (both in amplitude and in orientation relative to the incident laser wavevector) affects the onset intensity and threshold values for reflectivity. Without an external field, and for constant $k\lambda_D$, lower electron densities have lower reflectivities, since SRS saturates at amplitudes for which the detuning rate due to the nonlinear frequency shift is on the order of the growth rate. Lower reflectivities are also seen for shorter speckle lengths in multi-speckle ensembles. The sensitivity of SRS reflectivity to $B_0$ depends on the underlying kinetic physics, though we comment on generalities and the parameter regimes for which $B_0$ eliminates kinetic SRS reflectivity. [Preview Abstract] |
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NP10.00084: Measurements of Laser Plasma Instability (LPI) and Electron Density/Temperature Profiles in Plasmas Produced by the Nike KrF Laser Jaechul Oh, J. L. Weaver, V. Serlin, S. P. Obenschain We will present results of simultaneous measurements of LPI-driven light scattering and density/temperature profiles in CH plasmas produced by the Nike krypton fluoride laser ($\lambda= 248$ nm). The primary diagnostics for the LPI measurement are time-resolved spectrometers with absolute intensity calibration in spectral ranges relevant to the optical detection of stimulated Raman scattering or two plasmon decay. The spectrometers are capable of monitoring signal intensity relative to thermal background radiation from plasma providing a useful way to analyze LPI initiation. For further understanding of LPI processes, the recently implemented grid image refractometer (Nike-GIR)$^a$ is used to measure the coronal plasma profiles. In this experiment, Nike-GIR is equipped with a $5^{th}$ harmonic probe laser ($\lambda= 213$ nm) in attempt to probe into a high density region over the previous peak density with $\lambda= 263$ nm probe light ($\sim 4 \times 10^{21}$ cm$^{-3}$).$^a$ The LPI behaviors will be discussed with the measured data sets.$^a$ J. Oh, et al, Rev. Sci. Instrum. 86, 083051 (2015). [Preview Abstract] |
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NP10.00085: Controlling Laser Plasma Instabilities Using Temporal Bandwidth Frank Tsung, J. Weaver, R. Lehmberg We are performing particle-in-cell simulations using the code OSIRIS to study the effects of laser plasma interactions in the presence of temporal bandwidth under conditions relevant to current and future experiments on the NIKE laser. Our simulations show that, for sufficiently large bandwidth (where the inverse bandwidth is comparable with the linear growth time), the saturation level, and the distribution of hot electrons, can be effected by the addition of temporal bandwidths (which can be accomplished in experiments using beam smoothing techniques such as ISI). We will quantify these effects and investigate higher dimensional effects such as laser speckles. [Preview Abstract] |
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NP10.00086: Proton Energy Optimization and Spatial Distribution Analysis from a Thickness Study Using Liquid Crystal Targets Christopher Willis, Patrick Poole, Douglas Schumacher, Richard Freeman, Linn Van Woerkom Laser-accelerated ions from thin targets have been widely studied for applications including secondary radiation sources and cancer therapy, with recent studies trending towards thinner targets which can provide improved ion energies and yields. Here we discuss results from an experiment on the Scarlet laser at OSU using variable thickness liquid crystal targets. On this experiment, the spatial and spectral distributions of accelerated ions were measured along target normal and laser axes at varying thicknesses from $150\,nm$ to $2000\,nm$ at a laser intensity of $1\times10^{20}\,W/cm^2$. Maximum ion energy was observed for targets in the $600-800\,nm$ thickness range, with proton energies reaching $24\,MeV$. The ions were further characterized using radiochromic film, revealing an unusual spatial distribution on many laser shots. Here, the peak ion yield falls in an annular ring surrounding the target normal, with an increasing divergence angle as a function of ion energy. Details of these spatial and spectral ion distributions will be presented, including spectral deconvolution of the RCF data, revealing additional trends in the accelerated ion distributions. [Preview Abstract] |
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NP10.00087: Laser-Plasma Instability Control Using T$_{\mathrm{Pulse}}$ fixed vs I$_{\mathrm{max}}$ fixed Spike Trains of Uneven Duration and Delay: The Path to Green ICF Using STUD Pulses Bedros Afeyan, Stefan Hüller, Nathan Meezan, Jim Hammer, John Heebner We have studied the behavior of laser-plasma instabilities (LPI) as a function of seed noise (varied over seven orders of magnitude) and Rosenbluth gain exponent at the average intensity (varied over a decade) for structured laser beams with and without STUD pulse mitigation. We will show that for each section of the NIF ICF pulse, there are preferred configurations of STUD pulses, whether they be fixed duration of fixed peak intensity, so that maximum use is made of STUD pulse flexibility for LPI control. The duty cycle, hot spot scrambling rate, and cutting a hot spot into pieces (by switching the lasers on and off on the ps time scale), are the three main tools. We explore a variety of phase transitions in reflectivity behavior and in the amplification profile of plasma perturbations. We compare cases where amplification bursts are reinforced coherently or are healed, lead to brush fires or are tamed. The STUD pulse program is best suited for Green light implementation since Green offers higher bandwidth, more energy, and higher damage thresholds. We plan to test these ideas on the Jupiter Laser Facility at LLNL at the pair of 200J lasers level next. [Preview Abstract] |
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NP10.00088: Short-Pulse Amplification by Strongly-Coupled Stimulated Brillouin Scattering Matthew Edwards, Qing Jia, Julia Mikhailova, Nathaniel Fisch We examine the feasibility of strongly-coupled stimulated Brillouin scattering as a mechanism for the plasma-based amplification of sub-picosecond pulses. Fluid theory and particle-in-cell calculations are used to compare the relative advantages of Raman and Brillouin amplification over a broad range of parameters, with a focus on determining the maximum amplified pulse intensities and minimum durations that can be achieved. Amplification of short-wavelength pulses is considered in detail, with particular emphasis on the practical development of plasma-based x-ray amplifiers. Our results suggest that Brillouin scattering may allow amplification of shorter wavelength light than Raman scattering, but that at optical frequencies better performance is generally realized with Raman amplification, as strongly-coupled Brillouin scattering has limited capacity for amplifying sub-picosecond pulses. [Preview Abstract] |
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NP10.00089: Distinguishing mechanisms of plasma-based amplification for short laser pulses Qing Jia, Matthew Edwards, Ido Barth, Julia Mikhailova, Nathaniel Fisch Several plasma-based amplification mechanisms have been proposed to obtain short laser pulses with ultrahigh intensities beyond the damage threshold of solid-state devices, including Compton-like superradiant amplification, backward Raman amplification and strongly-coupled Brillouin amplification. These three mechanisms are all based on the periodic structure of particle (electrons for the former two and ions for Brillouin amplification) density fluctuations that function as a grating. By turning off the ion motion in particle-in-cell simulations, we can distinguish Brillouin from Raman, and show that Raman amplification is responsible for the main leading spike amplification of ultrashort pulses. By artificially turning off the longitudinal electric field ($E_x$) in simulations, we can distinguish Raman from Compton-like superradiant amplification. Interestingly, we find that the superradiant amplification in $E_x$-off simulation is similar to the amplification in pair plasmas, with roughly half amplification efficiency of the latter due to absence of equal contribution from positrons. In addition, we also discuss the competition between Brillouin amplification and superradiant amplification in pair plasmas by comparing the dominance of thermal pressure and ponderomotive force. [Preview Abstract] |
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NP10.00090: Inverse Bremsstrahlung Heating in Laser-Matter Interactions: the effects of other particles Rishi Pandit, Yasuhiko Sentoku, Edward Ackad The laser-matter interaction of inverse bremsstrahlung heating is studied via a particle in cell code, PICLS and via molecular dynamics code, MD. Inverse bremsstrahlung heating, an important process in the laser-matter interaction, involves three different kinds of interactions, i)the interaction of the electrons with the external laser field, ii) the electron–ion interaction and iii) the electron-electron interaction. In the interaction of atomic clusters with femtosecond laser pulses, nanoplasmas with high density are created. A new scaling for the rate of energy absorption in inverse bremsstrahlung heating has been derived which depends on the external laser field as well as electric field due to the other particles. Electric fields due to the particles depend on a parameter, the potential depth. Thus, inverse bremsstrahlung heating also depends on potential depth. We will discuss the particle in cell code results and molecular dynamics code results by varying laser intensities and potential depths to understand the effect of potential depth as well as the particle’s field’s dependence of inverse bremsstrahlung heating in laser-matter interaction. [Preview Abstract] |
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NP10.00091: Multiscale spectra of weak optical turbulence in random Kerr media Vladimir Malkin, Nathaniel Fisch A broad class of multiscale spectra of weak optical turbulence is found analytically within the kinetic equation describing nonlinear four-wave scattering combined with linear wave scattering in random statistically uniform Kerr media. This is accomplished by using the generalized Kolmogorov locality approach that enables expressing k-space integrals in the kinetic equation for waves by explicit formulas local in k-space. [Preview Abstract] |
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NP10.00092: WAKES: Wavelet Adaptive Kinetic Evolution Solvers Marine Mardirian, Bedros Afeyan, David Larson We are developing a general capability to adaptively solve phase space evolution equations mixing particle and continuum techniques in an adaptive manner. The multi-scale approach is achieved using wavelet decompositions which allow phase space density estimation to occur with scale dependent increased accuracy and variable time stepping. Possible improvements on the SFK method of Larson are discussed, including the use of multiresolution analysis based Richardson-Lucy Iteration, adaptive step size control in explicit vs implicit approaches. Examples will be shown with KEEN waves and KEEPN (Kinetic Electrostatic Electron Positron Nonlinear) waves, which are the pair plasma generalization of the former, and have a much richer span of dynamical behavior. WAKES techniques are well suited for the study of driven and released nonlinear, non-stationary, self-organized structures in phase space which have no fluid, limit nor a linear limit, and yet remain undamped and coherent well past the drive period. The work reported here is based on the Vlasov-Poisson model of plasma dynamics. [Preview Abstract] |
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NP10.00093: Critical Density Target Design for Ion Acceleration on the T-Cubed Laser Peter Kordell, Paul Campbell, Anatoly Maksimchuk, Louise Willingale, Karl Krushelnick The interaction of an intense laser pulse with a critical density target can form a high Mach number electrostatic shock. Recent experiments on CO2 lasers have demonstrated that such shocks can be used to produce directional, quasi-monoenergetic proton beams. PIC simulations indicate that the our single pulse system, the T-Cubed laser (1.053$\mu$m, 6J in 400fs), is both capable of both producing these shocks and accelerating protons to MeV energies. Shock formation and propagation with our system has challenging target peak density and density gradient requirements. We present our target design, an interferometric characterization of its density profile and preliminary experiments on T-Cubed. [Preview Abstract] |
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NP10.00094: ABSTRACT WITHDRAWN |
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NP10.00095: Microwave Probing of Air-Plasma and Plasma Metamaterials Katherine Schneider, Ben Rock, Mike Helle Plasma metamaterials are of recent interest due to their unique ability to be engineered with specific electromagnetic responses. One potential metamaterial architecture is based on a 'forest' of plasma rods that can be produced using intense laser plasma filaments. In our work, we use a continuous microwave source at 26.5 GHz to measure a single air plasma filament characteristics generated from a 5 mJ laser pulse within a cylindrical hole in a Ka-band waveguide. Preliminary results show the air plasma produces a strong shock and acts to reflect microwave radiation. A computational comparison using 3D EM modeling is performed to examine the reflection and transmission properties of a single plasma rod, and further, to investigate an array of plasma rods as a potential plasma based metamaterial. [Preview Abstract] |
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NP10.00096: Relativistic magnetic reconnection driven by intense lasers in preformed plasma Paul Campbell, A. Raymond, A. McKelvey, A. Maksimchuk, J. Nees, V. Yanovsky, K. Krushelnick, C. F. Dong, W. Fox, C. Zulick, M.S. Wei, H. Chen, V. Chvykov, C. Mileham, P.M. Nilson, C. Stoeckl, A.G.R. Thomas, L. Willingale Experiments were performed with the OMEGA EP laser system focusing the two short pulse beams to high intensities on foil targets. Relativistic electrons drive fast reconnection self-generated magnetic fields. To investigate the effects of a preformed plasma on this relativistic magnetic reconnection, a long pulse UV beam was used to ablate the front surface of layered targets. The density and reconnection dynamics in the preformed copper or CH plasma were diagnosed with a $4 \omega$ optical probe. A spherically bent crystal imaged characteristic copper $K_\alpha$ emission induced by fast electrons accelerated into the target in the reconnection diffusion region. [Preview Abstract] |
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NP10.00097: Kinetic Modeling of Ultraintense X-ray Laser-Matter Interactions Ryan Royle, Yasuhiko Sentoku, Roberto Mancini Hard x-ray free-electron lasers (XFELs) have had a profound impact on the physical, chemical, and biological sciences. They can produce millijoule x-ray laser pulses just tens of femtoseconds in duration with more than $10^{12}$ photons each, making them the brightest laboratory x-ray sources ever produced by several orders of magnitude. An XFEL pulse can be intensified to $10^{20}$ W/cm$^2$ when focused to submicron spot sizes, making it possible to isochorically heat solid matter well beyond 100 eV. These characteristics enable XFELs to create and probe well-characterized warm and hot dense plasmas of relevance to HED science, planetary science, laboratory astrophysics, relativistic laser plasmas, and fusion research. Several newly developed atomic physics models including photoionization, Auger ionization, and continuum-lowering have been implemented in a particle-in-cell code, PICLS, which self-consistently solves the x-ray transport, to enable the simulation of the non-LTE plasmas created by ultraintense x-ray laser interactions with solid density matter. The code is validated against the results of several recent experiments and is used to simulate the maximum-intensity x-ray heating of solid iron targets. [Preview Abstract] |
