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 CP10: Poster Session II (ICF Compression and ICF/HED Simulation; MFE: CTH, HSX and other Stellarators; MFE: Field-reversed Configurations and SPHEROMAKs; MFE: MST and Other Reversed-field Pinches; Plasma Sources and Diagnostic)Poster
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Room: Exhibit Hall 1 |
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CP10.00001: ICF COMPRESSION AND ICF/HED SIMULATIONS |
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CP10.00002: Investigation of the transition between hydrodynamic and kinetic regimes for DT exploding pushers at OMEGA and the NIF R. Simpson, N. Kabadi, J.A. Frenje, M. Gatu Johnson, C.K. Li, F.H. Seguin, H. Sio, R.D. Petrasso, M. Rosenberg, R. Betti, H. Rinderknecht, A. Nikroo, D.T. Casey, T. Kwan, A. Simakov, S. Atzeni, C. Bellei Previous experiments were conducted to study the transition from hydrodynamic-like to ion kinetic regimes for D$^{\mathrm{3}}$He exploding pushers [1], demonstrating the importance of an ion kinetic approach for formulating more robust predictions of implosion characteristics. This presentation details a series of planned experiments at the OMEGA Facility and the NIF using thin-glass exploding pushers with DT fuel. D and T ions have the same charge, unlike D and $^{\mathrm{3}}$He, yet their masses are unaltered from the D and $^{\mathrm{3}}$He case. This allows for the investigation of whether ion-thermal decoupling and species separation are largely a result of charge or mass. [2] The initial gas fill pressure will be varied in order to scan the transition from strongly hydrodynamic to strongly kinetic implosions, while leveraging the expansive diagnostic suite developed at NIF and OMEGA. [1] M. Rosenberg et al., PRL 112, 185001 [2] H. Rinderknecht et.al., PRL 114, 025001 [Preview Abstract] |
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CP10.00003: Using secondary nuclear products for inferring the fuel areal density, convergence, and electron temperatures of deuterium filled implosions on the NIF B. Lahmann, J.A. Frenje, M. Gatu Johnson, H. Sio, N.V. Kabadi, G. Sutcliffe, F.H. Seguin, C.K. Li, R.D. Petrasso, E.P. Hartouni, H.G. Rinderknecht, D.B. Sayre, C.B. Yeamans, S.F. Khan, G.A. Kyrala, S. LePape, L. Berzak-Hopkins, N. Meezan, R. Bionta, T. Ma In deuterium-filled inertial confinement fusion implosions, 0.82 MeV $^{\mathrm{3}}$He and 1.01 MeV T born from the primary DD reaction branches can undergo fusion reactions with the thermal deuterium plasma to create secondary D$^{\mathrm{3}}$He protons and DT neutrons respectively. In regimes of moderate fuel areal density ($\rho $R $\sim $ 5 - 100 mg/cm$^{\mathrm{2}})$ the ratio of both of these secondary yields to the primary yield can be used to infer the fuel $\rho $R, convergence, and an electron temperature ($T_{e})$ \quad simultaneously. This technique has been used on a myriad of deuterium filled implosion experiments on the NIF using the nuclear time of flight (NTOF) diagnostics to measure the secondary DT neutrons and CR-39 based wedge range filters (WRFs) to measure the secondary D$^{\mathrm{3}}$He protons. Additionally, a comparative study is conducted between the nuclear inferred convergence and x-ray inferred convergence obtained on these experiments. [Preview Abstract] |
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CP10.00004: Double-peaked proton spectra from shocks in D-$^{\mathrm{3}}$He ICF capsules D.C. Wilson, A.B. Zylstra, S.M. Sepke, H. Sio, B.J. Lahmann, E. Dewald, R. Tommasini, G.A. Kyrala, A. Yi, A.N. Simakov, J.L. Kline, R.D. Petrasso, S.H. Batha Proton production in D-$^{\mathrm{3}}$He gas filled ICF capsules peaks twice during an implosion, at ``shock flash'' and bangtime. Protons at peak production rate are often down-shifted too strongly to measure. In x-ray driven capsules at NIF we have observed two peaks in the proton spectra separated by about 1.8 MeV that are associated with shocks. Two capsules had copper doped beryllium ablators, but one had silicon doped GDP. The presence of the two peaks and their proton energies agree with calculations. The lower energy peak calculates to occur earlier in the implosion after the first shock reflects off capsule center, the ``shock flash''. The second, higher energy peak, occurs when the outward moving shock reaches the incoming shell about 0.5ns later. It is partially reflected, heating the fuel near the shell. The fuel has compressed more, causing protons emitted inward to be downshifted below the threshold of detection. The outward moving protons, created near the shell, are downshifted only by the shell, not the fuel, giving less down-shift than in the first peak. [Preview Abstract] |
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CP10.00005: Techniques for Enhancing Implosion Performance on High-Foot Ignition Capsules on NIF T. R. Dittrich, O. Hurricane, L. F. Berzak Hopkins, D. A. Callahan, D. Clark, S. W. Haan, D. E. Hinkel, T. Ma, A. Nikroo, A. E. Pak, H. S. Park, J. D. Salmonson, C. R. Weber Two options that have the potential to improve implosion performance in the High-Foot series of ignition capsules on NIF [1] will be presented. The first option explores changing the shape of the x-ray drive to include a 4$^{\mathrm{th}}$ and even a 5$^{\mathrm{th}}$ shock in the implosion. According to simulations, these extra shocks improve the configuration of the assembled fuel and lead to improved confinement and performance. A ``ramp compression'' between the foot of the drive and the main pulse is also investigated. The second option studies the effect of increasing the Si dopant in a thin-shell capsule. NIF shot N150211 produced relatively high fusion yield (7.6E15 neutrons) but may have suffered from shell burn through. Increasing the Si dopant may delay this burn through yet preserve high implosion velocity. [1] O. A. Hurricane, et al., Nature, 506, 343 (2014). [Preview Abstract] |
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CP10.00006: Simulations of Converging Shock Collisions for Shock Ignition Joshua Sauppe, Evan Dodd, Eric Loomis Shock ignition (SI) has been proposed as an alternative to achieving high gain in inertial confinement fusion (ICF) targets [Betti et. al. Phys. Rev. Lett. 103 045004 (2007)]. A central hot spot below the ignition threshold is created by an initial compression pulse, and a second laser pulse drives a strong converging shock into the fuel. The collision between the rebounding shock from the compression pulse and the converging shock results in amplification of the converging shock and increases the hot spot pressure above the ignition threshold. We investigate shock collision in SI drive schemes for cylindrical targets with a polystyrene foam interior using radiation-hydrodynamics simulations with the RAGE code. The configuration is similar to previous targets fielded on the Omega laser [Tubbs et. al. Laser and Particle Beams Vol. 17 No. 3 437-449 (1999)]. The CH interior results in a lower convergence ratio and the cylindrical geometry facilitates visualization of the shock transit using an axial X-ray backlighter, both of which are important for comparison to potential experimental measurements. One-dimensional simulations are used to determine shock timing, and the effects of low mode asymmetries in 2D computations are also quantified. [Preview Abstract] |
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CP10.00007: Results from the MARBLE Campaign on the National Ignition Facility: Implosion of Foam-Filled Capsules for Studying Thermonuclear Burn in the Presence of Heterogeneous Mix T. J. Murphy, M. R. Douglas, T. Cardenas, B. G. Devolder, J. R. Fincke, M. A. Gunderson, B. M. Haines, C. E. Hamilton, Y. H. Kim, M. N. Lee, J. A. Oertel, R. E. Olson, R. B. Randolph, R. C. Shah, J. M. Smidt The MARBLE\footnote{T. J. Murphy {\it et al} J Phys:Conf Series {\bf 717}, 012072 (2016).} campaign on NIF investigates the effect of heterogeneous mix on thermonuclear burn for comparison to a probability distribution function (PDF) burn model.\footnote{J. R. Fincke, unpublished.} MARBLE utilizes plastic capsules filled with deuterated plastic foam and tritium gas. The ratio of DT to DD neutron yield is indicative of the degree to which the foam and the gas atomically mix. Platform development experiments have been performed to understand the behavior of the foam and of the gas separately using two types of capsule. The first uses partially deuterated foam and hydrogen gas fill to understand the burn in the foam. The second uses undeuterated foam and deuterium gas fill to understand the dynamics of the gas. Experiments using deuterated foam and tritium gas are planned. Results of these experiments, and the implications for our understanding of thermonuclear burn in heterogeneously mixed separated reactant experiments will be discussed. [Preview Abstract] |
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CP10.00008: Kinetic simulation of hydrodynamic equivalent capsule implosions Thomas Kwan, Ari Le, Mark Schmitt, Hans Herrmann We have carried out simulations of direct-drive hydrodynamic equivalent capsule implosion experiments conducted on Omega laser facility at the Laboratory of Laser Energetics of the University of Rochester. The capsules had a glass shell (SiO$_{2})$ 4.87$\mu $m with an inner diameter of 1086$\mu $m. One was filled with deuterium (D) and tritium (T) at 6.635 and 2.475 atmospheric pressure respectively. The other capsule with D, T, and He-3 at 2.475, 2.475, and 5.55 atmospheric pressure respectively. The capsules were imploded with 60 laser beams with a square pulse length of 0.6ns of total energy of 15.6 kJ. One-dimensional radiation hydrodynamic calculations with HYDRA and kinetic particle/hybrid simulations with LSP are carried out for the post-shot analysis. HYDRA outputs at 0.6ns are linked to LSP, in which the electrons are treated as a fluid while all the ion dynamics is simulated by the standard particle-in-cell technique. Additionally, simulations with the new photon package in LSP are initiated at the beginning of the implosion to include the implosion phase of the capsule. The simulation results of density, temperature, and velocity profiles of the electrons, D, T, He-3, and SiO$_{2\, }$species are compared with HYDRA. Detail comparisons among the kinetic simulations, rad-hydro simulations, and experimental results of neutron yield, yield ratio, fusion burn histories, and shell convergence will be presented to assess plasma kinetic effects. [Preview Abstract] |
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CP10.00009: Overview of Double Shell Designs and Challenges for Fabrication and Diagnostics S.H. Batha, D.S. Montgomery, W.S. Daughton, E.C. Merritt, E.S. Dodd, D.C. Wilson, T. Cardenas, J.L. Kline Double shell capsules are predicted to ignite and burn at relatively low temperatures ($\sim$ 3 keV) via volume ignition, and are a potential low-convergence path to substantial $\alpha$-heating and ignition on NIF. Double shells consist of a dense, high-Z pusher, which first shock heats then performs $\emph{PdV}$ work on a DT fuel volume, bringing the entire fuel volume up to high pressure thermonuclear conditions near implosion stagnation. The high-Z pusher is accelerated via a shock and subsequent collision from an ablatively-driven low-Z outer shell. A broad capsule design parameter space exists due to the inherent flexibility of potential materials for the outer and inner shells and intervening foam cushion. This is narrowed down by design physics choices and the ability to fabricate and assemble a double shell capsule. We describe tradeoffs in various design choices for double shell capsules, and challenges for capsule fabrication. The dense, high-Z inner shell also presents diagnostic challenges and opportunities, which we will present. [Preview Abstract] |
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CP10.00010: Effect of non-local electron conduction in compression of solid ball target for fast ignition Hideo Nagatomo, Takashi Asahina, Philippe Nicolai, Atsushi Sunahara, Tomoyuki Johzaki In the first phase of the fast ignition scheme, fuel target is compressed by the implosion laser, where only achievement of high dense fuel is required because the increment of the temperature to ignite the fuel is given by heating lasers. The ideal compression method for solid target is isentropic compression with tailored pulse shape [1]. However, it requires the high laser intensity \textgreater 10$^{\mathrm{15}}$ W/cm$^{\mathrm{2}}$ which cause the hot electrons. Numerical simulation for these conditions non-local electron transport model is necessary. Recently, we have installed SNB model [2] to a 2-D radiation hydrodynamic simulation code. In this presentation, effect of hot electron in isentropic compression and optimum method are discussed, which may be also significant for shock ignition scheme. Also effect of external magnetic field to the hot electron [3] will be considered. [1] R.E. Kidder Nuclear Fusion 14 (1974). [2] G.P. Schultz et al, Phys. Plasmas 7 4238 (2000). [3] Ph. D. Nicolai et al, Phys. Plasmas 13 032701 (2007). [Preview Abstract] |
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CP10.00011: Kinetic studies of ICF target dynamics with ePLAS R.J. Mason The ePLAS code was recently used$^{\mathrm{1}}$ to show that a modeling change from artificial to real viscosity can result in a decrease of the predicted performance of ICF targets. This code typically follows either fluid or PIC electrons with fluid ions in self-consistent $E-$ and $B-$fields computed by the Implicit Moment Method$^{\mathrm{2}}$. For the present study the ions have instead been run as PIC particles undergoing Krook-like self-collisions. The ePLAS collision model continually redistributes the ion particle properties toward a local Maxwellian, while conserving the mean density, momentum and energy. Whereas the use of real viscosity captures large Knudsen Number effects as the active target dimensions shrink below the ion mean-free-path, the new kinetic modeling can manifest additional effects such as collisional shock precursors$^{\mathrm{3}}$ from the escape and streaming of the fastest particle ions. In 2D cylindrical geometry we will explore how such kinetic shock extensions might affect shell and core compression dynamics in ICF target implosions. 1. R. J. Mason, R. C. Kirkpatrick and R. J. Faehl, Phys. Plasmas \textbf{21}, 022705 and 039902 (2014). 2. R. J. Mason, J. Comp. Phys. \textbf{41}, 233 (1981). 3. R. J. Mason, Phys. Fluids \textbf{13,} 1467 (1970). [Preview Abstract] |
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CP10.00012: A molecular dynamics approach to barrodiffusion James Cooley, Mathieu Marciante, Michael Murillo Unexpected phenomena in the reaction rates for Inertial Confinement Fusion (ICF) capsules\cite{Rygg2006} have led to a renewed interest in the thermo-dynamically driven diffusion process for the past 10 years, often described collectively as barodiffusion\cite{Kagan2014}. In the current context, barodiffusion would manifest as a process that separates ions of differing mass and charge ratios due to pressure and temperature gradients set-up through shock structures in the capsule core. Barrodiffusion includes additional mass transfer terms that account for the irreversible transport of species due to gradients in the system, both thermodynamic and electric e.g, ${\bf i}=-\rho D [\nabla c +k_p\nabla ln (p_i) + k^{(i)}_T \nabla ln(T_i) + k^{(e)}_t \nabla ln(T_e) + \frac{e k_e}{T_i}\nabla \phi ]$. Several groups have attacked this phenomena using continuum scale models and supplemented with kinetic theory to derive coefficients for the different diffusion terms based on assumptions about the collisional processes\cite{Kagan2014}. In contrast, we have applied a molecular dynamics (MD) simulation to this system to gain a first-principle understanding of the rate kinetics and to assess the accuracy of the differin [Preview Abstract] |
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CP10.00013: Characterization of neutron scatter for the 25-m neutron time of flight detector at the Z Accelerator Edward Norris, Kelly Hahn, Gordon Chandler, Carlos Ruiz, Jedediah Styron, Gary Cooper, Brent Jones, Jose Torres, Decker Spencer, Alan Nelson We are investigating neutron scattering effects using Monte Carlo simulations for neutron time of flight (NTOF) detectors fielded at the Z Accelerator at Sandia National Laboratories. For the radial NTOF detector at 25 m, a large scatter distribution is observed during and after primary DD neutron signals produced during inertial-confinement fusion experiments which obfuscates inference of quantities such as ion temperature, yield, and liner areal density. We present comparisons of measurements with simulation results. We also propose improvements to this line-of-sight. [Preview Abstract] |
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CP10.00014: Plasma interfacial mixing layers: Comparisons of fluid and kinetic models Erik Vold, Lin Yin, William Taitano, B. J. Albright, Luis Chacon, Andrei Simakov, Kim Molvig We examine plasma transport across an initial discontinuity between two species by comparing fluid and kinetic models. The fluid model employs a kinetic theory approximation for plasma transport in the limit of small Knudsen number. The kinetic simulations include explicit particle-in-cell simulations (VPIC) and a new implicit Vlasov-Fokker-Planck code, iFP. The two kinetic methods are shown to be in close agreement for many aspects of the mixing dynamics at early times (to several hundred collision times). The fluid model captures some of the earliest time dynamic behavior seen in the kinetic results, and also generally agrees with iFP at late times when the total pressure gradient relaxes and the species transport is dominated by slow diffusive processes. The results show three distinct phases of the mixing: a pressure discontinuity forms across the initial interface (on times of a few collisions), the pressure perturbations propagate away from the interfacial mixing region (on time scales of an acoustic transit) and at late times the pressure relaxes in the mix region leaving a non-zero center of mass flow velocity. The center of mass velocity associated with the outward propagating pressure waves is required to conserve momentum in the rest frame. [Preview Abstract] |
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CP10.00015: Numerical Investigation of Kinetic Effects in ICF Capsules using iFP William Taitano, Luis Chac{\'o}n, Andrei Simakov, Brett Keenan Contrary to predictions of radiation-hydrodynamics design codes, the National Ignition Campaign and subsequent campaigns were not successful in achieving ignition of inertial confinement fusion (ICF) capsules. Recent experimental evidence suggests that plasma kinetic effects may play an important role during ICF capsules’ implosion. Consequently, kinetic models and simulations may need to be used to better understand experimental results and design ICF targets. We have developed a new, optimal, fully implicit, and fully conservative 1D2V Vlasov-Fokker-Planck code, iFP, to simulate ICF capsule implosions kinetically. Our approach uses an optimal, O(N), fully implicit temporal advance to step over stiff collision time-scales and optimal adaptive mesh to address grid resolution issues [1,2]. We have implemented a spherical geometry capability and performed an extensive suite of verification campaign (e.g. Guderley problem) as well as comparing against other kinetic codes (e.g. FPion [3]) in order to field the code for full spherical implosion. We present several preliminary results on kinetic effects in spherical geometry, relevant to ICF experiments.\\ [1] W.T. Taitano et al., JCP 297, 2015.\\ [2] W.T. Taitano et al., JCP 318, 2016.\\ [3] O. Larroche, PoP 19, 2012. [Preview Abstract] |
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CP10.00016: Shock wave, fluid instability and implosion studies with a kinetic particle approach Irina Sagert, Wesley P. Even, Terrance T. Strother Many problems in laboratory plasma physics are subject to flows that move between the continuum and the kinetic regime. The correct description of these flows is crucial in order to capture their impact on the system's dynamical evolution. Examples are capsule implosions in inertial confinement fusion (ICF). Although their dynamics is predominantly shaped by shock waves and fluid instabilities, non-equilibrium flows in form of deuterium/tritium ions have been shown to play a significant role. We present recent studies with our Monte Carlo kinetic particle code that is designed to capture continuum and kinetic flows in large physical systems with possible applications in ICF studies. Discussed results will include standard shock wave and fluid instability tests and simulations that are adapted towards future ICF studies with comparisons to hydrodynamic simulations. [Preview Abstract] |
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CP10.00017: ICF target 2D modeling using Monte Carlo SNB electron thermal transport in DRACO Jeffrey Chenhall, Duc Cao, Gregory Moses The iSNB (implicit Schurtz Nicolai Busquet$^{\mathrm{1,2}})$ multigroup diffusion electron thermal transport method is adapted into a Monte Carlo (MC) transport method to better model angular and long mean free path non-local effects. The MC model was first implemented in the 1D LILAC code to verify consistency with the iSNB model$^{\mathrm{3,4}}$. Implementation of the MC SNB model in the 2D DRACO code enables higher fidelity non-local thermal transport modeling in 2D implosions such as polar drive experiments on NIF. The final step is to optimize the MC model by hybridizing it with a MC version of the iSNB diffusion method. The hybrid method will combine the efficiency of a diffusion method in intermediate mean free path regions with the accuracy of a transport method in long mean free path regions allowing for improved computational efficiency while maintaining accuracy. Work to date on the method will be presented. This work was supported by Sandia National Laboratories and the Univ. of Rochester Laboratory for Laser Energetics. $^{\mathrm{1}}$Schurtz et. al. Phys. Plasmas 7, 4238 (2000) $^{\mathrm{2}}$Cao et. al. Phys. Plasmas 22, 082308 (2015) $^{\mathrm{3}}$Chenhall et.al. BAPS DPP15 TP12.24 (2015) $^{\mathrm{4}}$Moses et.al. BAPS DPP15 TP12.25 (2015) [Preview Abstract] |
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CP10.00018: Vlasov-Fokker-Planck Modeling of Ion Acoustic Waves in Inertial Confinement Fusion Plasmas Archis Joglekar, Ben Winjum, Adam Tableman, Michail Tzoufras, Warren Mori Using OSHUN, a 2D3P Vlasov-Fokker-Planck (VFP) code, we examine ion acoustic wave dynamics in regimes relevant to inertial confinement fusion. The VFP methodology is uniquely suited to modeling high energy density plasmas where Coulomb collisions can be important in mediating kinetic effects. A new capability has been added to OSHUN to model ions hydrodynamically. We examine ion acoustic wave physics for a range of electron temperatures and densities in the presence of inverse-bremsstrahlung heating and self-generated magnetic fields, and we comment on their relevance to ICF-relevant physics, such as in thermal transport and in the evolution of instabilities such as stimulated Brillouin scattering for which a thorough understanding of ion physics is essential. [Preview Abstract] |
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CP10.00019: Progress on 3-D ICF simulations and Ray-Traced Power Deposition Method Andrew J. Schmitt, David E. Fyfe We have performed 3D simulations of Omega-scale and NIF-scale spherical direct-drive targets with the massively parallel \textsc{fastrad3d} code. Of particular interest is the robustness of the targets to the low mode perturbations impressed on the target by the laser system and how it compares to the influence of the perturbations produced by laser imprinting. As part of this simulation capability, we have upgraded our smoothed 3D raytrace package to run in spherical geometry. This package, which connects rays to form bundles and performs power deposition calculations on the bundles, can decrease laser absorption noise while using fewer rays and less message passing. This model produces both the imprint and the low-mode asymmetry drive that we are interested in here. We show recent simulation results of directly-driven targets using conventional ignition drive, and report on the influences of the two sources -- low mode asymmetry and laser imprint -- as the pellet conditions (e.g. adiabat) are varied. [Preview Abstract] |
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CP10.00020: Additions and improvements to the high energy density physics capabilities in the FLASH code D. Q. Lamb, N. Flocke, C. Graziani, P. Tzeferacos, K. Weide FLASH is an open source, finite-volume Eulerian, spatially adaptive radiation magnetohydrodynamics code that has the capabilities to treat a broad range of physical processes. FLASH performs well on a wide range of computer architectures, and has a broad user base. Extensive high energy density physics (HEDP) capabilities have been added to FLASH to make it an open toolset for the academic HEDP community. We summarize these capabilities, emphasizing recent additions and improvements. In particular, we showcase the ability of FLASH to simulate the Faraday Rotation Measure produced by the presence of magnetic fields; and proton radiography, proton self-emission, and Thomson scattering diagnostics with and without the presence of magnetic fields. We also describe several collaborations with the academic HEDP community in which FLASH simulations were used to design and interpret HEDP experiments. [Preview Abstract] |
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CP10.00021: VISRAD, 3-D Target Design and Radiation Simulation Code Igor Golovkin, Joseph MacFarlane, Viktoriya Golovkina The 3-D view factor code VISRAD is widely used in designing HEDP experiments at major laser and pulsed-power facilities, including NIF, OMEGA, OMEGA-EP, ORION, LMJ, Z, and PLX. It simulates target designs by generating a 3-D grid of surface elements, utilizing a variety of 3-D primitives and surface removal algorithms, and can be used to compute the radiation flux throughout the surface element grid by computing element-to-element view factors and solving power balance equations. Target set-up and beam pointing are facilitated by allowing users to specify positions and angular orientations using a variety of coordinates systems (e.g., that of any laser beam, target component, or diagnostic port). Analytic modeling for laser beam spatial profiles for OMEGA DPPs and NIF CPPs is used to compute laser intensity profiles throughout the grid of surface elements. We will discuss recent improvements to the software package and plans for future developments. [Preview Abstract] |
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CP10.00022: Modeling of Dense Plasma Effects in Short-Pulse Laser Experiments Timothy Walton, Igor Golovkin, Joseph MacFarlane Warm and Hot Dense Matter produced in short-pulse laser experiments can be studied with new high resolving power x-ray spectrometers. Data interpretation implies accurate modeling of the early-time heating dynamics and the radiation conditions that are generated. Producing synthetic spectra requires a model that describes the major physical processes that occur inside the target, including the hot-electron generation and relaxation phases and the effect of target heating. An important issue concerns the sensitivity of the predicted K-line shifts to the continuum lowering model that is used. We will present a set of PrismSPECT spectroscopic simulations using various continuum lowering models: Hummer/Mihalas, Stewart-Pyatt, and Ecker-Kroll and discuss their effect on the formation of K-shell features. We will also discuss recently implemented models for dense plasma shifts for H-like, He-like and neutral systems. [Preview Abstract] |
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CP10.00023: Parallel Implementation of X-Ray Scattering Models in SPECT3D. James Sebald, Igor Golovkin, Joseph MacFarlane Spectrally resolved x-ray scattering has become a very effective method for diagnosing electron temperatures, densities, and average ionization in warm dense matter. The x-ray scattering modeling has been added to the multi-dimensional collisional-radiative spectral and imaging package SPECT3D. The ability to simulate the emissivity and attenuation of scattered photons within a multi-dimensional multi-volume-element plasma with non-uniform temperature and density distributions adds a major capability to existing spectroscopic models. We present parallel implementation of the scattering algorithms developed to support simulations with large-scale hydro grids. Various optimization options will also be discussed. [Preview Abstract] |
