Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session NP8: Poster Session V: NSTX-U, Advanced Ignition, Laser-Plasma Interaction and Instability, X-Ray-Laser Sources, ICF, Atomic, Basic and Partially Ionized |
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Room: Plaza ABC |
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NP8.00001: NSTX SPHERICAL TORUS |
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NP8.00002: Overview of NSTX Facility Upgrades Status and Research Plans Masayuki Ono NSTX-U is undergoing a major device upgrade as well as an addition of a second more tangential Neutral Beam Injection (NBI) heating and current drive system. NSTX-U will double the toroidal field from $\sim$ 0.5 T to 1 T, the plasma current from $\sim$ 1 MA to 2 MA, the NBI heating and current drive power from $\sim$ 7 MW to 14 MW, and increase the peak field plasma pulse length from 1 sec to 7 sec. More tangential NBI system is designed to achieve 100 {\%} non-inductive operation needed for a compact FNSF design. Innovative plasma start-up and ramp-up techniques without the central solenoid operation which is needed for a compact FNSF design will be explored. A major physics/technology goal for NSTX-U is to develop an attractive divertor solution for the very high steady-state divertor heat flux expected for FNSF. With doubling of the heat flux and plasma current, the peak divertor heat flux in NSTX-U could quadruple to $\sim$ 40 MW/m2 compared to $\sim$ 10 MW / m2 of NSTX. For divertor heat mitigation, snow-flake divertor configuration and liquid lithium divertor are being considered. The first plasma operation of NSTX-U is planned in October 2014. [Preview Abstract] |
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NP8.00003: The study of non-axisymmetric control coil applications in NSTX-U J.-K. Park, J.E. Menard, K. Kim, S.P. Gerhardt, R. Maingi, J.M. Bialek, S.A. Sabbagh, J.W. Berkery, A.H. Boozer, J.M. Canik, T.E. Evans As expanded 3D field capability is essential to meet NSTX-U programmatic goals and support ITER, non-axisymmetric control coil (NCC) configurations have been proposed and studied to assess potential physics applications. IPEC-NTV, POCA, and TRIP-3D code analysis show that NCC can provide a range of non-resonant error field control while minimizing resonant error field, and enhance NTV variability to better control rotation and shear, and also largely vary stochastic layers in the edge while maintaining similar plasma response characteristics. VALEN-3D analysis shows that RWM control performance increases with NCC and indicates even the possibility of operation near the ideal-wall limit. In addition, 3D analysis using stellarator codes such as COBRA indicates that NCC can directly broaden ballooning unstable region across radius and thus can be used to improve ELM pacing in NSTX-U. Relevant figures-of-merit are defined and used to quantify these NCC physics capabilities, as will be presented with future analysis plans. [Preview Abstract] |
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NP8.00004: Tokamak Simulation Code (TSC) and Free-Boundary TRANSP Modeling of NSTX-U C.E. Kessel, F.M. Poli, R. Raman, J.E. Menard The National Spherical Torus Experiment Upgrade (NSTX-U) will increase the plasma current and toroidal field by a factor of 2, and the off-axis neutral beam (NB) will significantly expand the high performance operating space from the NSTX. The plasma current can reach 2.0 MA with access to 5 s flattops, while the toroidal field can produce 1.0 T at the plasma center for 6.5 s. The NB power will increase to 10 MW for 5 s, which is expected to provide up to 1.0 MJ plasma stored energies. The additional NB will have off-axis steering that enhances the current drive efficiency and absorbed to injected power. Multiple divertor coils will enable greater flexibility in shaping and controlling divertor heat loads. In order to begin extensive discharge scenario evaluations a model is built for TSC including the solenoid coils, poloidal field coils, and conducting structures. Coordination of these simulations with the newly developed free-boundary TRANSP capability will be presented. Time-dependent free-boundary simulations of standard H-mode, advanced high non-inductive current fraction, and non-solenoidal startup are examined. Work supported by the US Department of Energy under DE-AC02-CH0911466. [Preview Abstract] |
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NP8.00005: NSTX-U Research Goals and Plans for Materials and Plasma-Facing Components R. Kaita, A.M. Capece, M.A. Jaworski, B.E. Koel, J.P. Roszell, C.H. Skinner, D.P. Stotler A major need for NSTX-U is plasma facing components (PFCs) that can survive heat and particle fluxes that result from increasing the maximum heating power to 19.2 MW, which leads to one of highest divertor PFC power densities in the world. This is expressible as the ratio of heating power to major radius of about 21 MW/m, which NSTX-U PFCs are expected to withstand for five to eight seconds. From the perspective of materials and PFCs, this challenge is being addressed through research in three major areas. 1) Understanding why lithium is effective for PFC conditioning, and determining its suitability for long-pulse discharges. Surface analytic techniques are thus being applied to study the complexes that are formed when lithium is deposited various substrates. 2) Investigating erosion and re-deposition of PFCs, including research on lithium-conditioned materials in linear plasma devices that simulate particle fluxes to tokamak walls. 3) Developing techniques for mitigating plasma-surface responsible for reducing wall lifetimes, such as continuous vapor shielding. Present plans are to change NSTX-U PFCs gradually from low-Z carbon to high-Z metallic PFCs. Liquid metals may provide the only long-term PFC solution, and a program to develop flowing lithium PFCs has been initiated. [Preview Abstract] |
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NP8.00006: High-temperature, liquid lithium plasma-facing component research for NSTX-U and next-step devices Michael Jaworski, Tyler Abrams, Robert Goldston, Robert Kaita, Andrei Khodak, Jon Menard, Masa Ono, Jacob Schwartz, Charles Skinner, Daren Stotler, Travis Gray, Greg De Temmerman, John Scholten, Miranda Van den Berg, Hennie Van der Meiden Liquid metals offer significant potential advantages as plasma-facing component materials including the elimination of net-reshaping due to plasma erosion. Large erosive fluxes may also result in a ``vapor-shielded'' regime to further reduce the incident heat flux. Engineering design studies indicate that near-term cooling technologies will enable cooling of liquid lithium PFC surfaces to temperatures of 700-900C with $10 MW/m^2$ incident where the lithium vapor pressure is comparable to scrape-off layer plasma pressures. Experiments conducted on the Magnum-PSI linear plasma device provide the first data in a quasi-steady, divertor-like plasma ($T_e$~2eV, $N_e~2\times10^{20}m^{-3}$). Lithium layers tested in the plasma indicate very high redeposition fractions exist near the target surface with lifetimes of 3-4s. These experiments provide the first feasibility assessments of a continuously-vapor shielded regime for use in the NSTX-U. [Preview Abstract] |
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NP8.00007: Study of lithium and carbon sputtering from lithium-coated graphite plasma facing components in the NSTX divertor Filippo Scotti, V.A. Soukhanovskii, J-W. Ahn, M.A. Jaworski, A. McLean, E.T. Meier, A.L. Roquemore, R.E. Bell, A. Diallo, S.P. Gerhardt, R. Kaita, B.P. LeBlanc, M. Podesta In this work, the behavior of lithium conditioned graphite PFCs in the NSTX divertor is characterized in terms of lithium and carbon sputtering yields and gross impurity influxes during H-mode ELM-free NBI-heated discharges. Impurity influxes and sputtering yield measurements in the NSTX divertor are derived from photometrically calibrated filtered cameras and divertor Langmuir probes via the S/XB method. Neutral lithium sputtering yield $Y_{Li}$ from solid lithium coatings in NSTX is found to be consistent with values reported from test stand experiments (with $Y_{Li} \sim 0.03-0.07$). Temperature-enhanced sputtering yield is generally observed for surface temperatures above the lithium melting point (with $Y_{Li} \sim 0.1-0.2$) in the proximity of the divertor strike point, leading to divertor gross lithium influxes of a few $10^{21}$ atoms/s. A moderate reduction of the carbon sputtering yield is observed with the application of lithium coatings with gross divertor carbon influxes of several $10^{20}$ atoms/s. [Preview Abstract] |
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NP8.00008: OEDGE modeling of tokamak-induced material migration in NSTX and NSTX-U J.H. Nichols, M.A. Jaworski, R. Kaita, T.W. Abrams, C.H. Skinner, D.P. Stotler As fusion reactors scale up in size, power, and duty cycle, the quantity of material eroded from the plasma-facing components (PFCs) will rise to levels far above those seen in prior experiments. Changes to PFC composition and topography due to global-scale material migration could have drastic effects on tokamak operation, especially in mixed-material machines such as NSTX-U and ITER. As a first step in understanding tokamak-induced material migration in a compact geometry, net erosion of carbon PFCs in NSTX is modeled using the OEDGE code suite (DIVIMP, EIRENE, and OSM) [P. Stangeby et al., J. Nucl. Mater. 313-316, 883 (2003)]. The sensitivity of erosion patterns to various core and divertor plasma parameters is examined. A realistic NSTX plasma background is then applied to the NSTX-U geometry in order to provide an estimate of net erosion patterns that will be seen in NSTX-U's all-carbon initial phase. These simulations are used as a guide for the optimal placement of erosion diagnostics (quartz crystal microbalances, witness samples, and marker tiles) for use in the NSTX-U startup campaign. [Preview Abstract] |
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NP8.00009: Measurements and Interpretive 2D Edge Modeling of Lithiated NSTX Discharges Travis Gray, Joon-Wook Ahn, John Canik, Michael Jaworski, Rajesh Maingi, Robert Goldston, Robert Kaita, Masa Ono, Filippo Scotti, Adam McLean, Vsevolod Soukhanovskii The National Spherical Torus Experiment (NSTX) has made extensive use of evaporative lithium coatings for improved discharge performance such as reduced divertor recycling, increased plasma stored energy and duration, and the elimination of Edge Localized Modes (ELMs). Measurements of divertor heat flux are accomplished with a unique dual-band IR (DBIR) thermography system to mitigate the effects of changing surface emissivity. Measurements from the DBIR system show reduced divertor surface temperature at the outer strike point for the case with 300 mg of lithium deposition. This results in the divertor heat flux being reduced from 5 to 2.5 MW/m$^{2}$. In turn, a reduction in divertor power accounting at the outer strike point is measured with increased lithium evaporation such that P$_{\mathrm{div}}^{\mathrm{IR}}$/P$_{\mathrm{loss}}$ $\sim$ 0.3 -- 0.5 for discharges with 150 mg of lithium and 0.12 -- 0.2 for discharges with 300 mg of lithium. The reduction in divertor power is correlated with an increase in divertor radiation for discharges with 300 mg of lithium evaporation. [Preview Abstract] |