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NP10.00098: All-optical radiation reaction in head-on laser electron interaction Marija Vranic, Thomas Grismayer, Joana L. Martins, Ricardo A. Fonseca, Luis O. Silva Radiation reaction (RR) accounts for the slowdown of a charged particle that occurs when a significant fraction of its kinetic energy is emitted as radiation. Here we show that this effect could be measured in an all-optical setup using a laser wakefield accelerated electron beam colliding with an intense laser pulse. We employ full-scale 3D~PIC simulations to show that~one can enter a radiation reaction dominated regime with a GeV electron beam and a 30 fs laser of I $=$ 10$^{\mathrm{21}}$W/cm$^{\mathrm{2}}$.~The electrons can lose up to 40{\%} of their initial energy, which can be used as an experimental signature in the spectra. Our results indicate that~modern laser facilities provide an exciting opportunity to explore classical RR and the near-future laser facilities can be employed to study the~RR beyond classical description.~By using higher laser intensities (10$^{\mathrm{22}}$-10$^{\mathrm{23}}$W/cm$^{\mathrm{2}})$, quantum effects such as Compton scattering and Breit-Wheeler pair production become relevant. We have~included these quantum effects in our PIC code OSIRIS through a Monte Carlo module, and performed a detailed numerical study of the transition from classical to quantum RR dominated regime. We identified the distinct features in the electron distribution function that could serve as signatures of quantum radiation reaction, and showed that large-scale infrastructures~(e.g. NIF and ELI ~and next generation of PW-class lasers~(e. g. CoReLS, Bella-i, Texas Petawatt, Apollon 10 PW)~could be employed to test the physics in these extreme scenarios. [Preview Abstract] |
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NP10.00099: Towards a more precise driving of capsules in ignition-scale hohlraums William Kruer, Cliff Thomas Precision Nova experiments$^{\mathrm{3}}$ demonstrated the importance of rather precisely irradiating the interior walls of a hohlraum, even for imploding capsules to a convergence ratio of \textasciitilde 20. Designs of precision implosions in a gas-filled ignition-scale hohlraum to even higher convergence would benefit from an improved understanding of both the plasma conditions and the laser energy deposition in time and space. The plasma conditions are sensitive to heat transport inhibition, which has been a recurrent theme in experiments with earlier lasers. Such inhibition has been proposed$^{\mathrm{4}}$ in order to better model NIF gas-filled hohlraums. An improved self-consistent model for the commonly invoked inhibition by two-stream turbulence is outlined, and some simple estimates made. These estimates suggest that the postulated reduction in heat transport may sometimes actually be due to other processes, such as self-generated magnetic fields. Several ways to reduce such B field effects are discussed, including greater temporal smoothing of speckle structure in the laser beams and reduction of intensity structure due to CBET and beam overlap on the hohlraum walls. 3. John D. Lindl, \textit{et. al}., Phys. Plasmas 11, 339 (2004) 4. Cliff A. Thomas (private communication) [Preview Abstract] |
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NP10.00100: Development of a Buried Layer Platform at the OMEGA laser to Study Coronal (nonLTE) Plasmas M. B. Schneider, E. V. Marley, G. V. Brown, R. F. Heeter, M. A. Barrios, M. E. Foord, W. J. Gray, L. C. Jarrott, D. A. Liedahl, C. W. Mauche, K. Widmann A buried layer platform is being developed at the OMEGA laser to study the radiative properties of coronal (non-LTE) plasmas (ne \textasciitilde few 10\textasciicircum 21 /cm\textasciicircum 3, Te \textasciitilde 1 -- 2 keV) of mid to high Z materials. In the current study, the target was a 200 $\mu $m square with equal atomic mixes of gold/iron/vanadium in the center of a 600 $\mu $m diameter, 10 $\mu $m thick beryllium tamper. The thickness of the buried layer was either 1200 A or 1800 A. Lasers heat the target from both sides for up to 4 ns. The size of the microdot vs time was measured with x-ray imaging (face-on) and x- ray spectroscopy (side-on). The radiant x-ray power was measured with a low-resolution absolutely calibrated x-ray spectrometer (DANTE). The temperature was measured from the Fe and V helium-beta complexes. The use of these measurements to deduce emissivity of the target in the 2-3 keV x-ray range and improvements for future experiments are discussed. [Preview Abstract] |
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NP10.00101: Numerical Modeling of LLNL's Au-Sphere experiments on the OMEGA Laser K.H. Ma, E. Raicher, Y. Frank, M. Fraenkel, E. Johnsen, R.P. Drake, D. Shvarts Experiments performed by LLNL on OMEGA studying X-ray conversion efficiencies for high-Z materials, aimed to confirm hohlraum modeling, resulted in a"liberal" flux limiter value of 0.15 to match simulations with these measurements[1]. This conclusion was re-examined and another model accounting for the effect of Ion Acoustic Turbulence on the thermal electron flux limitation was proposed[2]. Our work continues to explore relevant physical parameters in modeling these experiments using the HYADES and FLORENCE codes[3]. The sensitivity of laser absorption, X-ray emission and corona electron temperature to the electron flux limiter, inverse bremsstrahlung coefficient, resonant absorption in the critical layer, LTE and NLTE atomic physics and a numerical convergence study due to steep density and electron temperature profiles at the critical layer will be discussed. Additionally, alternative experimental designs, such as an ``onion'' configuration of plastic and gold as well as different laser illumination patterns, were studied. [1] Dewald, E.L., et al., \textit{Phys. of Plasmas 15}, 072706 (2008). [2] Rosen M.D. et al., presented at the 2015 APS/DPP conference. [3] Y. Frank et al., Phys. Rev. E 92, 053111 (2015) [Preview Abstract] |
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NP10.00102: HED AND PINCHES |
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NP10.00103: AmBe Radiological Source Replacement Using Dense Plasma Focus Z-Pinch Brian Shaw, Alexander Povilus, Steven Chapman, Yuri Podpaly, Christopher Cooper, Drew Higginson, Anthony Link, Andrea Schmidt A dense plasma focus (DPF) is a compact plasma gun that produces high energy ion beams up to several MeV through strong potential gradients formed from m=0 plasma instabilities. These ion beams can be used to replace radiological sources for a variety of applications. Americium-beryllium (AmBe) neutron sources are commonly used for oil well logging. An optimized DPF produces high energy helium ion beams of 2+ MeV which can interact with a beryllium target to produce neutrons. The alpha-Be interaction produces a neutron energy spectrum similar to the neutrons produced by the AmBe reaction. To demonstrate this concept experimentally a 2 kJ DPF is used to produce a beam of alpha particles which interacts with a beryllium target. We report on the improvements made to the DPF platform using He gas and the observation of $\sim 3.0\times10^{4}$ peak neutrons generated per shot. [Preview Abstract] |
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NP10.00104: Preionization Techniques in a kJ-Scale Dense Plasma Focus Alexander Povilus, Brian Shaw, Steve Chapman, Yuri Podpaly, Christopher Cooper, Steve Falabella, Rahul Prasad, Andrea Schmidt A dense plasma focus (DPF) is a type of z-pinch device that uses a high current, coaxial plasma gun with an implosion phase to generate dense plasmas. These devices can accelerate a beam of ions to MeV-scale energies through strong electric fields generated by instabilities during the implosion of the plasma sheath. The formation of these instabilities, however, relies strongly on the history of the plasma sheath in the device, including the evolution of the gas breakdown in the device. In an effort to reduce variability in the performance of the device, we attempt to control the initial gas breakdown in the device by seeding the system with free charges before the main power pulse arrives. We report on the effectiveness of two techniques developed for a kJ-scale DPF at LLNL, a miniature primer spark gap and pulsed, 255nm LED illumination. [Preview Abstract] |
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NP10.00105: Megajoule Dense Plasma Focus Solid Target Experiments Y.A. Podpaly, S. Falabella, A. Link, A. Povilus, D.P. Higginson, B.H. Shaw, C.M. Cooper, S. Chapman, N. Bennett, N. Sipe, R. Olson, A.E. Schmidt Dense plasma focus (DPF) devices are plasma sources that can produce significant neutron yields from beam into gas interactions. Yield increases, up to approximately a factor of five, have been observed previously on DPFs using solid targets, such as CD$_{2}$ and D$_{2}$O ice. In this work, we report on deuterium solid-target experiments at the Gemini DPF. A rotatable target holder and baffle arrangement were installed in the Gemini device which allowed four targets to be deployed sequentially without breaking vacuum. Solid targets of titanium deuteride were installed and systematically studied at a variety of fill pressures, bias voltages, and target positions. Target holder design, experimental results, and comparison to simulations will be presented. [Preview Abstract] |
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NP10.00106: Optimizing Dense Plasma Focus Neutron Yields with Fast Gas Jets Matthew McMahon, Christopher Kueny, Elizabeth Stein, Anthony Link, Andrea Schmidt We report a study using the particle-in-cell code LSP to perform fully kinetic simulations modeling dense plasma focus (DPF) devices with high density gas jets on axis. The high density jet models fast gas puffs which allow for more mass on axis while maintaining the optimal pressure for the DPF. As the density of the jet compared to the background fill increases we find the neutron yield increases, as does the variability in the neutron yield. Introducing perturbations in the jet density allow for consistent seeding of the m$=$0 instability leading to more consistent ion acceleration and higher neutron yields with less variability. Jets with higher on axis density are found to have the greatest yield. The optimal jet configuration is explored. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
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NP10.00107: Construction of a Compact, Low-Inductance, 100 J Dense Plasma Focus for Yield Optimization Studies Christopher Cooper, Alex Povilus, Steven Chapman, Steve Falabella, Yuri Podpaly, Brian Shaw, Jason Liu, Andrea Schmidt A new 100 J mini dense plasma focus (DPF) is constructed to optimize neutron yields for a variety of plasma conditions and anode shapes. The device generates neutrons by leveraging instabilities that occur during a z-pinch in a plasma sheath to accelerate a beam of deuterium ions into a background deuterium gas target. The features that distinguish this miniDPF from previous 100 J devices are a compact, engineered electrode geometry and a low-impedance driver. The driving circuit inductance is minimized by mounting the capacitors close to the back of the anode and cathode $<$ 20 cm away, increasing the breakdown current and yields. The anode can rapidly be changed out to test new designs. The neutron yield and 2D images of the visible light emission are compared to simulations with the hybrid kinetic code LSP which can directly simulate the device and anode designs. Initial studies of the sheath physics and neutron yields for a scaling of discharge voltages and neutral fill pressures are presented. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
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NP10.00108: Study on the Polarity Riddle of the Dense Plasma Focus Sheng Jiang, Anthony Link, Drew Higginson, Andrea Schmidt The dense plasma focus (DPF) Z-pinch devices are capable of producing intense pulses of X-rays and neutrons, thus can serve as portable sources for active interrogation. DPF devices are normally operated with the inner electrode as anode. It has been found that interchanging the polarity of the electrodes can cause orders of magnitude decrease in the neutron yield$^{\mathrm{1}}$. The reason for this severe decay remains unclear. Here we use the particle-in-cell (PIC) code LSP$^{\mathrm{2,3}}$ to model a portable DPF with both polarities. The filling gas is deuterium. The simulations are run in the fluid mode for the rundown phase and are switched to kinetic to capture the anomalous resistivity and beam acceleration process during the pinch. The difference in the shape of the sheath, the voltage and current traces, and the electric and magnetic fields in the pinch region due to different polarities all have great effects on the deuteron ion spectrum, which further determines the neutron yield. A detailed comparison will be presented. 1. G. Decker, W. Kies and G. Pross, Phys. Lett. 89A, 393 (1982) 2. D. R. Welch, D. V. Rose, R. E. Clark, T. C. Genoni, and T. P. Hughes, Comput. Phys. Commun. 164, 183 (2004) 3. A. Schmidt, V. Tang, D. Welch, Phys. Rev. Lett. 109, 205003 (2012) [Preview Abstract] |
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NP10.00109: Kinetic Modeling of Ion Beams in Dense Plasma Focus Z-Pinches A. Link, N. Bennett, S. Falabella, D.P. Higginson, R. Olsen, Y.A. Podpaly, A. Povilus, B. Shaw, N. Sipes, D.R. Welch, A. Schmidt Dense plasma focus (DPF) Z-pinches are compact devices capable of producing MeV ion beams, x-rays, and (for D or DT gas fill) neutrons. We report on predictions of ion beam generation using the particle-in-cell code LSP. These simulations include full-scale electrodes, an external pulse power circuit and model through the run-down phase as a fluid, transitioning to a fully kinetic simulation during the run-in phase and through the pinch. Simulations of a deuterium filled DPF predict a substantial number of ions accelerated to energies greater than 50 keV escape the dense plasma in the pinch region and could be used to enhance total neutron yield by employing a solid target. Results of the simulations will be presented and compared to experimental observations. LLNL-ABS-697617 [Preview Abstract] |
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NP10.00110: Experimental observation of discrete helical modes in imploding cylindrical liners D. A. Yager-Elorriaga, P. Zhang, A. M. Steiner, N. M. Jordan, P. C. Campbell, Y. Y. Lau, R. M. Gilgenbach The 1-MA Linear Transformer Driver at the University of Michigan was used to implode ultrathin (400 nm thick) cylindrical aluminum liners$^{1}$ that were pre-embedded with externally applied, axial magnetic fields of $B_{z} \quad =$ 0.2-2.0 T. Using 12-frame laser shadowgraphy and visible self-emission, helical striations were found that \textit{increased} in pitch angle during the implosion and \textit{decreased} in angle during the later time explosion, despite the relatively large, peak azimuthal magnetic field exceeding 40 T. The results are interpreted as a discrete, non-axisymmetric eigenmode of a helical instability that persists from implosion to explosion. The helical pitch angle $\varphi $ was found to obey the simple relation $\varphi \quad =$ \textit{m/kR}, where $m$, $k$, and $R$ are the azimuthal mode number, axial wavenumber, and radius of the helical instability. Analytic growth rates$^{2}$ for experimental parameters are presented, and show that early in the current pulse, axisymmetric modes ($m$ $=$ 0) are completely stabilized while non-axisymmetric modes ($m$ \underline {\textgreater } 1) are found to be unstable. [1] D. A. Yager-Elorriaga, et al., \textit{Rev. Sci. Instrum.} \textbf{86}, 113506 (2015). [2] M. R. Weis, P. Zhang, et al., \textit{Phys. Plasmas} \textbf{22}, 032706 (2015). [Preview Abstract] |