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CP10.00024: Comparing Particle-in-Cell QED Models for High-Intensity Laser-Plasma Interactions Scott V. Luedtke, Lance A. Labun, Bj\"{o}rn Manuel Hegelich High-intensity lasers, such as the Texas Petawatt, are pushing into new regimes of laser-matter interaction, requiring continuing improvement and inclusion of new physics effects in computer simulations. Experiments at the Texas Petawatt are reaching intensity regimes where new physics---quantum electrodynamics (QED) corrections to otherwise classical plasma dynamics---becomes important. We have two particle-in-cell (PIC) codes with different QED implementations. We review the theory of photon emission in QED-strong fields, and cover the differing PIC implementations. We show predictions from the two codes and compare with ongoing experiments. [Preview Abstract] |
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CP10.00025: Smoothed Particle Hydrodynamics for the Simulation of Laser Produced Plasmas Alec Griffith, Tyler Holladay, Michael S. Murillo To address the design and interpretation of experiments at next generation light sources such as at the SLAC LCLS and the LANL proposed MaRIE a simulation of the laser produced plasma targets has been developed. Smoothed particle hydrodynamics is used to capture the full experimental time and length scales, large degrees of deformation, and the experimental environment's open boundary conditions. Additionally the model incorporates plasma transport with thermal conduction, the electric potential, and a two species model of the electrons and ions. The electron and ion particle representations in SPH allow for time dependent ionization and recombination while addressing the disparate masses of the two species. To gain computational speedup our simulation takes advantage of parallelism, and to reduce computational cost we have explored using data structures such as the linked cell list and octree as well as algorithmic techniques such as the fast mutipole method. We will discuss the results of simulating several possible experimental configurations using our model. [Preview Abstract] |
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CP10.00026: Full-Pulse Particle-in-Cell Simulations of Hot-Electron Generation in OMEGA Experiments Eli Borwick, Jun Li, Chuang Ren, Rui Yan, Suxing Hu Using data from the \textit{LILAC} hydrocode in conjunction with the particle-in-cell code \textit{OSIRIS}, we now perform several simulations sampling a 1-ns pulse to determine the evolution of hot-electron generation as well as electron divergence during the pulse. The results will be compared with the OMEGA experiments that measured hot-electron generation and divergence.\footnote{ B. Yaakobi \textit{et al.}, Phys. Plasmas \textbf{20}, 092706 (2013).\par } [Preview Abstract] |
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CP10.00027: Hybrid simulations of weakly collisional plasmas Qian Xia, Brian Reville, Michail Tzoufras Laser produced plasma experiments can be exploited to investigate phenomena of astrophysical relevance. The high densities and velocities that can be generated in the laboratory provide ideal conditions to investigate weakly collisional or collisionless plasma shock physics. In addition, the high temperatures permit magnetic and kinetic Reynolds numbers that are difficult to achieve in other plasma experiments, opening the possibility to study plasma dynamo. Many of these experiments are based on a classic plasma physics problem, namely the interpenetration of two plasma flows. To investigate this phenomenon, we are constructing a novel multi-dimensional hybrid numerical scheme, that solves the ion distribution kinetically via a Vlasov-Fokker-Planck equation, with electrons providing a charge neutralizing fluid. This allows us to follow the evolution on hydrodynamic timescales, while permitting inclusion ofcollisionlesseffects on small scales. It also could be used to study the increasing collisional effects due to the stiff gradient and weakly anisotropic velocity distribution. We present some preliminary validation tests for the code, demonstrating its ability to accurately model key processes that are relevant to laboratory and astrophysical plasmas. [Preview Abstract] |
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CP10.00028: CTH, HSX AND OTHER STELLARATORS |
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CP10.00029: Overview of Compact Toroidal Hybrid research program progress and plans David Maurer, David Ennis, James Hanson, Gregory Hartwell, Jeffrey Herfindal, Stephen Knowlton, Xingxing Ma, Mihir Pandya, Nicholas Roberds, Kevin Ross, Peter Traverso disruptive behavior on the level of applied 3D magnetic shaping; (2) test and advance the V3FIT reconstruction code and NIMROD modeling of CTH; and (3) study the implementation of an island divertor. Progress towards these goals and other developments are summarized. The disruptive density limit exceeds the Greenwald limit as the vacuum transform is increased, but a threshold for avoidance is not observed. Low-$q$ disruptions, with 1.1 \textless $q(a)$ \textless 2.0, cease to occur if the vacuum transform is raised above 0.07. Application of vacuum transform can reduce and eliminate the vertical drift of elongated discharges that would otherwise be vertically unstable. Reconstructions using external magnetics give accurate estimates for quantities near the plasma boundary, and internal diagnostics have been implemented to extend the range of accuracy into the plasma core. Sawtooth behavior has been reproducibly modified with external transform and NIMROD is used to model these observations and reproduces experimental trends. An island divertor design has begun with connection length studies to model energy deposition on divertor plates located in an edge 1/3 island as well as the study of a non-resonant divertor configuration. [Preview Abstract] |
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CP10.00030: Control of Sawtooth Oscillation Dynamics using Externally Applied Stellarator Transform J.L. Herfindal, D.A. Maurer, G.J. Hartwell, D.A. Ennis Sawtooth instabilities have been observed in the Compact Toroidal Hybrid (CTH), a current-carrying stellarator/tokamak hybrid device. Experiments varying the vacuum rotational transform from 0.02 to 0.13 were conducted to explore the effect of 3D magnetic shaping on sawtooth dynamics by changing the amount of confining field from the external coils. The sawtooth period and amplitude are observed to decrease with increasing levels of 3D magnetic field from the external coils. The crash time of the sawtooth oscillation is not correlated with the amount of vacuum transform applied, indicating that the final nonlinear reconnection dynamics of the MHD kink-tearing instability are not affected. The observed decrease in both sawtooth period and amplitude is correlated with an estimation of the mean elongation of the last closed flux surface, rather than to core equilibrium changes such as a change in the central ohmic heating rate. Given that the kink-tearing mode is well known to be destabilized by elongation in tokamak plasmas, this observation supports an interpretation of the reduced sawtooth period being due to a change in the linear stability threshold for the kink-tearing mode responsible for the crash. [Preview Abstract] |
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CP10.00031: Density limit disruption suppression in a current-carrying stellarator Xinxing Ma, D.A. Ennis, J.D. Hanson, G.J. Hartwell, S.F. Knowlton, D.A. Maurer Density limit disruptions are frequently observed in current-driven discharges in the Compact Toroidal Hybrid (CTH). These disruptions are triggered by elevated densities achieved with edge fueling. The phenomenology of hybrid discharge terminations is similar to tokamak disruptions: progressive narrowing of the plasma current profile until the discharge is unstable to growing $m=2,n=1$ tearing modes, at which point there follows the negative loop voltage spike and positive current spike and subsequent rapid decay of the plasma current. As a result of the ability to adjust the vacuum rotational transform in CTH, i.e. the level of imposed 3D field strength, we have found the density limit at a given current linearly increases with the addition of vacuum transform. Consequently, plasmas with densities up to two times the Greenwald limit[1] are attained at the maximum vacuum transform of $0.22$. The results are similar in both hydrogen and deuterium plasma, although operation with deuterium extends achievable plasma currents and densities to higher values. 3D equilibrium reconstructions show these discharges disrupt at more peaked current profiles and lower plasma currents as the amount of 3D fields is increased. \\\\$[1]$M. Greenwald et al., Nucl. Fusion $\mathbf{28}$, (1988) [Preview Abstract] |
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CP10.00032: NIMROD Simulations of Low-q Disruptions in the Compact Toroidal Hybrid Device (CTH) E.C. Howell, M.D. Pandya, J.D. Hanson, D.A. Mauer, D.A. Ennis, G.J. Hartwell Nonlinear MHD simulations of low-q disruptions in the CTH are presented. CTH is a current carrying stellarator that is used to study the effects of 3D shaping. The application of 3D shaping stabilizes low-q disruptions in CTH [M. D. Pandya et al., POP 22, 2015]. The amount of 3D shaping is controlled by adjusting the external rotational transform, and it is characterized by the ratio of the external rotational transform to the total transform: $f ={ \iota_{vac}}/{\iota}$. Disruptions are routinely observed during operation with weak shaping ($f<0.05$). The frequency of disruptions decreases with increasing amounts of 3D shaping, and the disruptions are completely suppressed for $f> 0.1$. Nonlinear simulations are performed using the NIMROD code [C.R. Sovinec et al., JCP 195, 2004] to better understand how the shaping suppresses the disruptions. Comparisons of runs with weak ($f=0.04$) and strong ($f=0.10$) shaping are shown. [Preview Abstract] |
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CP10.00033: Coherence Imaging Measurements of Impurity Flow in the Compact Toroidal Hybrid Experiment D.A. Ennis, G.J. Hartwell, C.A. Johnson, D.A. Maurer, S.L. Allen, W.H. Meyer, C.M. Samuell Measurements of impurity ion emissivity and velocity in the Compact Toroidal Hybrid (CTH) experiment are achieved with a new optical coherence imaging diagnostic. The Coherence Imaging Spectroscopy (CIS) technique measures the spectral coherence of an emission line with an imaging interferometer of fixed delay. CIS has a number of advantages when compared to dispersive Doppler spectroscopy, including higher throughput and the capability to provide 2D spectral images, making it ideal for investigating the non-axisymmetric geometry of CTH plasmas. Furthermore, detailed measurements of the ion flow structure provided by CIS combined with predictive computational models could also provide spatially resolved images of complex flow structures, such as those associated with an island divertor. First CIS measurements of CTH plasmas reveal strong signals for C III (465 nm), He II (468 nm) and C II (513 nm) emission. Preliminary analysis of C III interferograms indicate a net toroidal flow on the order of 10 km/s during the time of peak current. Bench tests using Zn and Cd light sources reveal that the temperature of the instrument must be actively controlled to within 0.01\textdegree C to limit phase drift of the interferometer resulting in artificially measured flow. Results from this diagnostic will aid in characterizing the ion flow in planned island divertor and MHD mode-locking experiments. [Preview Abstract] |
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CP10.00034: Thomson scattering diagnostic on the Compact Toroidal Hybrid Experiment Peter Traverso, D.A. Maurer, D.A. Ennis, G.J. Hartwell A Thomson scattering system is being commissioned for the non-axisymmetric plasmas of the Compact Toroidal Hybrid (CTH), a five-field period current-carrying torsatron. The system takes a single point measurement at the magnetic axis to both calibrate the two color soft x-ray $T_e$ system and serve as an additional diagnostic for the V3FIT 3D equilibrium reconstruction code. A single point measurement will reduce the uncertainty in the reconstructed peak pressure by an order of magnitude for both current-carrying plasmas and future gyrotron-heated stellarator plasmas. The beam, generated by a frequency doubled Continuum 2 J, Nd:YaG laser, is passed vertically through an entrance Brewster window and a two-aperture optical baffle system to minimize stray light. The beam line propagates ~ 8 m to the CTH device mid-plane with the beam diameter < 3 mm inside the plasma volume. Thomson scattered light is collected by two adjacent f/2 plano-convex condenser lenses and focused onto a custom fiber bundle. The fiber is then re-bundled and routed to a Holospec f/1.8 spectrograph to collect the red-shifted scattered light from 535-565 nm. The system has been designed to measure plasmas with core $T_e$ of 100 to 200 eV and densities of $5\times10^{18}$ to $5\times10^{19}$ $m^{-3}$. [Preview Abstract] |
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CP10.00035: Design and Implementation of a 200kW, 28GHz gyrotron system for the Compact Toroidal Hybrid Experiment G.J. Hartwell, S.F Knowlton, D.A Ennis, D.A. Maurer, T. Bigelow The Compact Toroidal Hybrid (CTH) is an $\ell=2, m=5$ torsatron/tokamak hybrid ($R_0=0.75$\,m, $a_p\sim 0.2$\,m, and $|B|\leq 0.7$\,T). It can generate its highly configurable confining magnetic fields solely with external coils, but typically operates with up to 80\,kA of ohmically-generated plasma current for heating. New studies of edge plasma transport in stellarator geometries will benefit from CTH operating as a pure torsatron with a high temperature edge plasma. Accordingly, a 28\,GHz, 200\,kW gyrotron operating at 2nd harmonic for ECRH is being installed to supplement the existing 15\,kW klystron system operating at the fundamental frequency; the latter will be used to initially generate the plasma. Ray-tracing calculations that guide the selection of launching position, antenna focal length, and beam-steering characteristics of the ECRH have been performed with the TRAVIS code$[1]$. The calculated absorption is up to 95.7\% for vertically propagating rays, however, the absorption is more sensitive to magnetic field variations than for a side launch where the field gradient is tokamak-like. The design of the waveguide path and components for the top-launch scenario will be presented.\\ $[1]$N.B. Marushchenko, Y. Turkin, H. Maassberg, Comp. Phys. Comm. 185 165 (2014) [Preview Abstract] |
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CP10.00036: The present HSX program and path to a new mid-sized stellarator D.T. Anderson HSX has experimentally shown improved neoclassical confinement with quasisymmetry in low-collisionality hot-electron plasmas. The present program focuses on open issues in stellarator physics including the neoclassical radial electric field, impurity transport, edge magnetic field structure, turbulent transport and energetic ion confinement. GENE simulations are being used to identify configurations with varied turbulent transport levels. Doppler reflectometry, and new CECE and microwave scattering systems will provide data for comparison to the GENE calculations, with the ultimate goal of identifying means to optimize for turbulent transport reduction. Monitoring neutron production rates from a deuterium neutral beam (20 keV 25 A) into a deuterium plasma will provide data on energetic ion confinement as the magnetic configuration is varied. A pre-conceptual design of a new mid-sized stellarator to investigate quasisymmetry with higher ion temperatures and densities will be presented. The design will emphasize physics studies not attainable in W7-X including role of high effective transform and residual zonal flows, low flow damping, and good energetic particle confinement over a broad region of phase space. Inclusion of flexibility for divertor solutions is a requirement for good performance. [Preview Abstract] |
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CP10.00037: Impurity transport measurements in the HSX stellarator J.F. Castillo, K.M. Likin, D.T. Anderson, F.S.B. Anderson, J.N. Talmadge, S.T.A. Kumar, A. Bader The future design and operation of magnetic confinement fusion devices depend on accurate predictive models of impurity transport. Experiments are being conducted at HSX to measure the impurity transport diffusivity and convective velocity in order to advance the development of such models for stellarator devices. A laser blow-off impurity injection system is used to rapidly deposit a small, controlled quantity of aluminum into the confinement volume. Five AXUV photodiode arrays, some of which are equipped with filters that block the visible portion of the spectrum, are used to take time-resolved measurements of the impurity radiation. The spatially one-dimensional impurity transport code STRAHL is used to calculate a time-dependent plasma emissivity profile. A synthetic diagnostic code that integrates the resulting emissivity profile along the experimental lines of sight will provide modeled intensity signals. These modeled signals provide direct comparison between plasma simulation and experimental results. [Preview Abstract] |
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CP10.00038: Determination of radial electric field from Pfirsch-Schluter flows in a stellarator S. T. A. Kumar, J. N. Talmadge, T. J. Dobbins, F. S. B. Anderson, K. M. Likin, D. T. Anderson Inboard/outboard asymmetry in the impurity ion parallel flows is measured in the HSX stellarator using Charge Exchange Recombination Spectroscopy (CHERS). This observation is consistent with the Pfirsch-Schluter flows predicted by neoclassical theory. The asymmetry of the flow is used to calculate the magnitude and direction of the radial electric field (E\textunderscore r), as well as the mean flow, using computed magnetic geometry factors. This method enables measurement of E\textunderscore r near the core of the HSX plasma where the E\textunderscore r obtained from the radial force balance equation has large uncertainties due to the relatively large width of the diagnostic neutral beam and the smaller plasma radius. [Preview Abstract] |
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CP10.00039: The dependence of TEM turbulence on magnetic geometry in the HSX stellarator J. Smoniewski, B.J. Faber, M.J. Pueschel, K.M. Likin, C.B. Deng, J.N. Talmadge The Helically Symmetric eXperiment (HSX) is equipped with a set of auxiliary coils that can modify the designed Quasi-Helical Symmetry (QHS), and is well suited to probe the effect of geometry on turbulence. Configuration optimization to reduce turbulent transport could provide the next big confinement improvement in fusion devices. The first step towards optimization is to develop a predictive understanding of the relevant physics. Particularly in the case of density-gradient-driven TEM turbulence relevant to HSX scenarios, quasilinear models fail to predict transport. Here, moving beyond linear physics, nonlinear \textsc{Gene} simulations in the large density gradient region are compared in the QHS configuration, and in a configuration where the symmetry is broken by the addition of mirror terms to the magnetic spectrum. Investigations into zonal flow physics provide another perspective into differences between geometries. In parallel, power balance on experimental profiles in each configuration provides a scaling of electron thermal diffusivity $\chi_e$ with electron temperature and density gradient as a first step towards direct comparison to experimental fluctuation measurements. [Preview Abstract] |
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CP10.00040: Density Fluctuation measurement with Upgraded FIR System on the HSX Stellarator C.B. Deng, D.L. Brower, D.T. Anderson, F.S.B. Anderson, K.M. Likin, J.N. Talmadge Going forward, a primary physics goal for HSX is to study configuration optimization for reducing turbulence which requires measurement of turbulence with $k_{y}\rho_{s} $up to 1. For characteristic HSX parameters (Te \textasciitilde 200 eV at r/a \textasciitilde 0.5 where the density gradient peaks), this condition corresponds to $k_{y} $up to 7 cm$^{\mathrm{-1}}$. To accommodate this goal, the 9-chord HSX interferometer/far-forward scattering system (k\textless 2 cm$^{\mathrm{-1}})$ will be upgraded to measure density turbulence at higher k. The existing source (4 mW, 288 GHz) employing frequency modulation will be replaced with two high power (30 mW each, 320 GHz), solid-state sources with fixed frequency offset \textasciitilde 4 MHz. This will permit true heterodyne detection, thereby realizing faster measurement time response, increased bandwidth and reduced noise. High power sources and high sensitivity planar-diode mixers will allow us to reduce the aperture of the receiver optics to a few mm thereby increasing the maximum wavenumber to k\textasciitilde 15 cm$^{\mathrm{-1}}$. Reconfiguring the interferometer system into a finite-angle collective scattering arrangement is also planned as it will increase the measured k-spectrum up to 18 cm$^{\mathrm{-1}}$ with some spatial resolution (core or edge). [Preview Abstract] |
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CP10.00041: A MSE Polarimetry diagnostic for the measurement of radial electric fields on the HSX stellarator T. J. Dobbins, S. T. A. Kumar, F. S. B. Anderson, D. T. Anderson The radial electric field in HSX has been measured using the Charge Exchange Recombination Spectroscopy. These impurity ion flow measurements could not resolve a large positive radial electric field (40-50 kV/m) near the core of the HSX plasma, predicted by neoclassical codes. A dual PEM (Photo Elastic Modulator) MSE polarimetry system has been designed for direct measurement of the radial electric field in the HSX plasma. The polarimetry design has been optimized to get a maximum change in polarization angle from an electric field while still providing good spatial resolution. It is expected that a radial electric field as small as 7 kV/m can be detected. The initial results of the system on an available port will be presented. The choice and design of the optics for the optimal viewing port will also be presented. [Preview Abstract] |
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CP10.00042: Configuration Optimization for Turbulent Transport in HSX Joseph Talmadge, Jason Smoniewski, Konstin Likin, Chuanbao Deng, Josefine Proll, Harry Mynick, Samuel Lazerson Experimental measurements of the thermal diffusivity in the Quasihelically Symmetric (QHS) configuration have demonstrated that the neoclassical transport has been reduced so that the thermal diffusivity throughout the plasma is dominated by turbulence [1,2]. With advances in gyrokinetics and optimization codes, it has now become possible to assess whether 3D shaping can enhance or reduce turbulent transport in a computationally predictable manner [3]. Our starting point in this campaign is the experimental observation that the turbulent diffusivity in the region of maximum density gradient (0.5 \textless r/a \textless 0.7) is about a factor of two larger in a configuration in which the quasisymmetry has been intentionally degraded [2]. Preliminary results indicate that the numerically calculated heat flux using the GENE code is higher for the configuration with the degraded neoclassical transport. In this poster we will summarize the progress to date, discuss other possible magnetic configurations that have larger computed turbulent heat flux than the QHS geometry and lay out the near-term program in terms of diagnostic upgrades. [1] J. Canik et al., PoP 14, 056107 (2007) [2] Jeremy Lore, UW Ph.D. thesis [3] H. E. Mynick et al., PPCF 56, (2014) 094001 [Preview Abstract] |
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CP10.00043: High-beta extended MHD simulations of stellarators T. A. Bechtel, C. C. Hegna, C. R. Sovinec, N. A. Roberds The high beta properties of stellarator plasmas are studied using the nonlinear, extended MHD code NIMROD. In this work, we describe recent developments to the semi-implicit operator which allow the code to model 3D plasma evolution with better accuracy and efficiency. The configurations under investigation are an l=2, M=5 torsatron with geometry modeled after the Compact Toroidal Hybrid (CTH) experiment and an l=2, M=10 torsatron capable of having vacuum rotational transform profiles near unity. High-beta plasmas are created using a volumetric heating source and temperature dependent anisotropic thermal conduction and resistivity. To reduce computation expenses, simulations are initialized from stellarator symmetric pseudo-equilibria by turning on symmetry breaking modes at finite beta. The onset of MHD instabilities and nonlinear consequences are monitored as a function of beta as well as the fragility of the magnetic surfaces. [Preview Abstract] |
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CP10.00044: 3D low-beta magnetized plasma equilibria from external shaping A. Hassam, J. Tenbarge, M. Landreman, W. Dorland, W. Sengupta A 3D nonlinear dissipative MHD code is in development to allow relaxation to low-beta MHD equilibrium inside a shaped 3D conducting boundary with prescribed conserved axial magnetic flux and no external current. Formation of magnetic islands is expected. Heat sources would be eventually introduced to allow the possibility of non-stationary convection depending on the stability properties of the accessible MHD equilibria. The initial development will be done using the code UMHD (Guzdar et al, PF, 1993). The initial emphasis will be on recovering expected physics in simpler 3D geometries. A primary objective is to minimize numerical boundary noise. In particular, codes which specify the normal magnetic field B.n on bounding surfaces are prone to noise generation. We plan to shape the boundary to conform to the desired field shape so that B.n is zero on the boundary. Non-orthogonal coordinates will be chosen to effect this. We will test noise reduction within the tangential field approach. Results obtained to date support this conjecture. Initial results from simple 2D code equilibria have been verified against analytic solution of equilibria in weak shaping. Initial results also recover the expected features of the Hahm- Kulsrud island formation solution. Work supported by US DOE. [Preview Abstract] |
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CP10.00045: A comparison of quasi-symmetries in stellarators Alex Johnson, Andrew Ware This work explores the differences between the equilibrium, stability and transport properties of quasi-helically (QH) symmetric, quasi-axisymmetric (QA) and quasi-poloidally (QP) symmetric stellarator configurations with the same major radius, aspect ratio, average magnetic field strength, plasma $\beta$ and pressure profiles. Previous work on quasi-symmetry in stellarators has typically focused on one type of quasi-symmetry. Optimized stellarators have been developed for QH, QA and QP configurations but at very different plasma parameters. In this work computational studies of optimized cases of all three cases have been undertaken. Results of the studies will be presented. [Preview Abstract] |
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CP10.00046: Bootstrap current, magnetic islands and ballooning stability in W7-X Steve Morente, Andrew Ware, Steven Hirshman Computational modeling is used to calculate the expected bootstrap current in the Wendelstein 7-X stellarator and determine the impact of this current on the magnetic surfaces and ballooning stability. Pressure, density and temperature profiles have been selected to match the profiles anticipated in W7-X. Finite-$\beta$, free-boundary equilibria have been obtained with the VMEC code and analyzed for bootstrap current (BOOTSJ), ballooning stability (COBRAVMEC) and magnetic structure (SIESTA). The bootstrap current can impact the rotational transform profile and thus, the location of magnetic islands near the edge of W7-X. The possibility of controlling the rotational transform profile using planar coils is examined. [Preview Abstract] |
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CP10.00047: Testing the limits of quasi-symmetry in stellarator Andrew Ware This work explores the limits of quasi-symmetric stellarator configurations. Optimized quasi-symmetric stellarators have been developed that have improved neoclassical confinement relative to conventional stellarators. These quasi-symmetric configurations can also have stronger flows than conventional stellarators which may lead to reduced turbulent transport. In this work the range of flexibility of an optimized quasi-symmetric stellarator in parameter space is tested. Specifically, how much variation in the boundary and profiles is possible while still maintaining good neoclassical confinement? Here, an optimized case is tested to see how shallow the minimum is in parameter space. A shallower minimum allows for more flexibility in achieving other goals including stability and reducing the complexity of the configuration. [Preview Abstract] |