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NP8.00010: The Influence of Temperature and Oxygen Surface Contamination on the Adsorption of Deuterium on Lithium-Coated Molybdenum Substrates Angela Capece, John Roszell, Charles Skinner, Bruce Koel Lithium-conditioned plasma-facing components have improved plasma performance by reducing the recycling of hydrogenic species; however, this process is not well understood in the complex tokamak environment. UHV surface science experiments are used to investigate the processes that occur at the plasma-surface interface by probing surface chemistry and composition while independently controlling vacuum conditions, surface temperature, and D atom/ion flux. We report on the effects of temperature and surface oxygen contamination on D retention of lithiated TZM alloy and Mo(100) single crystal. The Mo(100) substrate is used for comparison with the TZM to determine the effects of impurities and grain boundaries. The Mo substrate is coated with a monolayer-scale Li film and then exposed to D atoms and 500 eV D$^+$ ions. Auger electron spectroscopy is used to probe the surface composition, and D retention is determined using temperature programmed desorption. Plots of D retention as a function of D atom/ion exposure will be presented for a range of surface temperatures. [Preview Abstract] |
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NP8.00011: Deuterium retention and hydride formation by low energy deuterium ions incident on lithium films on Mo(110) John Roszell, Angela Capece, Charles Skinner, Bruce Koel The presence of lithium on plasma facing components (PFCs) has been shown to improve plasma performance through the reduction of hydrogen recycling. Understanding the interactions between plasma species and lithium-conditioned PFCs is important to the successful implementation of lithium in a tokamak environment. Fundamental surface science experiments performed in a controlled UHV environment are used to investigate the interactions between deuterium ions and a lithium-coated Mo(110) crystal surface. The effects of deuterium ion bombardment on a Mo(110) substrate before and after lithium deposition are explored with a well characterized D$_{\mathrm{2}}^{\mathrm{+}}$ ion beam capable of achieving energies of \textless 10 eV/D$^{\mathrm{+}}$. Information about surface chemistry and composition is measured using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and temperature programed desorption (TPD) with a specific focus on deuterium retention and hydride formation in the lithium film. Data collected will be compared with results from Mo(100) as well as a TZM alloy in order to investigate the effects of surface structure, grain boundaries, and impurities. [Preview Abstract] |
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NP8.00012: The Dependence of H-mode Energy Confinement and Transport on Collisionality in NSTX Stanley Kaye, Stefan Gerhardt, Walter Guttenfelder, Rajesh Maingi, Ron Bell, Ahmed Diallo, Benoit LeBlanc, Mario Podesta A wide range of collisionality has been obtained in NSTX using two different wall conditioning techniques, one with boronization and between-shot helium glow discharge conditioning (HeGDC$+$B), and one with lithium evaporation (Li EVAP). Previous studies of HeGDC$+$B plasmas indicated a strong increase of normalized confinement with decreasing collisionality. Discharges with lithium conditioning achieved lower collisionality by a factor of two. While the confinement dependences on dimensional, engineering variables of the HeGDC$+$B and Li EVAP datasets differed, collisionality unified the trends, with the lower collisionality Li EVAP discharges also showing increasing normalized confinement time with decreasing collisionality when other dimensionless variables were held fixed. This increase of confinement with decreasing collisionality was driven by a large reduction in electron transport in the outer region of the plasma. This result is consistent with gyrokinetic calculations that show microtearing and Electron Temperature Gradient modes to be more stable for the lower collisionality discharges. Ion transport, near neoclassical at high collisionality, became more anomalous at lower collisionality due to the growth of hybrid TEM/KBM modes in the outer plasma region. [Preview Abstract] |
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NP8.00013: Studies of Electron-scale Turbulence and Thermal Transport in NSTX L-mode Plasmas Y. Ren, W. Guttenfelder, S.M. Kaye, B.P. LeBlanc, E. Mazzucato, W. Wang, J. Lang, K.C. Lee, C.W. Domier, H. Yuh Electron-scale turbulence and thermal transport in NSTX HHFW/NBI-heated L-mode plasmas are studied with a high-k scattering system and power balance analysis. One interesting observation is that measured turbulence spectral power is observed to reduce by almost an order of magnitude after HHFW heating is terminated in a set of NSTX L-mode plasmas with $I_P=300$ kA, $B_T=5.5$ kG and HHFW power about 1 MW. This sudden drop in the turbulence spectral power and the termination of HHFW are not exactly synchronized, and the drop in the spectral power happens approximately 1 ms after the HHFW termination. This correlation and the time delay indicate a causal relation between the measured turbulence and heat flux. However, we also found that such a sudden drop in turbulence spectral power is not obvious with HHFW termination in HHFW-heated L-mode plasmas with higher plasma current, e.g. 600 kA. Trying to explain this difference, we will compare global and local plasma parameters coupled with gyrokinetic simulations. In addition, we will present further gyrokinetic simulations on ExB shear-induced reduction in turbulence and thermal transport in NSTX NBI-heated L-mode plasmas reported in [1].\\[4pt] [1] Y. Ren et al., Nucl. Fusion 53 (2013) 083007. [Preview Abstract] |
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NP8.00014: Study of microtearing mode in NSTX with GEM Jugal Chowdhury, Yang Chen, Scott Parker, Weigang Wan, John Canik, Walter Guttenfelder, David Smith Anomalous thermal transport of electrons in conventional and spherical tokamaks is believed to be driven mainly by electron temperature gradient modes, and trapped electron modes. Another mode which is electromagnetic and sensitive to collisionality has emerged as another source of electron thermal loss. Earlier it has been believed that this instability is important only for spherical tokamaks both in core (Guttenfelder et al. Phys of Plasmas 19, 022506 (2012), as well as edge pedestal region (Dickinson et al., Plasma Phys. Control. Fusion 55 (2013) 074006). However, recent results indicate that it can equally affect the electron thermal loss in conventional tokamaks (Moradi et al., Nucl. Fusion 53 (2013) 063025)) also. The mode is characterized by the even parity in $A_\parallel$, and grows on the free energy provided by the electron temperature gradient. In the present work we will carry out a gyrokinetic study of the microtearing mode for NSTX parameters and compare properties of the mode in the core and edge using the nonlinear gyrokinetic electromagnetic code GEM. The scaling with collisionality in the two regimes will be studied. A preliminary comparison of the microtearing mode in local flux tube and global model of GEM will also be presented. [Preview Abstract] |
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NP8.00015: Gyrokinetic predictions of momentum and impurity transport in NSTX W. Guttenfelder, S.M. Kaye, Y. Ren, F. Scotti, W.M. Solomon, R.E. Bell, J. Candy, B.P. LeBlanc, H. Yuh Quasilinear predictions of core momentum transport in NSTX L-modes (unstable to low beta ITG,TEM modes) and H-modes (unstable to high beta ``hybrid-KBM'' modes) predict Prandtl numbers (Pr$=\chi_{\phi }$/$\chi_{i}\approx $0.2-6) very similar to experimental observations. However, the predicted momentum pinch (RV/$\chi_{\phi }\approx $-0.5-1) is much weaker than experiment (-1 to -6). In both L and H mode cases, the predicted momentum pinch is relatively insensitive to variations in density or temperature gradient, collisionality, safety factor, magnetic shear, or electron beta. A comparison will be made with DIII-D simulations at higher aspect ratio to illuminate possible causes for the differing strength of predicted momentum pinch. KBMs in the NSTX core (r/a\textless 0.8) are often predicted to be unstable simultaneously with microtearing (MT) modes, with mode dominance depending on local parameters. Nonlinear simulations are underway to predict the partition of ballooning and tearing mode turbulence under such mixed MT$+$KBM conditions, and the corresponding balance of energy, impurity and momentum transport. Similar quasilinear predictions of carbon impurity peaking (RV$_{c}$/D$_{c})$ from KBM modes will be shown for discharges that exhibit strong carbon accumulation. This work is supported by US DOE contract DE-AC02-09CH11466. [Preview Abstract] |
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NP8.00016: Progress in Understanding the Enhanced Pedestal H-mode in NSTX S. Gerhardt, D. Battaglia, R.E. Bell, A. Diallo, W. Guttenfelder, B.P. LeBlanc, R. Maingi, Y. Ren, J.M. Canik, P. Snyder, S. Kubota, D.R. Smith The enhanced pedestal H-mode (EP H-mode) is a high performance regime in NSTX, characterized by very steep ion temperature gradients and large edge flow shear. Recent research shows that the edge ion temperature shapes can very widely, from cases with the steep gradient region near the separatrix, to cases where the gradient is shifted inward by up to 10 cm. In all cases, however, the region of good confinement is coincident with a region of large toroidal flow shear. Typically 75{\%} of the stored energy increase following the EP H-mode transition is through the ion channel, with the remaining 25{\%} in the electron channel. While most EP H-mode examples are quite short, a number of quite quiescent long-pulse examples have been found in the NSTX database. Edge fluctuations in this regime have been assessed. There are some hints of an increase in magnetic fluctuations measured at the vessel wall, but no profound modifications in the turbulence measured by BES; reflectometer analysis is ongoing. Results of transport assessments with TRANSP and XGC-0, microstability calculations with GS-2, and edge macrostability with ELITE will be presented. This work was sponsored by the U.S. Department of Energy. [Preview Abstract] |
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NP8.00017: Effects of a GPI deuterium gas puff on the edge plasma in NSTX S.J. Zweben, R.E. Bell, W.M. Davis, S.M. Saye, S. Kubota, R. Maingi, T. Munsat, B.P. LeBlanc, R.J. Maqueda, Y. Sechrest, D.R. Smith, D.P. Stotler, V.A. Soukhanovskii Deuterium neutral gas puffs near the outer midplane of NSTX have been routinely used for the gas puff imaging (GPI) diagnostic to measure edge turbulence. These puffs can inject up to 3.5x10$^{20}$ D atoms over 100 msec from a manifold at the outer wall, with a maximum influx of 10$^{22}$ atoms/sec after 20 msec. The 3D shape and absolute brightness of the D-alpha emission cloud from this puff have previously been modeled using DEGAS 2 [1]. The effects of the GPI puff on the edge plasma are now evaluated using Thomson scattering and other edge diagnostics of NSTX. The time evolution of the radial profile of D-alpha emission from the GPI cloud itself can be used to infer local changes in density and/or temperature. These results will be compared with models for the expected density and temperature perturbations, including parallel and perpendicular transport, drifts, rotation, and energy loss from radiation and charge exchange. The edge turbulence seen by GPI does not change significantly vs. time during the GPI puff, and other measurements of edge turbulence will be evaluated across the time of this puff. \\[4pt] [1] B. Cao, D. Stotler et al, Fusion Sci. Tech. \textbf{64}, 29 (2013) [Preview Abstract] |