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NP10.00111: Study of Laser Ablation Plumes in 1-MA Z-Pinch Experiments Austin Anderson, Eric Dutra, Erik McKee, Cuyler Beatty, Timothy Darling, Vladimir Ivanov, Piotr Wiewior, Oleksandr Chalyy, Alexey Asttanovitskiy, Vidya Nalajala, Oleg Dmitriev, Aaron Covington Laser ablation plumes have been explored as a vehicle for pinch experiments and pulsed neutron production at the NTF research facility. The laser ablation plume is generated by striking a target with a 20J, 0.8ns laser pulse from the Leopard laser. The plume is allowed to expand and then pinched by a 1 MA current generated by the Zebra pulsed power machine. The plume is compact and pre-ionized, offering an advantage over neutral gas puffs and wire arrays. When used with deuterated-polyethylene targets, pinched ablation plumes can generate a pulse of \textasciitilde 10$^{\mathrm{11}}$ neutrons with a 35 ns pulse width. A laser-based 532 nm Mach-Zender interferometer and 16 frame imaging with 5 ns temporal resolution are used to characterize plasma density and observe implosion dynamics. Cathode activation was also measured post shot and has been used to determine the deuteron currents produced in the shots. Results and discussion are presented. [Preview Abstract] |
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NP10.00112: Image and Radiation Power Analysis Techniques for Determining Electron temperature, Liner Areal Density, and Radiated Energy in MagLIF Experiments Matthew Evans, Patrick Knapp, Matthew Gomez, Stephanie Hansen, Ryan McBride, L. Armon MacPherson, Pierre Gourdain We describe techniques developed to analyze filtered Time Integrated Pinhole Camera (TIPC) images to determine the axially resolved electron temperature and liner areal density at stagnation in MagLIF experiments conducted on the Z machine at Sandia National Laboratories. X-ray power detectors are analyzed to determine the absolute radiated energy. The TIPC images are co-registered using intensity based similarities. This technique is shown to provide accurate registration without the use of fiducial markings. A filtered 6-channel PCD array was used to record the radiated power at photon energies \textgreater 1 keV. A model for the x-ray emission is used with the data set to perform Bayesian parameter estimation to simultaneously determine the electron temperature, liner areal density and x-ray yield with uncertainties via $\chi^{\mathrm{2\thinspace }}$minimization. [Preview Abstract] |
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NP10.00113: Difference in Magnetic Field Threshold for Thermal Plasma Formation between Copper Alloys 145 and 101 Pulsed to Multi-Megagauss Surface Magnetic Field B.S. Bauer, S. Fuelling, V.V. Ivanov, T.M. Hutchinson, K.C. Yates, T.J. Awe Understanding the impact of choice of metal alloy on plasma formation is important for plasma physics and applications (e.g., fusion energy). Thermal ionization by pulsed ohmic heating of Cu-145 (99.5\% Cu, 0--0.7\% Te, 0--0.012\% P) has been compared with that of a purer alloy, Cu-101 ($>$99.99\% Cu), via well-characterized experiments that avoided contamination by arcing. Copper rods were pulsed to 1.0-MA peak current in 100\ ns, with the applied magnetic field rising linearly at 50--80\,MG/$\mu $s, depending on the rod initial diameter (0.49--1.59\,mm). The initial magnetic skin depth was much smaller than the rod radius, so surface plasma formation was observed while current was propagating into the conductor as a nonlinear diffusion wave. The rod surface finish was controlled (electropolished or not) and examined with optical and scanning electron microscopy. The expansion and ionization of the rod were observed with visible and EUV radiometry, time-resolved imaging, and laser shadowgraphy. Rods of both alloys explode when the applied magnetic field reaches 2\,MG, well before plasma formation. Rods of both alloys expand at 3.5\,km/s surface velocity. However, Cu-145 undergoes bulk surface ionization at 3.0\,MG, whereas Cu-101 only turns to plasma when the magnetic field exceeds 3.5\,MG. [Preview Abstract] |
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NP10.00114: Study of Thin Foil Explosion Using Point-Projection Radiography of Hybrid X-Pinch X-Rays T.A. Shelkovenko, S.A. Pikuz, L. Atoyan, D.A. Hammer, I.N. Tilikin The explosion of thin flat and cylindrical foils has been studied on the BIN (270 kA, 100 ns rise time) and COBRA (1.0 MA, 100 ns rise time) pulsed-power generators using X-ray point-projection imaging to investigate the exploded foil structure dependence on the foil material, thickness, geometry, and current through the foil. The geometry of the experiments and hybrid X-pinch source of soft X-ray radiation enabled better than 3 microns spatial resolution and less then 1 ns temporal resolution on both generators. On the BIN pulser the HXP was used as the main load and 1-15 microns thick and about 2-3 mm long foils were exploded in the return current circuit by 50-60 kA current. Al (4, 15 microns), Cu (1, 10 microns) and Ni (5 microns) foils were used in the experiments. Cylindrical foils 4 microns thick with 0.5 mm diameters were used as the main load on the COBRA generator with the HXP placed in one of two return current rods as the source of radiation for high resolution point-projection radiography. Some common features between explosion of fine wires and foils were observed. The exploded foil structure depended dominantly on the current through the foil. [Preview Abstract] |
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NP10.00115: Z-pinch Plasma Temperature and Implosion Velocity from Laboratory Plasma Jets using Thomson Scattering Jacob Banasek, Tom Byvank, Bruce Kusse, David Hammer We discuss the use of collective Thomson scattering to determine the implosion velocity and other properties of laboratory plasma jets. The plasma jet is created using a 1 MA pulsed power machine with a 15 $\mu$m Al radial foil load. The Thomson scattering laser has a maximum energy of 10~J at 526.5~nm with a pulse duration of 3~ns. Using a time gated ICCD camera and spectrometer system we are able to record the scattered spectrum from 9 or 18 regions along the laser path with sub-mm spatial resolution. Collecting scattered radiation from the same area at two different angles simultaneously enables determination of both the radial and azimuthal velocities. The scattered spectrum for non-magnetized jets indicates a radial implosion velocity of 27~km/s into the jets. A determination of ion and electron temperatures from the scattered spectrum is in progress. Comparing results using a laser energy of 10~J and 1~J shows noticeable effects on plasma jet properties when using 10~J. Therefore the lower laser energy must be used to determine the plasma properties. [Preview Abstract] |
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NP10.00116: Simulations of Ar gas-puff implosions on Z with a Xe dopant Varun Tangri, J. L. Giuliani, A. L. Velikovich, N. D. Ouart, A. Dasgupta, J. W. Thornhill, J. P. Apruzese, A. J. Harvey-Thompson, B. Jones, C. A. Jennings A recent experiment [1] on the Z machine at SNL indicated that the presence of a small fraction of Xe (0.8{\%} by number in the center jet) in a Ar gas puff shot had a significant effect on the emitted K-shell radiation. In presence of the Xe dopant, the Ar K-shell yield dramatically reduced from 373 \textpm 9 to 129 \textpm 9 kJ. The peak K-shell power was also significantly lower and accompanied by two nearly equal peaks. A second shot without the Xe dopant consisted of a single peak. We present radiation-magnetohydrodynamic simulations of these shots [Z2603 (with Xe) and Z2605 (without Xe)] using the using the Mach2-TCRE code with a tabulated collisional radiative equilibrium model. Detailed numerical simulations exploring the impact of the Xe dopant on the implosion dynamics and the resultant K-shell radiation will be presented. Analysis of a time- and space resolved synthetic K-shell spectra would also be presented. [1] Harvey-Thompson et al., Submitted to Phys. Plasmas, (2016) [Preview Abstract] |
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NP10.00117: Neutron Activation Diagnostics in Deuterium Gas-Puff Experiments on the 3 MA GIT-12 Z-Pinch J Cikhardt, D Klir, K Rezac, B Cikhardtova, J Kravarik, P Kubes, O Sila, A V Shishlov, R K Cherdizov, F I Fursov, V A Kokshenev, N E Kurmaev, A Yu Labetsky, N A Ratakhin, G N Dudkin, A A Garapatsky, V N Padalko, V A Varlachev, K Turek The experiments with a deuterium z-pinch on the GIT-12 generator at IHCE in Tomsk were performed in the frame of the Czech-Russian agreement. A set of neutron diagnostics included scintillation time-of-flight detectors, bubble detectors, and several kinds of threshold nuclear activation detectors in the order to obtain information about the yield, anisotropy, and spectrum of the neutrons produced by a deuterium gas-puff. The average neutron yield in these experiments was of the order of 10$^{\mathrm{12}}$ neutrons per a single shot. The energy spectrum of the produced neutrons was evaluated using neutron time-of-flight detectors and a set of neutron activation detectors. Because the deuterons in the pinch achieve multi-MeV energies, non-DD neutrons are produced by nuclear reactions of deuterons with a stainless steel vacuum chamber and aluminum components of diagnostics inside the chamber. An estimated number of the non-DD was of the order of 10$^{\mathrm{11}}$. [Preview Abstract] |
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NP10.00118: Thomson Scattering on exploding wires at 800 kA John Greenly, Charles Seyler, Jacob Banasek, William Potter Laser Thomson scattering measurements have been carried out on a single 0.25 mm diameter Al wire load driven with an 800 kA, 100 ns risetime pulse on the COBRA pulsed power facility. The 527 nm, 10 J, 5 ns laser is brought to a line focus on a chord across the unstable, roughly cylindrical plasma column of the wire, which reaches 8mm outer diameter at 100 ns. The laser path is either on axis or 2mm or 4mm off axis. Scattered signals are collected on a fiber array yielding data across the laser path through the plasma. The scattered light is easily visible over the wire plasma self-emission. The scattered spectra have highly complex structures comprised of as many as four distinct spectral peaks spread over \textasciitilde 1 nm in wavelength, both red-and blue-shifted. On axis, the laser does not reach the far side of the plasma, being totally absorbed and/or refracted out of its path. 2 mm off-axis the beam is severely refracted, probably from near the critical surface in the plasma, appearing in images taken with cameras \textasciitilde 45 degrees off its entering path. The scattering should be in the collective regime, and analysis is underway to extract information on flow velocities and temperatures within the volume, of 0.5mm radius, imaged by each fiber. [Preview Abstract] |
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NP10.00119: X-ray radiation from puff-on-wire implosion on the COBRA generator N. Ouart, J. Engelbrecht, P. de Grouchy, N. Qi, T. Shelkovenko, S. Pikuz, B. Kusse, D. Hammer, J. Giuliani, A. Dasgupta, A. Velikovich, J. Apruzese, R. Clark Substantial progress has been made in developing plasma radiation sources from Z-pinch implosions. University pulsed power machines provide a cost effective platform to study alternative mechanisms of producing x-rays that may provide guidance in search of further improvements on the larger machines. Radiation from puff-on-wire implosions were previously studied [1,2]. We report recent observations and modeling of puff-on-wire implosions using the 1 MA COBRA generator in the long pulse mode. The gas puff used Ne, Ar, or Kr and the wire material was either Cu or manganin 290 (84{\%} Cu, 12{\%} Mn, 4{\%} Ni). The diagnostics include time-integrated pinhole cameras and an axially resolved spectrometer, multiple filtered PCDs and Si-diodes, and time-gated XUV cameras. X-ray radiation from the gas puff and the K-alpha line from wire material was detected. A 1-D multi-zone non-LTE kinetics code with radiation transport will be used to model the radiation to infer the plasma conditions. [1] A. Chuvatin, P. Choi, and B. Ethlicher, Phys. Rev. Lett. \textbf{76}, 2282 (1996) [2] F.J. Wessel, P.L. Coleman, N. Loter, \textit{et al.}, J. Appl. Phys. \textbf{81}, 3410 (1997) [Preview Abstract] |
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NP10.00120: Striation Formation in Cylindrical Liners Made of Various Materials Driven by a 1 MA Pulsed Power Generator Levon Atoyan, Tom Byvank, Joseph Engelbrecht, John Greenly, Sergei Pikuz, William Potter, Tania Shelkovenko, Bruce Kusse, David Hammer Peterson \textit{et al.} found on the 20 MA Z machine that, without any applied external axial magnetic field, horizontal striations appear in radiographic images of a metal liner [\textit{Phys. Plasmas 19}, 092701, 2012], a result that has been reproduced on other pulsed power machines since. In this work we present experimental results of horizontal striations on the 1 MA, 100-200 ns COBRA pulsed power generator [T. A. Shelkovenko et al, \textit{Rev. Sci. Instrum. 77}, 10F521, 2006]. The pattern is observed in our experiments using extreme ultraviolet imaging, laser imaging, and X-ray backlighting. Using this combination of diagnostics, we were able to view simultaneously the pattern near the liner surface as well as in the higher density portion of the liner, displaying features with different wavelengths. Furthermore, materials such as Al, Cu, and Ti will be used for the liner to determine if the striation formation is affected by the nature of the material. [Preview Abstract] |
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NP10.00121: Radial and Azimuthal Velocity Profiles in Gas-Puff Z-Pinches Sophia Rocco, Joseph Engelbrecht, Jacob Banasek, Philip de Grouchy, Niansheng Qi, David Hammer The dynamics of neon, argon, and krypton (either singly or in combination) gas puff z-pinch plasmas are studied on Cornell's 1MA, 100-200ns rise-time COBRA pulsed power generator. The triple-nozzle gas puff valve, consisting of two annular gas puffs and a central jet, allows radial tailoring of the gas puff mass-density profile and the use of 1, 2 or 3 different gases at different pressures. Interferometry supplies information on sheath thickness and electron density, variously filtered PCDs and silicon diodes measure hard and soft x-ray production, and multi frame visible and extreme UV imaging systems allow tracking of the morphology of the plasma. A 527nm, 10J Thomson scattering diagnostic system is used to determine radial and azimuthal velocities. Implosion velocities of \textasciitilde 170km/s (Kr) and \textasciitilde 300km/s (Ne/Ar) are observed. We are investigating the correlations between instability growth, plasma density profile, velocity partitioning as a function of radius, and radiation production. [Preview Abstract] |
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NP10.00122: Diagnostics of Fast Axial Ions Produced in Deuterium Gas-Puff Z-Pinch K. Rezac, D. Klir, J. Cikhardt, P. Kubes, O. Sila, J. Kravarik, A.V. Shishlov, A.Yu. Labetsky, R.K. Cherdizov, N.A. Ratakhin, H. Orcikova, K. Turek, N. Dudkin, V.N. Padalko An unexpected advantage of some Z-pinch configurations is a possibility of an acceleration of ions to high energies. One of these configurations is a deuterium gas-puff with outer plasma shell, where hydrogen ions with energies up to 40 MeV has been observed during Z-pinch experiments on the GIT-12 generator since 2013. During the recent campaign in 2016, the source of high energetic ions and also parameters of ion pulses have been studied by various in-chamber diagnostics in 24 experimental shots on the current level below 3 MA. Principal aims were (i) to find a spatial distribution of ion sources, (ii) localization of ion sources on the z-axis and (iii) determine the ion energy spectra by an unfold technique. All of these has been done with the help of a new diagnostic setup consists of an ion pinhole camera, an ion 3-pinhole camera, a multi-pinhole camera and a detector of spatial ion beam profile. The ion diagnostics contained stacks with various absorbers, CR-39 track detectors, HD-V2 and EBT-3 radio-chromic films. One more aim, (iv) the study of a difference in production time of axial ion pulses with off-axis pulses, were accomplished by LiF samples and nTOF signals. [Preview Abstract] |