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CP10.00048: Progress on accelerated calculation of 3D MHD equilibrium with the PIES code Daniel Raburn, Allan Reiman, Donald Monticello Continuing progress has been made in accelerating the 3D MHD equilibrium code, PIES, using an external numerical wrapper. The PIES code (Princeton Iterative Equilibrium Solver) is capable of calculating 3D MHD equilibria with islands. The numerical wrapper has been demonstrated to greatly improve the rate of convergence in numerous cases corresponding to equilibria in the TFTR device where magnetic islands are present; the numerical wrapper makes use of a Jacobian-free Newton-Krylov solver along with adaptive preconditioning and a sophisticated subspace-restricted Levenberg backtracking algorithm. The wrapper has recently been improved by automation which combines the preexisting backtracking algorithm with insights gained from the stability of the Picard algorithm traditionally used with PIES. Improved progress logging and stopping criteria have also been incorporated in to the numerical wrapper. [Preview Abstract] |
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CP10.00049: Recent Advances in Stellarator Optimization David Gates, T. Brown, J. Breslau, M. Landreman, S. A. Lazerson, H. Mynick, G. H. Neilson, N. Pomphrey Computational optimization has revolutionized the field of stellarator design. To date, optimizations have focused primarily on optimization of neoclassical confinement and ideal MHD stability, although limited optimization of other parameters has also been performed. One criticism that has been levelled at this method of design is the complexity of the resultant field coils. Recently, a new coil optimization code, COILOPT$++$, was written and included in the STELLOPT suite of codes. The advantage of this method is that it allows the addition of real space constraints on the locations of the coils. As an initial exercise, a constraint that the windings be vertical was placed on large major radius half of the non-planar coils. Further constraints were also imposed that guaranteed that sector blanket modules could be removed from between the coils, enabling a sector maintenance scheme. Results of this exercise will be presented. We have also explored possibilities for generating an experimental database that could check whether the reduction in turbulent transport that is predicted by GENE as a function of local shear would be consistent with experiments. To this end, a series of equilibria that can be made in the now latent QUASAR experiment have been identified. [Preview Abstract] |
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CP10.00050: A new stellarator coil design tool using space curves Caoxiang Zhu, Stuart Hudson, Joshua Breslau, Samuel Lazerson, Yuntao Song, Yuanxi Wan Finding easy-to-build coils has always been critical for stellarator design. Conventional approaches assume a toroidal ``winding" surface. Either a surface current potential is constructed using a Green's function; or a discrete set of filamentary coils lying on the winding surface is non-linearly optimized. The winding surface concept ensures that the coils are separated from the plasma surface; however, requiring the coils lie on a given winding surface may overly constrain the coil optimization process. In this work, we investigate whether a winding surface is required. Our starting point is to represent each discrete coil as an arbitrary closed curve embedded in 3D space. From the Fundamental Theorem for Curves, such curves are uniquely described by the curvature and torsion functions. Our representation does not need a winding surface and can allow coils to evolve arbitrarily. We have constructed different penalty functions, F, that incorporate both the `physics' and `engineering' constraints. The first and second derivatives of F with respect to the parameters describing the coils are constructed analytically and are exploited to enable fast optimization algorithms for finding minima. Illustrations of coils for W7X and other stellarators will be presented. [Preview Abstract] |
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CP10.00051: Zero-plasma-current equilibria generated by tilted planar coils J. Li, B. Israeli, K. C. Hammond, F. A. Volpe It is known that a periodic toroidal arrangement of tilted planar coils, combined with vertical field coils, can generate a helical magnetic field. One question, though, is: is this coil-set a generator or an amplifier of rotational transform? In other words, is a finite plasma-current needed? A numerical scan of coil-currents shows that configurations exist, for which no plasma-current is needed, and yet torsatron plasmas of finite volume can be obtained. The case of six tilted circular coils has been examined in great detail because of its relevance to the CIRCUS device operated by Columbia, a generalization of the two-tilted-coil CNT stellarator, also at Columbia. More axisymmetric configurations featuring a higher number of tilted circular coils are also being investigated. The calculations are performed with the aid of a numerical field-line tracer and the VMEC equilibrium solver, slightly modified to reflect the simplicity of the coil geometry: the coils are not discretized; instead, their field is evaluated by means of analytical expressions. This allows for faster calculations and rapid, fine scans of large parameter spaces. [Preview Abstract] |
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CP10.00052: Neoclassical calculations for W7-X OP1.1 parameters Matt Landreman, A Alonso, C D Beidler, S Bozhenkov, A Dinklage, G Fuchert, J Geiger, M Hirsch, A Kr\"{a}mer-Flecken, A Langenberg, H Maassberg, A Moll\'{e}n, N Pablant, E Pasch, S Satake, H M Smith, P Traverso, Y Turkin, P Valson, J L Velasco, T Windisch, D Zhang Neoclassical calculations are carried out for W7-X OP1.1 plasmas with the SFINCS code, using experimental profiles of $n_{\mathrm{e}}$, $T_{\mathrm{e}}$, and $T_{\mathrm{i}}$. An electron root solution is found in the inner part of the plasma, consistent with measurements. Calculations are performed using a variety of assumptions for the plasma's impurity composition. Impurities modestly reduce the bootstrap current, and if a flat $Z_{\mathrm{eff}}$ profile is assumed, the impurity particle flux profile resembles the $E_{r}$ profile due to dominance of the associated thermodynamic force. Variations of quantities on flux surfaces computed with SFINCS are also presented. The incompressible-ExB approximation is found to be quite accurate for W7-X parameters. [Preview Abstract] |
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CP10.00053: Equilibrium Reconstructions with V3FIT and Current Evolution Modeling for 3-D Stellarator Plasmas J.C. Schmitt, M. Cianciosa, J. Geiger, S. Lazerson V3FIT is a powerful equilibrium reconstruction tool for magnetic confinement fusion experiments which are inherently 3-D in nature (i.e. stellarators) or have 3-D components (tokamaks with 3-D shaping, reversed field pinches with helical states, etc). Here, we present details of the diagnostic modeling, constraints and the user interface for reconstructions of W7-X plasmas. For typical discharges during the OP1.1 run campaign of W7-X, the net toroidal current and current density profile do not reach steady-state. When modeling the current evolution in 3-D plasmas, both poloidal and toroidal currents are linked with both poloidal and toroidal fluxes. In contrast, in toroidally axisymmetric plasmas, the poloidal flux is linked only with the toroidal current and the toroidal current is linked only with the poloidal flux. Compared to an equivalently-sized axisymmetric configuration, the current diffusion in 3-D plasmas is enhanced, leading to a faster relaxation of the current profile to its steady-state. Implications for the time-evolution of the current and rotational transform profiles in stellarator plasmas are discussed. This work is supported by DoE grant DE-SC00014529. [Preview Abstract] |
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CP10.00054: Limiter heat loads during the first operation of the W7-X stellarator Glen Wurden, Holger Niemann, Marcin Jakubowski, Sergey Bozhenkov, Christoph Biedermann, Stefan Marsen, Florian Effenberg, Laurie Stephey, Oliver Schmitz During the first operational phase (OP1.1) of the new W7-X stellarator, five poloidal graphite limiters served as the main boundary for the plasma. There was a dedicated set of diagnostics to observe the performance of the temporary poloidal limiters and infer basic transport behavior of the 3-D helical SOL plasma. We describe IR imaging of the limiters, which resulted in observations of 1) heat flux determination as a function of time and space, 2) total energy into the limiters, 3) high-frequency helical patterns of energy bursts onto the limiters, 4) changes in surface emissivity, and 5) detection of UFO's (small-to-large dusts). These measurements were made in 2 magnetic configuration discharges (differing iota), and in ones where the power loads to the limiters were systematically modified by the use of trim coils. Observed power fractions on the limiters ranged from 40{\%} to 20{\%} of the 0.6 to 4 MW ECRH input powers. [Preview Abstract] |
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CP10.00055: Observation of MHD fluctuation by ECE on W7-X first experimental campaign Hayato Tsuchiya, Matthias Hirsch, Gavin Weir, Udo Hofel, Marc Beurskens, Suguru Masuzaki Wendelstein 7-X is an optimized stellarator for ECRH high density steady-state discharges at reactor relevant collisionality regimes. The first experiment (OP1.1) was successfully conducted from Dec.2015. ECE (Electron Cyclotron Emission diagnostic) is one of the main diagnostic during the first experimental campaign. The 2$^{\mathrm{nd}}$ harmonic x-mode emission is obtained by outside-antenna and detected by 32-channel heterodyne radiometer. The frequency band is from 126GHz to 162GHz. Radiometers are calibrated by LN$_{\mathrm{2}}$ temperature and room temperature. The absolute calibration error was estimated to be \textasciitilde 10{\%}. The electron temperature radial profile obtained by ECE agrees the Thomson scattering and imaging X-ray spectroscopy result. The asymmetric profile is still indicated due to mix of O2-mode. Fluctuations derived from MHD instability are often observed by electron temperature and magnetic fluctuations. The radial mode structure is clearly identified by ECE. It indicates the existence of magnetic island and from its appearance on both sides of the X2 emission spectrum the knowledge on the localization of the ECE channels can be improved by symmetrization. [Preview Abstract] |
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CP10.00056: Conceptual Design of a Heavy Ion Beam Probe for W7-X P.J. Fimognari, T.P. Crowley, D.R. Demers, O. Grulke, R. Laube The conceptual design of a heavy ion beam probe (HIBP) for W7-X is in progress. The diagnostic is uniquely well suited to advancing understanding of particle and energy transport in optimized stellarators; it is able to simultaneously measure temporally and spatially resolved fluctuations of density and potential along with equilibrium electric potential and radial electric field from the plasma. The first steps in this design examine the inter-related issues of hardware interface and beam trajectory simulations. Beam trajectory simulations use the 3-D magnetic fields of W7-X to identify suitable beam energies, injection conditions, and cross-section of the plasma accessible to measurement. They also consider the available port pairs for the injection and detection of the ion beam, and location of the accelerator and energy analyzer. The implementation of the diagnostic on W7-X is anticipated during the second diagnostic phase (OP-2) and will use components of the 2MeV HIBP which was installed previously on the TEXT-U tokamak. [Preview Abstract] |
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CP10.00057: Anisotropic heat diffusion on stochastic magnetic field in the Large Helical Device Yasuhiro SUZUKI The magnetic topology is a key issue in fusion plasma researches. An example is the Resonant Magnetic Perturbation (RMP) to control the transport and MHD activities in tokamak and stellarator experiments. However, the physics how the RMP affects the transport and MHD is not clear. One reason is a role of the magnetic topology is unclear. That problem is connecting to the identification of the magnetic topology in the experiment. In the experiment, the finite temperature gradient is observed on the stochastic field where is stochastized by the theoretical prediction. In a classical theory, the electron temperature gradient should be zero on the stochastic magnetic field. We need to study the stochastic magnetic field can keep the finite temperature gradient or not. In this study, we study the anisotropic heat diffusion equation to simulate the heat transport on the stochastic magnetic field. Changing a ratio of $\kappa_{\parallel}$ and $\kappa_{\perp}$, the distribution of the temperature on the stochastic magnetic field is obtained. Hudson \textit{et al}. pointed out the KAM surface is a barrier to keep the finite temperature. We simulate those results in realistic magnetic field of the Large Helical Device. [Preview Abstract] |
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CP10.00058: Quantitative evaluation of wall heat loads by lost fast ions in the Large Helical Device Junki Morimoto, Yasuhiro Suzuki, Ryosuke Seki In fusion plasmas, fast ions are produced by neutral beam injections (NBI), ion cyclotron heating (ICH) and fusion reactions. Some of fast ions are lost from fusion plasmas because of some kinds of drift and instability. These lost fast ions may cause damages on plasma facing components such as divertors and diagnostic instruments in fusion reactors. Therefore, wall heat loads by lost fast ions in the Large Helical Device (LHD) is under investigation. For this purpose, we have been developing the Monte-Carlo code for the quantitative evaluation of wall heat loads based on following the guiding center orbits of fast ions. Using this code, we investigate wall heat loads and hitting points of lost fast ions produced by NBI in LHD. Magnetic field configurations, which depend on beta values, affect orbits of fast ions and wall heat loads. Therefore, the wall heat loads by fast ions in equilibrium magnetic fields including finite beta effect and magnetic islands are quantitatively evaluated. The differences of wall heat loads and particle deposition patterns for cases of the vacuum field and various beta equilibrium fields will be presented at the meeting. [Preview Abstract] |
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CP10.00059: Benchmark of MEGA Code on Fast Ion Pressure Profile in the Large Helical Device Ryosuke Seki, Yasushi Todo, Yasuhiro Suzuki, Masaki Osakabe As the first step for the analyses of energetic particle driven instabilities in the Large Helical Device (LHD) including the collisions of fast ions and the neutral beam injection, MEGA code is benchmarked on the classical fast ion pressure profile using the temperature and density profiles measured in the LHD experiments. In this benchmark, the MHD equilibrium is calculated with HINT code, and the beam deposition profile is calculated with HFREYA code. Since the equilibrium is not axisymmetric in LHD, the accuracy of orbit tracing is important for fast ion analyses. In the slowing down process of the MEGA code, the guiding center equation is numerically solved using the 4th order Runge-Kutta method and the linear interpolation. MEGA code is benchmarked against the results of MORH code, in which the 6th order Runge-Kutta and the 4th order spline interpolation are used. In LHD, the position of the loss boundary of fast ion is important because there are many “re-entering fast ions” which re-enter in plasma after they have once passed out of plasma. The effects of the position of the loss boundary on the fast ion pressure profile will be discussed, and a preliminary result of Alfven eigenmodes will be presented. [Preview Abstract] |
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CP10.00060: Bootstrap current calculations for TJ-II stellarator Julio J. Martinell, Katia Camacho Bootstrap current is stellarators is usually very small since they operate solely with the magnetic confinement provided by the external currents. Since plasma pressure gradients are always present the bootstrap current is always finite, but the magnetic design can be optimized to minimize it. In the helias configuration there is no optimization and therefore it is important to estimate the actual bootstrap current generated by given pressure profiles. Here, we use the configuration of the TJ-II helias to calculate the bootstrap current for various density regimes using the kinetic code DKES. We compute the monoenergetic transport coefficients $D_{11}$ and $D_{13}$ to find first the thermal ambipolar diffusion coefficients and the corresponding radial electric field and then the respective bootstrap current. This is made taking experimental density and electron and ion temperature profiles. In spite of the convergence problems of DKES at low collisionality, we can obtain bootstrap current values with acceptable uncertainties, without using Monte Carlo methods. The results are compared with axisymmetric neoclassical computations. The resulting rotational transform is used to obtain the rational surfaces location and predict the transport barriers observed in the experiments. [Preview Abstract] |
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CP10.00061: MFE: FIELD REVERSED CONFIGURATIONS AND SPHEROMAKS |
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CP10.00062: Overview of C-2U FRC Experimental Program and Plans for C-2W H. Gota, M.W. Binderbauer, T. Tajima, S. Putvinski, M. Tuszewski, S. Dettrick, S. Korepanov, A. Smirnov, M.C. Thompson, X. Yang, M. Cappello, A.A. Ivanov Tri Alpha Energy's experimental program has been focused on a demonstration of reliable field-reversed configuration (FRC) formation and sustainment, driven by fast ions via high-power neutral-beam (NB) injection. The world's largest compact-toroid experimental devices, C-2 [1] and C-2U [2], have successfully produced a well-stabilized, sustainable FRC plasma state with NB injection (input power, P$_{\mathrm{NB}}$ \textasciitilde 10$+$ MW; 15 keV hydrogen) and end-on coaxial plasma guns. Remarkable improvements in confinement and stability of FRC plasmas have led to further improved fast-ion build up; thereby, an advanced beam-driven FRC state has been produced and sustained for up to 5$+$ ms (longer than all characteristic system time scales), only limited by hardware and electric supply constraints such as NB and plasma-gun power supplies. To further improve the FRC performance the C-2U device is being replaced by C-2W featuring higher injected NB power, longer pulse duration as well as enhanced edge-biasing systems and substantially upgraded divertors. Main C-2U experimental results and key features of C-2W will be presented. \newline \newline [1] M.W. Binderbauer \textit{et al.}, Phys. Plasmas \textbf{22}, 056110 (2015). \newline [2] M.W. Binderbauer \textit{et al.}, AIP Conference Proceedings \textbf{1721}, 030003 (2016). [Preview Abstract] |
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CP10.00063: Overview of the Tri Alpha Energy Plasma Diagnostics Program Matthew Thompson, Hiroshi Gota, Sergei Putvinski, Michel Tuszewski, Michl Binderbauer Tri Alpha Energy (TAE) seeks to study the evolution of advanced beam-driven field-reversed configuration (FRC) plasmas sustained and heated by neutral beam (NB) injection [1,2]. Heating of FRCs is the focus of the upcoming C-2W program. Data on the FRC plasma performance is provided by a comprehensive suite of diagnostics including magnetic sensors, interferometry, Thomson scattering, spectroscopy, bolometry, reflectometry, and NB-related fast-ion/neutral diagnostics. While many of these diagnostic systems were first implemented for the earlier C-2 [3] and C-2U experiments, virtually all of them benefit from continuous improvement and upgrades. TAE maintains a large plasma diagnostics development program working on a variety of new systems for future devices including: far-infrared polarimetry, visible and infrared fast imaging cameras, proton detector arrays, end loss analyzers, impurity and majority ion CHERS, and 100-channel bolometer units with proprietary compact local data acquisition. In addition, extensive ongoing work focuses on developing advanced methods of measuring the internal magnetic fields of the FRC plasma. [1] M. W. Binderbauer et al., AIP Conference Proceedings \textbf{1721}, 030003 (2016). [2] M. W. Binderbauer et al., Phys. Plasmas \textbf{22}, 056110 (2015). [3] H. Gota et al., Rev. Sci. Instrum. \textbf{85}, 11D836 (2014). [Preview Abstract] |
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CP10.00064: C-2U Experimental Transport Analysis Erik Trask, M. Beall, N. Bolte, B. Deng, J. Douglass, H. Gota, E. Granstedt, D. Gupta, D. Osin, T. Roche, K. Zhai Upgrades of the experimental facility at Tri Alpha Energy have led to record Field Reversed Configuration (FRC) performance on the C-2U device[1]. Modifications to magnetic field characteristics and increases in neutral beam power were critical drivers in achieving sustained plasma targets for greater than 5ms. 0D power balance calculations detailing loss channel characteristics and plasma timescales will be presented demonstrating substantial improvements in equilibrium and transport parameters. [1]M.W. Binderbauer \textit{et al.}, AIP Conf. Proc. \textbf{1721}, 030003 (2016) [Preview Abstract] |
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CP10.00065: Neutral Beam Injection System for the C-2W Field Reversed Configuration Experiment Alexander Dunaevsky, Alexander Ivanov, Vyacheslav Kolmogorov, Artem Smirnov, Sergey Korepanov, Michl Binderbauer C-2U Field-Reversed Configuration (FRC) experiment proved substantial reduction in turbulence-driven losses via tangential neutral beam injection (NBI) coupled with electrically biased plasma guns at the plasma ends. Under such conditions, highly reproducible, advanced beam-driven FRCs were produced and sustained for times significantly longer (more than 5 ms) than all characteristic plasma decay times without beams. To further improve FRC sustainment and demonstrate the FRC ramp-up, the C-2U experimental device is undergoing a major upgrade. The upgrade, C-2W, will have a new NBI system producing a record total hydrogen beam power of 20$+$ MW in a 30ms pulse. The NBI system consists of eight positive-ion based injectors featuring flexible, modular design. Four out of eight NBI injectors have a capability to switch the beam energy during a shot from the initial 15 keV to 40 keV at a constant beam current. This feature allows to increase the beam energy and thereby optimize the beam-plasma coupling during the magnetic field ramp up. This presentation provides an overview of the C-2W NBI system, including the design of the switchable energy injectors, layout of the power supply system, and results of the prototype testing. [Preview Abstract] |
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CP10.00066: Observation of Super-thermal Plasma \& Beam Energization in a Beam Driven Field-reversed Configuration Ryan Clary, R Magee, A Roquemore, A Kolmogorov, A Ivanov, S Korepanov, S Medley, A Smirnov, M Tiunov C-2U is an advanced beam driven field-reversed configuration (FRC) plasma confinement experiment which sustains the configuration for $>5\ms$, in excess of typical MHD times as well as fast particle instability and slowing down times\footnote{M.~W. Binderbauer, et al. \emph{Physics of Plasmas}, 22(5):056110, 2015}. The dynamics of fast particles from hydrogen neutral beam injection are critical to the deuterium FRC performance. To improve our understanding of the effects of fast particles in this system an E$\parallel$B neutral particle analyzer \footnote{S.~S. Medley and A.~Roquemore. \emph{Review of Scientific Instruments}, 75(10):3625--3627, 2004} was acquired which simultaneously measures H$^0$ and D$^0$ flux with high energy resolution. In addition, a small, high purity, ion beam system was constructed and used to calibrate the absolute response of this device. Measurements from this NPA reveal a super-thermal population in the FRC (deuteron) plasma distribution as well as a fast ion (proton) distribution above the beam injection energy. [Preview Abstract] |
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CP10.00067: Phenomenology of beam driven modes in the field reversed configuration Richard Magee, Nathan Bolte, Ryan Clary, Ales Necas, Sergey Korepanov, Artem Smirnov, Matthew Thompson, Toshiki Tajima, THE TAE TEAM The C-2U experiment offers a unique plasma environment combining a high beta field reversed configuration (FRC) embedded in a low beta magnetic mirror with high power neutral beam injection. The beams are injected tangentially into a modest magnetic field so that the orbits of the resulting fast ions encircle the entire plasma. These large orbit particles sustain$^{\mathrm{1}}$ and stabilize$^{\mathrm{2}}$ the plasma and suppress turbulence. Measurements of magnetic fluctuations at the edge of the plasma reveal the presence of three coherent beam driven modes: a low frequency, chirping mode, a mode near the ion cyclotron frequency, and a high frequency compressional Alfven mode. Remarkably, none of these modes are observed to have a deleterious effect on global plasma confinement. In fact, the cyclotron mode has the beneficial effect of dramatically enhancing the DD fusion reaction rate by drawing a trail from the plasma ion energy distribution on a sub-collisional timescale. In this presentation, we experimentally characterize the beam driven modes in the C-2U FRC with data from multiple diagnostics including magnetics, spectroscopy, neutral particle analyzers and fusion product diagnostics. Results are compared to a particle-in-cell simulation in a simplified geometry. $^{\mathrm{1}}$ M. W. Binderbauer, et. al., AIP Conference Proceedings 1721, 030003 (2016) $^{\mathrm{2}}$ M. Tuszewski et. al, Phys. Rev. Lett 108, 255008 (2012) [Preview Abstract] |
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CP10.00068: EPOCH code simulation of a non-thermal distribution driven by neutral beam injection in a high-beta plasma A. Necas, T. Tajima, S. Nicks, R. Magee, R. Clary, T. Roche In Tri Alpha Energy's C-2U experiment, advanced beam-driven field-reversed configuration (FRC) plasmas were sustained via tangential neutral beam injection.$^{\mathrm{1}}$ The dominant fast ion population made a dramatic impact on the overall plasma performance. To explain an experimentally observed anomalous neutron signal (100x thermonuclear), we use EPOCH PIC code [2] to simulate possible beam driven non-destructive instabilities that transfer energy from fast ions to the plasma, causing phase space bunching. We propose that the hydrogen beam ion population drives collective modes in the deuterium target plasma, giving rise to the instability and increased fusion rate. The instability changes character from electrostatic in the low beta edge to fully electromagnetic in the core, with an associated reduction in growth rates. The DD reactivity enhancement is calculated using a two-body correlation function and compared to the experimentally observed neutron yield. The high-energy tails in the distributions of the plasma deuterons and beam protons are observed via a mass-resolving Neutral Particle Analyzer (NPA) diagnostic. This observation is qualitatively consistent with EPOCH [2] simulation of the beam-plasma instability. [1] M. Binderbauer et al. ``A high performance field-reversed configuration.'' \textit{Phys. Plasmas} 22.5 (2015): 056110. [2] T. D. Arber et al. ``Contemporary particle-in-cell approach to laser-plasma modeling.'' \textit{Plasma Phys. Control. Fusion} 57.11 (2015): 113001. [Preview Abstract] |
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CP10.00069: Instabilities in Beam-Plasma Waves in a Model of the Beam-Driven FRC Bradley Scott Nicks, Ales Necas, Toshi Tajima Using a semi-analytic solver, the kinetic properties of plasma waves are analyzed in various regimes in the presence of a beam. This analysis is done to model the strong beam-driven Field-Reversed Configuration (FRC) plasma kinetic instabilities in the neighborhood of the ion cyclotron frequency. As the frequency is relatively high, and wavelength small, the plasma is taken to be local and thus homogeneous, comprised of bulk ions, electrons, and beam ions, with a uniform background magnetic field. The beam ions are given an azimuthal drift velocity with respect to the magnetic field, but otherwise have various Maxwellian velocity distributions. First, the magnetic field is varied to create regimes of low and high $\beta $, and the mode structures are compared. The low-$\beta $ case (corresponding to the scrape-off layer and near the separatrix) features primarily the beam-driven ion Bernstein instability. The high-$\beta $ case (the core of FRC) is primarily electromagnetic and features the AIC instability when temperature anisotropy is included. The most unstable modes are incited by near-perpendicular beam injection with respect to the magnetic field. Finally, the results of the semi-analytic solver are compared with those from the EPOCH PIC code [1] to evaluate the influence of nonlinear effects. This theoretical modeling was used in conjunction with EPOCH to investigate the beam driven instabilities in Tri Alpha Energy's C-2U experiment [2]. [1] T.D. Arber \textit{et al}., Plasma Phys. Control. Fusion 57.11 (2015): 113001. [2] M.W. Binderbauer \textit{et al.}, AIP Conference Proceedings \textbf{1721}, 030003 (2016). [Preview Abstract] |