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NP8.00018: Investigations of pedestal turbulence and ELM bursts in NSTX H-mode plasmas D. Smith, R. Fonck, G. McKee, W. Wan, S. Parker, A. Diallo, W. Guttenfelder, S. Kaye The spherical torus H-mode pedestal is a challenging validation regime due to gradient scale lengths comparable to the ion gyro-radius, large $\rho $*, and strong shaping. Here, we investigate the spatial and temporal properties of pedestal turbulence and ELM bursts in NSTX H-mode plasmas. First, we present measurements, scalings, and simulations of pedstal turbulence correlation lengths and fluctuation amplitudes with k$\theta \rho $i\textless 1.5 and 0.8\textless r/a\textless 0.95. Fluctuation amplitudes are in the range $\delta $n/n$\approx $1-5{\%}. Parametric dependencies among turbulence quantities and transport-relevant plasma parameters indicate $\delta $n/n scales positively with $\nabla $ne, collisionality, and poloidal beta, and scales negatively with $\nabla $Ti. The scalings are most consistent with trapped electron mode, kinetic ballooning mode, or microtearing instabilities, but, notably, least consistent with ion temperature gradient turbulence. Gyrokinetic simulations with realistic pedestal profiles show collisional instabilities with growth rates that increase at higher $\nabla $ne and decrease at higher $\nabla $Ti, in qualitative agreement with observed scalings. Finally, we investigate the radial structure and temporal dynamics of ELM bursts in NSTX. Measurements show multiple intermittant radial structures that evolve on timescales of 10 $\mu $s during a single ELM burst. The observations illustrate the nonlinear dynamics of ELM bursts for simulation validation efforts. [Preview Abstract] |
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NP8.00019: Measurements of Magnetic Field in NSTX using the MSE-LIF Diagnostic Fred Levinton, Elizabeth Foley, Howard Yuh The motional Stark effect with laser-induced fluorescence diagnostic (MSE-LIF) was installed on NSTX in the 2011 run year. The MSE-LIF will enable radially resolved measurements of the magnetic field pitch angle and magnitude, both of which can be used to constrain plasma equilibrium reconstructions. A diagnostic neutral beam with low axial energy spread, low divergence, and high reliability has been developed. It operates routinely at 35 kV and 40 mA. A laser has been developed with high power ($\sim$10 W) and optimal linewidth match to the neutral beam ($\sim$6 GHz). The laser wavelength is near 651 nm for a match to the Doppler-shifted Balmer-alpha transition in the beam neutrals. The unique high-power, moderate linewidth laser system utilizes a 19 emitter diode laser bar and feedback from a volume holographic grating. A magnetic shield protects the ion source from the NSTX stray fields. Initial data in a gas-filled torus and low magnetic fields was taken on NSTX and is presented here. [Preview Abstract] |
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NP8.00020: Reflectometry Characterization of Edge Density Fluctuations During ELMy H-Modes in NSTX S. Kubota, N.A. Crocker, A. Diallo, S.M. Kaye, R. Maingi Understanding the role of instabilities that govern the properties of the H-mode pedestal is an important research topic for existing and future magnetic fusion devices. In a previous study (A.~Diallo et al., Phys.~Plasmas \textbf{20}, 012505), fluctuations in the pedestal of ELMy H-modes in NSTX were characterized using BES and reflectometry. With respect to the reflectometry analysis, the fluctuation model used for the 2-D full-wave analysis in that work assumed fully-developed turbulence characterized by Gaussian statistics. In the present work, data from the 30-75 GHz fixed-frequency reflectometer (16 channels, radially and toroidally separated) is used in conjunction with 3-D physical optics and full-wave calculations to determine the spatial structure and additional characteristics of the edge fluctuations. The analysis of the reflectometer data suggests that edge electron density fluctuations are dominated by long-lived ($\tau_{\mathrm{decorr}}$$\sim$several 100 $\mu$s) coherent structures. The largest structures are found to be poloidally localized (width $\sim$3 cm) with a dipole shape, and elongated in the direction of the magnetic field. Further information about the reflectometry analysis, as well a detailed description of the edge fluctuations, will be presented. [Preview Abstract] |
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NP8.00021: Direct Comparison of GPI and BES measurements of Edge Fluctuations in NSTX Y. Sechrest, D. Smith, T. Munsat, S.J. Zweben The Gas Puff Imaging (GPI) diagnostic has been used in numerous studies of turbulent fluctuations in the edge region of NSTX since its installation in 2001. Before the recent 2010 run campaign a Beam Emission Spectroscopy (BES) diagnostic was added on NSTX to study density fluctuations in the scrape-off layer (SOL), edge, and pedestal regions. Both diagnostics operate using similar principles to view visible light fluctuations from collisional excitation of neutral atoms, and the diagnostic views share coverage at some R and Z positions just above the outboard midplane. Making use of these commonalities, we conduct a cross-diagnostic comparison of fluctuation measurements of edge turbulence in NSTX including: poloidal correlation lengths, decorrelation times, probability density functions and their moments, and dominant poloidal wavenumber estimates. In addition, we explore cross-correlation and cross-spectral analyses between diagnostics. By characterizing the amount of shared information, it may be possible to use the two diagnostics collaboratively to effectively extend the diagnostic coverage. [Preview Abstract] |
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NP8.00022: First Doppler backscattering measurements in MAST J.C. Hillesheim, W.A. Peebles, H. Meyer, N.A. Crocker We present the first Doppler backscattering (DBS) measurements in the core of a spherical tokamak. Doppler backscattering (DBS) has become a well-established and versatile diagnostic technique for the measurement of intermediate-{\it k} ($k_{\theta} \rho_s \sim 1$, and higher) density fluctuations and flows in magnetically confined fusion experiments. The $180^{\circ}$ backscattering for DBS requires three dimensional wave-vector matching between the launched beam and the plasma fluctuations inducing the scattering, which are expected to be highly elongated along the magnetic field. The 3D alignment can be quantified in terms of a mismatch angle, $\hat{\bf k} \cdot \hat{\bf B} = \cos \left(\pi/2-\theta_{mis} \right)$, where $\theta_{mis}=0$ for accurately aligned $180^{\circ}$ backscattering. We report on ray tracing calculations to minimize $\theta_{mis}$ via a steerable mirror, description of the implementation, and present a survey of initial experimental data. [Preview Abstract] |
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NP8.00023: Comparison of resistive MHD simulations and experimental CHI discharges in NSTX E.B. Hooper, C.R. Sovinec, R. Raman, F. Fatima Resistive MHD simulations using NIMROD [1] simulate CHI discharges for NSTX startup plasmas [2]. Quantitative comparison with experiment ensures that the simulation physics includes a minimal physics set needed to extend the simulations to new experiments, e.g. NSTX-U. Important are time-varying vacuum magnetic field, ohmic heating, thermal transport, impurity radiation, and spatially-varying plasma parameters including density. Equilibria are compared with experimental injector currents, voltages and parameters including toroidal current, photographs of emitted light and measurements of midplane temperature profiles, radiation and surface heating. Initial results demonstrate that adjusting impurity radiation and cross-field transport yields temperatures and injected-current channel widths similar to experiment. These determine the plasma resistance, feeding back to the impedance on the injector power supply. \\[4pt] [1] E. B. Hooper, et al., to be published; F. Ebrahimi, et al., to be published.\\[0pt] [2] R. Raman et al., Phys. Rev. Letters \textbf{104}, 095003 (2010). [Preview Abstract] |
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NP8.00024: Characteristics for a Langmuir probe intercepting the HHFW RF power deposition spiral on NSTX J.C. Hosea, R.J. Perkins, M.A. Jaworski, G.J. Kramer, R.E. Bell, N. Bertelli, S. Gerhardt, B.P. LeBlanc, R. Maingi, C.K. Phillips, L. Roquemore, G. Taylor, J.R. Wilson, S. Zweben, J-W. Ahn, T.K. Gray, P.M. Ryan, S. Sabbagh, K. Tritz The HHFW RF power deposition spiral on the lower divertor of NSTX has been shown to result from power flow along the magnetic field lines in the scrape off layer (SOL) in front of the HHFW antenna [1]. It is noted in Ref. 1 that instrumented tile currents on the lower divertor exhibit an increase in electron current to the tiles when the deposition spiral is positioned over them. Similarly, a Langmuir probe is found to exhibit electron current at V$_{probe}\approx $ 0 (i.e. at the tile voltage) when it intercepts the RF deposition spiral. IV characteristics for this probe are evaluated in an attempt to determine whether RF electric field rectification at the probe or plasma heating away from the probe dominates the RF power deposition on the divertor. The IV data have a limited voltage scan and rather large current perturbations, presumably due to turbulence produced density fluctuations on the divertor plate [2]. Nevertheless, it appears that plasma heating dominates the IV characteristic, which would suggest a non-linear heating mechanism such as the two-stream instability of a current-carrying plasma could be active in the SOL plasma for the RF currents there.\\[4pt] [1] R.J. Perkins \textit{et al., PRL}\textbf{ 109}, (2012) 045001.\\[0pt] [2] R.J. Maqueda \textit{et al., NF} \textbf{50} (2010) 075002. [Preview Abstract] |
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NP8.00025: Modeling Results for 28 GHz Heating and Current Drive in the National Spherical Torus Experiment Upgrade (NSTX-U) G. Taylor, N. Bertelli, R.A. Ellis, S.P. Gerhardt, R.W. Harvey, J.C. Hosea, F. Poli, R. Raman, A.P. Smirnov A megawatt-level, 28 GHz electron heating system is being planned to heat non-inductive (NI) start-up plasmas and to provide radially localized electron heating and current drive during H-mode discharges in NSTX-U. NSTX-U will operate at axial toroidal fields of up to 1 T and plasma currents, I$_{\mathrm{p}}$, up to 2 MA. Development of fully NI plasmas is a critical long-term NSTX-U research goal that supports the design of a Fusion Nuclear Science Facility. 0.6 MW of 28 GHz electron cyclotron (EC) heating is predicted to increase the central electron temperature (T$_{\mathrm{e}}$(0)) of low density NI plasmas generated by Coaxial Helicity Injection (CHI) in NSTX-U from 10 eV to 400 eV in about 20 ms. The increased T$_{\mathrm{e}}$(0) will significantly reduce the plasma current decay rate of CHI plasmas, allowing the coupling of fast wave heating and neutral beam injection. Eventually 28 GHz electron Bernstein wave (EBW) heating and current drive will be used during the I$_{\mathrm{p}}$ flat top in NSTX-U H-mode discharges when the plasma is overdense. This paper will present numerical RF simulation results for 28 GHz EC and EBW heating and current drive for NSTX-U discharges and a conceptual design for the 28 GHz heating system. [Preview Abstract] |