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NP10.00123: University-scale pulsed-power system using linear transformer driver Po-Yu Chang, Mei-Feng Huang, Tzong Huan Iang, Yi-Liang Tsai Linear transformer driver (LTD) {[}A. A. Kim \textit{et al.}, Phys. Rev. ST Accel. Beams \textbf{12}, 050402 (2009){]} is a compact pulsed-power system suitable for x-ray sources or laboratory astrophysics and space research for university-scale laboratory. A LTD with $20$ bricks storing $8\;{\rm kJ}$ of total energy delivering $500\;{\rm kA}$ to the load with a $100\;{\rm ns}$ rise time is being built. It will be used for following two purposes: (1) gas-puff z pinches generating soft x-ray sources for bio-medical research in the future and (2) generating plasma jets to study interactions between plasma flows and unmagnetized/magnetized obstacles analogous to the interactions between solar winds and planetary magnetic fields or unmagnetized planets. One brick consisting of two $40\;{\rm nF}$ capacitors connected in series charged to $\pm100\;{\rm kV}$ and delivering a peak current of $25\;{\rm kA}$ to the load was built. The results of current measurement and circuit characteristics are shown. [Preview Abstract] |
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NP10.00124: Experimental Observations of the Electrothermal Instability on Thin Foils Adam Steiner, Paul Campbell, David Yager-Elorriaga, Nicholas Jordan, Y.Y. Lau, Ronald Gilgenbach The electrothermal instability (ETI) arises whenever a current-carrying material has a resistivity that depends on temperature. When resistivity increases with increasing temperature, ETI causes striations to form perpendicular to the direction of current. On pulsed-power-driven, ablating metallic loads, this process can cause sections of the target to ablate earlier than the bulk material, creating a macroscopic surface perturbation on the plasma surface that can seed hydrodynamic instabilities, such as the magneto Rayleigh-Taylor (MRT) instability. Experiments have been performed to observe the self-emission of ablating planar aluminum and tantalum foils using a 12-frame ultrafast ICCD camera with 2 $\mu $m spatial resolution and 5 ns time resolution. Other diagnostics include voltage, current, and optical spectroscopy. Ablations of foils ranging from 0.4 to 2 $\mu $m in thickness are driven with a 4 kA, 600 ns risetime pulse generator. Striations of hotter, brighter material forming perpendicular to current are consistently observed. These measurements provide experimental evidence of the growth of ETI as a temperature perturbation on initially solid metal loads. [Preview Abstract] |
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NP10.00125: High energy density laboratory astrophysics experiments with supersonic magnetized plasmas on the MAGPIE pulsed-power facility S.V. Lebedev, G.C. Burdiak, J.P. Chittenden, T. Clayson, C. Garcia, J.D. Hare, L.G. Suttle, F. Suzuki-Vidal, A. Frank, A. Ciardi, N.F. Loureiro The use of plasma flows generated by pulsed-power facilities provides a natural platform for designing magnetized HEDLA experiments. The plasma in this case is created and accelerated by the JxB force of the driving, \textasciitilde Mega-Ampere level currents, forming plasma flows with embedded, frozen-in magnetic fields. Here we present several recent experiments performed on the MAGPIE pulsed-power facility focusing on studies of the structure of magnetized bow shocks, the dynamics of counter-streaming plasma jets, the formation of shocks in inverse liners, and magnetic reconnection in colliding plasmas. The relatively large spatial and temporal scales characterizing these experimental platforms, together with excellent diagnostic access, allow detailed characterization of the key plasma parameters and quantitative comparison of the experimental results with numerical simulations. [Preview Abstract] |
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NP10.00126: Exploring Astrophysically Relevant Bow Shocks Using MIFEDS and the OMEGA Laser Joseph Levesque, Carolyn Kuranz, Rachel Young, Gennady Fiksel, Mario Manuel, Matthew Trantham, Sallee Klein, Patrick Hartigan, Andy Liao, Chikang Li We present current experiments using the Omega Laser Facility and their magneto-inertial fusion electrical discharge system (MIFEDS) to observe the effect of magnetic pressure on bow shock dynamics in an astrophysically relevant regime. Astrophysical bow shocks are an interesting phenomenon in which a shock forms when incident supersonic flow encounters a sufficiently magnetized medium surrounding an object. The most well-known example of this phenomena is the interaction of the solar wind with the Earth's magnetic field, which creates our magnetosphere. In our experiment the magnetosphere will be emulated by a current flowing through a curved wire to create an azimuthal magnetic field. To create the analogous solar wind, lasers rear-irradiate two opposing graphite targets so the plasma outflows collide and then expand along the collision plane toward the magnetized wires. We use the UV Thomson scattering diagnostic technique to determine plasma parameters along with optical imaging and proton radiography to characterize the plasma flow and the bow shock that forms. [Preview Abstract] |
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NP10.00127: Investigation of low-mode asymmetries in Omega direct-drive implosion stephane Laffite, C. Blancard, J. L. Bourgade, T. Caillaud, P. Cosse, G. Faussurier, F. Girard, O. Landoas, S. Lemaire, P. E. Masson-Laborde, F. Philippe, C. Reverdin, V. Tassin, G. Legay, L. Masse, J. Delettrez, V. Glebov, F. Marshall, T. Michel, W. Seka, J. Frenje, R. Mancini, T. Josgi We have investigated the evolution and the effect of low-mode asymmetries in direct-drive implosions. The experiments were carried out on the Omega facility. Two different pulse shapes, 1ns square pulse and 2-step pulse, were tested in order to vary the implosion stability of the same target whose the parameters, dimensions and composition, remained the same. For some of these shots, an artificially P4-mode asymmetry was imposed by lowering the energy of half the beams. For spectroscopy and x-ray imaging purpose, Ar tracer was added to the D2 fuel. A Ti tracer was also added to the CH ablator. Analysis of the spectra shows no mix between the fuel and the Ti layer. The core asymmetries, measured by x-ray and neutron imaging, clearly exhibit a P4 deformation. The correlation between asymmetries and pulse shape is investigated. [Preview Abstract] |
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NP10.00128: Modeling Turbulent Mixing Phenomena in the LLNL Reshocked Richtmyer‒Meshkov Instability Experimental Campaign Tanim Islam, Oleg Schilling The LLNL reshocked Richtmyer-Meshkov instability experimental campaign has been successfully designed and calibrated using high-resolution diagnostics to provide useful and reproducible data to study Richtmyer-Meshkov instability driven, plasma turbulent mixing. This work applies several Reynolds-averaged Navier‒Stokes mixing models in a radiation hydrodynamics code to a set of experiments fielded under this campaign in order to characterize the properties of turbulence and mixing. The evolution of the turbulent mixing layer width and of quantities such as the turbulent kinetic energy are investigated to determine how well the models predict the development and further growth of turbulence, especially following reshock of the layer. Flow features that can be more carefully explored using direct numerical simulation, large- eddy simulation and related numerical techniques that could potentially improve the modeling are also discussed. [Preview Abstract] |
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NP10.00129: Large eddy simulation of hydrodynamic instability growth in doubly-shocked plasmas at the National Ignition Facility Jason Bender, Kumar Raman, Britton Olson, Stephan MacLaren, Channing Huntington, Sabrina Nagel Richtmyer-Meshkov and Rayleigh-Taylor hydrodynamic instabilities play important roles in the behavior of high-energy-density (HED) plasmas, such as those considered in inertial confinement fusion research. Recent experiments at the National Ignition Facility have investigated instability growth at the irregular interface between two different-density fluids, following the impingement of two X-ray-driven shock waves. We discuss recent large eddy simulations of these ``re-shocked'' plasmas, with a focus on accurately modeling transition to turbulence. Various profiles are considered for the initial perturbation to the interface between the two fluids, including both sinusoidal (i.e., single-mode) and multimode profiles. We characterize nonlinear instability growth and turbulent-mixing-layer development in the simulations, and we compare our results with experimental data and with predictions from simple Reynolds-averaged Navier-Stokes models that are commonly employed to treat HED hydrodynamic turbulence. [Preview Abstract] |
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NP10.00130: Ablative Richtmyer-Meshkov instability with indirect drive. A. L. Velikovich, D. S. Clark, V. A. Smalyuk, O. L. Landen, K. O. Mikaelian, H. F. Robey, J. G. Wouchuk Ablative Richtmyer-Meshkov (ARM) instability develops while a strong radiation pulse, rapidly rising to its constant peak intensity, drives a constant-strength shock wave from the rippled irradiated surface of a solid target into its volume. For the direct laser irradiation, the theory, experiment, and simulations have demonstrated that the development of the ARM results in decaying oscillations of the areal mass/optical thickness modulation amplitude. Much less is known about the ARM with the indirect drive. This effect causing oscillations of the ablation front is the physical basis of the recently proposed [D. S. Clark \textit{et al}., Phys. Plasmas \textbf{21}, 112705 (2014)] and successfully demonstrated [H. F. Robey \textit{et al}., Phys. Plasmas \textbf{23}, 056303 (2016)] adiabat-shaping approach to improving the NIF target performance. We report a theoretical and numerical stability analysis of the indirectly-driven shock-piston flow performed to investigate the physical mechanism of the ablation-front oscillations detected in the simulations and the NIF experiments on adiabat shaping. [Preview Abstract] |
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NP10.00131: Modeling plasma plumes generated from laser solid interactions Scott C. Wilks, D. P. Higginson, A. J. Link, H.-S. Park, Y. Ping, H. G. Rinderknecht, J. S. Ross, C. Orban, R. Hua Laser pulses interacting with solid targets sitting in a vacuum form the basis for a large class of High Energy Density physics experiments. The resulting hydrodynamical evolution of the target during and after this interaction can be modeled using myriad techniques. These techniques range from pure particle-in-cell (PIC) to pure radiation-hydrodynamics, and include a large number of hybrid techniques in between. The particular method employed depends predominately on laser intensity. We compare and contrast several methods relevant for a large range of laser intensities (from $I\lambda^{2}\approx 1\times 10^{12}W\cdot \mu m^{2}/cm^{2}$ to $I\lambda^{2}\approx 1\times 10^{19}W\cdot \mu m^{2}/cm^{2})$ and energies (from $E\approx 100mJ$to $E\approx 100kJ$.) Density, temperature, and velocity profiles are benchmarked against recent experimental data. These experimental data include proton radiographs, time resolved x-ray images, and neutron yield and spectra. Methods to self-consistently handle backscatter and detailed energy deposition will also be discussed. LLNL-ABS-697767. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
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NP10.00132: Magnetized jet creation using a hollow ring of laser beams Yingchao Lu, Edison Liang, Lily Han, Lan Gao, Hantao Ji, Chi Kiang Li, Richard Petrasso, Dustin Froula, Russ Follett, Petros Tzeferacos, Don Lamb We report results of magnetized jet creation experiment using the Omega laser. Using 20 Omega beams to irradiate a flat plastic target in a hollow ring pattern, we demonstrated that the on-axis electron and ion density, temperature and velocity are higher than when all beams are focused onto a single spot. The jet created by the hollow ring laser is also more collimated. Proton radiography was used to probe the magnetic field in the jet. Proton images show ordered quasi-linear filaments whose divergence decreases with increasing ring radius. Proton density contrasts are consistent with filamentary field bundles with peak values in the tens of Teslas. These results demonstrate that magnetized jets created by a hollow ring of laser beams can become a versatile new platform for laboratory astrophysics. [Preview Abstract] |
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NP10.00133: Modeling of Late-Time Low Atwood Rayleigh-Taylor Experiments at OMEGA T. Handy, G. Malamud, L. Elgin, C.M. Huntington, M.R. Trantham, C.C. Kuranz, D. Shvarts, R.P. Drake Numerical simulations have shown a reacceleration phase of the Rayleigh-Taylor instability (RTI) at low Atwood numbers, during which the bubble and spike Froude numbers deviate from their expected asymptotic values [1]. Currently, there is no experimental validation of those results. Therefore, we propose a new HEDP experiment for late-time evolution of low Atwood number RTI. In this work, we present numerical simulations of our experimental RTI system. We compare our results to spike and bubble velocities provided by potential theory and buoyancy-drag models. Additionally, we identify aspects of the low Atwood RTI system that will influence experimental outcomes. [1] P. Ramaprabhu et al., Phys. Fluids 24, 074107 (2012) [Preview Abstract] |
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NP10.00134: Experimental Design: Rayleigh-Taylor Instability Growth to High Froude Number in the Non-Linear Regime at OMEGA Laura Elgin, G. Malamud, C.M. Huntington, T. Handy, M.R. Trantham, S.R. Klein, R.P. Drake, D. Shvarts, C.C. Kuranz Potential flow models predict that a Rayleigh-Taylor unstable system will reach a terminal velocity (and constant Froude number) at low Atwood numbers.~ Numerical simulations by Ramaprabhu et al. predict a reacceleration phase of Rayleigh-Taylor Instability (RTI) and higher Froude number at late times [1]. We are planning a series of experiments at OMEGA 60 to measure RTI growth at low and high Atwood numbers and late times in order to observe this effect. The evolution of this system will be diagnosed with dual, x-ray radiography. Experimental design and diagnostic challenges are discussed here. \newline \newline This work is supported by the U.S. DOE, through NNSA grants DE-NA0002956 (SSAA) and DE-NA0002719 (NLUF), by the LLE under DE-NA0001944, and by the LLNL under subcontract B614207 to DE-AC52-07NA27344. \newline [1] P. Ramaprabhu et al., "The late-time dynamics of the single-mode \newline Rayleigh-Taylor instability," Phys. Fluids 24, 074107 (2012). [Preview Abstract] |
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NP10.00135: Counter-streaming radiative shock experiments on the Orion laser F. Suzuki-Vidal, T. Clayson, G.F. Swadling, S. Patankar, G.C. Burdiak, S.V. Lebedev, R.A. Smith, C. Stehle, U. Chaulagain, R.L. Singh, J. Larour, M. Kozlova, C. Spindloe, J. Foster, J. Skidmore, E. Gumbrell, P. Graham, C. Danson The formation of radiative shocks, shocks in which the structure of density and temperature is affected by radiation from the shock-heated matter, is ubiquitous in many astrophysical scenarios. Experiments were performed at the Orion laser using a new target configuration that allows studying the formation of single and counter-streaming radiative shocks in gas-filled targets (Ne, Ar, Kr, Xe), with initial pressures \textasciitilde 0.1-1 bar and a driver intensity of \textasciitilde 6x10\textasciicircum 14 W/cm\textasciicircum 2. The shocks propagate at velocities \textgreater 60 km/s and were diagnosed with optical interferometry (streaked and time-resolved) and point-projection X-ray backlighting allowing to probe simultaneously the pre-shock radiative precursor and the shock front itself. Besides varying the extent of the radiative precursor the results show that different gases seem to have an effect on the shock front as evidenced by a number of spatial features. The results are compared with radiative hydrodynamics simulations in 1-D (HELIOS) and 2-D (NYM/PETRA). [Preview Abstract] |