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CP10.00070: Observation and suppression of a new fast ion driven micro burst instability in a field-reversed configuration plasma B.H. Deng, S. Korepanov, E. Belova, J. Douglass, M. Beall, M. Binderbauer, R. Clary, S. Detrick, E. Garate, H. Gota, E. Granstedt, R. Magee, A. Necas, S. Putvinski, T. Roche, A. Smirnov, T. Tajima, M Thompson, M. Tuszewski, A. Van Drie The C-2U experiment offers a unique plasma environment combining a high beta field reversed configuration (FRC) embedded in a low beta magnetic mirror with high power neutral beam injection. The beams are injected tangentially into a modest magnetic field so that the orbits of the resulting fast ions encircle the entire plasma. The dominant population of large orbit fast ions sustains and stabilizes the FRC, suppresses turbulence, and makes a dramatic beneficial impact on the overall plasma performance [1]. Abundant interesting new physics phenomena are observed in this high performance FRC operation regime, including micro bursts, which are benign, periodic bursting small amplitude down chirping fluctuations seen by several diagnostics. Detailed analysis of the micro bursts measurement data, bulk plasma equilibrium profiles, and fast ion orbit characteristics show that the micro bursts might be driven by a small number of resonant fast ions [2], and can be suppressed when the number of resonant particles is reduced. [1] M. Binderbauer \textit{et al}., Physics of Plasmas, 22, 056110 (2015). [2] E.V. Belova, Bull. of Am. Phys. Soc., \textbf{58}, p. 128, GP8 54 (2013). [Preview Abstract] |
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CP10.00071: Opto-Mechanical Design of FIR Diagnostic System for C-2W Michael Beall, B. H. Deng, G. Settles, M. Rouillard, J. Schroeder, H. Gota, M. Thompson, G. Snitchler, S. Ziaei The goal of the C-2W far-infrared (FIR) diagnostic system is to provide highly accurate, simultaneous polarimetry and interferometry information about the generation, equilibrium and time evolution of the advanced beam-driven field-reversed configuration (FRC). Thorough spatial coverage of the confinement vessel will be provided by a set of 14 chords at the central plane, with half of the chords tilted at a 15\textdegree angle to provide additional polarimetry information. Due to the very low (\textless .5\textdegree ) Faraday rotation expected in the field-reversed plasma, the system has a design goal of .25 $\mu $m maximum allowable vibration over the lifetime of the shot. Due to large eddy-current forces from simulation of magnetic-field ramp-up, a non-metallic canvas phenolic material has been selected for the primary breadboards, which are mounted on a rigid, sand-filled support structure. Given the size of the structure and the magnetic impact, the support structure does not use pneumatic or mechanical isolation. Dynamic vibration analysis with Ansys, based on measurements of local ground vibration and simulations of magnetic forces, predicts that the system will meet the design goal. [Preview Abstract] |
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CP10.00072: Design of C-2W Thomson Scattering System Kan Zhai, Tania Schindler, Helen Zhang, Kurt Walters, Matthew Thompson, The TAE Team A suite of multi-point Thomson scattering systems is now being designed and built in parallel with the construction of the C-2W FRC experimental device, which is expected to have a wide range of electron temperature T$_{\mathrm{e}}$ and density n$_{\mathrm{e}}$ from edge to center region at different operational phases. The suite consists of two sub-systems that measure T$_{\mathrm{e}}$ and n$_{\mathrm{e}}$ profiles at the C-2W central plane and at the jet region. A high-repetition rate Nd:YAG laser is planned for the central plane subsystem for time-resolved profile measurement at 1 kHz. The central plane and jet region subsystems have their own specially-designed collection optics that image 16 and 5 radial points along the laser-beam path onto corresponding surfaces of fiber bundles, which will then relay the collected laser light into dispersing polychromators. The polychromators are designed with five spectral channels with four channels optimized and dedicated to T$_{\mathrm{e}}$ measurement and one channel dedicated to Rayleigh scattering calibration for n$_{\mathrm{e}}$ measurement. Detail system design and layout of lasers, beam transportation and stray light control, collection optics and fiber optics, dispersion and detection system and its spectral calibration setup will be presented. [Preview Abstract] |
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CP10.00073: Absence of Ion-scale Core Turbulence and Transport Barrier Formation with Passive/active Divertor Biasing in the C-2/C-2U Field Reversed Configuration Michel Tuszewski, D. Fulton, C. Lau, I. Holod, Z. Lin, B.H. Deng, H. Gota, T. Tajima, M. Binderbauer, L. Schmitz Experimentally measured inverted core density fluctuations spectra show the absence of ion-scale modes in the FRC core, in agreement with linear, local gyrokinetic simulations. The absence of ion-scale core fluctuations is attributed to a combination of Finite Larmor radius effects, short fieldline connection length, and the radially increasing magnetic field gradient. In contrast, ion-scale modes driven unstable by the radial density and electron temperature gradients are observed in the FRC scrape-off layer (SOL) with characteristic wavenumbers 2 $\le \quad k\rho_{s} \quad \le $ 40, consistent with the unstable mode spectrum indicated by linear gyrokinetic calculations. Electostatic passive or active divertor biasing (via a large radius LaB$_{6}$ electron emitter) maintains sufficient \textbf{\textit{E}}\texttimes \textbf{\textit{B}} rotational shear just outside the FRC separatrix to establish an effective radial transport barrier, with a large critical density gradient comparable to or above the linear instability threshold from gyrokinetic simulations. An advanced Doppler Backscattering diagnostic design for C-2W will be shown. [Preview Abstract] |
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CP10.00074: Simulation of drift wave instability in field-reversed configurations using global magnetic geometry D. P. Fulton, C. K. Lau, Z. Lin, T. Tajima, I. Holod Minimizing transport in the field-reversed configuration (FRC) is essential to enable FRC-based fusion reactors. Recently, significant progress on advanced beam-driven FRCs in C-2 and C-2U (at Tri Alpha Energy) provides opportunities to study transport properties using Doppler backscattering (DBS) measurements of turbulent fluctuations and kinetic particle-in-cell simulations of driftwaves in realistic equilibria via the Gyrokinetic Toroidal Code (GTC) [1,2]. Both measurements and simulations indicate relatively small fluctuations in the scrape-off layer (SOL). In the FRC core, local, single flux surface simulations reveal strong stabilization, while experiments indicate quiescent but finite fluctuations. One possible explanation is that turbulence may originate in the SOL and propagate at very low levels across the separatrix into the core. To test this hypothesis, a significant effort has been made to develop A New Code (ANC) based on GTC physics formulations, but using cylindrical coordinates which span the magnetic separatrix, including both core and SOL. Here, we present first results from global ANC simulations. [1]D. P. Fulton et al, Phys. Plasmas 23, 012509 (2016). [2]D. P. Fulton et al, Phys. Plasmas 23, 056111 (2016). [Preview Abstract] |
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CP10.00075: Global simulation of field-reversed configuration using fully kinetic ion and drift kinetic electron Calvin Lau, Daniel Fulton, Animesh Kuley, Jian Bao, Zhihong Lin, Michael Binderbauer, Toshiki Tajima, Lothar Schmitz In the last several years, the C-2/C-2U advanced beam-driven field-reversed configuration (FRC) experiments at Tri Alpha Energy have progressed to consistent, reproducible plasma lifetimes of 10+ ms, i.e. FRCs have reached transport limited regimes. In FRC geometry, the thermal ion gyroradius is on the order of the size of the plasma near the magnetic null-point. Fast ion orbits intersect both the FRC core and the scrape-off layer (SOL) regions. Previous local simulations of electrostatic drift-wave instabilities using the Gyrokinetic Toroidal Code (GTC) find the core to be robustly stable with driftwave instability only in the SOL at frequencies approaching the ion cyclotron frequency\footnote{D. P. Fulton et al, Phys. Plasmas 23, 012509 (2016)}\footnote{D. P. Fulton et al, Phys. Plasmas 23, 056111 (2016)}. Therefore, FRC transport studies require fully kinetic ion simulations with cross-separatrix coupling between the core and SOL. Here we report progress of such global simulations using fully kinetic ions and drift kinetic electrons, including the implementation of the Boris push scheme for cyclotron motion and cylindrical coordinates for the separatrix. [Preview Abstract] |
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CP10.00076: First Fast-Ion D-alpha (FIDA) Measurements and Simulations on C-2U Nathan Bolte, Deepak Gupta, Luke Stagner, Marco Onofri, Sean Dettrick, Erik Granstedt In Tri Alpha Energy's C-2U experiment, advanced beam-driven field-reversed configuration (FRC) plasmas were sustained via tangential neutral beam injection$^{\mathrm{1}}$. The dominant fast ion population made a dramatic impact on the overall plasma performance. A fast-ion D-alpha (FIDA)$^{\mathrm{2}}$ diagnostic, which is based on the Doppler-shifted Balmer-alpha light from neutralized fast ions, was recently added to the C-2U fast-ion diagnostics suite. The first ever FIDA measurements on an FRC topology have been carried out. Bandpass-filtered FIDA measurements (\textgreater 6 keV ions) were made with a photomultiplier tube and are forward modeled by FIDASIM. Line-integrated signals were taken at eight radial locations and eight times during the FRC lifetime. While the measurements share some salient features with the simulation, they are 4.5x larger, suggesting a higher fast-ion content than the Monte Carlo distribution. Highly Doppler-shifted beam radiation is also measured with a high-speed camera and is spatially well-correlated with FIDASIM. Having shown the feasibility of FIDA on C-2U, we will further explore the use of FIDA on the upgraded C-2W machine to estimate fast-ion densities and to infer the local fast-ion distribution function. $^{\mathrm{1}}$M. Binderbauer \textit{et al}, Phys. of Plasmas \textbf{22}, 056110 (2015) $^{\mathrm{2}}$W.W. Heidbrink., Rev. Sci. Instr. \textbf{81,} 10D727 (2010) [Preview Abstract] |
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CP10.00077: Fast imaging measurements and modeling of neutral and impurity density on C-2U Erik Granstedt, B. Deng, S. Dettrick, D.K. Gupta, D. Osin, T. Roche, K. Zhai The C-2U device\footnote{M.~Binderbauer, et~al. Physics of Plasmas \textbf{22}, 056110 (2015)} employed neutral beam injection and end-biasing to sustain an advanced beam-driven Field-Reversed Configuration plasma for 5+ ms, beyond characteristic transport time-scales. Three high-speed, filtered cameras observed visible light emission from neutral hydrogen and impurities, as well as deuterium pellet ablation and compact-toroid injection which were used for auxiliary particle fueling. Careful vacuum practices and titanium gettering successfully reduced neutral recycling from the confinement vessel wall. As a result, a large fraction of the remaining neutrals originate from charge-exchange between the neutral beams and plasma ions. Measured H/D-$\alpha$ emission is used with DEGAS2 neutral particle modeling to reconstruct the strongly non-axissymmetric neutral distribution. This is then used in fast-ion modeling to more accurately estimate their charge-exchange loss rate. Oxygen emission due to electron-impact excitation and charge-exchange recombination has also been measured using fast imaging. Reconstructed emissivity of O$^{4+}$ is localized on the outboard side of the core plasma near the estimated location of the separatrix inferred by external magnetic measurements. [Preview Abstract] |
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CP10.00078: Measurements of energy loss in the scrape-off layer of C-2U M.E. Griswold, S. Korepanov, M.C. Thompson We report on measurements of energy transport in the scrape-off layer (SOL) plasma that surrounds the advanced beam-driven field reversed configuration (FRC) of the C-2U experiment at Tri Alpha Energy. The SOL plasma is trapped on mirror-like open field lines outside of the FRC separatrix that connect to material surfaces at both ends of the vacuum vessel. Heat transport in this region is expected to be convective, like in mirror machines, and can be characterized by the amount of energy lost per electron-ion pair. We measured this value with an end loss analyzer system that consists of gridded ion energy analyzers that measure ion current density and pyroelectric crystal bolometers that measure total particle power flux. [Preview Abstract] |
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CP10.00079: Integrated Electron Density Measurements of End Loss Plasma in the C-2U Divertor Daniel Sheftman, Lothar Schmitz, Matthew Thompson Experiments demonstrating sustainment of field-reversed configuration (FRC) plasmas via neutral beam injection have been carried out on C-2U [1]. Accurate design and operation of an end-loss plasma divertor is of crucial importance to the sustainment of the FRC plasma, and the increase of electron and ion temperature. A single-chord, mono-static, homodyne 94 GHz microwave interferometer was developed and installed on the C-2U divertor, and was used to measure the integrated electron density of the end-loss plasma. Results of these measurements as well as an initial design of a multi-chord microwave interferometer, planned to be used on the divertor of the advanced C-2W experiment, will be presented. This diagnostic will provide a full radial electron-density profile of the end-loss plasma in the divertor. [1] M. W. Binderbauer et al., AIP Conference Proceedings \textbf{1721}, 030003 (2016). [Preview Abstract] |
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CP10.00080: Spectroscopic observations of scrape-off layer electron temperature and density in an advanced beam-driven field-reversed configuration Dmitry Osin, Tania Schindler A Scrape-Off Layer (SOL) of an advanced beam-driven Field-Reversed Configuration (FRC)\footnotemark[1] was experimentally studied employing a compact dual-wavelength imaging system. A nearly 10~cm SOL plasma region adjacent to the FRC core was examined with relatively high spatial 0.2~cm and temporal 25~$\mu$s resolutions. Injection of a helium jet into the C-2U confinement vessel ensured local measurements in the observed SOL region. Time-resolved electron temperature and density were determined from a ratio of the obtained 2D spectral images of neutral He lines. The SOL plasmas were studied in two different configurations of edge-biasing systems\footnotemark[2]$^,$ \footnotemark[3]: passively biased electrode plates and active LaB6 cathode. Observed electron temperatures and densities are presented for both passive and active configurations of edge-biasing systems.\\ % % References. % 1. M.W. Binderbauer et al., Phys. Plasmas 22, 056110 (2015).\\ 2. M. Tuszewski et al, Phys. Rev. Lett. 108, 255008 (2012).\\ 3. M. Thompson et al, Bull. Am. Phys. Soc. 59, UP8.00011 (2014). [Preview Abstract] |
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CP10.00081: Possible role of external radial electric field on ion heating in an FRC Deepak Gupta, E Trask, S Korepanov, E Granstedt, D Osin, T Roche, B Deng, M Beall, K Zhai, The TAE Team In C-2/C-2U FRCs [1], a radial electric field is applied by either plasma guns or biased electrodes inside the divertors, at both ends of the machine. The electric field plays an important role in stabilizing the FRC; thus, providing a favorable target condition to a neutral beam injection. In addition, it is also observed that the application of radial electric field may lead to a heating of ions. Radial profile of impurity ion emission, azimuthal velocity and temperature are measured under different configurations. The conditions and evidences of ion heating due to the electric field biasing will be presented and discussed. Radial momentum balance equation of oxygen impurity ions is used with these measurements to estimate the radial electric field profile. Parameters affecting the ion heating due to biasing will also be discussed with some correlations. The external radial electric field is planned to be applied by biased electrodes and plasma guns in C-2W inner/outer divertors. [1] M. W. Binderbauer et al., Phys. Plasmas \textbf{22}, 056110 (2015). [Preview Abstract] |
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CP10.00082: Edge/SOL Plasma Parameter/Magnetic Field Profile and Fluctuation Measurements at C-2U Mid-plane T. Roche, A. Necas, B. Deng, D. Osin, K. Zhai, K. Knapp C-2U is the premier advanced beam-driven FRC at Tri Alpha Energy. It has been found that edge interaction plays a significant role in core stability. In particular, the radial electric field in the scrape-off-layer (SOL), generated by applying a voltage to the central electrode panel of the divertors, can suppress $n=$1 and $n=$2 MHD modes [1]. The conduction path for the currents that generate this field is very long. Measurements of plasma parameters, including n$_{\mathrm{e}}$, T$_{\mathrm{e}}$ and V$_{\mathrm{f}}$, in the edge region at the mid-plane of the C-2U confinement vessel have been performed with an insertable triple probe. In conjunction with other diagnostics these measurements allow us to generate complete radial profiles of these parameters. In addition to the absolute values of said parameters, fluctuations have been analyzed. Radial profiles of magnetic-field fluctuations have been measured by a new insertable 3D B-dot probe. The data indicate the presence of the Alfv\'{e}n Ion Cyclotron (AIC) instability. Profile measurements and fluctuation analysis will be presented. [1] M. Binderbauer \textit{et al}., Phys. Plasmas \textbf{22}, 056110 (2015). [Preview Abstract] |
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CP10.00083: Behavior of Compact Toroid Injected into C-2U Confinement Vessel Tadafumi Matsumoto, T. Roche, I. Allrey, J. Sekiguchi, T. Asai, M. Conroy, H. Gota, E. Granstedt, C. Hooper, J. Kinley, T. Valentine, W. Waggoner, M. Binderbauer, T. Tajima The compact toroid (CT) injector system [1] has been developed for particle refueling on the C-2U device [2]. A CT is formed by a magnetized coaxial plasma gun (MCPG) and the typical ejected CT/plasmoid parameters are as follows: average velocity $\sim $100 km/s, average electron density $\sim 1.9\times 10^{15}$ cm$^{\mathrm{-3}}$, electron temperature 30-40 eV, mass $\sim 12\mu g$. To refuel particles into FC plasma the CT must penetrate the transverse magnetic field that surrounds the FRC. The kinetic energy density of the CT should be higher than magnetic energy density of the axial magnetic field, i.e., $\rho v^{2}/2\ge B^{2}/2\mu_{0} $, where $\rho $, v, and B are mass density, velocity, and surrounded magnetic field, respectively. Also, the penetrated CT's trajectory is deflected by the transverse magnetic field (B$_{\mathrm{z}}\sim $1 kG). Thus, we have to estimate CT's energy and track the CT trajectory inside the magnetic field, for which we adopted a fast-framing camera on C-2U: framing rate is up to 1.25 MHz for 120 frames. By employing the camera we clearly captured the CT/plasmoid trajectory. Comparisons between the fast-framing camera and some other diagnostics as well as CT injection results on C-2U will be presented. [1] T. Matsumoto et.al., Rev. Sci. Instrum. \textbf{87}, 053512 (2016). [2] M. Binderbauer et. al., Phys. Plasmas \textbf{22}, 056110 (2015). [Preview Abstract] |
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CP10.00084: Performance improvement of magnetized coaxial plasma gun by magnetic circuit on a bias coil Takahiro Edo, Tadafumi Matsumoto, Tomohiko Asai, Yasuhiro Kamino, Michiaki Inomoto, Hiroshi Gota A magnetized coaxial plasmoid accelerator has been utilized for compact torus (CT) injection to refuel into fusion reactor core plasma. Recently, CT injection experiments have been conducted on the C-2/C-2U facility at Tri Alpha Energy [1,2]. In the series of experiments successful refueling, i.e. increased particle inventory of field-reversed configuration (FRC) plasma, has been observed. In order to improve the performance of CT injector and to refuel in the upgraded FRC device, called C-2W, with higher confinement magnetic field, magnetic circuit consisting of magnetic material onto a bias magnetic coil is currently being tested at Nihon University. Numerical work suggests that the optimized bias magnetic field distribution realizes the increased injection velocity because of higher conversion efficiency of Lorenz self force to kinetic energy. Details of the magnetic circuit design as well as results of the test experiment and field calculations will be presented and discussed. [1] T. Matsumoto \textit{et al}., Rev. Sci. Instrum. \textbf{87}, 053512 (2016). [2] T. Roche \textit{et al}., Bull. Am. Phys. Soc. \textbf{60}, BP12.00023 (2015). [Preview Abstract] |
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CP10.00085: A Pre-ionization System to Limit Neutral Gas in a Compact Toroid Injector Ian Allfrey, Thomas Roche, Tadafumi Matsumoto, Eusebio Garate, Hiroshi Gota, Tomohiko Asai Fusion plasmas require long lifetimes and high temperatures, both of which are limited by particle loss, among other factors. Therefore, refueling a long-lived advanced beam-driven field-reversed configuration (FRC) plasma in C-2U [1] is necessary, and injecting a supersonic compact toroid (CT) is an effective means of introducing particles into the FRC core [2]. However, neutral gas that trails the CT into the target chamber cools the FRC. Pre-ionization (PI) system assists the break down between electrodes of the CT injector (CTI), so the amount of introduced gas can be lowered by up to a factor of two, effectively increasing the ionization fraction; thus, reducing the amount of neutral gas in the system. Additionally, the PI decreases the delay in CTI breakdown so a highly reproducible operation is achievable. The PI system consists of a fast, high voltage, pulse discharge circuit coupled to a Teflon insulated semi-rigid coaxial cable inserted into the CTI. System details and experimental data will be presented, in addition to issues such as the introduction of impurities and pre-ionizer lifetime. [1] M. Binderbauer \textit{et al}., Phys. Plasmas 22, 056110 (2015). [2] T. Matsumoto \textit{et al}., Rev. Sci. Instrum. 87, 053512 (2016). [Preview Abstract] |
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CP10.00086: Simulation Overview of High-Performance Beam-Driven FRCs S.A. Dettrick, D.C. Barnes, E. Belova, F. Ceccherini, D.P. Fulton, L. Galeotti, S. Gupta, H.J. Monkhorst, Y. Mok, A. Necas, M. Onofri, L.C. Steinhauer, T. Tajima The C-2U experiment [1] presents a unique combination of challenges to simulation: a dynamic formation process, high beta ($\sim 85\%$ average) and large ion orbits, neutral beam heating and energetic particles, coupling of core transport with the SOL, and electrode biasing of the SOL. These challenges have been addressed with a suite of codes, including extended MHD simulation of dynamic theta-pinch formation, translation, and collision; Monte Carlo simulation of Neutral Beam heating; 3D hybrid PIC code simulation of the influence of neutral beams and end-biasing on macrostability; 3D PIC simulation of turbulent transport; PIC simulation of beam driven plasma modes; hybrid fluid/particle transport simulation of heating, fueling, and current drive; kinetic simulation of parallel electron transport in the Scrape Off Layer; and neutral particle transport. \newline$[1]$ M.W. Binderbauer et al., Phys. Plasmas 22, 056110 (2015). [Preview Abstract] |
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CP10.00087: FPIC Study of the n$=$1 Toroidal Mode in FRC Plasmas Francesco Ceccherini, Sean Dettrick, Dan Barnes, Laura Galeotti The near term goal of TAE's experiments is to sustain through the of use of neutral beam heating and edge biasing an advanced beam-driven FRC for many milliseconds, i.e., well beyond the growth times of common instabilities. Here we apply FPIC, a quasineutral hybrid code with fully kinetic ions, to study the growth rate of the n$=$1 mode and related kinetic effects versus different equilibrium parameters in configurations of interest for upcoming TAE's experiments. In particular we address how the n$=$1 growth rate scales versus S*/E and E. The mode strength is computed taking into account a full 3D mode decomposition of the FRC separating the physically distinct contributions, namely tilt, radial shift and interchange modes and we study the role of each one of them. [Preview Abstract] |
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CP10.00088: Transport simulations of the C-2 and C-2U Field Reversed Configurations with the Q2D code. Marco Onofri, Sean Dettrick, Daniel Barnes, Toshiki Tajima The Q2D code is a 2D MHD code, which includes a neutral fluid and separate ion and electron temperatures, coupled with a 3D Monte Carlo code, which is used to calculate source terms due to neutral beams. Q2D has been benchmarked against the 1D transport code Q1D and is used to simulate the evolution of the C-2 and C-2U field reversed configuration experiments [1]. Q2D simulations start from an initial equilibrium and transport coefficients are chosen to match C-2 experimental data. C-2U is an upgrade of C-2, with more beam power and angled beam injection, which demonstrates plasma sustainment for 5$+$ ms. The simulations use the same transport coefficients for C-2 and C-2U, showing the formation of a steady state in C-2U, sustained by fast ion pressure and current drive. [Preview Abstract] |
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CP10.00089: FRC equilibrium reconstruction by Bayesian evaluation of Monte Carlo transport simulations Nikolaus Rath, M. Onofri, E. Trask Beam-driven field reversed configurations (FRCs) can be sustained
for multiple ms. Many important properties of such FRCs can not be
measured directly. When such properties are needed to guide
experiments, they are either substituted by proxies (e.g. the
excluded flux radius $r_{\Delta \Phi}$ is used instead of the
separatrix radius $r_\text{s}$), or derived from other measurements by
imposing specific models (e.g. $\left |
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CP10.00090: Hybrid reconstruction of field-reversed configurations Loren Steinhauer Field-reversed configurations (FRC) are poorly represented by fluid-based models and require instead an ion-distribution function. Two such populations are needed since ``core'' ions are roughly restricted to the region inside the separatrix, whereas ``periphery'' ions can escape along open field lines. The Vlasov equation governs the distribution, the general solution to which is an arbitrary function of the constants of motion (Hamiltonian, canonical angular momentum). Only a small subset of such distributions are realistic in view of collisions, which smooth the distribution, and instabilities, which reorganize the field structure. Collisions and end loss are included if the distribution is a solution to the Fokker-Planck (FP) equation. Vlasov and FP solutions are nearly identical in weakly-collisional plasmas. Numerical construction of such equilibria requires solving both Ampere's law for the magnetic flux variable and the ponderous task of a full velocity-space integration at each point. The latter can be done analytically by expressing the distribution as the superposition of simple basis elements. This procedure allows rapid reconstruction of evolving equilibria based on limited diagnostic observables in FRC experiments. [Preview Abstract] |
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CP10.00091: UEDGE modeling of a small-s FRC reactor's asymmetric scrape-off-layer (SOL) Nicholas McGreivy, Amir Raja, Eugene Evans, Olivier Izacard, Tom Rognlien, Samuel Cohen The field-reversed configuration (FRC) is being considered for use as a terrestrial power plant and as a direct-fusion-drive rocket engine for future space missions in the solar system. To produce thrust for a rocket or extract energy for electricity production, an asymmetric SOL has been proposed in which coolant/propellant gas is injected into a gas box at one end of the SOL. Plasma formed in the gas box flows along the SOL and its electrons are heated as they pass near the FRC core. The heated plasma is then ejected out a magnetic nozzle at the opposite end. We have used a 2D fluid code, UEDGE, to conduct numerical simulations of this FRC's SOL. We have examined the effects of power input (1-10~MW) and gas flow (10-200~kA-equiv) on the rocket’s thrust (1-100~N) and specific impulse (\textit{ca}. 1.5e5~s) as well as on the power flow. One important result is that the high plasma flow out of the gas box and the cold plasma within it reduce the power flow into the gas box well below 50\% of the input power. Plasma dynamics of the gas box region have been investigated to assess the degree of detachment that can be obtained for given performance requirements. [Preview Abstract] |