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NP8.00026: Effect of Array Phasing on the HHFW Power Coupling Through the NSTX and NSTX-U Edge Plasmas P.M. Ryan, L.A. Berry, D.L. Green, E.F. Jaeger, N. Bertelli, J.C. Hosea, R. Perkins, C.K. Phillips, G. Taylor, J.R. Wilson The plasma load presented to the HHFW system in the upcoming NSTX-U experiment will differ from that of NSTX due to increased operational parameters: B$_{\mathrm{T}}$ increased from 0.55 to 1.0 T, I$_{\mathrm{P}}$ from 1 MA to 2 MA, and the addition of a second neutral beam (7 MW to 14 MW). Changes in the ICH harmonics, beta, and the fast ion population are expected to change the power division between the electrons, thermal and fast D$^{\mathrm{+}}$, and H$^{\mathrm{+}}$ minorities; these are being calculated with the AORSA code, recently extended to include the edge plasma [1]. Power coupling through and propagation within the edge plasma are being analyzed as a function of plasma gap, edge density profile, and array phasing. The optimal launched spectrum for core power absorption may well differ from that needed for effective power transmission though the edge plasma, and is the primary focus of this study. Collisional damping is being used as a proxy dissipation mechanism to study the anomalous power loss associated with normal modes in the scrape-off layer [2]. The AORSA analysis is being guided by recent experimental measurements of HHFW power deposition in the NSTX scrape off layer [3].\\[4pt] [1] D. L. Green, et al., \textit{Phys. Rev. Lett.,} \textbf{107} (2011), 145001\\[0pt] [2] N. Bertelli, invited paper, this conference\\[0pt] [3] R. J. Perkins et al, \textit{Phys. Rev. Lett.}, \textbf{109} (2012), 045001 [Preview Abstract] |
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NP8.00027: Effects of TAE Avalanches on the neutral beam driven current profile in NSTX and NSTX-U plasmas Douglass Darrow, Alessandro Bortolon, Neal Crocker, Eric Fredrickson, Nikolai Gorelenkov, Marina Gorelenkova, Shigeyuki Kubota, Mario Podest\`a, David Smith, Roscoe White Strong bursts of TAEs with multiple n numbers present, termed TAE avalanches, are observed in NSTX plasmas, including early in the discharge, when the plasma current is being ramped up. These avalanches cause radial redistribution, rapid slowing down, and often loss of beam ions from the plasma. Loss measurements show particularly pronounced losses during these events. All of these changes in the beam ion distribution can affect the beam driven current profile. Measurements and modeling of the beam ion population and driven currents in NSTX will be compared, including the effects of avalanches and associated MHD activity during the current ramp up phase. In addition, since NSTX-U will utilize additional neutral beams with different orientations from the set on NSTX, modeling of the effects of avalanches on the distribution of beam ions and associated beam driven current from the new beam lines will also be discussed. [Preview Abstract] |
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NP8.00028: Investigation of Alfv\'{e}n eigenmode structure in NSTX and MAST N.A. Crocker, J.C. Hillesheim, W.A. Peebles, E.D. Fredrickson, M. Podest\`a, K. Tritz, S. Kubota, H. Meyer Alfv\'{e}n eigenmodes (AEs) play critical roles in fusion research plasmas. Low frequency (f $\la$ 100 kHz) toroidicity-induced (TAE) and reverse-shear (RSAE) AEs cause significant fast-ion transport. High frequency (f $\gg$ 100 kHz) global (GAE) and compressional (CAE) AEs are correlated with enhanced thermal electron transport. Investigation of these modes in the National Spherical Torus eXperiment (NSTX) and the Mega Amp Spherical Tokamak (MAST) have been facilitated by a 16 channel, 30 -- 75 GHz fixed-frequency quadrature system implemented for reflectometry in NSTX and recently adapted for reflectometry and Doppler backscattering (DBS) in MAST. As a reflectometer array, the system has probed the spatial structure of AE density perturbations. As a DBS array, it has provided localized phase velocity measurements of the intermediate-k turbulence (k $\sim $ 4 -- 12 cm$^{-1})$, which are expected to yield the fluctuating ExB velocities, and thus the structure of the E perturbation, associated with AEs. [Preview Abstract] |
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NP8.00029: Energy channeling from trapped to passing fast ions mediated by GAE/CAE activity in NSTX S.S. Medley, E. Belova, G. Kramer, M. Podesta, D. Liu In the National Spherical Torus Experiment, an increased charge exchange neutral flux localized at the neutral beam full injection energy is measured by the E\textbar \textbar B Neutral Particle Analyzer. Termed the High-Energy Feature (HEF), it appears on the beam-injected energetic ion spectrum in discharges where NTM or kink modes (f \textless\ 10 kHz) are absent, TAE activity (f $\sim$ 10-150 kHz) is weak and CAE/GAE activity (f $\sim$ 400 -- 1200 kHz) is robust. The HEF exhibits a growth time of t $\sim$ 20 - 80 ms and develops a slowing down distribution that continues to evolve over periods \textgreater\ 100 ms. HEFs are observed only in H-mode discharges with NB power P$_{\mathrm{b}} \ge $ 4 MW and in the pitch range v$_{\mathrm{\vert \vert }}$/v $\sim$ 0.7 -- 0.9. The HEF appears to be caused by a CAE/GAE wave-particle interaction that modifies the fast ion distribution, f$_{\mathrm{i}}$(E,v$_{\mathrm{\vert \vert }}$/v,r). This mechanism was studied using the SPIRAL code that evolves an initial TRANSP-calculated f$_{\mathrm{i}}$(E,v$_{\mathrm{\vert \vert }}$/v,r) distribution in the presence of background plasma profiles under drive from wave-particle resonances with CAE/GAE Alfv\'{e}n eigenmodes. [Preview Abstract] |
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NP8.00030: Development of a reduced model for resonant fast ion transport in TRANSP Mario Podesta, M. Gorelenkova, R.B. White A new reduced fast ion transport model that captures the physics of resonant wave-particle interaction is being developed for the tokamak transport code TRANSP. The model is based on a probability distribution function to reproduce changes in phase space coordinates (energy, toroidal canonical momentum and possibly magnetic moment) of fast ions interacting with a given set of instabilities. The probability function can be derived from analytical theories as well as from particle-following codes such as ORBIT or SPIRAL. The basic principles of the new model and the progress in its implementation in TRANSP will be reported. Examples of the initial verification of the model for a specific NSTX scenario with multiple unstable Toroidal Alfven Eigenmodes, leading to enhanced fast ion transport, will be then discussed. [Preview Abstract] |
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NP8.00031: Modeling NSTX Snowflake Divertor Experiments E.T. Meier, V.A. Soukhanovskii, A.G. McLean, T.D. Rognlien, D.D. Ryutov, R.E. Bell, A. Diallo, R. Kaita, B.P. LeBlanc, M. Podesta, F. Scotti Experiments on the National Spherical Torus Experiment (NSTX) have demonstrated the potential of the snowflake divertor to alleviate the tokamak power exhaust challenge. The NSTX snowflake configuration induced partial detachment and reduced heat flux approximately fivefold. To explore snowflake physics, the multi-fluid edge transport code, {\sf UEDGE}, has been used to compare standard and snowflake configurations. Radial profiles of anomalous perpendicular transport coefficients (assumed to be poloidally uniform) are constrained by requiring solutions to match ion and electron temperature and density data at the outer midplane. Divertor recycling and separatrix location are constrained by matching $D_\alpha$ emission and heat flux at the outer target. Good agreement with heat flux data is achieved, and partial detachment is captured in the snowflake case. Increased snowflake divertor volume and connection length result in higher radiation which, in tandem with direct flux-expansion profile broadening, leads to heat flux reduction. [Preview Abstract] |
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NP8.00032: Scrape-off layer turbulence (SOLT) simulations of effects of lithium deposition on heat flux characteristics observed in NSTX D.A. Russell, D.A. D'Ippolito, J.R. Myra, J.M. Canik, T.K. Gray Established benefits of lithium conditioning observed in experiments at NSTX [1] include reduced ELMs, improved energy confinement and lowered H-mode thresholds. Recent measurements of the power exhaust channel [2] found reduced heat flux width ($\lambda_{\mathrm{q}})$ at the divertor, compared to experiments with no lithium coating. Gradients of plasma energy and particle density as well as density fluctuations are observed to be reduced at the edge in the presence of lithium [3], suggesting a role for interchange-driven turbulence in setting heat flux characteristics in these experiments. To explore this possibility, we simulate the edge turbulence, in the outboard midplane, driven by plasma profiles measured in two NSTX experiments, with and without lithium, using the SOLT model code, [4] newly expanded [5] to include \textit{self-consistent} ion diamagnetic drift evolution. Simulated and experimentally measured heat flux and $\lambda_{\mathrm{q}}$ are compared, and the underlying (simulated) turbulence is characterized in detail.\\[4pt] [1] R. Maingi et al., Nucl. Fusion \textbf{52}, 083001 (2012). [2] T.K. Gray et al., IAEA 2012, San Diego, paper EX/P5-27. [3] J.M. Canik et al., Phys. Plasmas \textbf{18}, 056118 (2011). [4] J.R. Myra et al., Phys. Plasmas \textbf{18}, 012305 (2011). [5] D.A. Russell et al., Bull. Am. Phys. Soc., DPP 2012, \textbf{57} (12), BP8-159. [Preview Abstract] |
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NP8.00033: Characterization of a Liquid Lithium Pellet Delivery (LLPD) System for ELM Pacing and Lithium Replenishment in NSTX-U Daniel Andruczyk, Lane Roquemore, Peter Fiflis, David Ruzic A new liquid lithium pellet delivery system has been developed that will be used in several Li delivery schemes in NSTX-U. The LLPD is based on dripping Li out of a 300 $\mu $m diameter nozzle. A modulating current and permanent magnets provide a J $\times$ B force that drives the formation of the droplets. It is suggested that the current amplitude sets the size of the drops and the modulation sets the frequency that the drops come out of the nozzle. An alternative method for drop production uses high pressure gas where the pressure sets the frequency, up to 2 kHz, and drop size, 0.6 \textless\ $d_{drop}$ \textless\ 2.0 mm. It's intended to use LLPD to deliver solid Li pellets for the granular injector system, fill the upward facing LITER (U-LITER) being developed and maintain fresh Li during a discharge by injecting Li drops into the plasma, ablating the Li and replenishing the coatings on the PFCs. [Preview Abstract] |
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NP8.00034: The Structure of Flux Tubes for Generating Toroidal Asymmetry in the Tokamak Scrape-Off-Layer (SOL) Hironori Takahashi, Eric Fredrickson Creating and maintaining a tokamak discharge involve toroidally localized operations, including particle and heat inputs through gas puffing and neutral beam and pellet injections. In the main plasma, the injected particle and heat distributions become toroidally symmetric through rapid transport along infinitely long field lines forming irrational magnetic surfaces. But in the SOL, rapid transport along open finite-length field lines, which end on a structural component, can result in a toroidally asymmetric region (flux tube) with properties distinguishable from those of its surroundings. Of particular interest is a flux tube carrying field-aligned current, thermoelectrically driven by an electron temperature difference between its two ends. This work investigates the efficacy of such Scrape-Off-Layer Current (SOLC) in generating error field in an otherwise magnetically symmetric tokamak as a function of the flux tube structure, and explores the possibility that SOLC-generated error field contributes to strong plasma rotation braking often observed when the SOL magnetic structure rapidly evolves in an early discharge phase. [Preview Abstract] |