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NP10.00136: Momentum transport and non-local transport in heat-flux-driven magnetic reconnection in HEDP Chang Liu, Will Fox, Amitava Bhattacharjee Strong magnetic fields are readily generated in high-energy-density plasmas and can affect the heat confinement properties of the plasma. Magnetic reconnection can in turn be important as an inverse process, which destroys or reconfigures the magnetic field. Recent theory [1] has demonstrated a novel physics regime for reconnection in high-energy-density plasmas where the magnetic field is advected into the reconnection layer by plasma heat flux via the Nernst effect. In this work we elucidate the physics of the electron dissipation layer in this heat-flux-driven regime. Through fully kinetic simulation and a new generalized Ohm's law, we show that momentum transport due to the heat-flux-viscosity effect [2] provides the dissipation mechanism to allow magnetic field line reconnection. Scaling analysis and simulations show that the characteristic width of the current sheet in this regime is several electron mean-free-paths. These results additionally show a coupling between non-local transport and momentum transport, which in turn affects the dynamics of the magnetic field.\\ $^1$A.S. Joglekar, A.G.R. Thomas, W. Fox, and A. Bhattacharjee, Phys. Rev. Lett. 112, 105004 (2014).\\ $^2$C. Liu, W. Fox, and A. Bhattacharjee, Phys. Plasmas 22, 053302 (2015). [Preview Abstract] |
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NP10.00137: Nonlocal thermal conduction due to laser-generated nonthermal electrons under strong magnetic fields Takashi Asahina, Hideo Nagatomo, Atsushi Sunahara, Tomoyuki Johzaki, Masayasu Hata, Yasuhiko Sentoku Strong magnetic field application has been considered to be an attractive method of electron beam guiding in fast ignition scheme after the successful generation of kilotesla-order magnetic field by means of intense laser\footnote{S. Fujioka, et al., \textbf{Sci. Rep.} 3, 1170}. However, such strong magnetic fields may violate the implosion symmetry due to anisotropic thermal conduction\footnote{H. Nagatomo, et al., \textbf{Nucl. Fusion} 55, 093028}. Although nonlocal thermal conduction is an implosion-driving process of critical importance, the effect of magnetic field is poorly known. In this study, nonlocal thermal conduction is investigated in the presence of external magnetic fields using 1D3V Particle-In-Cell simulation. It is found that nonthermal electrons are generated near the critical surface with the intense laser irradiation unlike the case without external magnetic field. Their effect on nonlocal thermal conduction is under discussion. [Preview Abstract] |
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NP10.00138: Temporally Resolved Ion Fluorescence Measurements of the Interaction of a Field-Parallel Laser Produced Plasma and an Ambient Magnetized Plasma R. S. Dorst, P. V. Heuer, A. S. Bondarenko, D. B. Shaffer, G. Contantin, S. Vincena, S. Tripathi, W. Gekelman, M. Weidl, D. Winske, C. Niemann We present measurements of the collisionless coupling between an exploding laser-produced plasma (LPP) and a large, magnetized ambient plasma. The LPP was created by focusing the Raptor laser (400J, 40ns) on a planar plas- tic target embedded in the ambient Large Plasma Device (LAPD) plasma at the University of Californa, Los Angeles. The resulting ablated material moved parallel to the background magnetic field, interacting with the ambient plasma along the full 17m length of the LAPD. A high temporal and spectral resolution monochrometer measured fluorescence from debris and ambient ions to deter- mine the debris velocity distribution by charge state and study the fast electron precursor to the LPP. Measurements are compared to hybrid simulations of quasi-parallel shocks. [Preview Abstract] |
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NP10.00139: Collisionless and Kinetic Phenomena of Interpenetrating Plasma Streams via Neutron Self-Emission J.S. Ross, D.P. Higginson, R. Hatarik, A. Link, H.-S. Park, D.D. Ryutov, S.V. Weber, S.C. Wilks, F. Fiuza, C.K. Li, H. Sio, A.B. Zylstra Recent NIF experiments focus on the generation and diagnosis of collisionless shocks relevant to astrophysical phenomena such as supernova remnants and gamma ray bursts. In the experiments, two opposing CD laser-generated plasmas flow into each other at high velocity (\textasciitilde 1000 km/s). As the ion-ion collisional mean-free-path is near to or greater than the system size, the flows interpenetrate and neutrons are generated via beam-beam deuteron interactions. We model this system using the hybrid particle-in-cell code LSP with electric and magnetic fields suppressed to capture the full temporal and spatial size of the experiment. These simulations show good agreement with the yield, spectrum and spatial/temporal profiles of the neutrons observed in the experiment. When one CD foil is replaced with CH an asymmetry develops in the neutron spectrum that is caused by the Doppler shift related to the flow velocity. Additionally, in this case the neutron yield is found to be lower in the simulations than is observed experimentally, which indicates that the deuterons thermalize more efficiently in the experiment. This suggests that another mechanism is responsible for this yield enhancement other than small angle scattering since it is included in the simulations. Possible mechanisms such as scattering across Weibel-mediated magnetic filaments and large-angle Coulomb scattering will be evaluated and discussed. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
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NP10.00140: Weibel instability mediated collisionless shocks using intense laser-driven plasmas. Sasi Palaniyappan, Chengkun Huang, Donald Gautier, Juan Fernandez, Wenjun Ma, Jorg Schreiber The origin of cosmic rays remains a long-standing challenge in astrophysics and continues to fascinate physicists. It is believed that ``collisionless shocks'' -- where the particle Coulomb mean free path is much larger that the shock transition -- are a dominant source of energetic cosmic rays. These shocks are ubiquitous in astrophysical environments such as gamma-ray bursts, supernova remnants, pulsar wind nebula and coronal mass ejections from the sun. Several spacecraft observations have revealed acceleration of charged particles, mostly electrons, to very high energies with in the shock front. There is now also clear observational evidence that supernova remnant shocks accelerate both protons and electrons. The understanding of the microphysics behind collisionless shocks and their particle acceleration is tightly related with nonlinear basic plasma processes and remains a grand challenge. In this poster, we will present results from recent experiments at the LANL Trident laser facility studying collisionless shocks using intense ps laser (80J, 650 fs -- peak intensity of 10$^{\mathrm{20}}$ W/cm$^{\mathrm{2}})$ driven near-critical plasmas using carbon nanotube foam targets. A second short pulse laser driven protons from few microns thick aluminum foil is used to image the laser-driven plasma. [Preview Abstract] |
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NP10.00141: Weibel magnetic field competes with Biermann fields in laser-solid interactions Nitin Shukla, Kevin Schoeffler, Jorge Vieira, Ricardo Fonseca, Luis Silva Biermann battery induced magnetic fields caused by non-parallel density and temperature gradients, first investigated experimentally [ J. A. Stamper et al, PRL 26, 1012 (1971)], continue to be measured in many current experiments. A detailed study of Biermann generated magnetic fields in collisionless systems has been carried out [K. M. Schoeffler et al, POP 23, 056304 (2016)], showing that for large system sizes $(L/d_e \ge 100)$, where de is the electron inertial length, the Weibel instability dominates as the major source of magnetic field. In this work, we demonstrate the possibility of experimentally generating this strong Weibel magnetic field. We model, using ab initio PIC [R. A Fonseca et al, Lec. Notes Comput. Sci. 2331, 342 (2002)] simulations, the interaction of a short (~ps) high intensity $(a_0 \ge 1)$ laser pulse, with a target of sufficiently large gradient scale length, L. The expanding hot energetic electron population generated by the laser produces an anisotropy in the velocity distribution. This anisotropy provides the free energy that drives the Weibel instability that appears on the surfaces of the target and dominates over the Biermann battery field. [Preview Abstract] |
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NP10.00142: Electron acceleration in collisionless shocks and magnetic reconnection by laser-produced plasma ablation Jaehong Park, Anatoly Spitkovksy, Will Fox, Amitava Bhattacharjee We perform particle-in-cell simulations of collisionless shocks and magnetic reconnection generated by ablated plasma expanding into a magnetized background plasma. We find: (1) The simulated proton radiography produces different morphology of the shock structure depending on the orientation of the magnetic field and can be used to identify a shock in the experiment. Electrons are accelerated by the whistler waves generated at oblique sites of the shock. (2) Forced collisionless magnetic reconnection is induced when the expanding plumes carry opposite magnetic polarities and interact with a background plasma. Electrons are accelerated at the reconnection X line and reveal a power-law distribution as the plasma beta is lowered, {\$}$\backslash $beta$=$0.08{\$}. As the plasma beta is increased, {\$}$\backslash $beta$=$0.32{\$}, the 1st order Fermi mechanism against the two plasma plumes contributes to the electron acceleration as well as the X line acceleration. Using 3-D simulations, we also explore the effect of 3-D instabilities (Weibel instability or drift-kink) on particle acceleration and magnetic field annihilation between the colliding magnetized plumes [Preview Abstract] |
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NP10.00143: A multiscale strength model in HYDRA M. M. Marinak, N. R. Barton We describe a multiscale strength model recently implemented in HYDRA. The model incorporates results from a hierarchy of methods which span from the atomistic to the continuum level. Those are obtained from focused physics codes that treat density functional theory, molecular statics, molecular dynamics, dislocation dynamics and continuum mechanics. The model is designed to handle extreme pressures and temperatures, and is especially appropriate for strain rates in excess of 10$^{\mathrm{4}}$ s$^{\mathrm{-1}}$. As such it can be used to provide insight into HEDP experimental observations. The model has demonstrated success in capturing planar Rayleigh-Taylor growth for \textasciitilde 1 Mbar shocks in Ta and V. [N. R. Barton and M. Rhee, Journal of Applied Physics \textbf{114}, 123507 (2013)] *This work was performed under the auspices of the Lawrence Livermore National Security, LLC, (LLNS) under Contract No. DE-AC52-07NA27344 [Preview Abstract] |
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NP10.00144: Measuring the properties of shock released Quartz and Parylene-N James Hawreliak, Max Karasik, Jaechul Oh, Yefim Aglitskiy The high pressure and temperature properties of Quartz and hydrocarbons are important to high energy density (HED) research and inertial confinement fusion (ICF) science. The bulk of HED material research studies the single shock Hugoniot. Here, we present experimental results from the NIKE laser where quartz and parylene-N are shock compressed to high pressure and temperature and the release state is measured through x-ray imaging. The shock state is characterized by shock front velocity measurements using VISAR and the release state is characterized by using side-on streaked x-ray radiography. [Preview Abstract] |
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NP10.00145: Experimental design to measure oxygen opacity at high density and temperature Paul Keiter, S. Turck-Chieze, J. Colgan, J.-E. Ducret, C. J. Fontes, J. A. Guzik, D. P. Kilcrease, M. La Pennec, R. C. Mancini, K. Mussack, C. Orban, T. S. Perry, P. L. Poole, Matt Trantham In recent years, there has been a debate over the abundances of heavy elements (Z \textgreater 2) in the solar interior. Recent solar atmosphere models [Asplund 2009] find a significantly lower abundance for C, N, and O compared to models used roughly a decade ago. This discrepancy has led to an investigation of opacities through laboratory experiments and improved opacity models for many of the larger contributors to the sun's opacity, including iron and oxygen. Recent opacity measurements of iron disagree with opacity model predictions [Bailey et al, 2015]. Although these results are still controversial, repeated scrutiny of the experiment and data has not produced a conclusive reason for the discrepancy. New models have been implemented in the ATOMIC opacity code for C, O and Fe to address the solar abundance issue [Colgan, 2013]. Armstrong et al [2014] have also implemented changes in the ATOMIC code for low-Z elements. However, no data currently exists to test the low-Z material models in the regime relevant to the solar convection zone. We present an experimental design using the opacity platform developed at the National Ignition Facility to study the oxygen opacity at densities and temperatures near the solar convection zone conditions.\\ This work is~funded~by the~U.S. DOE, through the~NNSA-DS and SC-OFES Joint Program in HEDPLP, grant No.~DE-NA0001840, and the NLUF Program, grant No.~DE-NA0000850, and~through LLE, University of Rochester~by the NNSA/OICF under Agreement No. DE-FC52-08NA28302. [Preview Abstract] |
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NP10.00146: Ionization effects and modeling considerations for sudden viscous dissipation in compressing plasma turbulence Seth Davidovits, Nat Fisch Turbulent plasma flow, amplified by rapid 3D compression, can be suddenly dissipated under continuing compression.\footnote{S. Davidovits and N. J. Fisch PRL \textbf{116}, 105004 (2016)} This sudden dissipation comes about because the plasma viscosity is very sensitive to temperature, $\mu \sim T^{5/2}$. We discuss approaches to constructing simple models to capture the turbulence energy growth and dissipation during rapid plasma compressions. Additionally, we explore the effects on compressing turbulence of plasma ionization during compression, to which the viscosity is also very sensitive. We show plasma ionization during compression enables larger turbulence growth, compared to when there is no plasma ionization. Further, ionization during compression can prevent the sudden dissipation effect, and can also make the difference between increasing and decreasing turbulence energy under compression. The influence exerted by ionization opens up the prospect for control of turbulence growth and sudden dissipation timing through choice of the plasma ion species. [Preview Abstract] |