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CP10.00092: Observations of FRC Trapped Flux Lifetime Relative to Its Prolateness Chris Grabowski, James Degnan, Matthew Domonkos, David Amdahl, Edward Ruden, Glen Wurden, Thomas Weber The Field-Reversed Configuration Heating Experiment (FRCHX) explored scientific issues associated with HED laboratory plasmas (HEDLPs) and phenomena relevant to magneto-inertial fusion in a closed-field-line plasma. To create the HEDLP conditions, a field-reversed configuration (FRC) of moderate density was formed via reversed-field theta pinch, translated into a solid liner where it was trapped between two magnetic mirrors, and then adiabatically compressed by solid liner implosion. Shortly following formation, the FRCs typically had a separatrix radius of 3\textasciitilde 3.5 cm, peak density of \textasciitilde 10$^{\mathrm{17}}$ cm$^{\mathrm{-3}}$, and temperature of \textasciitilde 200 eV. The lifetime of trapped flux within the plasma was initially 13-16 $\mu $s following formation, or 8-11 $\mu $s once the FRC settled within the capture region. This was too short to allow complete compression by the solid liner, even when starting implosion before FRC formation. By moving the mirror coils 10 cm further apart, the magnetic well width increased by 6\textasciitilde 8 cm, which resulted in an increase in the trapped flux lifetime by 4\textasciitilde 5 $\mu $s. This presentation describes characteristics of the FRC plasmas prior to and following the lengthening of the capture region. From the literature, conclusions are made linking FRC stability and prolateness to FRC trapped flux lifetime. [Preview Abstract] |
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CP10.00093: Overview of HIT-SI3 experiment: Simulations, Diagnostics, and Summary of Current Results James Penna, Thomas Jarboe, Brian Nelson, Aaron Hossack, Derek Sutherland, Kyle Morgan, Chris Hansen, Thomas Benedett, Chris Everson, Brian Victor The Helicity Injected Torus - Steady Inductive 3(HIT-SI3)experiment forms and maintains spheromaks via Steady Inductive Helicity Injection (SIHI). Three injector units allow for continuous injection of helicity into a copper flux conserver in order to sustain a spheromak. Firing of the injectors with a phase difference allows finite rotation of the plasma to provide a stabilizing effect. Simulations in the MHD code NIMROD and the fluid-model code PSI-TET provide validation and a basis for interpretation of the observed experimental data. Thompson Scattering (TS) and Far Infrared (FIR) Interferometer systems allow temperature and line-averaged density measurements to be taken. An Ion Doppler Spectroscopy (IDS) system allows measurement of the plasma rotation and velocity. HIT-SI3 data has been used for validation of IDCD predictions, in particular the projected impedance of helicity injectors according to the theory. The experimental impedances have been calculated here for the first time for different HIT-SI3 regimes. Such experimental evidence will contribute to the design of future experiments employing IDCD as a current-drive mechanism. [Preview Abstract] |
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CP10.00094: The inductive, steady-state sustainment of stable spheromaks A. C. Hossack, T. R. Jarboe, K. D. Morgan, D. A. Sutherland, C. J. Hansen, C. J. Everson, J. M. Penna, B. A. Nelson Inductive helicity injection current drive with imposed perturbations has led to the breakthrough of spheromak sustainment while maintaining stability. Sustained spheromaks show coherent, imposed plasma motion and low plasma-generated mode activity, indicating stability. Additionally, record current gain of 3.9 has been achieved with evidence of pressure confinement. The Helicity Injected Torus - Steady Inductive (HIT-SI) experiment studies efficient, steady-state current drive for magnetic confinement plasmas using a novel experimental method which is ideal for low aspect ratio, toroidal geometries and is compatible with closed flux surfaces. Analysis of surface magnetic probes indicates large $n=$ 0 and 1 toroidal Fourier mode amplitudes and little energy in higher modes. Biorthogonal decomposition shows that almost all of the $n=$ 1 energy is imposed by the injectors, rather than plasma-generated. Ion Doppler spectroscopy (IDS) measurements show coherent, imposed plasma motion of $+$/-2.5 cm in the region inside $r\approx $ 10 cm ($a=$ 23 cm) and the size of the separate spheromak is consistent with that predicted by Imposed-dynamo Current Drive (IDCD). Coherent motion indicates that the spheromak is stable and a lack of plasma-generated $n=$~1 energy indicates that the maximum $q$ is maintained below 1 for stability during sustainment. [Preview Abstract] |
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CP10.00095: Optical Diagnostics on HIT-SI3 Christopher Everson, Thomas Jarboe, Kyle Morgan Interferometry and Thomson Scattering are implemented on the HIT-SI3 (Helicity Injected Torus - Steady Inductive 3) device to provide time resolved measurements of electron density and spatially resolved measurements of electron temperature, respectively. HIT-SI3 is a modification of the original HIT-SI apparatus that uses three injectors instead of two. The scientific aim of HIT-SI3 is to develop a deeper understanding of how injector behavior and interactions influence current drive and spheromak stability. The interferometer system makes use of an intermediate frequency between two parallel 184.3 $\mu$m Far-Infrared (FIR) laser cavities which are optically pumped by a $CO_{2}$ laser. The phase shift in this beat frequency due to the plasma index of refraction is used to calculate the line-integrated electron density. To measure the electron temperature, Thomson Scattered light from a 20 J (1 GW pulse) Ruby laser off of free electrons in the HIT-SI3 plasma is measured simultaneously at four locations across the spheromak (nominally 23 cm minor radius). Polychromators bin the collected light into 3 spectral bands to detect the relative level of scattering. [Preview Abstract] |
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CP10.00096: NIMROD Modeling of HIT-SI and HIT-SI3 Kyle Morgan, Tom Jarboe, Aaron Hossack The HIT-SI and HIT-SI3 devices are spheromaks formed and sustained via a set of Steady Inductive Helicity Injectors (SIHI) that are operated in AC. The experiment explores the formation and sustain of stable spheromaks with a variety of perturbation mode structures. The HIT-SI device consisted of two injectors with primarily $n=1$ toroidal symmetry while the HIT-SI3 device has three injectors capable of a mixture of $n=1$ and $n=2$ perturbations or a primarily $n=3$ perturbation, depending on the relative phase of the injectors. Using the NIMROD code to model these devices, we are able to validate with experimental results (previously only done on HIT-SI) and examine the interaction between the injectors and the spheromak. Simulations are performed with both finite and zero-$\beta$ models to gain an understanding of the thermal properties of the device. Additionally, a set of extrapolation simulations has been performed illustrating the spontaneous formation of closed flux surfaces at high current amplification. [Preview Abstract] |
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CP10.00097: Applying the new HIT results to tokamak and solar plasmas Thomas Jarboe, Derek Sutherland, Aaron Hossack, Brian Nelson, Kyle Morgan, Hansen Chris, Thomas Benedett, Chris Everson, James Penna Understanding sustainment of stable equilibria with helicity injection in HIT-SI has led to a simple picture of several tokamak features. Perturbations cause a viscous-like force on the current that flattens the $\lambda $ profile, which sustains and stabilizes the equilibrium. An explanation of the mechanism is based on two properties of stable, ideal, two-fluid, magnetized plasma. First, the electron fluid is frozen to magnetic fields and, therefore, current flow is also magnetic field flow. Second, for a stable equilibrium the structure perpendicular to the flux surface resists deformation. Thus toroidal current is from electrons frozen in nested, rotating resilient flux surfaces. Only symmetric flux surfaces allow free differential current flow. Perturbations cause interference of the flux surfaces. Thus, perturbations cause forces that oppose differential electron rotation and forced differential flow produces a symmetrizing force against perturbations and instability. This mechanism can explain the level of field error that spoils tokamak performance and the rate of poloidal flux loss in argon-induced disruptions in DIII-D. This new understanding has led to an explanation of the source of the solar magnetic fields and the power source for the chromosphere, solar wind and corona. [Preview Abstract] |
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CP10.00098: An Overview of Research and Design Activities at CTFusion D.A. Sutherland, T.R. Jarboe, A.C. Hossack CTFusion, a newly formed company dedicated to the development of compact, toroidal fusion energy, is a spin-off from the University of Washington that will build upon the successes of the HIT-SI research program. The mission of the company to develop net-gain fusion power cores that will serve as the heart of economical fusion power plants or radioactive-waste destroying burner reactors. The overarching vision and development plan of the company will be presented, along with a detailed justification and design for our next device, the HIT-TD ({\textbf{T}}echnology {\textbf{D}}emonstration) prototype. By externally driving the edge current and imposing non-axisymmetric magnetic perturbations, HIT-TD should demonstrate the sustainment of stable spheromak configurations with Imposed-Dynamo Current Drive (IDCD), as was accomplished in the HIT-SI device, with higher current gains and temperatures than previously possible. HIT-TD, if successful, will be an instrumental step along this path to economical fusion energy, and will serve as the stepping stone to our {\textbf{P}}roof- {\textbf{O}}f- {\textbf{P}}rinciple device (HIT-PoP). Beyond the implications of higher performance, sustained spheromaks for fusion applications, the HIT-TD platform will provide a unique system to observe plasma self-organizational phenomena of interest for other fusion devices, and astrophysical systems as well. Lastly, preliminary nuclear engineering design simulations with the MCNP6 code of the HIT-FNSF ({\textbf{F}}usion {\textbf{N}}uclear {\textbf{S}}cience {\textbf{F}}acility) device will be presented. [Preview Abstract] |
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CP10.00099: Plasmoid formation in the elongated current sheet during transient CHI on HIST Masayoshi Nagata, Akihiro Fujita, Takahiro Matsui, Yusuke Kikuchi, Naoyuki Fukumoto, Takashi Kanki The Transient-Coaxial Helicity Injection (T-CHI) is a promising candidate for the non-inductive plasma start-up on Spherical Torus (ST). The problem of the flux closure in the T-CHI is important and related to understand the physics of fast magnetic reconnection. The recent MHD simulation (F. Ebrahimi and R. Raman, Phys. Rev. Lett. 114, 205003 (2015)) on T-CHI for NSTX predicts the formation and breakup of an elongated Sweet-Parker (S-P) current sheet and a transient to plasmoid instability. According to this simulation, the reconnection rate based on the plasmoid instability is faster than that by S-P model and becomes nearly independent of the Lundquist number S. In this meeting, we will present that the formation of multiple X-points and plasmoids has been observed in T-CHI start-up plasmas on HIST. The stronger external guide (toroidal) magnetic field makes plasma less compressible, leading to slower reconnection time and longer current sheet. The experimental observation shows that 2/3 plasmoids are generated in the elongated current sheet with the narrow width comparable to the ion skin depth or the ion sound gyro-radius. The small plasmoids develop to a large-scale flux structure due to a current inward diffusion during the decay phase. [Preview Abstract] |
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CP10.00100: Developing a compact toroid injector in the ThermoElectric driven Liquid metal plasma facing Structures device Michael Christenson, Matthew Szott, Kishor Kalathiparambil, Carl Sovinec, David Ruzic The ThermoElectric-driven Liquid-metal plasma-facing Structures (TELS) device at the University of Illinois is a theta-pinched, plasma-material interaction test stand used to simulate extreme events in the edge and divertor regions of a tokamak plasma. Previous measurements of the electron and ion temperatures have shown that the isotropic heat load on target ranges between 0.1 and 0.2 MJ m$^{\mathrm{-2}}$ over a pulse lasting 0.2 ms. While this compares well to the heat loads from Type 1 ELMs in larger toroidal devices, it is still much less than the energy deposition from Type 1 ELMs expected in ITER, which are in excess of 1 MJ m$^{\mathrm{-2}}$. To this end, a compact toroid (CT) injector has been proposed as a modification to the existing TELS device. By using an externally applied bias field to force reconnection at the muzzle of the coaxial plasma accelerator source that drives ionization, NIMROD MHD simulations have shown a peak magnetic flux of 3.5 mWb is reached 0.025 ms into the pulse - more than sufficient to form a CT. Early calorimetry and magnetic field measurements indicate that a new plasma structure has been formed in the magnetized coaxial plasma source. This work presents the current results of CT generation with respect to the bias field strength as well as the coaxial source geometry. [Preview Abstract] |
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CP10.00101: A Study of the Topology of Magnetic Helicity and its Application to the Spheromak Roselyn Williams, Ronald Williams The purpose of our study is to develop the mathematical theory to describe and analyze the behavior of a spheromak plasma. This is a collaboration effort with the Spheromak Turbulent Physics Experiment (STPX) located at Florida A. and M. University. We present the results of the topology of magnetic helicity in order to develop mathematical models to describe the helicity, reconnection and confinement of the magnetic field of the spheromak plasma. We investigate the structures of the homotopy groups of the tori, knots, braids, and their relation to twisting, knotting, braiding and tangle of the magnetic fields. [Preview Abstract] |
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CP10.00102: The STPX Spheromak System: Recent Measurements and Observations R. L. Williams, J. Clark, M. Richardson, R. E. Williams We present results of recent measurements made to characterize the plasma formed in the STPX* Spheromak plasma device installed at the Florida A. and M University. The toroidal plasma is formed using a pulsed cylindrical gun discharge and, when fully operational, is designed to approach a density of 10$^{\mathrm{21}}$ /m$^{\mathrm{3}}$ and electron temperatures in the range of 100-350 eV. The diagnostic devices used for these recent measurements include Langmuir probes, electrostatic triple probes, optical spectrometers, CCD detectors, laser probes and magnetic field coils. These probes have been tested using both a static and the pulsed discharges created in the device, and we report the latest measurements. The voltage and current profiles of the pulsed discharge as well as the pulsed magnetic field coils are discussed. Progress in modeling this spheromak using NIMROD and other simulation codes will be discussed. Our recent results of an ongoing study of the topology of magnetic helicity are presented in a separate poster. (*Spheromak Turbulent Physics Experiment) [Preview Abstract] |
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CP10.00103: Acoustically Driven Magnetized Target Fusion At General Fusion: An Overview Peter O'Shea, M. Laberge, M. Donaldson, M. Delage Magnetized Target Fusion (MTF) involves compressing an initial magnetically confined plasma of about 1e$^{23}$ m$^{-3}$, 100eV, 7 Tesla, 20 cm radius, \textgreater 100 $\mu $sec life with a 1000x volume compression in \textasciitilde 100 microseconds. If near adiabatic compression is achieved, the final plasma of \textasciitilde 1e$^{26}$ m$^{-3}$, 10keV, 700 Tesla, 2 cm radius, confined for 10$\mu $sec would produce interesting fusion energy gain. General Fusion (GF) is developing an acoustic compression system using pneumatic pistons focusing a shock wave on the CT plasma in the center of a 3 m diameter sphere filled with liquid lead-lithium. Low cost driver, straightforward heat extraction, good tritium breeding ratio and excellent neutron protection could lead to a practical power plant. GF (65 employees) has an active plasma R{\&}D program including both full scale and reduced scale plasma experiments and simulation of both. Although acoustic driven compression of full scale plasmas is the end goal, present compression studies use reduced scale plasmas and chemically accelerated Aluminum liners. We will review results from our plasma target development, motivate and review the results of dynamic compression field tests and briefly describe the work to date on the acoustic driver front. [Preview Abstract] |
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CP10.00104: Results of subscale MTF compression experiments Stephen Howard, A. Mossman, M. Donaldson In magnetized target fusion (MTF) a magnetized plasma torus is compressed in a time shorter than its own energy confinement time, thereby heating to fusion conditions. Understanding plasma behavior and scaling laws is needed to advance toward a reactor-scale demonstration. General Fusion is conducting a sequence of subscale experiments of compact toroid (CT) plasmas being compressed by chemically driven implosion of an aluminum liner, providing data on several key questions. CT plasmas are formed by a coaxial Marshall gun, with magnetic fields supported by internal plasma currents and eddy currents in the wall. Configurations that have been compressed so far include decaying and sustained spheromaks and an ST that is formed into a pre-existing toroidal field. Diagnostics measure B, n$_{\mathrm{e}}$, visible and x-ray emission, T$_{\mathrm{i}}$ and T$_{\mathrm{e}}$. Before compression the CT has an energy of \textasciitilde 10kJ magnetic, \textasciitilde 1 kJ thermal, with T$_{\mathrm{e}}$ of 100 - 200 eV, n$_{\mathrm{e}}$ \textasciitilde 5x10$^{\mathrm{20}}$ m$^{\mathrm{-3}}$. Plasma was stable during a compression factor R$_{\mathrm{0}}$/R \textgreater 3 on best shots. A reactor scale demonstration would require \textasciitilde 10x higher initial B and n$_{\mathrm{e}}$ but similar T$_{\mathrm{e}}$. Liner improvements have minimized ripple, tearing and ejection of micro-debris. Plasma facing surfaces have included plasma-sprayed tungsten, bare Cu and Al, and gettering with Ti and Li. [Preview Abstract] |
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CP10.00105: Magnetic Compression Experiment at General Fusion Carl Dunlea, Stephen Howard, Kelly Epp, Wade Zawalski, Charlson Kim The magnetic compression experiment at General Fusion was designed as a repetitive non-destructive test to study plasma physics applicable to Magnetic Target Fusion compression. A spheromak compact torus (CT) is formed with a co-axial gun into a containment region with an hour-glass shaped inner flux conserver, and an insulating outer wall. The experiment has external coils to keep the CT off the outer wall (levitation) and then rapidly compress it inwards. Experiments used a variety of levitation/compression field profiles. The optimal configuration was seen to improve levitated CT lifetime by around 50{\%} over that with the original design field. Suppression of impurity influx to the plasma is thought to be a significant factor in the improvement, as supported by spectrometer data. Improved levitation field may reduce the amount of edge plasma and current that intersects the insulating outer wall during the formation process. Higher formation current and stuffing field, and correspondingly higher CT flux, was possible with the improved configuration. Significant field and density compression factors were routinely observed. The level of MHD activity was reduced, and lifetime was increased further by matching the decay rate of the levitation field to that of the CT fields. Details of experimental results and comparisons to equilibrium models and MHD simulations will be presented. [Preview Abstract] |
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CP10.00106: Plasma Studies in the SPECTOR Experiment as Target Development for MTF Russ Ivanov, William Young General Fusion (GF) is developing a Magnetized Target Fusion (MTF) concept in which magnetized plasmas are adiabatically compressed to fusion conditions by the collapse of a liquid metal vortex. To study and optimize the plasma compression process, GF has a field test program in which subscale plasma targets are rapidly compressed with a moving flux conserver. GF has done many field tests to date on plasmas with sufficient thermal confinement but with a compression geometry that is not nearly self-similar. GF has a new design for our subscale plasma injectors called SPECTOR (for SPhErical Compact TORoid) capable of generating and compressing plasmas with a more spherical form factor. SPECTOR forms spherical tokamak plasmas by coaxial helicity injection into a flux conserver (a$=$ 9 cm, R$=$ 19 cm) with a pre-existing toroidal field created by \textasciitilde 0.5 MA current in an axial shaft. The toroidal plasma current of 100 - 300 kA resistively decays over a time period of \textasciitilde 1.5 msec. SPECTOR1 has an extensive set of plasma diagnostics including Thomson scattering and polarimetry. MHD stability and lifetime of the plasma was explored in different magnetic configurations with a variable safety factor q($\Psi )$.~ Relatively hot (T$_{e\, }\ge $ 350 eV) and dense (\textasciitilde 10$^{20}$ m$^{-3})$ plasmas have achieved energy~confinement times $\tau _{E}\ge $100$\mu $sec and are now ready for field compression tests. [Preview Abstract] |
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CP10.00107: Thomson Scattering Results from General Fusion's SPECTOR William Young General Fusion has been characterizing and optimizing a new spherical tokamak based device, SPECTOR, which has demonstrated electron temperatures as high as 350 eV. This new device is intended for testing of spherically symmetric compression. Thomson scattering diagnostic is installed on an uncompressed, but heavily diagnosed version of the device, as the compression method precludes some diagnostics. Temperature and density measurements are made at four spatial positions with upcoming plans to expand to six spatial positions. The diagnostic uses a 532 nm Nd:YAG laser and an imaging spectrometer with photomultiplier tube based detector. Other planned upgrades include camera and fiber based alignment monitoring, and multi-pass configuration reusing the laser pulse to increase the scattered light signal. [Preview Abstract] |
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CP10.00108: Simulation of MTF experiments at General Fusion Meritt Reynolds, Aaron Froese, Sandra Barsky, Peter deVietien, Gabor Toth, Dylan Brennan, Bick Hooper General Fusion (GF) aims to develop a magnetized target fusion (MTF) power plant based on compression of magnetically-confined plasma by liquid metal. GF is testing this compression concept by collapsing solid aluminum liners onto spheromak or tokamak plasmas. To simulate the evolution of the compressing plasma in these experiments, we integrated a moving-mesh method into a finite-volume MHD code (VAC). The single-fluid model includes temperature-dependent resistivity and anisotropic heat transport. The trajectory of the liner is based on experiments and LS-DYNA simulations. During compression the geometry remains axially symmetric, but the MHD simulation is fully 3D to capture ideal and resistive plasma instabilities. We compare simulation to experiment through the primary diagnostic of Mirnov probes embedded in the inner coaxial surface against which the magnetic flux and plasma are compressed by the imploding liner. The MHD simulation reproduces the appearance of n=1 mode activity observed in experiments performed in negative D-shape geometry (MRT and PROSPECTOR machines). The same code predicts more favorable compression in spherical tokamak geometry, having positive D-shape (SPECTOR machine). [Preview Abstract] |
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CP10.00109: MFE: MST AND OTHER REVERSED FIELD PINCHES |
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CP10.00110: Overview of MST Research J.S. Sarff MST progress in advancing the RFP for (1) fusion plasma confinement with ohmic heating and minimal external magnetization, (2) predictive capability in toroidal confinement physics, and (3) basic plasma physics is summarized. Validation of key plasma models is a program priority. Programmable power supplies (PPS) are being developed to maximize inductive capability. Well-controlled flattops with current as low as 0.02 MA are produced with an existing PPS, and $I_{p }\le $0.8 MA is anticipated with a second PPS under construction. The Lundquist number spans S =10$^{\mathrm{ (4-9)}}$ for 0.02-0.8 MA, allowing nonlinear MHD validation using NIMROD and DEBS at low S to be connected to highest S experiments. The PPS also enables MST tokamak operation for studying transients and runaway electron suppression with RMPs. Gyrokinetic modeling with GENE predicts unstable TEM in improved-confinement plasmas. Fluctuations are measured with TEM properties including a density-gradient threshold larger than for tokamak plasmas. Probe measurements hint that drift waves are also excited via the turbulent cascade in standard RFP plasmas. Turbulent energization of an electron tail occurs during sawtooth reconnection. New diagnostics are being developed to measure the energetic ion profile and transport from EP instabilities with NBI. Supported by US DoE and NSF [Preview Abstract] |
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CP10.00111: Validation of Extended MHD Models using MST RFP Plasmas C.M. Jacobson, B.E. Chapman, D. Craig, K.J. McCollam, C.R. Sovinec Significant effort has been devoted to improvement of computational models used in fusion energy sciences. Rigorous validation of these models is necessary in order to increase confidence in their ability to predict the performance of future devices. MST is a well diagnosed reversed-field pinch (RFP) capable of operation over a wide range of parameters. In particular, the Lundquist number $S$, a key parameter in resistive magnetohydrodynamics (MHD), can be varied over a wide range and provide substantial overlap with MHD RFP simulations. MST RFP plasmas are simulated using both DEBS, a nonlinear single-fluid visco-resistive MHD code, and NIMROD, a nonlinear extended MHD code, with $S$ ranging from $10^4$ to $5\times10^4$ for single-fluid runs, with the magnetic Prandtl number $Pm=1$. Experiments with plasma current $I_\mathrm{P}$ ranging from 60~kA to 500~kA result in $S$ from $4\times10^4$ to $8\times10^6$. Validation metric comparisons are presented, focusing on how magnetic fluctuations $\tilde{b}$ scale with $S$. Single-fluid NIMROD results give $S\sim\tilde{b}^{-0.21}$, and experiments give $S\sim\tilde{b}^{-0.28}$ for the dominant $m=1$, $n=6$ mode. Preliminary two-fluid NIMROD results are also presented. [Preview Abstract] |
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CP10.00112: Measuring viscosity with a resonant magnetic perturbation in the MST RFP Richard Fridstr\"{o}m, Stefano Munaretto, Lorenzo Frassinetti, Brett Chapman, Per Brunsell, John Sarff Application of an $m =$ 1 resonant magnetic perturbation (RMP) causes braking and locking of naturally rotating $m =$ 1 tearing modes (TMs) in the MST RFP. The experimental TM dynamics are replicated by a theoretical model including the interaction between the RMP and multiple TMs [Fridstr\"{o}m PoP \textbf{23}, 062504 (2016)]. The viscosity is the only free parameter in the model, and it is chosen such that model TM velocity evolution matches that of the experiment. The model does not depend on the means by which the natural rotation is generated. The chosen value of the viscosity, about 40 m$^{\mathrm{2}}$/s, is consistent with separate measurements in MST using a biased probe to temporarily spin up the plasma. This viscosity is about 100 times larger than the classical prediction, likely due to magnetic stochasticity in the core of these plasmas. Viscosity is a key parameter in visco-resistive MHD codes like NIMROD. The validation of these codes requires measurement of the viscosity over a broad parameter range, which will now be possible with the RMP technique that, unlike the biased probe, is not limited to low-energy-density plasmas. Estimation with the RMP technique of the viscosity in several MST discharges suggests that the viscosity decreases as the electron beta increases. Work supported by USDOE. [Preview Abstract] |