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NP8.00035: Midplane Neutral Density Profiles in NSTX D.P. Stotler, F. Scotti, R.E. Bell, B.P. LeBlanc, R. Raman The experimental determination of neutral densities in tokamak plasmas from line radiation is only accurate in the narrow region in which both the excitation rate and neutral density are significant; elsewhere the result is dominated by noise. We propose an alternative, simulation based inversion procedure utilizing tools developed in the validation of the DEGAS 2 Monte Carlo neutral transport code against Gas Puff Imaging camera data.\footnote{B. Cao, et al., Fusion Sci. Tech. {\bf 64}, 29 (2013).} Here, the Balmer-$\beta$ emission rate recorded by an absolutely calibrated tangentially viewing camera is used to quantify a simulated, ad hoc neutral gas source at the vacuum vessel wall. This procedure yields absolute radial profiles of deuterium atoms and molecules at midplane. The validity of this characterization of the gas source is confirmed by the similarity of the shapes of the simulated and observed light emission profiles. We also compare the resulting neutral pressures at the vessel walls with data from midplane micro-ion gauges. [Preview Abstract] |
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NP8.00036: Safety and diagnostic systems on the Liquid Lithium Test Stand (LLTS) J.A. Schwartz, M.A. Jaworski, R. Ellis, R. Kaita, R. Mozulay The Liquid Lithium Test Stand (LLTS) is a test bed for development of flowing liquid lithium systems for plasma-facing components at PPPL. LLTS is designed to test operation of liquid lithium under vacuum, including flowing, solidifying (such as would be the case at the end of plasma operations), and re-melting. Constructed of stainless steel, LLTS is a closed loop of pipe with two reservoirs and a pump, as well as diagnostics for temperature, flow rate, and pressure. Since liquid lithium is a highly reactive material, special care must be taken when designing such a system. These include a permanent-magnet MHD pump and MHD flow meter that have no mechanical components in direct contact with the liquid lithium. The LLTS also includes an expandable 24-channel leak-detector interlock system which cuts power to heaters and the pump if any lithium leaks from a pipe joint. Design for the interlock systems and flow meter are presented. [Preview Abstract] |
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NP8.00037: Upgrade for High-k Scattering on NSTX-U Robert Barchfeld, Chris Muscatello, Calvin Domier, Neville Luhmann, Yang Ren, Robert Kaita A major upgrade to the high-k scattering system for NSTX-U is currently under development. This system measures electron-scale density fluctuations through collective scattering and will be comprised of four major components: 1) a 700 GHz FIR laser probe beam, 2) remote controlled, steerable launching optics on Bay G, 3) receiving optics on Bay L, 4) and a 4-channel receiver. An FIR laser is under construction to meet the needs of the high-k scattering system. A hybrid output coupler optimizes FIR power, while blocking IR pump wavelengths. Corrugated waveguide will deliver the probe beam to NSTX-U with minimal attenuation. At the launching port, remote controlled optics will focus and steer the beam $\pm$3 degrees toroidally and poloidally. With the FIR laser fully characterized, the receiver system can be designed and fabricated including quasi-optical subharmonic mixers. [Preview Abstract] |
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NP8.00038: Refurbishment of the FIR Tangential Interferometer/Polarimeter Diagnostic on NSTX-U Calvin Domier, Robert Barchfeld, Christopher Muscatello, Neville Luhmann, Jr., Robert Kaita The addition of a 2nd neutral beam injector on NSTX during the current shut-down period necessitates reconfiguration of the far-infrared tangential interferometer/polarimeter (FIReTIP) system. During this shut-down period, the laser systems have been returned to UC Davis for laboratory characterization, maintenance, and upgrade. FIReTIP will be reconfigured as a 3-channel system, employed for core density monitoring/feedback control as well as core/edge fluctuation measurements. Besides the spatial rearrangement of the laser system, optics, and electronics, a significant upgrade to FIReTIP involves modification of its constituent Stark laser, in particular, the shape of its waveguide and electrodes. With the new design, simulations indicate improved mode quality and power output which directly translates to improved FIReTIP signal-to-noise ratios. [Preview Abstract] |
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NP8.00039: Multi-Energy Soft X-ray Measurements of the Electron Temperature Profile K. Tritz, D.J. Clayton, D. Stutman, R.E. Bell, A. Diallo, B.P. LeBlanc, M. Podesta, S. Sabbagh A novel analysis method using a neural network algorithm has been used to reconstruct the electron temperature profile for high power NSTX discharges with fast time resolution (\textgreater\ 10kHz) using multi-energy soft X-ray (ME-SXR) arrays both alone and in combination with low time-resolution space-resolved spectroscopy and a single chord of line-integrated density. This fast profile reconstruction uses the measured electron temperature profile from the 60Hz multipoint Thomson scattering (MPTS) diagnostic to train the neural network with the high time resolution ME-SXR data. Comparisons using cross-validation between the neural network reconstruction and the measured T$_{e}$ profiles from MPTS show agreement within 5{\%} over the profile radius. The accuracy of the neural network reconstruction demonstrates the ability to use ME-SXR diagnostics for high time resolution electron temperature measurements, and will be available on both EAST and NSTX-U for studies of electron heat transport as well as other general studies including impurity/particle transport, ELM profile dynamics, and MHD. [Preview Abstract] |
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NP8.00040: Modeling and Control of Plasma Rotation for the NSTX using the Neoclassical toroidal viscosity Imene Goumiri, Steven Sabbagh, David Gates, Clarence Rowley, Stefan Gerhardt A one-dimensional plasma model and its controller for a magnetically confined fusion device are developed in an effort to assist the continuous extraction of fusion energy. In particular, this study is based on the experimental measurements from the National Spherical Torus Experiment (NSTX) and is aimed to capture the rotation (toroidal) momentum transport inside the tokamak. The neutral beam injection being fixed, the neoclassical toroidal viscosity will be considered in our model as the actuator. Based on the proposed model, a feedback controller is designed to sustain the toroidal momentum of the plasma in a stable fashion and to achieve desirable plasma geometry. [Preview Abstract] |
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NP8.00041: Physics-based Control-oriented Modeling of the Current Profile Evolution in NSTX-Upgrade Zeki Ilhan, Justin Barton, Wenyu Shi, Eugenio Schuster, David Gates, Stefan Gerhardt, Egemen Kolemen, Jonathan Menard The operational goals for the NSTX-Upgrade device include non-inductive sustainment of high-$\beta $ plasmas, realization of the high performance equilibrium scenarios with neutral beam heating, and achievement of longer pulse durations. Active feedback control of the current profile is proposed to enable these goals. Motivated by the coupled, nonlinear, multivariable, distributed-parameter plasma dynamics, the first step towards feedback control design is the development of a physics-based, control-oriented model for the current profile evolution in response to non-inductive current drives and heating systems. For this purpose, the nonlinear magnetic-diffusion equation is coupled with empirical models for the electron density, electron temperature, and non-inductive current drives (neutral beams). The resulting first-principles-driven, control-oriented model is tailored for NSTX-U based on the PTRANSP predictions. Main objectives and possible challenges associated with the use of the developed model for control design are discussed. [Preview Abstract] |
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NP8.00042: FAST AND SHOCK IGNITION |
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NP8.00043: Fast Ignition Thermonuclear Fusion: Enhancement of the Pellet Gain by the Colossal-Magnetic-Field Shells V. Alexander Stefan The fast ignition fusion\footnote{M. Tabak, D. Clark, R. P. J. Town et. all, \textit{Features of a Point Design for Fast Ignition}, Keynote Talk, Conf. on Res. Trends in Laser Matter Interaction, (Bulletin of the Stefan University, Series B, Stefan Frontier Conferences, Vol. 22, May 2010; ISSN:1938-3967).} pellet gain\footnote{S. A. Slutz\textbf{,} M. C. Herrmann, R. A. Vesey, et. all., Phys. Plasmas, 1\textbf{7}, 056303 (2010)]} can be enhanced by a laser generated B-field shell. The B-field shell, (similar to Earth's B-field, but with the alternating B-poles), follows the pellet compression in a frozen-in B-field regime. A properly designed laser-pellet coupling \footnote{V. Alexander Stefan, Bulletin of the American Physical Society, Vol. 55, No.15, (2010); Stefan, Laser Thermonuclear Fusion: Research Review, [Vol. 1, (1963-1983); Vol.2, (1984-2008)], on Generation of Suprathermal Particles, Laser Radiation Harmonics, and Quasistationary B-Fields; Stefan-U Graduate Courses, ISSN: 1543-558X, (2008).} can lead to the generation of a B-field shell, (up to 100 MG), which inhibits electron thermal transport and confines the alpha-particles. In principle, a pellet gain of few-100s can be achieved in this manner. [Preview Abstract] |
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NP8.00044: Study of Pre-Plasma Effects on Fast Electron Generation and Transport Using the 1.5kJ, 10ps OMEGA EP Laser J. Peebles, C. Mcguffey, L.C. Jarrott, A. Sorokovikova, S. Krasheninnikov, F.N. Beg, M.S. Wei, R.B. Stephens, C. Chen, H.S. McLean, P.K. Patel The efficient coupling of high intensity laser energy to fast electrons and their subsequent transport is of great interest to fast ignition inertial confinement fusion. Recent 10-ps experiments performed on the OMEGA EP laser show significantly different dynamics of fast electrons depending on pre-pulse level [1]. Here we present the detailed analysis of bremsstrahlung data used to understand the fast electron energy deposition in the target. The fast electron transport is modeled with the Monte-Carlo code package ITS 3.0 and with the hybrid-PIC code Zuma, which includes particle induced fields. We show increase in electron temperature and conversion efficiency and decrease in divergence when pre-pulse is reduced. We also show that fields play a role in electron transport with 10-ps pulses.\\[4pt] [1] M.S. Wei, ``Investigation of the Dependence of Fast-Electron Generation and Transport on Laser Pulse-Length and Preplasma,'' SSAP Symposium (2013) [Preview Abstract] |
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NP8.00045: Magnetic field compression in an implosion for fast ignition Hideo Nagatomo, Tomoyuki Johzaki, Atsushi Sunahara, Hitoshi Sakagami, Kunioki Mima In the Fast Ignition research, reduction of the divergence angle of heating electron beam is urgent issue. In the recent researches suggest that the magnetic field plays an important role in the problem, especially for the controlling of the high energy electron transport toward the compressed core plasma. At ILE, Osaka University, generation of a strong external magnetic field ($\sim$10kT) is demonstrated successfully using a laser-driven capacitor-coil target [1]. In such a strong magnetic field we cannot ignore the effect to the reduction of thermal conduction which is strongly related to implosion dynamics, as well as the effect to the hot electron transport. Therefore, magnetic field transport code must be solved with two-dimensional radiation hydrodynamic code simultaneously or strongly coupled with each other when we investigate the effect to the implosion dynamics. Especially, the high Hall parameter effect and the Nernst effect are our most interest [2]. Our simulation code will be extended to simulate the transport of external magnetic field in axial direction. The electron beam reflection due to the mirroring effect in the strong magnetic field can be estimated. Also implosion dynamics and electron transport under such a strong magnetic field will be discussed. Finally, optimum applied magnetic field is proposed for the Fast Ignition. [1] Fujioka S. \textit{et al Plasma Phys. Control. Fusion} \textbf{54} 124042, (2012). [2] Nishiguchi A. \textit{et al Phys. Fluid} \textbf{28}, 3683, (1985). [Preview Abstract] |