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NP10.00147: Detailed opacity calculations for stellar models Jean-Christophe Pain, Franck Gilleron We present a state of the art of precise spectral opacity calculations illustrated by stellar applications. The essential role of laboratory experiments to check the quality of the computed data is underlined. We review some X-ray and XUV laser and Z-pinch photo-absorption measurements as well as X-ray emission spectroscopy experiments of hot dense plasmas produced by ultra-high-intensity laser interaction. The measured spectra are systematically compared with the fine-structure opacity code SCO-RCG. Focus is put on iron, due to its crucial role in the understanding of asteroseismic observations of Beta Cephei-type and Slowly Pulsating B stars, as well as in the Sun. For instance, in Beta Cephei-type stars (which should not be confused with Cepheid variables), the iron-group opacity peak excites acoustic modes through the kappa-mechanism. A particular attention is paid to the higher-than-predicted iron opacity measured on Sandia's Z facility [J. Bailey et al., Nature 517, 56 (2015)] at solar interior conditions (boundary of the convective zone). We discuss some theoretical aspects such as orbital relaxation, electron collisional broadening, ionic Stark effect, oscillator-strength sum rules, photo-ionization, or the ''filling-the-gap'' effect of highly excited states. [Preview Abstract] |
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NP10.00148: DIAGNOSTIC TECHNIQUES, FUZE, ZAP-HD, MIRRORS AND OTHER MAGNETIC CONFINEMENT |
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NP10.00149: Design of a phase contrast imaging diagnostic for the Wendelstein 7-X stellarator E. M. Edlund, M. Porkolab, O. Grulke, L.-G. B{\"o}ttger, C. Sehren The Wendelstein 7-X stellarator at IPP Greifswald commenced operation in 2015, and while its design has been aimed at minimizing neoclassical transport, turbulent transport is expected to be strongly affected by the magnetic geometry. With this in mind, MIT and IPP-Greifswald scientists have undertaken a project to design and implement a phase contrast imaging (PCI) diagnostic to measure turbulence in W7-X in the OP1.2 operating phase starting in 2017. The principle and design aspects of the PCI method have been described in numerous past publications [1,2]. In W7-X the PCI system will have two imaging systems differing only in the angle of the spatial mask that selects for magnetic pitch angle, and will produce measurements of poloidal and radial correlations. A series of remotely controllable optics will allow the beam size and image magnification to be adjustable. We expect sensitivity to fluctuations in the range of 2 kHz to approximately 2 MHz and wavenumbers in the range of 1 cm$^{-1}$ to 30 cm$^{-1}$ which should allow us to detect ITG, TEM and possibly ETG turbulence.\\ $[1]$ M. Porkolab et al, IEEE Trans. Plasma Sci. 34, 229 (2006).\\ $[2]$ J. Dorris et al, Rev. Sci. Instrum. 80, 023503 (2009). [Preview Abstract] |
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NP10.00150: Bayesian Gaussian Process Tomography of W7-X bolometers using the Minerva framework Jakob Svensson, Daihong Zhang We develop a new Bayesian tomographic method based on Gaussian processes (Gaussian Process Tomography, GPT) where the model complexity is adjusted automatically, varying between 1D flux surface constancy and full 2D using a Bayesian Occam's razor criteria. The GPT method for non-flux surface constrained tomography has been prevously developed and used for soft x-ray, bolometer, interferometer and current tomography problems. In this paper we present an extension of this method which allows for a probabilistic flux surface constraint, that finds the most probable underlying complexity of the emission distribution. The distribution is defined in 2D flux coordinates, where the poloidal coordinate is described by a periodic Gaussian process. As with the standard GPT method, this method also gives uncertainties of the tomographic reconstruction that includes uncertainties both from measurements and from intrinsic ambiguities of the ill-posed tomography problem. The method has been applied to the bolometer system for the first experimental phase of W7-X and results will be shown here. The model has been implemented in the Minerva Bayesian modeling framework, which is used for a number of W7-X diagnostics. [Preview Abstract] |
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NP10.00151: Fractional pressure measurements in the W7-X startup phase with a spectroscopically assisted Penning gauge Thierry Kremeyer, Oliver Schmitz, Uwe Wenzel, Kurt Flesch Studies of helium exhaust from stellarator divertors is important to qualify for maintaining low overall helium concentrations for future reactors. Penning gauges assisted by spectroscopy were used to measure total neutral pressure and to resolve the D and He partial pressures [T. Denner et al. RSI 67 (1996) 3515]. A similar system was installed on an outboard vacuum flange as a generic feasibility test for W7-X. In this contribution, initial results from the W7-X startup phase are shown. A compact CCD spectrometer with a range from 500 to 1000 nm was used to observe the Penning discharge. With a long integration time of 25s, He lines can be seen down to 10-5 mbar and Hα lines down to 10-6 mbar. The impact of He cleaning discharges on the in-situ neutral gas spectrum was measured and will be discussed in comparison to the mass spectrometers on the device. This initial test is the basis for developing optimized Penning gauges, which will be deployed to measure the D/He pressure ratios inside of the island divertor. The status of this development will be discussed. [Preview Abstract] |
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NP10.00152: Results from Three Extreme Ultraviolet Spectrometers for Impurity Monitoring on NSTX-U M.E. Weller, P. Beiersdorfer, V.A. Soukhanovskii, E.W. Magee, F. Scotti, M.L. Reinke The National Spherical Torus Experiment -- Upgrade (NSTX-U) has been performing plasma operations for over a year and one of the focuses of study has been to monitor the levels of impurities emitting from plasma facing components (PFC). To monitor the levels of impurities three high resolution flat field grazing incident extreme ultraviolet (EUV) spectrometers capable of time resolution below 10 ms have been implemented on NSTX-U. The spectrometers are dubbed the X-ray and Extreme Ultraviolet Spectrometer (XEUS, 8 -- 70 {\AA}), the Metal Monitor and Lithium Spectrometer Assembly (MonaLisa, 50 -- 220 {\AA}), and the Long-Wavelength and Extreme Ultraviolet Spectrometer (LoWEUS, 190 -- 440 {\AA}). Confirmed lines measured by the spectrometers emit from He, Li, B, C, O, Fe, and Ni. Results of trends of various lines and line ratios measured are presented, including the Lyman-$\alpha $ (2p $\to $ 1s) transitions of O VIII at 18.9 {\AA}, C VI at 33.7 {\AA}, B V at 48.6 {\AA}, Li III at 134.9 {\AA}, and He II at 303.8 {\AA}. Future plans include utilizing high-Z PFCs made of molybdenum and tungsten and also implementing a new laser blow-off (LBO) system. [Preview Abstract] |
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NP10.00153: High-k Scattering Receiver Mixer Performance for NSTX-U Robert Barchfeld, Paul Riemenschneider, Calvin Domier, Neville Luhmann, Yang Ren, Robert Kaita The High-k Scattering system detects primarily electron-scale turbulence k$\theta $ spectra for studying electron thermal transport in NSTX-U. A 100 mW, 693 GHz probe beam passes through plasma, and scattered power is detected by a 4-pixel quasi optical, mixer array. Remotely controlled receiving optics allows the scattering volume to be located from core to edge with a k$\theta $ span of 7 to 40 cm-1. The receiver array features 4 RF diagonal input horns, where the electric field polarization is aligned along the diagonal of a square cross section horn, at 30 mm channel spacing. The local oscillator is provided by a 14.4 GHz source followed by a x48 multiplier chain, giving an intermediate frequency of \textasciitilde 1 GHz. The receiver optics receive 4 discreet scattering angles simultaneously, and then focus the signals as 4 parallel signals to their respective horns. A combination of a steerable probe beam, and translating receiver, allows for upward or downward scattering which together can provide information about 2D turbulence wavenumber spectrum. IF signals are digitized and stored for later computer analysis. The performance of the receiver mixers is discussed, along with optical design features to enhance the tuning and performance of the mixers. [Preview Abstract] |
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NP10.00154: Progress on MSE-LIF diagnostic for NSTX-U Y. Sechrest, F. M. Levinton, M. Galante, D. DiCicco, H. La Fleur, D. Cylinder, H. Yuh The motional Stark effect with laser-induced fluorescence diagnostic (MSE-LIF) was installed on NSTX during the 2011 run year. The MSE-LIF will enable radially resolved measurements of the magnetic field pitch angle and magnitude, both of which can be used to constrain plasma equilibrium reconstructions. A diagnostic neutral beam with low axial energy spread, low divergence, and high reliability has been developed. It operates routinely at 35 kV and 40~mA. A laser has been developed with high power (~10 W) and optimal linewidth matched to the energy spread of the neutral beam (~6\,GHz). The laser wavelength is near 651 nm for a match to the Doppler-shifted Balmer-alpha transition in the beam neutrals. The unique high-power, moderate linewidth laser system utilizes a 19 emitter diode laser bar and feedback from a volume Bragg grating for line width narrowing. Progress on the development of the MSE-LIF diagnostic toward initial operation during NSTX-U plasma operations will be presented, and highlights from the first run year will be discussed. [Preview Abstract] |
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NP10.00155: Long-Pulse Integrator Testing with DIII-D Magnetic Diagnostics Ilia Slobodov, Kenneth Miller, Timothy Ziemba, James Prager, John Carscadden, Eric Hanson Eagle Harbor Technologies (EHT), Inc. has developed a high-gain integrator for magnetic diagnostics that meets ITER specifications including integration time and integration error limits. EHT has conducted testing of this long-pulse integrator at DIII-D with existing DIII-D magnetic probes. The EHT long-pulse integrator was operated for several hours up to a full day. During a single period of EHT integrator operation, DIII-D was pulsed multiple times. The multiple pulses from the DIII-D magnetic diagnostics can be clearly resolved in the integrator signal output. The results are compared to DIII-D measurements. EHT also operated the long pulse integrator in High Dynamic Range Mode (HDRM), which effectively allows for a dramatic increase in measurement bit depth for higher resolution signal acquisition with the same diagnostic and digitizers presently available on DIII-D. Additionally, EHT has tested a new microprocessor and FPGA-based digitizer, which can be included on the integrator PCB, for a single board magnetic diagnostic solution. [Preview Abstract] |
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NP10.00156: Magnetic field amplitude and pitch angle measurements using Spectral MSE on EAST and Alcator C-Mod Ken Liao, William Rowan, Jia Fu, Bo Lyu, Ying-Ying Li, Oleksandr Marchuk, Yuri Ralchenko Magnetic field amplitude and pitch angle measurements follow from the analysis of the Motional Stark Effect spectrum emitted by high energy neutral beam emission in tokamaks. Here we focus on deriving these quantities on EAST and Alcator C-Mod. These measurements provide a cross check for the polarimetry MSE diagnostics, and also act as a proof of principle for a spectral MSE diagnostic, which could potentially provide real-time measurements of the magnetic field and be used to increase the accuracy of equilibrium reconstruction. Measurement uncertainty is evaluated using the NBASS synthetic diagnostic. The same code allows design of measurements with improved accuracy such as spectral measurements techniques which take advantage of polarization. Accurate fitting of the MSE spectrum requires taking into account non-statistical beam excited state populations. The spectral MSE analysis techniques have applications to measurement of the beam density, which allows for improved analysis of the charge exchange recombination spectroscopy diagnostic. Resolution of beam components improves with increased beam energy and magnetic field, so these techniques have high applicability to future fusion devices. [Preview Abstract] |
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NP10.00157: Analysis of Al and Fe transport coefficients in LHD core plasmas based on space-resolved EUV spectroscopy Xianli Huang, Shigeru Morita, Tetsutarou Oishi, Izumi Murakami, Motoshi Goto, Hongming Zhang, Yang Liu With impurity pellet injection and a one-dimensional transport code, the transport of Al and Fe ions has been analyzed in the Large Helical Device (LHD) based on extreme ultraviolet (EUV) space-resolved spectroscopy. The total density of Fe ions in the plasma core has been analyzed from the radial profiles of the Fe n$=$3-2 L$_{\mathrm{\alpha }}$ emissions. When the n$_{\mathrm{e}}$ profile is peaked or hollow, the Fe density also exhibits a peaked or hollow profile, respectively. Fe transport analysis shows that the convective velocity (V) is outward in the plasma core and inward near the edge when the n$_{\mathrm{e}}$ profile is hollow. On the other hand, the V takes negative value over the whole radial range in the peaked n$_{\mathrm{e}}$ profile. Therefore, the different Fe density profiles between peaked and hollow n$_{\mathrm{e}}$ profiles can be explained by the significant difference in the V profile. Comparison of the transport coefficients between Al and Fe shows the magnitude of V for Al ions seems to be smaller than that for Fe ions while the difference in the diffusion coefficient profile is not significant. The difference in the decay time of line emissions between the two speices is attributed to the weaker convection for the Al. [Preview Abstract] |
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NP10.00158: Observation of the electron density fluctuations by using the O-mode Microwave Imaging Reflectometry in LHD Yoshio Nagayama, Soichiro Yamaguchi, Hayato Tsuchiya, Daisuke Kuwahara Visualization of local electron density fluctuations will be very useful to study the physics of confinement and instabilities in fusion plasma. In the Large Helical Device (LHD), the O-mode microwave imaging reflectometry (O-MIR) has been intensively developed in order to visualize the electron density fluctuations. The frequency is 26 -- 34 GHz. This corresponds to the electron density of 0.8 -- 1.5 × 10$^{\mathrm{19}}$ m$^{\mathrm{-3}}$. The plasma is illuminated by the Gaussian beam with four frequencies. The imaging optics make a plasma image onto the newly developed 2D (8×8) Horn-antenna Millimeter-wave Imaging Device (HMID). In HMID, the signal wave that is accumulated by the horn antenna is transduced to the micro-strip line by using the finline transducer. The signal wave is mixed by the double balanced mixer with the local wave that is delivered by cables. By using O-MIR, electron density fluctuations are measured at the H-mode edge and the ITB layer in LHD. [Preview Abstract] |
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NP10.00159: Experimental validation of non-uniformity effect of the radial electric field on the edge transport barrier formation in JT-60U H-mode plasmas Kensaku Kamiya, Kimitaka Itoh, Sanae-I Itoh Non-uniformity effects of the edge radial electric field (Er) on the edge transport barriers (ETBs) formation have been identified with high-spatiotemporal resolution spectroscopic measurement. We found the decisive importance of Er-curvature (2nd derivative of Er) on ETB formation during ELM-free H-mode phase, but there is only a low correlation with the Er-shear (1st derivative of Er) value at the peak of normalized ion temperature gradient in the pedestal region. Observation of a uniform toroidal MHD oscillation (i.e. Geodesic Acoustic Mode having toroidal mode number n $=$ 0) during the ETBs formation can also support the hypothesis of turbulence suppression in association with Zonal-flow (and/or Er-curvature). Furthermore, in the ELMing phase, the effect of curvature is also quantified in terms of the relationship between pedestal width and thickness of the layer of inhomogeneous Er. This is the fundamental basis to understand the structure of transport barriers in fusion plasmas. [Preview Abstract] |