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CP10.00113: Two-fluid Magnetic Relaxation in the MST Reversed Field Pinch Joseph Triana, AF Almagri, KJ McCollam, JS Sarff, CR Sovinec Recent measurements and extended MHD simulations expose the importance of two-fluid physics in the relaxation and self-organization of the current and momentum profiles in RFP plasmas. A hallmark of relaxation is that the inductive electric field is not balanced by resistive dissipation, prompting the study of fluctuation-induced emfs in the generalized Ohm's law, E-$\eta $J$=$-\textless vxb\textgreater $+$\textless jxb\textgreater /ne, the two terms on the right known as the MHD and Hall dynamo terms, respectively. The Hall emf is measured in the outer half of the MST plasma minor radius using an armored deep-insertion probe. The emf matches previous measurements in the edge (r/a\textgreater 0.8) but in the new region examined (0.8\textgreater r/a\textgreater 0.6) it is much larger than E-$\eta $J, implying the MHD dynamo must also be large and oppositely directed. Recent nonlinear simulations that include two-fluid effects using the extended-MHD NIMROD code show complex radial structure for the emf terms, but the size of the measured Hall emf is much larger than predicted by the simulations. In the two-fluid model, the Hall dynamo couples to the parallel momentum as the mean-field Maxwell stress. The simulations predict relaxation of the parallel flow profiles that is also qualitatively consistent with measurements in MST plasmas. [Preview Abstract] |
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CP10.00114: Dynamics of m$=$0 Modes in the RFP D. Craig, A.M. Futch, R. Hesse, C.M. Jacobson In the reversed field pinch (RFP), poloidal mode number m$=$0 fluctuations are driven by nonlinear coupling in standard sawtoothing plasmas but are unstable in improved confinement plasmas. We explore how the rise and fall time of these m$=$0 fluctuations depends on the type of event and the resistivity and viscosity. Visco-resistive MHD simulations using the DEBS code reveal that the resistivity, the viscosity, and the radial profiles of these all play a role. Analysis of standard sawteeth in MST experiments shows that the rate of rise observed in experiment is consistent with the code results but that the rate of decay is faster in experiment and more weakly dependent on S. The rise of the m$=$0 modes at the sawtooth crash is well described by nonlinear, visco-resistive MHD and primarily determined by the initial conditions heading into the crash rather than the dissipation during the crash. In contrast, the duration and decay of the elevated m$=$0 mode amplitude during a crash event is more dependent on the amount and profile of dissipation. The rise and fall time for unstable m$=$0 modes during improved confinement discharges in MST is faster than for stable m$=$0 modes in standard plasmas. Events of this type have yet to be observed in MHD computation and the dependence on resistivity and viscosity in these cases is therefore less fully explored. This work has been supported by the U.S.D.O.E. [Preview Abstract] |
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CP10.00115: Studying Filamentary Currents with Thomson Scattering on MST D. J. Den Hartog, W. C. Young, S. Z. Kubala The MST reversed-field pinch plasma generates bursts of toroidally localized magnetic activity associated with $m$~$=$ 0 modes resonant at the reversal surface near the plasma edge. Previously, using data from an array of edge magnetic probes, these bursts were connected to poloidal current filaments. Now the MST Thomson scattering diagnostic is being used to measure the net drift in the electron distribution due to these currents. An additional long-wavelength spectral bin has been added to several Thomson scattering polychromators, in addition to 5-7 pre-existing short wavelength spectral bins, to improve discrimination between shifted vs. broadened spectra. The bursts are examined in plasma conditions that display spontaneous periods of low tearing-mode activity, with higher confinement and higher temperatures that improve Thomson scattering measurement performance. [Preview Abstract] |
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CP10.00116: Maximizing MST's inductive capability with a Bp programmable power supply B.E. Chapman, D.J. Holly, C.M. Jacobson, K.J. McCollam, J.C. Morin, J.S. Sarff, A. Squitieri A major goal of the MST program is the advancement of inductive control for the development of both the RFP's fusion potential and, synergistically, the predictive capability of fusion science. This entails programmable power supplies (PPS's) for the Bt and Bp circuits. A Bt PPS is already in place, allowing advanced RFP operation and the production of tokamak plasmas, and a Bp PPS prototype is under construction. To explore some of the new capabilities to be provided by the Bp PPS, the existing Bt PPS has been temporarily connected to the Bp circuit. One key result is new-found access to very low Ip (20 kA) and very low Lundquist number, S (10\textasciicircum 4). At this low S, simulation of RFP plasmas with the MHD code NIMROD is readily achievable, and work toward validation of extended MHD models using NIMROD is underway with direct comparisons to these MST plasmas. The full Bp PPS will also provide higher Ip and S than presently possible, allowing MST to produce plasmas with S spanning as much as five orders of magnitude, a dramatic extension of MST's capability. In these initial tests, the PPS has also increased five-fold MST's Ip flattop duration, to about 100 ms. This, coupled with the recently demonstrated PPS ability to drive large-amplitude sinusoidal oscillations in Ip, will allow tests of extended-duration oscillating field current drive, the goal of which is ac sustainment of a quasi-dc plasma current. Work supported by US DOE. [Preview Abstract] |
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CP10.00117: Runaway electrons and mitigation studies in MST tokamak plasmas J.A. Goetz, B.E. Chapman, A.F. Almagri, B.S. Cornille, A. DuBois, K.J. McCollam, S. Munaretto, C.R. Sovinec Studies of runaway electrons generated in low-density MST tokamak plasmas are being undertaken. The plasmas have $B_{t} \le $\textit{ 0.14 T, I}$_{p} \le $\textit{ 50 kA, q(a) }$=$\textit{ 2.2,} and an electron density and temperature of about \textit{5x10}$^{17} m^{-3}$ and \textit{150 eV}. Runaway electrons are detected via x-ray bremsstrahlung emission. The density and electric field thresholds for production and suppression have been previously explored with variations in gas puffing for density control. Runaway electrons are now being probed with resonant magnetic perturbations (RMP's). An $m =$\textit{ 3} RMP strongly suppresses the runaway electrons and initial NIMROD modeling shows that this may be due to degradation of flux surfaces. The RMP is produced by a poloidal array of 32 saddle coils at the narrow vertical insulated cut in MST's thick conducting shell, with each RMP having a single $m$ but a broad $n$ spectrum. While a sufficiently strong $m =$\textit{ 3 }RMP \quad suppresses the runaway electrons, an RMP with $m =$\textit{ 1} and comparable amplitude has little effect. The impact of the RMP's on the magnetic topology of these plasmas is being studied with the nonlinear MHD code NIMROD. With an $m =$\textit{ 3} RMP, stochasticity is introduced in the outer third of the plasma but no such flux surface degradation is observed with an $m =$\textit{ 1} RMP. NIMROD also predicts regularly occurring MHD activity similar to that observed in the experiment. These studies have also been done in $q(a) =$\textit{ 2.7} plasmas and analysis and modeling is ongoing. This work supported by USDoE. [Preview Abstract] |
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CP10.00118: Anisotropic electron tail generation during tearing mode magnetic reconnection Ami DuBois, Abdulgader Almagri, Jay Anderson, Daniel Den Hartog, John Sarff Magnetic reconnection (MR) plays an important role in particle transport, energization, and acceleration in space, astrophysical, and laboratory plasmas. In MST RFP plasmas, strong ion energization occurs during discrete MR events associated with the release of large amounts of equilibrium magnetic field energy, while the thermal electron temperature decreases due to enhanced stochastic transport. Recent high-speed x-ray spectrum measurements imply the generation of a non-Maxwellian electron tail during MR, characterized by a power-law spectral index decreasing from 4.15 to 2.15, and then increasing rapidly to 6.77 after MR due to stochastic transport. The x-ray emission peaks in the radial direction and is symmetric in the toroidal direction, indicating an anisotropic electron tail is generated. Fokker-Planck simulations predict higher x-ray emission in the parallel versus anti-parallel direction if runaway were active, consistent with the mean electric field during MR being 10X smaller than the Dreicer field. Hence the measurements of anisotropic energetic x-ray flux imply that the electron tail formation during MR results from a turbulent process. [Preview Abstract] |
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CP10.00119: Drift wave turbulence in the edge region of MST reversed field pinch plasmas D.J. Thuecks, A.F. Almagri, J.S. Sarff, P.W. Terry Measurements of electric field fluctuations reveal activity consistent with drift waves in the edge region of standard-confinement MST plasmas. The fluctuations are broadband and strongly anisotropic, with a power spectral index that is steeper in the direction parallel to the mean magnetic field direction than it is in the perpendicular direction. The power in the fluctuating kinetic energy, $\frac{1}{2}m_{i}n_{i}v_{\tilde{E}\times B_{0}}^{2}$, exceeds the power in magnetic fluctuation energy for frequencies above 80 kHz. At lower frequencies (20-40 kHz), magnetic energy associated with unstable global tearing modes dominates. A lack of equipartition in the turbulent cascade coincides with measured signatures of independent fluctuation activity broadly consistent with drift-wave fluctuations. Statistical coherence measurements reveal mode activity at high frequencies ($\ge $80 kHz) that is compressive, has high coherence in regions of the plasma with strong density gradients, and has a phase speed comparable to the electron drift speed. Elevated coherency associated with this fluctuation feature of the drift wave fluctuations return more quickly following magnetic reconnection events than corresponding coherence associated with the tearing activity. This suggests the drift-wave fluctuations may be excited by the large edge-localized thermal pressure gradient, but they could also be excited nonlinearly in a turbulent cascade driven by the tearing modes. [Preview Abstract] |
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CP10.00120: Measurements of drift-wave-induced density and velocity fluctuations using high-speed passive impurity spectroscopy Takashi Nishizawa, D. Craig, D.J. Den Hartog, M.D. Nornberg Passive impurity spectroscopy is used to study high frequency (\textasciitilde 100 kHz) electron density and ion velocity fluctuations in the edge of MST reversed field pinch plasmas. When tearing modes are suppressed, stochastic transport is greatly reduced and microturbulence is anticipated to become important. Gyrokinetic simulations predict unstable trapped electron modes (TEM) in the edge region of these improved-confinement MST plasmas. Interferometry measurements reveal electron density fluctuations with wavenumbers, propagation direction, and a density-gradient threshold in good agreement with predictions for TEMs. These density fluctuations are also observed as emission fluctuations using a recently upgraded Ion Dynamics Spectrometer (IDS II) through edge passive C$+$2 measurements. The particle transport associated with TEMs will be evaluated directly by correlating the IDS-measured ion velocity and density fluctuations. The measurement is localized to the C$+$2 emission shell in the edge of the plasma, which is determined by a coronal charge-state balance model using ADAS. We used a large-throughput spectrometer originally developed for fast CHERS measurements and PMTs for light detection to achieve high time resolution. This work is supported by the US DOE. [Preview Abstract] |
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CP10.00121: Characterization and initial results from the upgraded MST interferometer-polarimeter W.X. Ding, E. Parke, D.L. Brower, J. Duff The FIR interferometer-polarimeter diagnostic on MST is a high-bandwidth system with unique capabilities for measuring high-frequency density and internal magnetic fluctuations. Installation of new planar-diode mixers improves both the signal strength and the noise floor compared to the corner-cube mixers previously used. Measurements of density and Faraday rotation angle in standard reversed-field pinch (RFP) plasmas and plasmas with improved, tokamak-like confinement are presented. The noise floor in the Faraday rotation power spectrum is reduced by nearly an order of magnitude, with fluctuations observed up to 250 kHz. Cross-correlation between multiple mixers is an additional, novel technique for reducing the noise floor and improving the resolution of high-frequency, small-amplitude magnetic and density fluctuations. Correlation of signals from two independent mixers viewing the same chord reduces the noise floor by another order of magnitude. High wavenumber resolution may be possible when operating without focusing elements, using only the 2-3 mm aperture on the mixer to determine the sampled chord width. This configuration will provide better resolution of small-scale fluctuations observed in the RFP during periods of improved confinement. Work supported by U.S. D.O.E. [Preview Abstract] |
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CP10.00122: Measurements of beam-driven instabilities with the upgraded MST interferometer-polarimeter E. Parke, J. K. Anderson, D. L. Brower, W. X. Ding Neutral beam injection (NBI) in MST produces a core-localized fast ion population that modifies the plasma equilibrium and drives a rich variety of instabilities. These instabilities include energetic particle modes (EPMs) and Alfv\'{e}nic modes that can produce an avalanche process leading to significantly enhanced fast ion transport, as well as chirping modes. The MST FIR interferometer-polarimeter system has high-bandwidth and low phase noise and has previously been used to characterize density fluctuations correlated with many of these instabilities, as well as internal magnetic fluctuations associated with the dominant, n = 5 EPM. However, many of the Alfv\'{e}nic and chirping modes were too weak to observe in polarimetry measurements in previous studies. Recent upgrades to the interferometer-polarimeter have further reduced the noise floor and extended the accessible bandwidth for fluctuation measurements. Initial measurements of fast ion driven instabilities with the upgraded system will be presented. Improved measurements of internal structure may be important for understanding mode dynamics and particle transport. [Preview Abstract] |
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CP10.00123: Observation of Electron Bernstein Wave Heating in the MST Reversed Field Pinch Andrew Seltzman, Jay Anderson, Ami DuBois, Abdulgader Almagri, Paul Nonn, Karsten McCollam, Brett Chapman, John Goetz, Cary Forest We report the first observation of electron Bernstein wave heating in the MST RFP. Similar to a high density stellarator, the RFP is inaccessible to electromagnetic ECRH. The plasma current and \textbar B\textbar operating range of MST allows a 5.5 GHz RF source (100kW, 4ms pulse) to heat on the fundamental and up to 4th harmonic EC resonances. With an x-ray diagnostic most sensitive to edge electrons located $+$12 degrees toroidally from the antenna, the measured emission is a strong function of predicted heating inside versus outside the Bt$=$0 reversal layer of the RFP. Measured during a scan of plasma current, distinct edges in a plot of emissivity versus predicted deposition layer align with the deposition layers crossing of this reversal layer and confirm EBW heating on the fundamental through 4th EC harmonic. Additional confirmation of the absorption location has been demonstrated by using auxiliary poloidal current drive to reduce electron diffusion rates and sweep the location of the Bt$=$0 surface across a static RF absorption location in RFP discharges. In these discharges EBW enhancement of the 15-40keV x-ray energies has been observed. Work supported by USDOE. [Preview Abstract] |
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CP10.00124: Damping Rates of Energetic Particle Modes and Stability With Changing Equilibrium Conditions in the MST Reversed-Field Pinch S.H. Sears, A.F. Almagri, J.K. Anderson, P.J. Bonofiglo, W. Capecchi, J. Kim The damping of Alfvenic waves is an important process, with implications varying from anomalous ion heating in laboratory and astrophysical plasmas to the stability of fusion alpha-driven modes in a burning plasma. With a 1 MW NBI on the MST, a controllable set of energetic particle modes (EPMs) and Alfvenic eigenmodes can be excited. We investigate the damping of these modes as a function of both magnetic and flow shear. Typical EPM damping rates are -10$^{\mathrm{4}}$ s$^{\mathrm{-1}}$ in standard RFP discharges. Magnetic shear in the region of large energetic ion density is -2 cm$^{\mathrm{-1}}$ and can be increased up to -2.5 cm$^{\mathrm{-1}}$ by varying the boundary field. Continuum mode damping rates can be reduced up to 50{\%}. New experiments use a bias probe to control the rotation profile. Accelerating the edge plasma relative to the rapidly rotating NBI-driven core decreases the flow shear, while decelerating the edge plasma increases the flow shear in the region of strong energetic ion population. Mode damping rates measured as a function of the local flow shear are compared to ideal MHD predictions. Work supported by US DOE. [Preview Abstract] |
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CP10.00125: Fast Ion Transport in the MST Reversed Field Pinch P. J. Bonofiglo, J. K. Anderson, W. Capecchi, J. Kim, S. H. Sears, J. Egedal The reversed field pinch (RFP) provides a unique environment to study fast ion confinement and transport. The magnetic topology of the RFP establishes guiding center drifts along flux surfaces, resulting in naturally well-confined fast ions. Past experiments reveal reduced confinement and a redistribution of fast ions with beam-driven instabilities or transition to a 3D equilibrium state. A fast ion transport model characterized by a temporally and spatially dependent diffusion profile describes the fast ion evolution. The diffusion coefficient varies as the square of the measured mode amplitude, and the width is inferred from comparison with correlated density fluctuations. In studying multiple interacting modes, the model reproduces the dynamic NPA-measured $\sim 20 \%$ drop in core fast ion concentration. In the case of long-lived frequency chirping modes, there is a consistent time evolution of the fast ion distribution and measured mode frequency on a spatially varying Alfven continuum. Additional studies probe the dynamics of energetic particle modes (EPMs) during the growth of the core-localized kink mode and the rapid loss of fast ion confinement as a transition to a 3D equilibrium occurs. This research is supported by US DOE. [Preview Abstract] |
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CP10.00126: Fast ion generation and runaway through magnetic reconnection events in MST Jungha Kim, Jay Anderson, William Capecchi, Phillip Bonofiglo, Stephanie Sears Fokker-Planck and full orbit modeling are used to investigate how global reconnection events in MST plasmas generate an anisotropic fast ion distribution. A multi-step process is hypothesized. First, thermal ions are heated by a perpendicular heating mechanism, possibly a stochastic process that relies on turbulent diffusion and strong radial electric fields, or ion cyclotron damping in the tearing-driven turbulent cascade. Second, a small fraction of the heated ions have sufficient speed to develop substantial guiding center drifts that are relatively immune to stochastic magnetic transport. In the RFP, these fast ion drift orbits are favorable to confinement. Finally, these fast ions are accelerated by a parallel inductive electric field (up to \textasciitilde 80 V/m) associated with the abruptly changing magnetic equilibrium. This strong impulsive field does not include any magnetic-fluctuation-based contribution as experienced by thermal particles or electrons, which do not run away like fast ions. CQL3D, a Fokker-Planck solver, and RIO, a full orbit tracing code, are used to model this multi-step process that is responsible for anisotropy in fast ion distribution in MST. Work supported by US DOE. [Preview Abstract] |
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CP10.00127: Ion heat transport in improved confinement MST plasmas Zichuan Xing, Mark Nornberg, Daniel J. Den Hartog, Santhosh Kumar, Jay K. Anderson Ion power balance in improved confinement (PPCD) plasmas in MST is dominated by electron collisional heating balanced by charge exchange transport. Neoclassical effects on ions in the RFP are inherently small and PPCD plasmas have reduced turbulence and stochasticity. Thus PPCD plasmas provide a good starting point for a transport model developed to account for collisional equilibration between species, classical conductive energy transport, and energy loss due to charge exchange collisions. This model also allows a possible noncollisional anomalous term to be isolated for study, and correlations between residual magnetic fluctuations during PPCD plasmas and anomalous heating and transport will be investigated. Recent modeling with DEGAS2 Monte Carlo neutral simulation suggests higher core neutral temperature than previously estimated with more simplistic assumptions. However, the working model does not fully account for the electron density increase in the core during PPCD, which is higher than expected from classical particle transport, and neutral and impurity ionization. Other possible mechanisms are considered and analyzed, including more complex impurity charge-state balance and pinch effects. Work supported by the US DOE. DEGAS2 is provided by PPPL. [Preview Abstract] |
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CP10.00128: Testing the role of shear in the Quasi Single Helicity state in MST RFP plasmas J. Boguski, M.D. Nornberg, S. Munaretto, B.E. Chapman, M. Cianciosa, P.W. Terry, D.J. Den Hartog, K.J. McCollam, J.D. Hanson In high current and low density (large Lundquist number) RFP plasmas, the island associated with the innermost resonant tearing mode can grow to sufficient width that it envelops the magnetic axis, resulting in a helical axis and a 3D equilibrium. This Quasi Single Helicity (QSH) state has improved core particle and energy confinement and reduced (secondary) tearing mode amplitudes at larger radii. One possible explanation for the decrease in secondary mode amplitudes is the decoupling of secondary modes by the shear (either magnetic or flow) associated with the dominant mode. Analysis of magnetic shear is being pursued through time series reconstructions of QSH plasmas in MST using V3FIT-VMEC, a non-axisymmetric MHD equilibrium solver being applied to stellarators and 3D RFP and tokamak plasmas. Charge Exchange Recombination Spectroscopy will provide the poloidal flow associated with the helical structure. Of interest are correlations between the magnetic and flow shear and the persistence of the QSH state. Both the flow and magnetic field profiles are also important for understanding the non-axisymmetric contribution to the net electric field that may arise as a single-mode dynamo. This material is based upon work supported by the U.S. DOE. [Preview Abstract] |
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CP10.00129: 3-D Measurement of Recycling and Radiation in MST Ryan Norval, John Goetz, Oliver Schmitz The MST reversed-field pinch (RFP) can undergo spontaneous transition to a helical core state, associated with the growth of the innermost resonant magnetic mode. Currently multiple 2-D imaging cameras are in place allowing for nearly full vessel viewing and measurement of recycling and impurities fluxes. The transition from the standard to helical RFP causes an observable change in edge plasma. While in the helical state the plasma wall interaction (PWI) on MSTs poloidal limiter strongly correlates with the helicity of the core mode. PWI on the toroidal limiter overall is reduced, with the remaining PWI sites corresponding the the helicity of the core mode, or the locations of diagnostic limiters and the error fields they create. EIRENE, a neutral particle code use for modeling edge plasmas, is used to compute the neutral profiles based on measured recycling fluxes. EIRENE computes the radiative and charge exchange power losses. Comparison is made between the standard and helical RFP plasmas. Bolometer measurements of total radiation are currently in progress to supplement the modeling. This work is supported by the U.S. Department of Energy [Preview Abstract] |
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CP10.00130: Prospects for measuring shifted- and non-Maxwellian electron distributions with Thomson scattering on MST S.Z. Kubala, D.J. Den Hartog, A.M. DuBois, L.A. Morton, W.C. Young Recent measurements using a high-time-resolution soft x-ray spectrometer on MST suggest that a non-Maxwellian, energetic electron tail is generated during magnetic reconnection events. This has motivated the addition to the Thomson scattering (TS) diagnostic of the capability to measure shifted- and non-Maxwellian distribution functions. To that end, an 1140 nm centerline filter with 80 nm bandwidth has been installed in nine of 21 polychromators. This filter supplements a filter set that covers from approximately 715 nm to 1065 nm, used to measure Thomson scattered light from the 1064 nm YAG laser line. Simulations being performed will assess whether the TS diagnostic with the upgraded filter set will be capable of detecting a small population (around 5\% of the electron density) of energetic electrons, and whether an eight-channel polychromator, which has increased resolution but also is inherently more noisy, provides a better fit than a six-channel polychromator. [Preview Abstract] |
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CP10.00131: Incorporating beam attenuation into an Integrated Data Analysis model to determine $Z_{\rm eff}$ M.D. Nornberg, L.M. Reusch, D.J. Den Hartog Determining the resistive dissipation of current in hot plasmas requires knowledge of the effective ionic charge, $Z_{\rm{eff}}$. Typically $Z_{\rm{eff}}$ is determined from visible bremsstrahlung emission, but in limited plasmas with relatively high edge neutral density, the neutrals contribute as much to the visible spectrum as do the impurities. Using techniques from integrated data analysis (IDA), measurements of soft-x-ray emission from a region of the spectrum dominated by bremsstrahlung and impurity recombination were combined with individual impurity density profile measurements from charge exchange recombination spectroscopy, enabling determination of $Z_{\rm eff}$ in MST. Attenuation of the diagnostic neutral beam used to determine those impurity densities depends on $Z_{\rm eff}$. In order to further enhance the analysis, measurements of beam attenuation are incorporated into the IDA framework. The cross sections for attenuation are determined using the Atomic Data Analysis and Structure (ADAS) code suite. This measurement takes advantage of recent detailed calibrations performed during refurbishment of our 50 kV diagnostic neutral beam. [Preview Abstract] |
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CP10.00132: Prototype detectors for measuring poloidal magnetic flux with an ion beam probe T. P. Crowley, D. R. Demers, P. J. Fimognari, T. D. Kile Development of a detector and associated techniques to determine the localized magnetic flux, and therefore poloidal magnetic field and current density profile, in an axisymmetric plasma device is underway. This will provide invaluable information on equilibrium, transport and stability studies of fusion plasmas. A singly charged ion beam is injected into the plasma and the detector located outside the plasma measures doubly charged ions created within a cm-scale sample volume of the plasma. The ions are split into beamlets at the detector. The toroidal angle of the beam's velocity is determined by measuring the fraction of the beamlets that strike detection plates and wires. The corresponding angle is used to determine the beam's toroidal velocity component. Due to canonical momentum conservation, that toroidal velocity is proportional to the poloidal flux function in the sample volume. We have built several prototype detectors and measured the angle of a 45 keV potassium ion beam. The cross-section of the plasma that can be studied will be maximized and system costs will be minimized if the detector has a direct view of the plasma and is operated close to it. However, this subjects the detector to noise due to UV-induced photoelectrons and plasma particles. We have conducted experiments that demonstrate reductions of this noise to facilitate measurement of ion beam signals. Experimental and design results will be presented. (This work is supported by US DoE award no. DE-SC0006077.) [Preview Abstract] |