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NP8.00046: Laser-plasma interactions and hot electron generation in shock ignition Chuang Ren, Rui Yan, Jun Li We present 2D Particle-in-cell (PIC) simulations, including electron-ion collisions and lasting more than 10 ps, on laser-plasma interactions for two sets of shock ignition (SI) parameters. The first is for conditions relevant to the Omega laser facility with a spike intensity of $I=2\times {10}^{15}$W/cm$^{2}$ and the density scale length at the quarter critical surface of L $\sim$ 170microns. The second is relevant to NIF conditions with $I=5\times {10}^{15}$W/cm$^{2}$ and L$\sim$400microns. Under the Omega conditions, the simulations show a bursting pattern in both plasma waves and hot electron fluxes, which is attributed to the interplay between stimulated Raman scattering (SRS) and two-plasmon decay (TPD) instabilities. The observed hot electron temperatures compare favorably to those measured in the 40$+$20 spherical SI experiments (Theobald et al. 2012). SRS is the main source for hot electrons but TPD can produce \textgreater 100 keV ones. Similar bursting patterns are also observed in the NIF-relevant simulations. However, these simulations show strong SBS in rather low density region ($\sim$ 0.1$n_{cr})$. [Preview Abstract] |
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NP8.00047: Generation of narrow divergence electron beams in relativistic laser-plasma interactions H. Nakamura, G. Hicks, Z. Najmudin, M. Vranic, L.O. Silva, M. Borghesi, D. Doria, Satya Kar, G. Sarri, R. Heathcote, R. Scott, R. Trines, E. Guillaume, E. Higson, J. Swain, K. Tang, J. Weston, P. Zak, K.A. Tanaka, Y. Amano, H. Habara, M. Skramic, B. Bingham, P.A. Norreys The evacuation of plasma from channels formed during the interaction of intense laser pulses with under-dense plasma is attractive for a number of applications, particularly fast ignition inertial fusion. We investigated the channel formation using proton radiography as the diagnostic tool. We observed the interactions of ultra-intense laser pulse (120J/ 15ps/ 1053nm) with a large scale-length plasma which was formed by the expansion of a plastic foil target by preheating with a laser pulse comprising 200J/ 1ns/ 527nm, focused to 400-diameter. This experiment was set-up to mimic the coronal plasma experienced during the compression phase of a directly driven implosion. The results showed that laser-induced electron beam were guided by self-generated magnetic field. [Preview Abstract] |
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NP8.00048: Numerical study of fast electron transport and core heating in cone-guiding fast ignition Ryan Royle, Yasuhiko Sentoku A critical issue for the fast ignition (FI) of inertial fusion targets---where the compressed fuel is to be ignited by fast electrons generated by an ultrashort, ultraintense laser pulse---is whether the electrons will reach the core region and deposit sufficient energy to ignite fusion burn. In the cone-guiding FI scheme, a gold cone is embedded into the target capsule to keep the ignition beam path clear of coronal plasma and bring the critical surface closer to the core. This work presents the results of comprehensive two-dimensional particle-in-cell simulations of cone-guided FI using the PICLS code, which includes collisional, ionization and radiative processes. We study the impact of various gold cone tip geometries on the fast electron transport and generation of resistive magnetic fields that are supposedly excited in the core region. The details of simulation results and scaling of resistive magnetic field guiding are presented. [Preview Abstract] |
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NP8.00049: LASER-PLASMA INTERACTIONS |
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NP8.00050: Spectrum and Angular Distribution of $\gamma$-rays from Radiative Damping in Extremely Relativistic Laser-Plasma Interaction Rishi Pandit, Yasuhiko Sentoku Effects of the radiative damping in the interaction of extremely intense laser ($>$ $10^{22}$W/cm$^{2}$) with dense plasma is studied via a relativistic collisional particle-in-cell simulation, PICLS. When the laser intensity is getting close to $10^{24}$ W/cm$^{2}$, the effect of quantum electrodynamics (QED) appears. We had calculated $\gamma$-rays from the radiative damping processes based on the classical model [1], but had taken into account the QED effect [2] in the spectrum calculation. In ultra-intense laser-plasma interaction, electrons are accelerated by the strong laser fields and emit $\gamma$-ray photons mainly via two processes, namely, Bremsstrahlung and radiative damping. Such relativistic $\gamma$-ray has wide range of frequencies and the angular distribution depends on the hot electron source. Comparing the details of $\gamma$-rays from the Bremsstrahlung and the radiative damping in simulations, we will discuss the laser parameters and the target conditions (geometry and material) to distinguish the photons from each process and also the QED effect in the $\gamma$-rays spectrum at the extremely relativistic intensity. [Preview Abstract] |
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NP8.00051: ABSTRACT WITHDRAWN |
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NP8.00052: ABSTRACT WITHDRAWN |
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NP8.00053: Modeling target normal sheath acceleration using handoffs between multiple simulations Matthew McMahon, Christopher Willis, Robert Mitchell, Frank King, Douglass Schumacher, Kramer Akli, Richard Freeman We present a technique to model the target normal sheath acceleration (TNSA) process using full-scale LSP PIC simulations. The technique allows for a realistic laser, full size target and pre-plasma, and sufficient propagation length for the accelerated ions and electrons. A first simulation using a 2D Cartesian grid models the laser-plasma interaction (LPI) self-consistently and includes field ionization. Electrons accelerated by the laser are imported into a second simulation using a 2D cylindrical grid optimized for the initial TNSA process and incorporating an equation of state. Finally, all of the particles are imported to a third simulation optimized for the propagation of the accelerated ions and utilizing a static field solver for initialization. We also show use of 3D LPI simulations. Simulation results are compared to recent ion acceleration experiments using SCARLET laser at The Ohio State University. This work was performed with support from ASOFR under contract {\#} FA9550-12-1-0341, DARPA, and allocations of computing time from the Ohio Supercomputing Center. [Preview Abstract] |
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NP8.00054: Design of a liquid membrane target for high repetition rate neutron generation Patrick Poole, C. David Andereck, Mike Storm, Douglass Schumacher Ultra-bright, pulsed, spatially-small sources of energetic neutrons have applications in radiography and non-destructive remote sensing. Neutrons can be generated by a process wherein ions accelerated from a laser-irradiated primary target subsequently bombard a converter material, causing neutron-producing nuclear reactions, such as $^{7}$\textit{Li(d,n)}$^{8}$\textit{Be}. Deuterons from this process are suppressed by contamination that builds up on the rear of the solid primary target. To eliminate this issue we propose a self-replenishing liquid membrane target consisting of heavy water and deuterated surfactant, formed in-vacuum within a moveable wire frame. In addition to removing issues associated with solid target positioning and collateral damage, this apparatus provides flow rate and target thickness control, and allows for the high repetition rates required to generate desired neutron fluxes with a portable laser-based system. The apparatus design will be presented, as well as a novel interferometric method that measures the membrane thickness using tightly-focused light. [Preview Abstract] |
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NP8.00055: Backward Raman amplification of intense hard X-ray pulses Vladimir Malkin, Nathaniel Fisch Backward Raman amplification of laser pulses in plasmas can produce intensities far greater than the current state of the art. However, the backward Raman amplification techniques are thought not to extend to the hard x-ray range of wavelengths. The theoretical short-wavelength limit is imposed primarily by the inverse bremsstrahlung absorption of laser pulses in plasmas. This process can either overheat the plasma up to the extinguishing the resonant Langmuir wave branch mediating energy transfer from the laser pump to the seed pulse, or can deplete the laser pump pulse even before it reaches the counter-propagating seed pulse. To overcome the short-wavelength limit, it is proposed here to use laser pump pulses consisting of multiple separate spikes each of which encounters the seed pulse in a separate plasma layer sufficient for the pump depletion, yet thin enough to avoid an excessive inverse bremsstrahlung of the pump. The inverse bremsstrahlung of the seed, amplified to intensities much higher than the pump, is suppressed by higher electron quiver velocities. [Preview Abstract] |
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NP8.00056: Investigation of energy partitioning from Leopard short-pulse laser interactions in mass limited targets B. Griffin, H. Sawada, T. Yabuuchi, H. McLean, P. Patel, F. Beg The energy distribution in the interaction of a high-intensity, short-pulse laser with a mass limited target was investigated by simultaneously collecting x-ray and particle data. The Leopard laser system at the Nevada Terawatt Facility delivered 15 J of energy in a 350 fs pulse duration. With a beam spot size limited to within 8$\mu $m, the target interaction achieved a peak intensity of 10$^{19}$ W/cm$^{2}$ at 20$^{\circ}$ incidence. The size of the Cu foil targets was varied from 2 -20 $\mu $m in thickness and from 50 by 50 $\mu $m to 2000 by 2000 $\mu $m in surface area. A Bragg crystal x-ray spectrometer and a spherical crystal imager were used to measure 7.5 -9.5 keV x-rays and 8.05 keV monochromatic x-ray images respectively. The escaping electrons and protons in the rear were monitored with a magnet-based electron spectrometer and radiochromic film. Preliminary results show both a decrease of the K$\beta $/K$\alpha $ ratio\footnote{P. Nilson et al., PRE \textbf{79, }016406 (2009).} and a stronger He$\alpha $ emission for smaller sized targets, less than 250 by 250 $\mu $m. The detailed analyses of the K$\alpha $ images and particle data will be presented. [Preview Abstract] |
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NP8.00057: Ion acceleration during isothermal expansion of plasma slab into vacuum Evgeny Govras, Valery Bychenkov The interaction of short intense laser pulses with solid targets allows
record-breaking ion energies to be attained at the laboratory scale.