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NP10.00160: A polarimeter for JT-60SA: chords layout study with V3FIT for q profile reconstruction David Terranova, Alexandru Boboc, Christophe Gil, Sorin Soare, Francesco Orsitto, Ryota Imazawa JT-60SA is the new tokamak device that is being built in Japan under the Broader Approach Satellite Tokamak Programme and the Japanese National Programme [JT-60SA Research Plan, Version 3.3, March 2016, www.jt60sa.org/pdfs/JT-60SA\textunderscore Res\textunderscore Plan.pdf] and will operate as a satellite machine for ITER. To provide valuable information for the steady state scenario for ITER and the design of DEMO, a high $\beta_{\mathrm{N}}$ scenario is included in the program, where the real-time control of the q-profile is needed. In this work we present a study of the geometry of the polarimetry chords, derived from a true realistic CAD-driven feasibility study, aiming at an optimization in terms of q-profile reconstruction, using the V3FIT code [J.D. Hanson et al., Nucl. Fusion 49 (2009) 075031]. Some magnetic and kinetic measurements are considered along with the FIR poloidal polarimeter in order to assess the possibility of estimating q in the core with the required accuracy (around 10{\%}) providing a diagnostic for a continuous measurement useful in high density pulses where MSE measurements would not have adequate time resolution. [Preview Abstract] |
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NP10.00161: Study of the harmonic oscillation on EAST by an eight-channel Doppler Backscattering (DBS) system. C. Zhou, A.D. Liu, M. Y. Wang, J. Q. Hu, J. Zhang, H. Li, T. Lan, J. L. Xie, W. D. Liu, C. X. Yu, E. J. Doyle The eight-channel DBS system has been installed for turbulence measurements in such plasmas. The frequency range is 55 to 75 GHz, covering the entire H-mode pedestal, with a turbulence wavenumber range of 4-12/cm.. A harmonic oscillation has been observed by DBS on EAST during ELMy-free H mode. The fundamental frequency of the coherent oscillation is 12-20 kHz and 2nd-8th harmonic are observed, and the radial coverage is from the edge to rho\textasciitilde 0.85. [Preview Abstract] |
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NP10.00162: A novel approach to estimate the Doppler shift frequency from quadrature mixer output M.Y. Wang, A.D. Liu, C. Zhou, J.Q. Hu, J. Zhang, H. Li, T. Lan, J.X. Xie, W.D Liu, C.X. Yu Doppler backscattering system (DBS) has been widely used in Magnetic confinement fusion devices to measure the density fluctuations and propagation velocity of the turbulence. However, the received signals of DBS usual include both forward scattering and backscattering components, which caused interference as calculating the Doppler shift frequency from the backscattering components. A novel method will be introduced here to estimate the Doppler shift frequency by separating the forward scattering and backscattering components though the cross-phase spectrum between I-signal and Q-signal from quadrature mixer, based on the difference of symmetrical characteristic between forward scattering and backscattering signal spectrum. It is proven that this method is more effective than the tradition approached such as the Center of Gravity of the spectrum and the Gaussian fitting method. *Work supported by the Natural Science Foundation of China (NSFC) under 11475173, 11505184, National Magnetic Confinement Fusion Energy Development Program of China under 2013GB106002 and 2014GB109002. [Preview Abstract] |
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NP10.00163: Development of Local Oscillator Integrated Antenna Array for Electron Cyclotron Emission Imaging Diagnostics. Daisuke Kuwahara, Naoki Ito, Yoshio Nagayama, Hayato Tsuchiya, Masayuki Yoshikawa, Junko Kohagura, Tomokazu Yoshinaga, Soichiro Yamaguchi, Yuichiro Kogi, Atsushi Mase Microwave imaging systems include difficulties in terms of multi-channelization and cost. Our group solved these problems by developing a Horn-antenna Mixer Array (HMA), a 50 - 110 GHz 1-D heterodyne-type antenna array, which can be easily stacked as a 2-D receiving array. However, the HMA still evidenced problems owing to the requirement for local oscillation (LO) optics and an expensive high-power LO source. To solve this problem, we have developed an upgraded HMA, named the Local Integrated Antenna array (LIA), in which each channel has an internal LO supply using a frequency multiplier integrated circuit. Therefore, the proposed antenna array eliminates the need for both the LO optics and the high-power LO source. However, the LIA still has problems, that the instabilities of the sensitivity and poor channel isolation. This paper describes the principle of the LIA, and solutions of above-mentioned problems. [Preview Abstract] |
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NP10.00164: Hybrid model for simulation of plasma jet injection in tokamak Sergei A. Galkin, I.N. Bogatu Hybrid kinetic model of plasma treats the ions as kinetic particles and the electrons as charge neutralizing massless fluid. The model is essentially applicable when most of the energy is concentrated in the ions rather than in the electrons, i.e. it is well suited for the high-density hyper-velocity C60 plasma jet. The hybrid model separates the slower ion time scale from the faster electron time scale, which becomes disregardable. That is why hybrid codes consistently outperform the traditional PIC codes in computational efficiency, still resolving kinetic ions effects. We discuss 2D hybrid model and code with exact energy conservation numerical algorithm and present some results of its application to simulation of C$_{\mathrm{60}}$ plasma jet penetration through tokamak-like magnetic barrier. We also examine the 3D model/code extension and its possible applications to tokamak and ionospheric plasmas. [Preview Abstract] |
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NP10.00165: Fast, deep record length, time-resolved visible spectroscopy of plasmas using fiber grids Samuel Brockington, Andrew Case, Edward Cruz, F. Douglas Witherspoon, Robert Horton, Ruth Klauser, D. Q. Hwang HyperV Technologies is developing a fiber-coupled, deep-record-length, low-light camera head for performing high time resolution spectroscopy on visible emission from plasma events. New solid-state Silicon Photo-Multiplier (SiPM) chips are capable of single photon event detection and high speed data acquisition. By coupling the output of a spectrometer to an imaging fiber bundle connected to a bank of amplified SiPMs, time-resolved spectroscopic imagers of 100 to 1,000 pixels can be constructed. Target pixel performance is 10 Megaframes/sec with record lengths of up to 256,000 frames yielding 25.6 milliseconds of record at10 Megasamples/sec resolution. Pixel resolutions of 8 to 12 bits are pos- sible. Pixel pitch can be refined by using grids of 100 µm to 1000 µm diameter fibers. A prototype 32-pixel spectroscopic imager employing this technique was constructed and successfully tested at the University of California at Davis Compact Toroid Injection Experiment (CTIX) as a full demonstration of the concept. Experimental results will be dis-cussed, along with future plans for the Phase 2 project, and potential applications to plasma experiments . [Preview Abstract] |
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NP10.00166: Microwave Imaging Radar Reflectometer System Utilizing Digital Beam Forming Fengqi Hu, Meijiao Li, Calvin W. Domier, Xiaoguang Liu, Neville C. Luhmann, Jr. Microwave Imaging Reflectometry is a radar-like technique developed to measure the electron density fluctuations in fusion plasmas. Phased Antenna Arrays can serve as electronically controlled ``lenses'' that can generate the required wavefronts by phase shifting and amplitude scaling, which is being realized in the digital domain with higher flexibility and faster processing speed. In the transmitter, the resolution of the phase control is 1.4 degrees and the amplitude control is 0.5 dB/ step. A V-band double-sided, printed bow tie antenna which exhibits 49{\%} bandwidth (46 - 76 GHz) is employed. The antenna is fed by a microstrip transmission line for easy impedance matching. The simple structure and the small antenna are suitable for low cost fabrication, easy circuit integration, and phased antenna array multi-frequency applications. In the receiver part, a sub-array of 32 channels with 200 mil spacing is used to collect~the scattered reflected signal from one unit spot on the plasma cutoff surface. Pre-amplification is used to control the noise level of the system and wire bondable components are used to accommodate the small spacing between each channel. After down converting, base band signals are digitized and processed in an FPGA module. [Preview Abstract] |
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NP10.00167: Microwave Imaging Reflectometer (MIR) Development for the EAST Tokamak Calvin Domier, Xing Hu, Alexander Spear, Yilun Zhu, Jinlin Xie, Neville Luhmann An upgraded MIR system is being developed for the EAST tokamak based on the successful DIII-D MIR system. The EAST MIR system has 8 radial channels consisting of 8 independent probing frequencies ranging from 75 to 103 GHz, driven by fast tuning synthesizers and active frequency multipliers. There are 12 poloidal channels in the heterodyne down-conversion receiver system, with each channel corresponding to a separate poloidal position inside the tokamak. The down-conversion electronics are designed to optimize signal to noise ratio and are embedded with a microcontroller to realize remote computer control. Considerable improvements are also seen in the front-end plasma facing optics. This new optical system provides features including focusing, zoom, field curvature adjustment, and incident angle adjustment. These functions can be realized together or independently depending on the configuration setup of the large aperture lenses. This MIR system is expected to be installed on the EAST tokamak in December 2016, co-located with the Electron Cyclotron Emission Imaging (ECEI) system, to simultaneously measure electron density and temperature fluctuations. [Preview Abstract] |
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NP10.00168: LH and ICRH RF electric field measurements using Doppler-free Saturation Spectroscopy E. H. Martin, A. Zafar, J. B. O. Caughman, R. C. Isler, G. L. Bell The physics mechanisms of wave heating and current drive processes in the bulk hot plasma are generally well identified, however, details of the wave-plasma interaction in the cold plasma edge are still not fully understood. To investigate the alluding physics non-perturbative diagnostics are required due to the large energy flux traversing the space associated with the corresponding RF antenna/launcher. A spectroscopic diagnostic, based on Doppler-free saturation spectroscopy, is currently under development at ORNL that will be capable of measuring RF electric fields with high precision (20 V/cm). The RF electric field is determined by systematically fitting a Balmer series spectral line profile obtained via DFSS using a previous validated non-perturbative quantum mechanically model. The spectral line profile is measured using Doppler-free saturation spectroscopy (DFSS). DFSS is a laser-based technique involving two counter-propagating beams, referred to as the pump and probe, which are made to overlap at a single point in space. The frequency of the laser is swept over that associated with the electronic transition of interest and the probe beam absorption intensity is measured. In this presentation an active spectroscopic technique allowing for measurements of the RF electric field driving wave-plasma interactions for lower hybrid (LH) and ion cyclotron resonance heating (ICRH) systems, based on DFSS, will be discussed. Initial measurements of the electric field in the magnetized capacitively coupled RF sheath obtained on a laboratory test stand will be presented. [Preview Abstract] |
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NP10.00169: Reconstruction of full electron energy distributions by Poisson-regularized spectral inversion of x-ray Bremsstrahlung emissions in the PFRC device Charles Swanson, Peter Jandovitz, Alexandra Bosh, Samuel Cohen The PFRC is an odd-parity Rotating Magnetic Field (RMF) driven Field-Reversed Configuration plasma confinement experiment equipped with Si-PIN and SDD x-ray detectors. It is predicted that the electron energy distribution is non-thermal when the RMF is active. Using a novel inversion technique, we present full electron distribution functions as reconstructed (``spectrally inverted'') from the x-ray Bremsstrahlung emissions. This method regularizes the inverse treating the measurement as a Poisson random variable, as opposed to state-of-the-art methods which assume a Normal random variable. The method maximizes the log-likelihood of the solution, determined from Bayes' Theorem. [Preview Abstract] |
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NP10.00170: Modeling and Simulation for Nanoparticle Plasma Jet Diagnostic Probe for Runaway Electron Beam-Plasma Interaction I. N. Bogatu, S. A. Galkin The C$_{\mathrm{60}}$ nanoparticle plasma jet (NPPJ) rapid injection into a tokamak major disruption is followed by C$_{\mathrm{60}}$ gradual fragmentation along plasma-traversing path. The result is abundant C ion concentration in the core plasma enhancing the potential to probe and diagnose the runaway electrons (REs) during different phases of their dynamics. A C$_{\mathrm{60}}$/C NPPJ of \textasciitilde 75 mg, high-density (\textgreater 10$^{\mathrm{23}}$ m$^{\mathrm{-3}})$, hyper-velocity (\textgreater 4 km/s), and uniquely fast response-to-delivery time (\textasciitilde 1 ms) has been demonstrated on a test bed. It can rapidly and deeply deliver enough mass to increase electron density to \textasciitilde 2.4x10$^{\mathrm{21}}$ m$^{\mathrm{-3}}$, \textasciitilde 60 times larger than typical DIII-D pre-disruption value. We will present the results of our on-going work on: 1) self-consistent model for RE current density evolution (by Dreicer mechanism and ``avalanche'') focused on the effect of fast and deep deposition of C ions, 2) improvement of single C$_{\mathrm{60}}^{\mathrm{q+}}$ fragmenting ion penetration model through tokamak B(R)-field and post-TQ plasma, and 3) simulation of C$_{\mathrm{60}}^{\mathrm{q+}}$ PJ penetration through the DIII-D characteristic \textasciitilde 2 T B-field to the RE beam central location by using the Hybrid Electro-Magnetic 2D code (HEM-2D. [Preview Abstract] |
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NP10.00171: Utilization of Double Langmuir Probes on Proto-MPEX Nischal Kafle, John B. Caughman, Juan F. M. Caneses, Richard H. Goulding, Elijah. H. Martin, David. C. Donovan Langmuir probes (LP) are a robust, simply constructed, and inexpensive diagnostic tool. They are routinely used to measure the electron temperature and density in plasmas. However, the uncompensated single-tip LP has demonstrated limitations in time fluctuating plasma potential. The measurement quality can be improved by implementing compensation or by using a double-tipped probe. Double Langmuir probes (DLPs) are referenced against each other instead of the device vessel and therefore are less susceptible to fluctuations in RF plasmas. DLPs are being used to measure plasma parameters at multiple locations in the Proto-MPEX experiment at Oak Ridge National Laboratory. Proto-MPEX is a linear plasma device that combines a helicon plasma source with additional microwave and radio frequency heating to deliver a high plasma heat flux at a target. An electron temperature of 3-6 eV and density of 3e19 - \textgreater 5e19 m$^{\mathrm{-3}}$ has been measured near the target in Proto-MPEX for different magnetic field configurations, with peak magnetic fields \textgreater 1 T. Plasma density and temperature tend to be higher closer to the plasma source and are strongly dependent on operating pressure. This presentation will give an overview of DLP and will provide results from multiple locations and for different operating conditions. [Preview Abstract] |