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CP10.00133: Impact of diagnostic neutral beam optimization on active spectroscopy in MST Xiande Feng, Mark. D. Nornberg, Daniel. J. Den hartog, Steven. P. Oliva, Darren Craig The hydrogen diagnostic neutral beam on MST provides local measurements of impurity ion emission through charge exchange recombination spectroscopy (CHERS) and of core-localized magnetic field through the motional Stark effect (MSE). The beam has been optimized to operate at 50kV, 4A steady beam current with 20ms beam pulse and 75{\%} primary energy ion fraction. It's achieved by tuning the beam voltage, arc current, fuel line pressure, arc and high voltage module timing, and the magnetic isolation field. Electron density measurements in the ion source revealed that ion extraction is maximized under low density conditions which are thought to affect the shape of the ion sheath at the extraction grid. The sheath may be transitioning from a planar or convex shape at high density to one which is concave which helps focus the ion trajectories and produce higher beam current. With the improvements in beam operation, the CHERS signal is expected to increase by 20{\%}-30{\%}, and the Stark broadening is expected to increase by 10{\%}. These signal increases will help resolve convolved fine-structure components in both analyses. Beam voltage ripple is also measured to better quantify the accuracy of spectral MSE and CHERS measurement. This work is supported by the U.S. DOE. [Preview Abstract] |
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CP10.00134: Improvements to the Interpretation and Understanding of SXR Tomography Measurements on MST Patrick VanMeter, Paolo Franz, Lisa Reusch, John Goetz, Daniel Den Hartog The soft x-ray (SXR) tomography system on MST uses four cameras in a double foil configuration to determine the emissivity and temperature structures of the plasma. The emissivity is due to a combination of bremsstrahlung, recombination, and line radiation due to impurities in the plasma. At higher energies recombination steps and line radiation are no longer present and can therefore be removed using thick filters. However, this limits the range of measurements to high temperature, high performance plasmas. Recent analysis focuses on including these additional sources of radiation in order to extend the effective range of SXR measurements and to explore the agreement of SXR measurements with other diagnostics like the external magnetic sensing coils. The SXR emissivity structure should directly correspond to the structure of the magnetic field; however, there is a discrepancy between the phase of the emissivity reconstructions and magnetic field reconstructions when using a cylindrical approximation to interpret the magnetic signals. This discrepancy was measured for each SXR camera viewing angle and for two distinct plasma conditions, with results supporting the interpretation that it emerges from physical effects of the toroidal geometry. Improving the understanding of these toroidal effects and the effects of radiation from impurity ions will aid in the interpretation of all SXR measurements. Supported by the US DOE. [Preview Abstract] |
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CP10.00135: Expanded Ion Energy Distribution Measurements on MST RFP Plasmas Jerry Clark, J.B. Titus, E.D. Mezonlin, J.K. Anderson, A.F. Almagri The Compact Neutral Particle Analyzer (CNPA) is a low energy (0.34 -- 5.2 keV), high energy resolution (25 channels) neutral particle analyzer for ion energy distribution and temperature measurements on the Madison Symmetric Torus (MST). In MST plasmas during neutral beam injection, deuterium ions are known to have energies out to 40 keV. A retarding potential was built, installed, and calibrated to allow CNPA measurements to explore this region with high energy resolution, expanding ion energy distribution measurements, allowing us to better understand the dynamics of the bulk and fast ion populations during global magnetic reconnection events. [Preview Abstract] |
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CP10.00136: Two-dimensional visible light and SXR imaging studies for impurity influx in a low-aspect-ratio RFP Akio Sanpei, Sadao Masamune, Shun Nakanobo, Ryosuke Tsuboi, Satoshi Kunita, Haruka Makizawa, Haruhiko Himura, Satoshi Ohdachi, Naoki Mizuguchi, Tsuyoshi Akiyama The measurement of visible light emission and bremsstrahlung soft X-ray (SXR) radiation are useful passive methods for diagnosing high-temperature plasmas, because these emissivity distributions correspond to plasma density, temperature and impurities. In a low-aspect-ratio (low-$A$) RFP machine RELAX ($R$ = 0.51 m/$a$ = 0.25 m ($A$ = 2)), two dimensional (2D) high-speed visible light imaging using a high-speed camera and SXR imaging diagnostics were developed to identify the emission structures associated with dominant MHD instabilities in the RFP. In the present study, impurity release associated with phase locking of the dominant $m$ =1 modes has been studied using 2D imaging diagnostics. A rotating helical structure has been revealed by both 2D images of visible light and SXR. The rotating helical structure tends to be wall-locked during phase locking of the dominant modes. The appearance of the rotating helical structure of H$_{\alpha}$ emission has shown to be correlated with an increase in emission intensity of iron impurity. Moreover, hollow SXR emissivity distribution has been observed in deeper reversal discharge ($F \sim$-1.5). The appearance of the hollow and helical structure of SXR image may be an indication of an increase in iron impurity influx. [Preview Abstract] |
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CP10.00137: Comparing nonlinear MHD simulations of low-aspect-ratio RFPs to RELAX experiments K.J. McCollam, D.J. Den Hartog, C.M. Jacobson, C.R. Sovinec, S. Masamune, A. Sanpei Standard reversed-field pinch (RFP) plasmas provide a nonlinear dynamical system as a validation domain for numerical MHD simulation codes, with applications in general toroidal confinement scenarios including tokamaks. Using the NIMROD code, we simulate the nonlinear evolution of RFP plasmas similar to those in the RELAX experiment. The experiment's modest Lundquist numbers $S$ (as low as a few times $10^4$) make closely matching MHD simulations tractable given present computing resources. Its low aspect ratio ($\approx 2$) motivates a comparison study using cylindrical and toroidal geometries in NIMROD. We present initial results from nonlinear single-fluid runs at $S=10^4$ for both geometries and a range of equilibrium parameters, which preliminarily show that the magnetic fluctuations are roughly similar between the two geometries and between simulation and experiment, though there appear to be some qualitative differences in their temporal evolution. Runs at higher $S$ are planned. This work is supported by the U.S. DOE and by the Japan Society for the Promotion of Science. [Preview Abstract] |
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CP10.00138: The impact of boundary shaping on reverse-field-pinch equilibria Carmen Miele, Andrew Ware This work explores the impact of boundary shaping on access into and out of quasi-single helicity states in reverse-field-pinch (RFP) plasmas. Experiments have shown that RFP plasmas can self-organize to a quasi-single helicity equilibrium with a helical axis. These states have improved confinement and lower magnetic turbulence levels compared to a standard RFP plasma [D.F. Escande, et al., {\it Phys. Rev. Lett.}~{\bf 85}, 1662 (2000)]. The VMEC code can obtain these similar equilibria with a helical axis and a symmetric boundary [J.D. Hanson, et al., {\it Nucl. Fusion} ~{\bf 53}, 083016 (2013)]. These equilibria all have circular, or nearly-circular cross-sections. In this work we analyze the impact of 2D-shaping of the boundary on RFP equilibria. Particular attention is paid to the impact on access to quasi-single helicity states. [Preview Abstract] |
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CP10.00139: Spectral properties of VMEC equilibria I Predebon, B Momo, D Terranova, P Innocente The aim of this work is to clarify some aspects related to the mutual spectral properties of MHD equilibria with different symmetries defined by action-angle coordinates. We refer mostly to the helical states occurring in high current RFP plasmas, reconstructed by means of the equilibrium code VMEC. The existence of such equilibria is the basis on which several stability and transport studies can be established. Tearing instabilities are one of the most dangerous sources of particle and energy transport, so an important question to be addressed is the occurrence of resonant tearing instabilities in helical plasmas and particularly what they look like when seen by the (toroidally symmetric) set of edge magnetic probes. In order to clarify the mutual spectral properties of axisymmetric and helical MHD equilibria we deal with the problem of how magnetic spectra (Fourier analysed in helical coordinates) transform under a change of coordinates, from helical action-angle coordinates to toroidal ones, and viceversa. Since the method can be applied to any VMEC equilibria, similar considerations can be made for the other toroidal configurations, for example for the study of the effect of resonant or non-resonant magnetic perturbations in tokamak plasmas. [Preview Abstract] |
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CP10.00140: Electrical modeling of the Reversed Field Pinch configuration. Roberto Cavazzana Starting from the Poynting theorem, a two port equivalent formulation for the Reversed Field Pinch (RFP) is obtained. At first a general formulation applicable to any sort of underlying MHD physics is derived. Then its specialization is discussed, showing that: i) the toroidal field reversal is guided from outside the plasma by the external imposed boundary conditions; ii) the classic textbook RFP derivation with the toroidal flux $\Phi_t$ conserved is only a particular choice among the many possible. Here a parametric force free MHD family of equilibria is used to derive the two port equation of a realistic RFP boundary condition. The key master parameter turns out to be the edge safety factor $q(a)= a/R \cdot B_{t}(a)/B_{p}(a)$, whereas $\Phi_t$ becomes a free variable determined by the RFP self-organization processes. As a by product a correct expression for the resistive component of the toroidal loop voltage is given. The two port model obtained is finally closed by adding the poloidal and toroidal power supply networks and evolved by means of a SPICE simulator. The results enlighten some peculiarities found in the RFP transient operations: RFP startup and formation, pulsed poloidal current drive (PPCD) and oscillating field current drive (OFCD). [Preview Abstract] |
(Author Not Attending)
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CP10.00141: Resistive MHD Simulation of Quasi-Single-Helicity State on KTX Bing Luo, Ping Zhu, Hong Li, Wandong Liu The potential formation of quasi-single-helicity (QSH) state on Keda Torus eXperiment (KTX) is evaluated in resistive MHD simulations using the NIMROD code. In this work, we focus on the effects of finite resistivity on the mode structure and characteristics of the dominant linear and nonlinear resistive tearing-mode instability in a finite $\beta$, cylindrical reversed field pinch model configuration for KTX. In the typical resistivity regimes of KTX where Lundquist number $S=10^5$, the plasma reaches a steady QSH state after the initial transient phase of multiple helicities. The dominat mode of the QSH state is developed from the dominat linear tearing mode instability. The conditions for and the variations of the formation of QSH states in different resistivity regimes of KTX will be reported and discussed. [Preview Abstract] |
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CP10.00142: Modeling of Resistive Wall Modes in Tokamak and Reversed Field Pinch Configurations of KTX Rui Han, Ping Zhu, Wei Bai, Tao Lan, Wandong Liu Resistive wall mode is believed to be one of the leading causes for macroscopic degradation of plasma confinement in tokamaks and reversed field pinches (RFP). In this study, we evaluate the linear RWM instability of Keda Torus eXperiment (KTX) in both tokamak and RFP configurations. For the tokamak configuration, the extended MHD code NIMROD is employed for calculating the dependence of the RWM growth rate on the position and conductivity of the vacuum wall for a model tokamak equilibrium of KTX in the large aspect-ratio approximation. For the RFP configuration, the standard formulation of dispersion relation for RWM based on the MHD energy principle has been evaluated for a cylindrical $\alpha$-$\Theta$ model of KTX plasma equilibrium, in an effort to investigate the effects of thin wall on the RWM in KTX. Full MHD calculations of RWM in the RFP configuration of KTX using the NIMROD code are also being developed. [Preview Abstract] |
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CP10.00143: The effect of shaping on Reversed Field Pinch dynamics Robert Chahine, Jorge A. Morales, Kai Schneider, Wouter J. T. Bos Reversed Field Pinch fusion devices (RFPs) are inevitably plagued by magnetohydrodynamic (MHD) instabilities. High resolution numerical simulations of fully nonlinear visco-resistive magnetohydrodynamics using a Fourier pseudo-spectral method with volume penalization [Morales et al. JCP, 2014] are performed. Results of RFP simulations in toroidal geometry were reported in [Morales et al. PPCF, 2014]. Here we consider a cylindrical domain with elliptical cross-section for different aspect ratios. The results illustrate a notable influence of the shape of the cross-section on the nonlinear dynamics of RFPs. The axial mode-spectrum is qualitatively changed in cylinders with elliptic cross-section. The results suggest that shaping could change, and possibly improve the confinement of RFPs. It is certainly possible that specific helical modes can be promoted, approaching thereby a QSH state. [Preview Abstract] |
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CP10.00144: PLASMA SOURCES AND DIAGNOSTIC |
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CP10.00145: Characterization of a small railgun-based plasma jet source Maximilian Schneider, Colin Adams, Marius Popescu, Joshua Korsness, Michael Sherburne Experimental characterization of a small plasma jet source has been undertaken at Virginia Tech's Center for Space Science and Engineering Research (Space@VT). The plasma-armature railgun features a square bore approximately $0.5 \times 0.5$~cm and a rail length of $\approx 10$~cm. Fed by an $\approx 100$~psi- gas manifold and powered by an LC pulse-forming network capable of delivering $\approx 100$~kA current on timescales of several microseconds, jet velocities in the 10-20~km/s range are predicted. A modular design, the insulators and rails are readily swappable for investigation the interaction of the plasma armature with plasma-facing components fabricated with different materials and geometry. The plasma jet is characterized by a suite of diagnostics including a multichord Mach-Zehnder interferometer, spectrometer, photodiode array, and fast photography. Diagnostics planned for the near future include plasma laser-induced fluorescence and particle energy analyzers. The railgun source described is envisioned as a future platform for basic science experiments on topics ranging from plasma-material interaction to plasma shocks. [Preview Abstract] |
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CP10.00146: Development of a long pulse plasma gun discharge for magnetic turbulence studies David Schaffner A long pulse ($\approx 300~\mu s$) plasma gun discharge is in development at the Bryn Mawr College Plasma Laboratory for the production of sustained magnetized plasma injection for magnetohydrodynamic (MHD) turbulence studies. An array of eight $0.5~mF$ parallel capacitors are used to create a pulse-forming-network (PFN) with a plateaued current output of $\approx 50~kA$ for at least $200$ of the $300~\mu s$ pulse. A $24~cm$ inner diameter plasma gun provides stuffing flux fields at the stuffing threshold in order to allow for the continuous injection of magnetic helicity. Plasma is injected into a $24~cm$ diameter flux-conserving aluminum chamber with a high density port array for fine spatial resolution diagnostic access. Fluctuations of magnetic field and saturation current are measured using pickup probes and Langmuir probes respectively. [Preview Abstract] |
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CP10.00147: Modeling nitrogen plasmas produced by intense electron beams Justin Angus, Steve Swanekamp, Andrew Richardson, Joseph Schumer, David Mosher, Paul Ottinger The Gamble II generator at the Naval Research Laboratory produces $\sim$100ns pulse duration, relativistic-electron beams with peak energies on the order of 1MV and peak currents of about 800kA with annular beam areas between 40-80cm$^2$. This gives peak current densities $\sim$10 kA/cm$^2$. For many different applications, a nitrogen gas in the 1Torr range is used as a charge- and current-neutralizing background to achieve beam transport. For these parameter regimes, the gas transitions from a weakly-ionized molecular state to a strongly-ionized atomic state on the time scale of the beam pulse. A detailed gas-chemistry model is presented for a dynamical description of the nitrogen plasmas produced in such experiments. The model is coupled to a 0D circuit model representative of annular beams, and results for 1Torr nitrogen are in good agreement with experimental measurements of the line-integrated electron density and the net current. It is found that the species are mostly in the ground and metastable states during the atomic phase, but that ionization proceeds predominantly through thermal ionization of the higher-lying optically-allowed states with excitation energies close to the ionization limit. [Preview Abstract] |
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CP10.00148: Low Temperature Plasmas Generated and Sustained Indefinitely Using a Focused Microwave Beam Remington Reid, Brad Hoff, Paul Lepell The Air Force Research Laboratory has constructed a device that can initiate a plasma discharge in a focused microwave beam and sustain it indefinitely. A 10 kW, 4.5 GHz beam is passed through a vacuum chamber outfitted with pressure windows that are transparent to 4.5 GHz radiation. The pressure windows are large enough in diameter to prevent any interactions between the beam and the metallic chamber. The entire experiment is housed inside an anechoic chamber to minimize reflections. This novel plasma source generates low temperature, low density plasmas that have no contact with the walls which minimizes contamination and sheath formation. [Preview Abstract] |
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CP10.00149: Updates to the Development of Low Pressure High Density Plasmas on the Helicon Plasma Experiment (HPX) Royce James, Phil Azzari, Paul Crilly, Omar Duke-Tinson, Jackson Karama, Richard Paolino, Carter Schlank, Justin Sherman, Tooran Emami, Jeremy Turk The small Helicon Plasma Experiment (HPX) at the Coast Guard Academy Plasma Lab (CGAPL), continues to progress toward utilizing the reputed high densities (10$^{\mathrm{13}}$ cm$^{\mathrm{-3}}$ and higher) at low pressure (.01 T) [1] of helicons, for eventual high temperature and density diagnostic development in future laboratory investigations. HPX is designed to create repeatedly stable plasmas (\textasciitilde 20 - 30 ns) induced by an RF frequency in the 10 to 70 MHz range. HPX is constructing RF field corrected Langmuir probe raw data will be collected and used to measure the plasma's density, temperature, and potentially the structure and behavior during experiments. Our 2.5 J YAG laser Thomson Scattering system backed by a 32-channel Data Acquisition (DAQ) system is capable 12 bits of sampling precision at 2 MS/s for HPX plasma property investigations are being developed and tested. Progress on the construction of the RF coupling system, Helicon Mode development, and magnetic coils, along with observations from the Thomson Scattering, particle, and electromagnetic scattering diagnostics will be reported. [Preview Abstract] |
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CP10.00150: Electron Heating in Microwave-Assisted Helicon Plasmas John McKee, Umair Siddiqui, Andrew Jemiolo, Julianne McIlvain, Earl Scime The use of two (or more) rf sources at different frequencies is a common technique in the plasma processing industry to control ion energy characteristics separately from plasma generation. A similar approach is presented here with the focus on modifying the electron population in argon and helium plasmas. The plasma is generated by a helicon source at a frequency f 0 $=$ 13.56 MHz. Mcrowaves of frequency f 1 $=$ 2.45 GHz are then injected into the helicon source chamber perpendicular to the background magnetic field. The microwaves damp on the electrons via X-mode Electron Cyclotron Heating (ECH) at the upper hybrid resonance, providing additional energy input into the electrons. The effects of this secondary-source heating on electron density, temperature, and energy distribution function are examined and compared to helicon-only single source plasmas as well as numeric models suggesting that the heating is not evenly distributed but spatially localized. Optical Emission Spectroscopy (OES) is used to examine the impact of the energetic tail of the electron distribution on ion and neutral species via collisional excitation. Large enhancements of neutral spectral lines are observed with little to no enhancement of ion lines. [Preview Abstract] |
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CP10.00151: Full Wave Modeling of Helicon Operation in Proto-MPEX Pawel Piotrowicz, Juan Caneses, Green David, Cornwall Lau, John Caughman, Richard Goulding, David Ruzic An improved ``high density mode'' of operation of the helicon plasma source on Proto-MPEX has been observed recently. The high density mode is characterized by an increase in on-axis electron density (\textgreater 5e19 m$^{\mathrm{-3}})$ and a flat electron temperature (2 - 3 eV) profile during a helicon pulse. Presently, this transition has only been observed when deuterium gas is puffed downstream of the helicon antenna and the delivered RF power exceeds 110 kW. Establishing plasma densities and magnetic field strengths under the antenna that support a stable resonant helicon mode are believed to be the reason for the improved mode of operation. A full wave model of the helicon antenna has been made using finite element analysis software, COMSOL Multiphysics. This model is used to investigate the wave fields produced by the helicon antenna before and after the high density transition occurs. The investigation of the wave fields will be used in identifying the experimental conditions that are necessary for the high density mode transition and the resonant helicon mode responsible for the transition. Simulation results will be compared to radial B-dot probe measurements at multiple axial locations. [Preview Abstract] |
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CP10.00152: Upgrades to the MARIA Helicon Experiment at UW-Madison Jonathan Green, Noah Hershkowitz, Oliver Schmitz, Greg Severn, Victoria Winters The MARIA helicon plasma device at UW Madison is setup to investigate the neutral particle fueling of helicon discharges. Following initial results from the 668.614nm diode laser LIF system, the active spectroscopy diagnostic suite was expanded by establishing a 1.4J pulsed Nd:YAG pumped dye laser. To verify the new laser system, a comparison of measured ion velocities near a target plate was made between the diode based and dye based LIF systems. Additionally, theory and further verification of a new technique for measuring ion velocities leveraging Zeeman splitting is presented. During a campaign with $\le $ 750W RF power, densities in the range of 1x10$^{\mathrm{18}}$ m$^{\mathrm{-3}}$ and 2 eV electron temperature were achieved with 4.1 mTorr of argon and a magnetic field of 750G. To achieve higher densities and explore the physics of neutral depletion, the available RF power was increased from 750W to 2kW, with further expansion to 4kW on a single antenna planned. For both power levels a clear helicon mode can be reliably established and its extension increases with increasing RF power. Basic plasma characterization at the higher RF power, such as electron density vs magnetic field scans, will be presented. [Preview Abstract] |
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CP10.00153: ABSTRACT WITHDRAWN |
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CP10.00154: Single-shot measurements of laser-induced avalanche breakdown demonstrating spatial and temporal control by an external source Daniel Woodbury, Jared Wahlstrand, Andy Goers, Linus Feder, Bo Miao, George Hine, Fatholah Salehi, Howard Milchberg We report on the use of single-shot supercontinuum spectral interferometry (SSSI) to make temporally and spatially resolved measurements of laser-induced avalanche breakdown in ambient air by a 200 ps pulse. By seeding the breakdown using an external ~100 fs pulse, we demonstrate control over the timing and spatial characteristics of the avalanche. In addition, we calculate the collisional ionization rates at various laser intensities and demonstrate seeding of the avalanche breakdown both by multiphoton ionization and by photodetaching ions produced from a radioactive source. These observations provide proof-of-concept support for recent proposals to remotely measure radioactivity using laser-induced avalanche breakdown. [Preview Abstract] |
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CP10.00155: Heating, Hydrodynamics, and Radiation From a Laser Heated Non-LTE High-Z Target William Gray, M.E Foord, M.B Schneider, M.A Barrios, G.V Brown, R.F Heeter, L.C. Jarrott, D.A Liedahl, E.V Marley, C.W Mauche, K Widmann We present 2D R-z simulations that model the hydrodynamics and x-ray output of a laser heated, tamped foil, using the rad-hydro code LASNEX. The foil consists of a thin (2400 A) cylindrical disk of iron/vanadium/gold that is embedded in a thicker Be tamper. The simulations utilize a non-LTE detailed configuration (DCA) model, which generates the emission spectra. Simulated pinhole images are compared with data, finding qualitative agreement with the time-history of the face-on emission profiles, and exhibiting an interesting reduction in emission size over a few ns time period. Furthermore, we find that the simulations recover similar burn through times in both the target and Be tamper as measured by a time-dependent filtered x-ray detector (DANTE). Additional results and characterization of the experimental plasma will be presented. This work performed under the auspices of U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
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CP10.00156: Negative ion studies on the RF plasma device MAGPIE Hannah Willett, Jesse Santoso, Cormac Corr, Kieran Gibson Neutral beam injection (NBI) systems provide both heating and current drive in tokamak fusion reactors. High energy ($> 1$\,MeV) neutral beams are produced by neutralising accelerated ions, for which negative ions are used; the neutralisation cross section for positive ions becomes negligible at these energies. This requires very high throughput negative ion sources. Currently this is achieved using inductively coupled plasma sources, which incorporate caesium to improve the production rate. It has been proposed that helicon plasma sources could provide a more efficient, higher throughput method of producing negative ions for NBI, possibly even removing the need for caesium\footnote{S. Briefi and U. Fantz. AIP Conference Proceedings, 1515:278–283, 2013}. We report on studies of the negative hydrogen ion population in the MAGPIE helicon device (Australian National University)\footnote{B.D. Blackwell et al. Plasma Sources Sci. Technol., 21:055033, 2012} under a variety of operating conditions. The probe-based laser photodetachment method and Langmuir probes are employed to estimate the negative hydrogen ion density throughout the device. Initial results support the viability of helicon-based negative ion sources. [Preview Abstract] |