Quasineutral plasma outflow and the regime of plasma expansion with charge
separation effects in collisionless isothermal expansion of a semi-bounded
plasma have been theoretically studied in great detail. However, at high
electron energy (temperature) the model of semi-bounded plasma becomes
inapplicable as far as the electron Debye length, $\lambda_{D_e}$ approaches
the foil thickness, $L$. Also, analytically well studied
regime of ion acceleration from plasma foil is the Coulomb explosion. Going
beyond previous studies we have developed a theory of plasma slab expansion into
a vacuum where the electrons follow Boltzmann distribution with an arbitrary
temperature. The electron temperature, $T_e$, is a controlling
parameter of our theory and matches laser intensity. By increasing $T_e$
($0 |
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NP8.00058: Selective emission of low frequency electromagnetic wave due to an interaction between strong laser field and single-walled carbon nanotubes Toshihiro Taguchi, Thomas Antonsen, Masahiko Inoue Interaction between an intense laser light and carbon nanotubes (CNT) is one of hot topics in laser plasma research. A large number of vertically synthesized CNT, which is called a single-walled carbon nanotube (SWNT) is often used as a target with absorption coefficient much larger than a flat material. We have been investigating the laser-SWNT interaction in order to develop a low frequency coherent radiation source. According to our 2D PIC simulation, when the laser intensity becomes high enough, nonlinear interaction between the strong laser and CNT has a capability to generate sub-harmonics of the laser wavelength. When the height of a nanotube is longer than the laser wavelength, the electromagnetic wave can penetrate through void between nanotubes and will be trapped in the void. When the distance between two neighboring nanotubes is adjusted to the wavelength emitted by the nonlinear process, the nanostructure is expected to work as a selective emitter of a low frequency electromagnetic wave. We will present recent studies about the interaction between strong laser and long carbon nanotubes analyzed by an electromagnetic PIC code. In the presentation, we will show the electromagnetic field excited inside the nanotube region and emission spectrum observed outside. [Preview Abstract] |
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NP8.00059: Pre-pulse energy suppression for high-energy ultrashort pulses by liquid jet based plasma mirrors Jungmoo Hah, Kirk Liberty, John Nees, Bixue Hou, Karl Krushelnick, Alexander Thomas Currently, the peak intensity that can be achieved in the laser focus, with intensities in excess of 1021 Wcm$^{-2}$ creates pre-pulse problem. The most successful way to eliminate pre-pulse is to use plasma mirrors. Plasma mirror contrast enhancement at relativistic intensities has been demonstrated by using pulses in the high contrast plasma mirror (HCPM) with a solid target (anti-reflection coated optics). However, every single shot requires high-quality optical surfaces. Therefore, the plasma mirror must be replaced and aligned before each shot. To overcome the issue, a liquid-based concept is attempted, and experiments are performed on the high repetition rated (500 Hz) Lambda Cubed laser (duration 30 fs). A continuously flowing fluid jet can be used as an optically flat surface that regenerates itself in less than a millisecond, which makes it possible to use high repetition rate laser pulse systems. [Preview Abstract] |
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NP8.00060: Electron beam optimization using 3D printed gas cells in a laser-plasma accelerator Keegan Behm, Michael Vargas, William Schumaker, Zhen Zhao, Vladimir Chvykov, Anatoly Maksimchuk, Victor Yanovsky, Alexander Thomas, Karl Krushelnick Laser driven tabletop accelerators have made it possible to produce tunable relativistic beams of electrons. One of the ways in which these electron beams can be optimized is by changing the plasma environment that creates and accelerates the electrons. Using a rapid prototyped gas cell built with a 3D printer to create a relatively contained environment for the plasma has increased the electron beam pointing stability and has created more monoenergetic beams than what was achieved with a gas jet. Several different gas cell designs have been studied and tested to determine the optimum configuration and gas mixture for stable, monoenergetic electron beams. Two-staged gas cells have produced the highest quality electron beams with greatest pointing and beam stability. The purpose of the two-staged gas cell is to divide the laser wakefield acceleration process into two steps, an injection stage, where a helium-nitrogen mixture is used to inject more charge into the wake of the laser, and an acceleration stage where pure helium is used to create a plasma conducive for accelerating the electrons captured in the first stage. [Preview Abstract] |
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NP8.00061: Direct laser field electron generation using a tight focused high intensity laser pulse Sylvain Fourmaux, Antoine Bois, Sylvain De Leseleuc, Marc Glesser, Kazuto Otani, Stephane Payeur, Mark Quinn, Arpit Saraf, Bruno Schmidt, Patrizio Antici, Francois Legare, Steve Maclean, Michel Piche, Jean-Claude Kieffer Energetic electrons generation (around 20 keV) by longitudinal field acceleration from a laser pulse has been demonstrated by focusing a radially polarized TM01 ultrashort laser pulse (1,8 microns, 550 microJ, 15 fs) with a high numerical aperture parabola. We extend this technique using a 100 TW laser system. The expected intensity is around 10E22 W/cm2. Preliminary results about energetic electrons generation (MeV range) by focusing a linearly polarized TEM00 ultrashort pulse (800 nm, 3 J, 30 fs) are presented. [Preview Abstract] |
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NP8.00062: Electrical conductivity and Equation of State from Measurements of a Tamped Electrically Exploded Foil Edward Ruden, David Amdahl, Rufus Cooksey, Matthew Domonkos, Paul Robinson, Francis Analla, Darwin Brown, Mark Kostora, Frank Camacho Results are presented for an experiment that produces and diagnoses dynamic surface conditions of homogeneous warm dense matter (WDM) to infer intrinsic bulk properties such as density, pressure, temperature, specific energy, electrical conductivity, and emissivity in the ranges of up to few eV and down to 0.1 solid density - typical of those encountered in single shot pulsed power device electrodes. The goal is to validate ab initio models of matter encountered for predictive modeling of such devices. In the test whose results are presented here, the WDM is produced by Ohmically heating and exploding an 80 $\mu$m Al foil placed between two fused quartz tampers by the discharge of a 36 $\mu$F capacitor bank charged to 30.1 kV and discharged in 2.55 $\mu$s to a peak load current of 460 kA. Measurements are presented from two division of amplitude polarimeters which operate at 532 nm and 1064 nm, a complementary pyrometer which measures the spectral radiance ratio at those wavelengths, a long-range 660 nm photonic Doppler velocimeter, and a B-dot probe array from which the aforementioned intrinsic properties may be inferred. Available results are compared to a 3-D MHD ALEGRA simulation of the full dynamic load and return conductor geometry with a two-loop external coupled circuit. [Preview Abstract] |
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NP8.00063: Status of the proposed Helmholtz International Beamline for Extreme Fields (HIBEF) at the European XFEL Thomas Cowan HIBEF will establish multi-purpose high-power and ultra-intense lasers, as well as high-field magnets, at the HED end-station of the European XFEL. This will significantly extend the scope of research that can be carried out at the European XFEL in the areas of strong-field physics, high energy density science, relativistic laser-plasma physics, ultra high-pressure planetary and astrophysics, dynamic materials research, and magnetic phenomena in condensed matter. The proposed laser systems include: an ultra-intense PW-class Ti:Sapphire laser operating with full energy at 1 Hz, and at 200 TW at 10 Hz; high-energy few ns-pulse (shaped) lasers operating at 100 J with Hz rep-rate, upgradable to kJ-class; and a 1.5 MJ pulse generator to drive pulsed high-field magnets (50 T, $\sim$ 1ms) for condensed matter and magnetized HED-plasma research. HIBEF is an international, multi-institution initiative, with primary funding applied for from the German Helmholtz Association, within the scope of an European XFEL User Consortium. Present status and future plans will be presented. [see, www.hzdr.de/hgfbeamline] [Preview Abstract] |
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NP8.00064: Physical effects on visible temperature measurements of shocked foams J. Benage, K. Falk, C. Fryer, C. Greef, J. Williams, D. Schmidt, C. McCoy, T. Boehly We have conducted a series of experiments measuring the temperature of shock compressed foams at pressures from $\sim$ 50 to \textgreater~300 GPA. These experiments were carried out at the OMEGA laser facility and utilized the streaked optical pyrometer (SOP) to measure the optical emission from the shocked foam targets and infer the temperature. A comparison of our results to both standard equation of state (EOS) tables and to quantum molecular dynamics (QMD) simulations result in temperatures that are below these theoretical predictions, especially at the higher pressures. This indicates either an error in these models or a limitation with this type of measurement technique. In order to estimate whether the emission from the shock front is consistent with the bulk temperature of the shocked material, a series of high-resolution hydrodynamic simulations were performed. We find these simulations predict emission more consistent with the measurements at high pressures, similar to previous results for shocked Silicon [1]. The effect of specific physics issues at the shock front, including temperature relaxation, electron thermal conduction, and radiation transport are evaluated to determine the magnitude of their affect on the emission. Both experimental and simulations results will be presented. \\[4pt] [1] A. Ng, P. Celliers, G. Xu, and A. Forsman, Phys. Rev. E., \textbf{52}, 4299 (1995). [Preview Abstract] |
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NP8.00065: Enhanced ion heating and ion dynamics in short-pulse laser-driven buried-layer targets L.G. Huang, M. Bussmann, T. Kluge, W. Yu, T.E. Cowan We analyze the dynamics of ion heating in ultra-intense laser-driven buried layer targets. By using layers of significantly different electron density, heating from the bulk return currents can produce extreme pressure gradients, with corresponding expansion/compression inside the solid target. Rapid collisional coupling efficiently heats the ions in the compressed regions, leading to extreme temperatures at about solid density. Particle-in-Cell simulations will be presented which all a study of the precise ion heating dynamics. The prospect of directly probing the buried layer dynamics with coherent diffraction techniques at hard x-ray FELs will be discussed. [Preview Abstract] |
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NP8.00066: X-RAY LASER SOURCES |
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NP8.00067: ABSTRACT WITHDRAWN |
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NP8.00068: The Influence of Nonlinear Optical Processes on Intense X-ray Laser Driven Ionization Dynamics Kenneth Whitney, Tzvetelina Petrova, Jack Davis The non-equilibrium dynamics in which hole states are created and destroyed when an incident coherent, high intensity, x-ray laser pulse impacts planar gold targets is modeled. Two aspects of the modeling are investigated. On the one hand, sufficiently low laser intensities lead to ionization rates that depend linearly on laser intensity and ionization levels are limited by the strength of ionization cross sections, the x-ray pulse duration, and the pulse intensity. On the other hand, at sufficiently high laser intensities, ionization rates with a quadratic and higher dependence on laser intensity begin to compete with linear rates. Where this transition is expected to occur is investigated along with its effect on ion populations. [Preview Abstract] |
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NP8.00069: Development of a shortpulse laser-driven 15.7 keV x-ray probe for bent-crystal imaging and spectroscopy M. Schollmeier, M. Geissel, P.K. Rambo, J. Schwarz, A.B. Sefkow, M. Vargas, J.L. Porter High energy x-rays above 10 keV are needed to probe HEDP experiments with dense, high-Z samples. Shortpulse lasers were shown to be more efficient to generate above-10 keV x-rays than ns lasers. We have used Sandia's Z-Petawatt laser to drive a 15.7 keV, Zr K-alpha x-ray source. A set of bent-crystal spectrometers and imagers was characterized for their throughput and spectral or spatial resolution. Ray-tracing with a newly developed, GPU-accelerated Monte-Carlo code has been done to evaluate the measurements. Estimates of the system performance at the kJ level have been made to evaluate its potential application for bent-crystal backlighting or x-ray Thomson scattering at Sandia's Z-machine. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