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NP10.00172: Optimized and Automated design of Plasma Diagnostics for Additive Manufacture James Stuber, Morgan Quinley, Paul Melnik, Paul Sieck, Trevor Smith, Katherine Chun, Simon Woodruff Despite having mature designs, diagnostics are usually custom designed for each experiment. Most of the design can be now be automated to reduce costs (engineering labor, and capital cost). We present results from scripted physics modeling and parametric engineering design for common optical and mechanical components found in many plasma diagnostics and outline the process for automated design optimization that employs scripts to communicate data from online forms through proprietary and open-source CAD and FE codes to provide a design that can be sent directly to a printer. As a demonstration of design automation, an optical beam dump, baffle and optical components are designed via an automated process and printed. [Preview Abstract] |
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NP10.00173: Additive Manufacture (3D Printing) of Plasma Diagnostic Components and Assemblies for Fusion Experiments Morgan Quinley, Katherine Chun, Paul Melnik, Paul Sieck, Trevor Smith, James Stuber, Simon Woodruff, Carlos Romero-Talamas, William Rivera, Alexander Card We are investigating the potential impact of additive manufacturing (3D printing) on the cost and complexity of plasma diagnostics. We present a survey of the current state-of-the-art in additive manufacture of metals, as well as the design of diagnostic components that have been optimized for and take advantage of these processes. Included among these is a set of retarding field analyzer probe heads that have been printed in tungsten with internal heat sinks and cooling channels. Finite element analysis of these probe heads shows the potential for a 750K reduction in peak temperature, allowing the probe to take data twice as often without melting. Results of the evaluation of these probe heads for mechanical strength and outgassing, as well as their use on Alcator C-Mod will be presented. [Preview Abstract] |
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NP10.00174: Overview of the Fusion Z-Pinch Experiment FuZE T.R. Weber, U. Shumlak, B.A. Nelson, R.P. Golingo, E.L. Claveau, H.S. McLean, K.K. Tummel, D.P. Higginson, A.E. Schmidt Previously, the ZaP device, at the University of Washington, demonstrated sheared flow stabilized (SFS) Z-pinch plasmas [1]. Instabilities that have historically plagued Z-pinch plasma confinement were mitigated using sheared flows generated from a coaxial plasma gun of the Marshall type. Based on these results, a new SFS Z-pinch experiment, the Fusion Z-pinch Experiment (FuZE), has been constructed. FuZE is designed to investigate the scaling of SFS Z-pinch plasmas towards fusion conditions. The experiment will be supported by high fidelity physics modeling using kinetic and fluid simulations. Initial plans are in place for a pulsed fusion reactor following the results of FuZE. Notably, the design relies on proven commercial technologies, including a modest discharge current (1.5 MA) and voltage (40 kV), and liquid metal electrodes. [1] U. Shumlak et al., Nucl. Fusion 49 (2009) 075039. [Preview Abstract] |
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NP10.00175: Initial results of Fusion Z-Pinch Experiment, FuZE, and magnetic field topology analysis through data driven modeling E.L. Claveau, U. Shumlak, R.P. Golingo, B.A. Nelson, T.R. Weber, H. McLean The FuZE project is a sheared flow stabilized (SFS) Z-pinch experiment that investigates scaling the SFS Z-pinch to fusion conditions. FuZE will generate a 1 mm radius Z-pinch with a 300 kA plasma current. An array of 94 surface-mounted magnetic field probes that are embedded in the outer copper electrode provide the primary measure of the time-dependent magnetic topology of the pinch plasma. Azimuthal field measurement provide instantaneous information about the magnitude and position of the plasma current. The initial results obtained in the form of magnetic field topology are compared to previous ZaP experimental results, an experiment that investigated shear flow stabilization of Z-pinch at lower current. The magnetic field topology evolution is investigated through data-driven modeling of the characteristic dynamics. The modeling provides time evolution of large-scale structures and dynamics quantified over multiple plasma pulses. These properties can give insight about spatial and temporal propagation of fluctuations to better characterize the plasma evolution. [Preview Abstract] |
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NP10.00176: 2D Kinetic Particle in Cell Simulations of a Shear-Flow Stabilized Z-Pinch Kurt Tummel, Drew Higginson, Andrea Schmidt, Anthony Link, Harry McLean, Uri Shumlak, Brian Nelson, Raymond Golingo, Elliot Claveau The Z-pinch is a relatively simple and attractive potential fusion reactor design, but attempts to develop such a reactor have consistently struggled to overcome Z-pinch instabilities. The "sausage" and "kink" modes are among the most robust and prevalent Z-pinch instabilities, but theory and simulations suggest that axial flow-shear, $dv_{z}/dr \neq 0$, can suppress these modes. Experiments have confirmed that Z-pinch plasmas with embedded axial flow-shear display a significantly enhanced resilience to the sausage and kink modes at a demonstration current of 50kAmps. A new experiment is under way to test the concept at higher current, and efforts to model these plasmas are being expanded. The performance and stability of these devices will depend on features like the plasma viscosity, anomalous resistivity, and finite Larmor radius effects, which are most accurately characterized in kinetic models. To predict these features, kinetic simulations using the particle in cell code LSP are now in development, and initial benchmarking and 2D stability analyses of the sausage mode are presented here. These results represent the first kinetic modeling of the flow-shear stabilized Z-pinch. \\ Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
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NP10.00177: Parameter scaling toward high-energy density in a quasi-steady flow Z-pinch M.C. Hughes, U. Shumlak, B.A. Nelson, R.P. Golingo, E.L. Claveau, S.A. Doty, E.G. Forbes, B. Kim, M.P. Ross Sheared axial flows are utilized by the ZaP Flow Z-Pinch Experiment to stabilize MHD instabilities. The pinches formed are 50 cm long with radii ranging from 0.3 to 1.0 cm. The plasma is generated in a coaxial acceleration region, similar to a Marshall gun, which provides a steady supply of plasma for approximately 100 us. The power to the plasma is partially decoupled between the acceleration and pinch assembly regions through the use of separate power supplies. Adiabatic scaling of the Bennett relation gives targets for future devices to reach high-energy density conditions or fusion reactors. The applicability of an adiabatic assumption is explored and work is done experimentally to clarify the plasma compression process, which may be more generally polytropic. The device is capable of a much larger parameter space than previous machine iterations, allowing flexibility in the initial conditions of the compression process to preserve stability. [Preview Abstract] |
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NP10.00178: Z-pinch equilibrium and instability analysis with digital holographic interferometry M.P. Ross, U. Shumlak, B.A. Nelson, R.P. Golingo, M.C. Hughes, E.L. Claveau, E.G. Forbes, S. Doty, B. Kim The ZaP-HD Flow Z-Pinch project provides a platform to explore how shear flow stabilized Z-pinches could scale to high-energy-density plasma and fusion reactor conditions. ZaP-HD generates shear stabilized, axisymmetric Z-pinches with stable lifetimes approaching $60$ $\mu$s. The goal of the project is to increase the plasma density and temperature compared to the previous ZaP project by compressing the plasma to smaller radii ($\approx 1$ mm). Radial and axial plasma electron density structures are measured using digital holographic interferometry (DHI), which provides the necessary fine spatial resolution. ZaP-HD's DHI system uses a 2 ns Nd:YAG laser pulse with a second harmonic generator ($\lambda = 532$ nm) to produce holograms recorded by a Nikon D3200 digital camera. The holograms are numerically reconstructed with the Fresnel transform reconstruction method to obtain the phase shift caused by the interaction of the laser beam with the plasma. This provides a two-dimensional map of line-integrated electron density, which can be Abel inverted to determine the local number density. The DHI resolves line-integrated densities down to $3\times 10^{20}$ m$^{-2}$ with spatial resolution near $10$ $\mu$m. [Preview Abstract] |
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NP10.00179: Time-resolved Spectroscopy of a Sheared Flow Stabilized Z-pinch Plasma Eleanor Forbes The ZaP Flow Z-pinch Project investigates the use of sheared-axial flows to stabilize an otherwise unstable plasma configuration. Diagnostics with sub-microsecond resolution are required to obtain accurate time-resolved data since the plasma pulse is approximately 100 $\mu $s. Analyzing the Doppler shift of impurity line radiation from the pinch provides a measure of the velocity profile and is a reliable method of determining the plasma sheared flow. The velocity profile is spatially resolved through the use of a 20-chord fiber bundle. The ZaP-HD experiment has used a PI-MAX intensified CCD array to record a single time-resolved spectrum per plasma pulse. Obtaining the evolution of the velocity profile using this method required spectra acquired over hundreds of pulses with identical initial parameters and varying acquisition times. The use of a Kirana 05M ultra-fast framing camera is investigated for recording time-resolved velocity profiles during a single pulse. The Kirana utilizes an ultraviolet intensifier to record 180 frames of UV light at up to 2 million frames per second. An ultraviolet optics system is designed to couple the exit port of an Acton SP-500i spectrometer to the Kirana UV intensifier and focus spectra at the camera detector plane. [Preview Abstract] |
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NP10.00180: Overview of the Lockheed Martin Compact Fusion Reactor (CFR) T4B Experiment Thomas McGuire The Lockheed Martin Compact Fusion Reactor (CFR) Program endeavors to quickly develop a compact fusion power plant with favorable commercial economics and military utility. The CFR uses a diamagnetic, high beta, magnetically encapsulated, linear ring cusp plasma confinement scheme. The goal of the T4B experiment is to demonstrate a suitable plasma target for heating experiments and to characterize the behavior of plasma sources in the CFR configuration. The design of the T4B experiment will be presented, including discussion of predicted behavior, plasma sources, heating mechanisms, diagnostics suite and relevant numerical modeling. \copyright 2016 Lockheed Martin Corporation. All Rights Reserved. [Preview Abstract] |
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NP10.00181: High Power LaB$_{\mathrm{6}}$ Plasma Source Performance for the Lockheed Martin Compact Fusion Reactor Experiment Jonathon Heinrich Lockheed Martin's Compact Fusion Reactor (CFR) concept is a linear encapsulated ring cusp. Due to the complex field geometry, plasma injection into the device requires careful consideration. A high power thermionic plasma source (\textgreater 0.25MW; \textgreater 10A/cm$^{\mathrm{2}})$ has been developed with consideration to phase space for optimal coupling. We present the performance of the plasma source, comparison with alternative plasma sources, and plasma coupling with the CFR field configuration. \copyright 2016 Lockheed Martin Corporation. All Rights Reserved. [Preview Abstract] |
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NP10.00182: Development of plasma sources for ICRF heating experiment in KMAX mirror device Xuan Sun, Ming Liu, Hongshen Yi, Munan Lin, Peiyun Shi KMAX, Keda Mirror with AXisymmeticity, is a tandem mirror machine with a length of \textasciitilde 10 meters and diameters of 1.2 meters in the central cell and 0.3 meters in the mirror throat. In the past experiments, the plasma was generated by helicon wave launched from the west end. We obtained the blue core mode in argon discharge, however, it cannot provide sufficient plasma for hydrogen discharge, which is at least 10\textasciicircum 12cm-3 required for effective ICRF heating. Several attempts have thus been tried or under design to increase the central cell's plasma density: (1) a washer gun with aperture of 1cm has been successfully tested, and a plasma density of 10\textasciicircum 13 cm-3 was achieved in the west cell near the gun, however, the plasma is only \textasciitilde 10\textasciicircum 11 cm-3 in the central cell possible due to the mirror trapping and/or neutral quenching effect (2) a larger washer gun with aperture of 2.5 cm and a higher power capacitor bank are being assembled in order to generate more plasmas. In addition, how to mitigate the neutrals is under consideration (3) A hot cathode is been designed and will be tested in combination with plasma gun or alone. Preliminary results from those plasma sources will be presented and discussed. [Preview Abstract] |
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NP10.00183: Measurements and modeling of mid- and high-Z coronal plasmas from laser-irradiated planar foils Jeffrey Fein, Matt Trantham, Yechiel Frank, Erez Raicher, Moshe Fraenkel, James P. Holloway, Paul Keiter, Dov Shvarts, R. Paul Drake Plasmas generated from mid- to high-Z elements with intense (10$^{\mathrm{15}}$ W/cm$^{\mathrm{2}})$ lasers have applications as x-ray drive for indirect-drive fusion, as means to mitigate laser-plasma instabilities in direct-drive fusion, and as bright sources of x-rays for imaging dense plasmas. However, modeling of these plasmas is difficult due to the wide range of densities and temperatures present, and the resulting wide range of physical processes that determine their energetics. Measurements of electron density profiles from irradiated planar foils show a shortening gradient length scale when increasing the foil material Z. We present analytical models and Rad-Hydro simulations to assess the role of Z-dependent processes, such as distributed laser absorption and nLTE atomic physics [see Y. Frank et al., Phys. Rev. E 92, 053111 (2015)] in determining the observed profiles.* * Supported through the NNSA-DS and SC-OFES Joint Program in HED Laboratory Plasmas, by grant number~DE-NA0002956, the NLUF grant number DE-NA0002719, by the DTRA, grant number DTRA-1-10-0077 and by the NSF Graduate Research Fellowship. [Preview Abstract] |
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NP10.00184: Growth of interfacial perturbations driven by blast waves Marc Henry de Frahan, Eric Johnsen, Dov Shvarts, R. Paul Drake Hydrodynamic instabilities play important roles in a variety of high-energy-density physics flows, including problems in astrophysics and inertial confinement fusion. While classical Richtmyer-Meshkov (RM) and Rayleigh-Taylor (RT) unstable interfacial flows are relatively well understood, less is known about interactions of blast waves with interfaces. Using a new 2D high-order Discontinuous Galerkin multifluid hydro code, we simulate the interaction of a blast, modeled as a shock followed by a finite-length rarefaction, with a single-mode, perturbed interface separating heavy and light fluids. This model allows us to control, independently, the shock strength, rarefaction strength and length. Starting the blast in the heavy material gives rise to an RT-unstable configuration (driven by the rarefaction). Our findings indicate that the time-evolution of the perturbation growth can be described as a succession of three phases corresponding to different mechanisms (linear RM, combined decompression and RT with time-varying Atwood number and acceleration, and circulation-driven), which we will explain in detail and relate to the blast properties. [Preview Abstract] |
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