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CP10.00157: Continuous emission of keV x-rays from low-pressure, low-field, low-power-RF plasma columns and significance to mirror confinement P. Jandovitz, C. Swanson, A. Glasser, S.A. Cohen We report on observations of a continuous stream of 0.8-6.0 keV x-rays emitted from cool (bulk $T_e$ $\sim$ 4 eV), tenuous ($n_e$ $\sim$ $10^{10}$ cm$^{−3}$), 4-cm-diameter hydrogen or argon plasma columns generated in an axisymmetric, high-mirror-ratio, tandem mirror machine heated in one end cell by an external RF (27 MHz) antenna operating at low power, 20–600 W. The continuous emission of x-rays is evidence of the steady production of energetic electrons. The source appears to be ion-induced secondary electron emission from a floating carbon cup in the vacuum system about 2 cm from the RF antenna. The cup is charged to a high negative potential, perhaps by other secondary electrons emitted from the self-biased Pyrex vessel under the antenna. X-ray emission in the central cell increases as the mirror ratio increases, an effect we attribute to increased trapping of passing particles due to non-adiabatic scattering at the midplane of the central cell. [Preview Abstract] |
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CP10.00158: ECRH launching scenario in FFHR-d1 Kota Yanagihara, Shin Kubo, Takashi Shimozuma, Yasuo Yoshimura, Hiroe Igami, Hiromi Takahashi, Tohru Tsujimura, Ryohhei Makino ECRH is promising as a principal heating system in a prototype helical reactor FFHR-d1 where the heating power of 80 MW is required to bring the plasma parameter to break even condition. To generate the plasma and bring it to ignition condition in FFHR-d1, it is effective to heat the under/over-dense plasma with normal ECRH or Electron Bernstein Wave (EBW). Normal ECRH is well established but heating via EBW need sophisticated injection control. EBW can be excited via the O(ordinary)-X(extraordinary)-B(EBW) mode conversion process by launching the ordinary wave from the low field side to plasma cut-off layer with optimum injection angle, and the range of injection angle to get high OXB mode conversion rate is called OXB mode conversion window. Since the window position can change as the plasma parameter, it is necessary to optimize the injection angle so as to aim the window in response to the plasma parameters. Candidates of antenna positions are determined by optimum injection points on the plasma facing wall calculated by the injection angle. Given such picked up area, detailed analysis using ray-tracing calculations and engineering antenna design will be performed. [Preview Abstract] |
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CP10.00159: Radiation Power as Function of Current in Wall-stabilized AC Arc of Water-cooled Vortex Type with Small Caliber Toru Iwao, Yuto Naito, Yuta Shimizu, Shinji Yamamoto The problem of an emergency large-scale lighting with the high-intensity discharge (HID) lamp is the lack of radiation intensity because of inappropriate energy balance. Some researchers have researched that the radiation power depended on the arc temperature increases with increasing the current. However, the heat loss and the erosion of the electrode as well as the radiation power increases with increasing the current excessively. AC current replaces alternately the cathode and the anode. Thus, it is possible to avoid the concentration of the heat transfer to the anode. Moreover, the lamp efficiency decreases with increasing the current excessively because of ultra violet rays increment. It is necessary to control the temperature distribution with controlling the current and radius. In this paper, the radiation power as a function of the current in the wall-stabilized AC arc of water-cooled vortex type with small caliber was measured. As a result, the radiation power increased with increasing the current and appropriate wall radius. The radiation of AC arc is smaller than it of DC arc. And, the erosion of electrode decreases. [Preview Abstract] |
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CP10.00160: MOVED TO GP10.171 |
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CP10.00161: Arc Deflection Length Affected by Transverse Rotating Magnetic Field with Lateral Gas Toru Shiino, Yoko Ishii, Shinji Yamamoto, Toru Iwao Gas metal arc welding using shielding gas is often used in the welding industry. However, the arc deflection affected by lateral gas is problem because of inappropriate heat transfer. Shielding gas is used in order to prevent the instability affected by the arc deflection. However, the shielding gas causes turbulence, then blowhole of weld defect occurs because the arc affected by the instability is contaminated by the air. Thus, the magnetic field is applied to the arc in order to stabilize the arc using low amount of shielding gas. The method of applying the transverse rotating magnetic field (RMF) to the arc is one of the methods to prevent the arc instability. The RMF drives the arc because of electromagnetic force. The driven arc is considered to be prevented to arc deflection of lateral gas because the arc is restrained by the magnetic field because of the driven arc. In addition, it is assume the RMF prevented to the arc deflection of lateral gas from the multiple directions. In this paper, the arc deflection length affected by the RMF with lateral gas was elucidated in order to know the effect of the RMF for arc stabilization. Specifically, the arc deflection length affected by the magnetic frequency and the magnetic flux density is measured by high speed video camera. As a result, the arc deflection length decreases with increasing magnetic frequency, and the arc deflection length increases with increasing the magnetic flux density. [Preview Abstract] |
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CP10.00162: Shape reconstruction of merging spherical tokamak plasma in UTST device Tomohiko Ushiki, Masafumi Itagaki, Michiaki Inomoto Spherical tokamak (ST) merging method is one of the ST start-up methods which heats the plasma through magnetic reconnection. In the present study reconstruction of eddy current profile and plasma shape was performed during spherical tokamak merging only using external sensor signals by the Cauchy condition surface (CCS) method. CCS method have been implemented for JT-60 (QST), QUEST (Kyushu University), KSTAR (NFRI), RELAX (KIT), and LHD (Nifs). In this method, CCS was assumed inside each plasmas, where both flux function and its normal derivative are unknown. Effect of plasma current was replaced by the boundary condition of CCS, assuming vacuum field everywhere. Also, the nodal points for the boundary integrals of eddy current density were set using quadratic elements in order to express the complicated vacuum vessel shape. Reconstructed profiles of the eddy current and the magnetic flux were well coincided with the reference in each phase of merging process. Magnetic sensor installation plan for UTST was determined from these calculation results. “Acknowledgements” This work was supported by the JSPS A3 Foresight Program “Innovative Tokamak Plasma Startup and Current Drive in Spherical Torus”. [Preview Abstract] |
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CP10.00163: Anode Spot Formation in Low Pressure and Temperature He Plasma Brett Scheiner, Edward Barnat, Matthew Hopkins, Scott Baalrud, Benjamin Yee When a small electrode is biased sufficiently above the plasma potential in a low temperature plasma, the electron impact ionization of neutral species near the electrode becomes significant. At neutral gas pressures of \textasciitilde 1-100mTorr, it has been previously observed that if this ionization rate is sufficiently high, a double layer may form near the electrode. In some cases the double layer will move outward, separating a high potential plasma attached to the electrode surface from the bulk plasma. This phenomenon is known as an anode spot. A model has been developed describing the formation of anode spots based on observations from 2D particle-in-cell simulations. In this model ionization leads to the buildup of an ion rich region adjacent to the electrode, which modifies the potential structure in a way that traps electrons near the electrode surface. This leads to the formation of a quasineutral plasma near the electrode surface. When the density of this plasma is large enough, the double layer expands due to a pressure imbalance. Observations from PIC simulations were found to be consistent with time resolved measurements of the electron density from laser collision induced fluorescence, and with plasma emission measurements. [Preview Abstract] |
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CP10.00164: Ion flow and sheath structure near positively biased electrodes Ryan Hood, Brett Scheiner, Scott Baalrud, Matthew Hopkins, Ed Barnat, Benjamin Yee, Robert Merlino, Fred Skiff Measurements of the ion velocity distribution function (IVDF) and plasma potential were made near small positively biased electrodes using laser-induced fluorescence (LIF) and an emissive probe. The effect of dielectric around the electrode was tested and compared with a 2D particle-in-cell (PIC) simulation. Both measurements and simulation reveal that if the electrode is embedded within a surrounding dielectric, ions are accelerated toward the electrode to approximately 0.5 times the ion sound speed before being deflected radially by the electron sheath potential barrier. The axial potential profile in this case contains a virtual cathode. In comparison, when the surrounding dielectric is removed, both the ion flow and virtual cathode depth are dramatically reduced. These measurements suggest that the ion presheath from the dielectric may enclose the electron sheath of the electrode resulting in a virtual cathode that substantially influences the local ion flow profile. [Preview Abstract] |
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CP10.00165: ABSTRACT WITHDRAWN |
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CP10.00166: Investigation of inversion methods for calculation of soft X-ray-based plasma position and its consistency with magnetically determined position on tokamaks Martin Imrisek, Jan Mlynar, Tomas Odstrcil, Michal Odstrcil, Jakub Svoboda, Viktor Loffelmann, Ondrej Ficker, Vladimir Weinzettl, Martin Hron, Radomir Panek Distribution of soft X-ray (SXR) radiation in tokamak plasma can be reconstructed via tomography from line integrated measurements. The center of mass of the SXR distribution gives reliable information about plasma position assuming a primary source being bremsstrahlung. The Tikhonov regularization constrained by the minimum Fisher information is often applied to solve the ill-posed and underdetermined plasma tomography. There are two ways how to apply this method on large datasets: to use either a rapid version that process all time slices simultaneously or linear inversion methods which provide the center of radiation as a scalar product of the measured data and precomputed weight vector. Here, robustness, reliability and computational time of various methods of the calculation of the center of SXR emissivity are compared on modeled and real data. Furthermore, consistency of the magnetic and SXR plasma position influenced by the plasma shape and impurities is analyzed and discussed. [Preview Abstract] |
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CP10.00167: Upgrades to the C-Mod FIR Polarimeter Sameer Abraham, Jim Irby, Reich Watterson, Rui Vieira, Rick Leccacorvi, William Parkin, Rick Murray, Earl Marmar The 3-Chord FIR Polarimeter presently deployed on C-Mod is capable of responding to both fast changes in the plasma equilibrium and high frequency fluctuations. Two FIR lasers locked together with a slight frequency offset provide a signal IF at 4 MHz, which allows for the fast response of the system. Recently implemented upgrades including relocation of the laser table from the C-Mod experimental cell to a more shielded location, the design and installation of a humidity controlled beam-line to convey the FIR beams across the cell, and improved collimation optics will be discussed. Results from initial testing of the system during C-Mod operation, as well as fluctuation data from the most recent and previous campaigns will be presented and compared. [Preview Abstract] |
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CP10.00168: Probing of high-frequency coherent fluctuations by using a two-channel microwave reflectometer with antenna switching R. Ikezoe, M. Ichimura, J. Itagaki, M. Hirata, S. Sumida, S. Jang, K. Izumi, A. Tanaka, M. Yoshikawa, J. Kohagura, M. Sakamoto, Y. Nakashima A two-channel microwave reflectometer with capability of fast switching of microwave antennas in array was developed and applied to a hot linear plasma produced in GAMMA 10 to study the behavior of Alfv\'{e}n waves in a collisionless bounded plasma. High-frequency fluctuations associated with Alfv\'{e}n-ion-cyclotron (AIC) waves were successfully measured at multi points using this system. It is found that coherent phase fluctuations are obtainable at wide radial and axial region for the AIC waves. In addition, measured phase-difference profile clearly shows standing wave structures. Signature of movement of these nodes is also obtained. These results demonstrate applicability of the developed two-channel reflectometer for assessment of spatial structure of high-frequency waves and also verifies globally expanded coherent structure of the AIC waves in GAMMA 10. Two-point correlation analysis in conjunction with multi-point measurements using antenna switching turns out to be a powerful tool for investigating spatial structure of waves in a hot plasma where traditional solid probes are inadequate. [Preview Abstract] |
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CP10.00169: High Temporal and Spatial Resolution Electron Density Diagnostic for the Edge Plasma based on Stark Broadening Abdullah Zafar, Elijah Martin, Steve Shannon, Ralph Isler, John Caughman Passive spectroscopic measurements of Stark broadening have been reliably used to determine electron density for decades. However, a low-density limit (\textasciitilde 10$^{\mathrm{14}}$ cm$^{\mathrm{-3}})$ exists due to Doppler and instrument broadening of the spectral line profile. A synthetic electron density diagnostic capable of high temporal (ms) and spatial (mm) resolution is currently under development at Oak Ridge National Laboratory. The diagnostic is based on measuring the Stark broadened, Doppler-free, spectral line profile of a Balmar series transition by using an active laser based technique. The diagnostic approach outlined here greatly reduces both of these broadening contributions using Doppler-free saturation spectroscopy (DFSS), allowing access to lower density regimes. The measured profile is then fit to a fully quantum mechanical model including the appropriate electric and magnetic field operators. The modeling and experimental results for this active spectroscopic technique are presented for a magnetized ($\le $5 T), low-density (10$^{\mathrm{11}}$-10$^{\mathrm{13}}$ cm$^{\mathrm{-3}})$ plasma. Details of applying DFSS to the plasma edge are also discussed. [Preview Abstract] |
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CP10.00170: Vacuum Compatibility of Laser-Sintered Metals W.F. Rivera, C. A. Romero-Talamas, E. M. Bates, W.J. Birmingham, M. Quinley, S. Woodruff, J.E. Stuber, P.E. Sieck, P.A. Melnik We present the design and results of a mass spectrometry system used to assess vacuum compatibility of selective laser-sintered parts. The parts are disks with a thickness of 0.20 cm and a diameter of 8.25 cm, and are made of aluminum, stainless steel, inconel, and titanium. From preliminary results, titanium had the lowest partial pressure for hydrogen. Outgassing from laser-sintered parts is compared against parts with similar surface area that are manufactured with traditional methods. Outgassing is also measured while the part is heated, emulating the conditions at the edge of high temperature plasma confinement chambers. Each part is placed on a heated container that can vary in temperature inside the mass spectrometer’s vacuum chamber. The partial pressures of elements up to 200 atomic mass units are analyzed to obtain outgassing data from each sample. [Preview Abstract] |
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CP10.00171: Laser Induced Fluorescence (LIF) Data for Neutral Argon (ArI) in a Large Scale Helicon Plasm Ralph Kelly, Mark Gilmore, Kevin Meany, Yue Zhang, Tiffany Desjardins When neutral and ion densities are spatially non-uniform, neutral-ion collisions can exert a torque on a magnetized plasma column via the FxB force, where F is the force exerted on ions by neutrals. This FxB force may have a significant effect on the dynamics of plasma instabilities and flows. In order to investigate the role of neutral dynamics in helicon discharges in the HelCat (\underline {Hel}icon-\underline {Cat}hode) plasma device at U. New Mexico, an Ar I Laser Induced Fluorescence (LIF) system has been developed. The LIF system is based on a \textgreater 250 mW, tunable solid state laser. The laser will pumps the metastable ($^{\mathrm{2}}$P$^{\mathrm{0}}_{\mathrm{3/2}})$4s level to the ($^{\mathrm{2}}$P$^{\mathrm{0}}_{\mathrm{1/2}})$4p level using 696. 7352 nm light, and fluorescence radiation from decay to the ($^{\mathrm{2}}$P$^{\mathrm{0}}_{\mathrm{1/2}})$4s level at 772. 6333 nm is observed. The system design and initial data will be presented. [Preview Abstract] |
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CP10.00172: Modeling Laser-induced Fluorescence Measurements of Plasma Ion Temperature and Mean-field Waves using a Lagrangian Approach Feng Chu, Fred Skiff Laser-induced fluorescence (LIF) is a nonintrusive diagnostic technique that has found applications in the study of a wide range of fundamental and applied problems. Thus it is important to make a correct interpretation of LIF signals. We adopt a Lagrangian approach to compute LIF signals by introducing a non-linear conditional probability function P(x,v,t;x',v',t'). These simulations show agreement with experimental measurements of the ion velocity distribution function (IVDF) as conditions are altered such as laser intensity, collision rate, metastable state quench rate, and collisional excitation rate. In addition, with the presence of mean-field waves, we investigate the dependence of LIF signals on wave frequency and quench rate. [Preview Abstract] |
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CP10.00173: Laser-induced incandescence (LII) diagnostic for in situ monitoring of nanoparticle synthesis in a high-pressure arc discharge. Shurik Yatom, Vladislav Vekselman, James Mitrani, Brentley Stratton, Yevgeny Raitses A DC arc discharge is commonly used for synthesis of carbon nanoparticles, including buckyballs, carbon nanotubes, and graphene flakes. In this work we show the first results of nanoparticles monitored during the arc discharge. The graphite electrode is vaporized by high current (60 A) in a buffer Helium gas leading to nanoparticle synthesis in a low temperature plasma. The arc was shown to oscillate, which can possibly influence the nano-synthesis. To visualize the nanoparticles in-situ we employ the LII technique. The nanoparticles with radii \textgreater 50 nm, emerging from the arc area are heated with a short laser pulse and incandesce. The resulting radiation is captured with an ICCD camera, showing the location of the generated nanoparticles. The images of incandescence are studied together with temporally synchronized fast-framing imaging of C2 emission, to connect the dynamics of arc instabilities, C2 molecules concentration and nanoparticles. The time-resolved incandescence signal is analyzed with combination of ex-situ measurements of the synthesized nanoparticles and LII modeling, to provide the size distribution of produced nanoparticles. [Preview Abstract] |
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CP10.00174: New level-resolved collision data for neutral argon, benchmarked against the ALEXIS plasma experiment. Nicholas Arnold, Stuart Loch, Connor Ballance, Ed Thomas Performing spectroscopic measurements of emission lines in low temperature laboratory plasmas is challenging because the plasma is often neutral-dominated~and~not in thermal equilibrium. The densities and temperatures are such that coronal models do not apply; meaning that generalized collisional-radiative (GCR) methods must be employed to theoretically analyze atomic processes. However, for most noble gases, detailed, level-resolved atomic data for neutral and low-charge states does not exist in the literature. We report on a new project, where we use existing atomic physics codes to calculate level-resolved atomic data for neutral and low charge states of argon and compare with previously published, term-resolved theoretical results. In addition, we use the Atomic Structure and Data Analysis (ADAS)~suite of codes to calculate a GCR model for low temperature neutral argon, which we compare to published measurements of argon optical emission cross sections. Finally, we compare synthetic spectra generated from our data with observations taken from the Auburn Linear Experiment for Instability Studies (ALEXIS) in an attempt to develop new optical plasma diagnostics for electron temperature and plasma density measurements. [Preview Abstract] |
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CP10.00175: Azimuthal Doppler shift of absorption spectrum in optical vortex laser absorption spectroscopy Shinji Yoshimura, Mitsutoshi Aramaki, Naoya Ozawa, Kenichiro Terasaka, Masayoshi Tanaka, Kenichi Nagaoka, Tomohiro Morisaki Laser spectroscopy is a powerful diagnostic tool for measuring the mean flow velocity of plasma particles. We have been developing a new laser spectroscopy method utilizing an optical vortex beam, which has helical phase fronts corresponding to the phase change in the azimuthal direction. Because of this phase change, a Doppler effect is experienced even by an atom crossing the beam vertically. The additional azimuthal Doppler shift is proportional to the topological charge of optical vortex and is inversely proportional to the distance from the beam axis in which the beam intensity is vanished by destructive interference or the phase singularity. In order to detect the azimuthal Doppler shift, we have performed a laser absorption spectroscopy experiment with the linear ECR plasma device HYPER-I. Since the azimuthal Doppler shift depends on a position in the beam cross section, the absorption spectra at various positions were reconstructed from the transmitted beam intensity measured by a beam profiler. We have observed a clear spatial dependence of the Doppler shift, which qualitatively agreed with theory. Detailed experimental results, as well as remaining issues and future prospect, will be discussed at the meeting. [Preview Abstract] |
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CP10.00176: Phase Contrast Imaging on the HL-2A Tokamak Yi Yu, Shaobo Gong, Min Xu, Wei Jiang, Wulv Zhong, Zhongbin Shi, Huajie Wang, Yifan Wu, Boda Yuan, Tao Lan, Minyou Ye, Xuru Duan In this article we present the design of a phase contrast imaging (PCI) system on the HL-2A tokamak. This diagnostic is developed to infer line integrated plasma density fluctuations by measuring the phase shift of an expanded CO$_{\mathrm{2}}$ laser beam passing through magnetically confined high temperature plasmas. This system is designed to diagnose plasma density fluctuations with the maximum wavenumber of 66 cm$^{\mathrm{-1}}$. The designed wavenumber resolution is 2.09cm$^{\mathrm{-1}}$, and the time resolution is higher than 0.2 $\mu $s. The broad $k\rho_{s} $ ranging from 0.34 to 13.37 makes it suitable for turbulence measurement. An upgraded PCI system is also discussed, which is designed for the HL-2M tokamak. [Preview Abstract] |
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CP10.00177: On Floating Potential of Emissive Probes in a Partially-Magnetized Plasma Yevgeny Raitses, Brian Kraus We compare measurements of plasma potential in a cross-field Penning discharge from two probes: swept biased Langmuir probe and floating emissive probe. The plasma potential was deduced from the first derivative of the Langmuir probe characteristic. In previous studies [1], the emissive and swept biased probes were placed at the channel exit of a Hall thruster (HT). Measurements showed that the emissive probe floats below the plasma potential, in agreement with conventional theories. However, recent measurements in the Penning discharge indicate a floating potential of a strongly-emitting hot probe above the plasma potential. In both probe applications, xenon plasmas have magnetized electrons and non-magnetized ions with similar plasma densities (10$^{\mathrm{10}}$ - 10$^{\mathrm{11}}$ cm$^{\mathrm{-3}})$. Though their electron temperatures differ by an order of magnitude (Penning 5 eV, HT 50 eV), this difference cannot explain the difference in measurement values of the hot floating potential because both temperatures are much higher than the emitting wire. In this work, we investigate how the ion velocity and other plasma parameters affect this discrepancy between probe measurements of the plasma potential. [1] J. P. Sheehan, Y. Raitses, N. Hershkowitz, I. Kaganovich, and N. J. Fisch, Phys. Plasmas 18, 073501 (2011). [Preview Abstract] |
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CP10.00178: Transport Processes in a Cylinder Immersed in a Weakly Collisional, Magnetized Plasma Andrew Alt, Yevgeny Raitses Plasma-immersed wall experiments were performed in a magnetized xe plasma in a cross-field penning configuration with density and electron temperature around 10\textasciicircum 12 cm-3 and 2ev [1]. A cylinder with an open end and diameter of 1.4mm was placed across field lines so that electrons were blocked from reaching a wire recessed behind the shield while Ions were unimpeded. This is the configuration of a magnetically insulated baffled probe, a diagnostic for passively measuring plasma potential [2,3]. The reduction of electron current to the wire causes it to float close to the plasma potential [3]. Electrons have been observed further behind the baffle than expected and possible mechanisms for this have been studied, including a hall effect at the entrance, exb drift inside the volume, and other collisional effects. \textsc{[1] raitses, et. Al., 34th iepc, kobe, japan (2015).} \textsc{[2] }\textsc{katsumata, contrib. Plasma phys. }\textsc{\textbf{s}}\textsc{, 73 (1996).} \textsc{[3] demidov et. Al., rev. Sci. Instrum. 81 10e129}(2010). [Preview Abstract] |
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CP10.00179: The Diagnostics Of Hydrogen-Cesium Plasma Using Fully Relativistic Electron Impact Cross Sections Priti Priti, Dipti Dipti, Reetesh Gangwar, Rajesh Srivastava Electron excitation cross-sections and rate coefficients have been calculated using fully relativistic distorted wave theory$^{\mathrm{1}}$for several fine-structure transitions from the ground as well as excited states of cesium atom in the wide range of incident electron energy. These processes play dominant role in low pressure hydrogen-cesium plasma relevant to the negative ion based neutral beam injectors for the ITER project$^{\mathrm{2}}$.The calculated cross-sections are used to construct a reliable collisional radiative (CR) model to characterize the hydrogen-cesium plasma$^{\mathrm{3}}$. The calculated plasma parameters are compared with the available experimental and theoretical results$^{\mathrm{4}}$.\newline R. K. Gangwar, L. Sharma, R. Srivastava and A. D. Stauffer, \textit{Phys. Rev. A} \textbf{81}, 05270 (2010). \underline {http://www.iter.org/newsline/139/330} \newline R.~K.~Gangwar, Dipti, R.~Srivastava and L.~Stafford, \textit{Plasma Sources Sci. Technol. }\textbf{25}, 035025 (2016). \newline D. W\"{u}nderlich, C. Wimmer and R. Friedl, \textit{J. Quant. Spectrosc. Radiat. Transf.} \textbf{149}, 360 (2014). [Preview Abstract] |
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CP10.00180: Plasma Stratification by Planar Shocks Brett Keenan, Andrei Simakov, William Taitano, Luis Chacon, William Daughton A number of experimental observables in neutron yield and capsule compression [Li et al., PRL 100, 225001 (2008)] in Inertial Confinement Fusion (ICF) experiments have been evading explanation by standard, single-fluid, hydrodynamic (hydro) numerical simulations. Fuel stratification -- resulting from particle diffusion, multi-ion temperature separation (in the case of OMEGA experiments with gas filled capsules), and certain kinetic effects -- is a likely culprit. As a preliminary step to elucidate these effects, we consider mass fraction and temperature stratification by shocks in plasmas with two ion species. We present an analytical solution valid for very weak shocks ($M-1 \ll 1$). Employing the state-of-the-art Vlasov-Fokker-Planck code, iFP [W.T. Taitano et al., JCP 318 (2016)], we describe the stratification for shocks with arbitrary mach number, and we rigorously delineate the kinetic and hydrodynamic regimes in terms of Mach number, relative species concentration, and ion species mass ratio -- thereby clarifying the conditions under which significant departure from single-fluid hydrodynamics may occur. [Preview Abstract] |
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