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NP8.00070: Energetics Measurements of Silver Halfraum Targets at the National Ignition Facility M.J. May, K.B. Fournier, C.G. Brown, W.H. Dunlop, J.O. Kane, P.B. Mirkarimi, R. Patterson, M. Schneider, K. Widmann, R. Guyton, E. Giraldez The energetics of silver halfraum targets are presented from laser plasma experiments at the National Ignition Facility (NIF). Four beams from the NIF laser were used to heat the halfraum targets with $\sim$ 10 kJ of energy in a 1 ns square laser pulse. The silver halfraum targets were spheres 2 mm in diameter with an 800 $\mu$m laser entrance hole (LEH). Targets with different sphere wall thicknesses (8 to 16 $\mu$m) were characterized. The energetics and the laser coupling to the targets were determined to be 0.92 by using the NIF X-ray (Dante) and optical backscatter diagnostics (NBI and FABS). The energy losses from the targets were through X-ray radiation and backscatter from laser plasma instabilities (SRS and SBS) from the LEH. As expected the different wall thickness had different levels of burn through emission. The thickest walled target ($\sim$ 15.9 $\mu$m) had very low radiative losses through the target wall. The thinnest walled targets ($\sim$ 8 $\mu$m) radiated about 0.2 of the input energy into the X-ray region. This work was done under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
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NP8.00071: Investigation of a laser-produced bismuth plasma soft X-ray source Takamitsu Otsuka, Paddy Hayden, Fergal O'Reilly, Emma Sokell, Padraig Dunne, Gerry O'Sullivan, Noboru Yugami The emission spectra of many high-$Z$ plasmas, with $Z$ close to or greater than 50, are dominated in the soft X-ray region by a bright spectral feature known as an Unresolved Transition Array (UTA). This feature is attributed to hundreds of thousands of near-degenerate resonance lines from $4d-4f$ and $4p-4d$ type transitions. According to previous work, it was shown that the UTA peak wavelength depends on atomic number and will therefore extend down towards the water window region for high atomic numbers. In our previous paper, emission spectra from Bi plasma were observed to have a UTA peak around 4 nm. Calculated results also showed that there is strong UTA emission at 3.3 nm for plasma temperatures higher than 900 eV. However, this UTA emission has yet to be observed. Possible reasons for not observing this feature are the optical thickness of the Bi plasma itself and an inability to reach a sufficiently high plasma temperature. In order to obtain information on the optical thickness, absorption spectroscopy was carried out by way of the dual laser plasma photoabsorption technique. Two Nd:YAG lasers were focused on planar W and Bi targets. Emission from the W plasma was used to backlight Bi plasmas. Both emission and absorption spectra will be shown in the presentation. [Preview Abstract] |
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NP8.00072: Simulations for the Elucidation of Electron Beam Properties in Laser-Wakefield Acceleration Experiments via Betatron and Synchrotron-Like Radiation Paul Cummings, Alec Thomas A promising application of laser-wakefield acceleration (LWFA) technology is as a tunable source of x-ray and gamma radiation via synchrotron radiation. Such a source could have many potential applications. Consequently, the generation of synchrotron radiation in LWFA experiments is investigated computationally using the particle-in-cell simulation code OSIRIS 2.0. In LWFA systems, electrons are accelerated by the wake structure produced by a propagating laser pulse up to the dephasing length. The wake fields not only accelerate, but axially focus the beam; consequently the electron beam naturally undergoes betatron motion while being accelerated. Once the beam passes the dephasing length, it slows and re-enters the plasma, undergoing a hosing instability. This dramatically increases the magnitude of the beam's betatron oscillations. Consequently, radiation emission in LWFA experiments can be controlled by varying the propagation distance of the laser pulse. The particle-in-cell code OSIRIS 2.0 was modified with a novel model for explicitly simulating synchrotron radiation to investigate this phenomena computationally. A parameter sweep was performed, varying the propagation distance of the simulation. The results of this parameter sweep are presented and discussed. [Preview Abstract] |
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NP8.00073: Production of K-$\alpha$ Emission X-Rays from Laser Solid Interactions for use in Phase Contrast Imaging Thomas Batson, Zhen Zhao, Bixue Hou, John Nees, A.G.R. Thomas, Karl Krushelnick Phase contrast X-ray imaging represents significant potential for radiographical medical applications. The relativistic lamba cubed laser at the University of Michigan produces 30 fs laser pulses at a peak intensity of $5\times10^{18} W/cm^{2}$. When focused onto solid targets, the resulting plasma formation produces small source size and monochromatic K-$\alpha$ X-rays well suited to the purpose of imaging, since the spatial coherence of the X-ray source is inversely dependent on source size. In addition, the laser's repetition rate of 500 Hz allows long time scale production of the X-rays. Currently work is underway to accurately measure the X-ray source size, which will precisely determine the spatial coherence of the X-ray pulses. Radiographic images will then be taken with a range of coated and uncoated metal laser targets and studied for phase contrast characteristics. [Preview Abstract] |
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NP8.00074: Investigating the influence of overdense plasma surfaces in high harmonic generation from high-intensity laser irradiation Anthony Raymond, Calvin Zulick, Paul Cummings, Franklin Dollar, Vladimir Chvykov, Louise Willingale, Victor Yanovsky, Anatoly Maksimchuk, Alexander Thomas, Karl Krushelnick In recent experimental campaigns and computational surveys, high harmonic generation (HHG) has found applications as a diagnostic tool, revealing information regarding pre-plasma scale-length and in extension laser contrast [1], in addition to many applications such as a direct means by which to produce trains of attosecond pulses [2]: via filtering lower-ordered multiples of the fundamental frequency. Additional flexibility and utility may be derived by pre-shaping the target-area of the material undergoing irradiation on the micron scale, as the results of 2D PIC simulations carried out at the University of Michigan's High Field Science group imply. Specifically, micron-scale parabolic and spherically concave target geometries are investigated in regard to their ability to collimate and further refocus the reflected harmonic beam, respectively. Additionally, results are summarized from experimental investigations carried out at the same research facility with ultrafast, ultra-relativistic, and high-contrast pulses regarding the effect of the target's pre-plasma scale-length on the efficacy of the resultant reflected beam's harmonic content. [1] F. Dollar, et al., Phys. Rev. Lett. 110, 175002 (2013) [2] George D Tsakiris, et al., New Journal of Physics 8 (2006) 19 [Preview Abstract] |
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NP8.00075: Spectroscopic Analysis of High Intensity Laser Beam Jets Interaction Experiments on the Leopard Laser at UNR E.E. Petkov, M.E. Weller, V.L. Kantsyrev, A.S. Safronova, J.J. Moschella, I. Shrestha, V.V. Shlyapsteva, A. Stafford, S.F. Keim Results of Ar gas-puff experiments performed on the high power Leopard laser at UNR are presented. Flux density of laser radiation in focal spot was up to 2x10$^{16}$ W/cm$^{2}$ (pulse duration was 0.8 ns and laser wavelength was 1.057 $\mu m)$. Specifically, spectroscopic analysis of K-shell Ar spectra are investigated and compared as functions of the orientation of the laser beam to linear gas jet. The laser beam axis was positioned either along the jet plane or orthogonal to it at a distance of 1 mm from the nozzle output. The diagnostics used included a time-integrated x-ray spectrometer along with a set of filtered Si diodes with various cutoff energies. In order to identify lines, a non-local thermodynamic equilibrium (non-LTE) kinetic model was utilized and was also used to determine plasma parameters such as electron temperature and density. The importance of the spectroscopic study of high intensity laser beam-jets interaction experiments is discussed. [Preview Abstract] |
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NP8.00076: Efficient X-ray generation from volumetrically heated aligned nanowire arrays R.C. Hollinger, C. Bargsten, M. Purvis, V.N. Shlyaptsev, A. Pukhov, A. Townsend, D. Keiss, A.L. Prieto, Y. Wang, S. Wang, L. Yin, B. Luther, M. Woolston, J.J. Rocca The trapping of femtosecond laser pulses of relativistic intensity deep within ordered nanowire arrays can volumetrically heat dense matter into an ultra-hot, near-solid density plasma in which the increased hydrodynamic-to-radiative lifetime ratio results in very efficient X-ray generation. Using high contrast (\textgreater~1x10$^{11})$ pulses of 60fs FWHM duration from a frequency doubled ($\lambda =$ 400 nm) high power Ti:Sa laser, arrays of 55nm and 80nm targets with 12{\%} of solid density were irradiated with pulses of 5x10$^{18}$ Wcm$^{-2}$. The Ni nanowire target produced strong He-like line emission that surpasses the K$\alpha $ emission. Conversion efficiency of \textgreater~5{\%} into photons with energy \textgreater~0.9 KeV in a hemisphere was measured. The Au nanowire spectrum displays strong Au M-shell emission with unresolved 4-3 lines from ions ranging from Co-like (Au$^{+52})$ to Ga-like Au (Au$^{+48})$. Filtered photodiode measurements show a $\sim$ 100X emission increase respect to smooth solid targets for photon energies \textgreater~9 keV. [Preview Abstract] |
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NP8.00077: INERTIAL CONFINEMENT |
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NP8.00078: The Nike Laser Facility and its Capabilities V. Serlin, Y. Aglitskiy, L.Y. Chan, M. Karasik, D.M. Kehne, J. Oh, S.P. Obenschain, J.L. Weaver The Nike laser is a 56-beam krypton fluoride (KrF) system that provides 3 to 4~kJ of laser energy on target. The laser uses induced spatial incoherence to achieve highly uniform focal distributions. 44 beams are overlapped onto target with peak intensities up to $10^{16}$~W/cm$^2$. The effective time-averaged illumination nonuniformity is $< 0.2$\%. Nike produces highly uniform ablation pressures on target allowing well-controlled experiments at pressures up to 20~Mbar. The other 12 laser beams are used to generate diagnostic x-rays radiographing the primary laser-illuminated target. The facility includes a front end that generates the desired temporal and spatial laser profiles, two electron-beam pumped KrF amplifiers, a computer-controlled optical system, and a vacuum target chamber for experiments. Nike is used to study the physics and technology issues of direct-drive laser fusion, such as, hydrodynamic and laser-plasma instabilities, studies of the response of materials to extreme pressures, and generation of X rays from laser-heated targets. Nike features a computer-controlled data acquisition system, high-speed, high-resolution x-ray and visible imaging systems, x-ray and visible spectrometers, and cryogenic target capability. Work supported by DOE/NNSA. [Preview Abstract] |
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NP8.00079: Optimal Path to a Laser Fusion Energy Power Plant Stephen Bodner There was a decision in the mid 1990s to attempt ignition using indirect-drive targets. It is now obvious that this decision was unjustified. The target design was too geometrically complex, too inefficient, and too far above plasma instability thresholds. By that same time, the mid 1990s, there had also been major advances in the direct-drive target concept. It also was not yet ready for a major test. Now, finally, because of significant advances in target designs, laser-target experiments, and laser development, the direct-drive fusion concept is ready for significant enhancements in funding, on the path to commercial fusion energy. There are two laser contenders. A KrF laser is attractive because of its shortest wavelength, broad bandwidth, and superb beam uniformity. A frequency-converted DPSSL has the disadvantage of inherently narrow bandwidth and longer wavelength, but by combining many beams in parallel one might be able to produce at the target the equivalent of an ultra-broad bandwidth. One or both of these lasers may also meet all of the engineering and economic requirements for a reactor. It is time to further develop and evaluate these two lasers as rep-rate systems, in preparation for a future high-gain fusion test. [Preview Abstract] |
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NP8.00080: Advantages of a KrF laser driver to achieve robust fusion ignition and high yield Stephen Obenschain, Jason Bates, Max Karasik, David Kehne, Andrew Schmitt, Victor Serlin, John Sethian, James Weaver, Frank Hegeler, Jaechul Oh, Yefim Aglitskiy The krypton-fluoride (KrF) laser has substantial target physics and technological advantages towards achieving robust direct-drive implosions that ignite and provide high gain. The potential physics advantages arise from its shorter wavelength (248nm), capability for more uniform target illumination, and broader bandwidth than existing frequency tripled glass lasers. These features can increase target performance and reduce the risk from both hydrodynamic and laser plasma instabilities. KrF's ISI beam smoothing technology allows easy implementation of focal diameter zooming that increases absorption efficiency and reduces risk from cross beam energy transfer. We will outline the current understanding of laser-target interactions with the KrF laser and present a phased path to a high-shot-rate high-performance inertial fusion facility that employs the KrF laser. [Preview Abstract] |
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NP8.00081: StarDriver: a Novel, Flexible, Solid-State Laser-Based Architecture for Inertial Confinement Fusion (ICF) D. Eimerl, E.M. Campbell, J. Zweiback, W.F. Krupke, J. Zuegel, J. Myatt, J. Kelly, D. Froula, R.L. McCrory, W.L. Kruer We introduce StarDriver, a novel, flexible, solid-state laser-based architecture for ICF, HED science and possible future energy production. StarDriver is a new approach to ICF that minimizes laser-plasma instabilities and improves laser-plasma coupling by the use of a multi-beam laser architecture with system bandwidth equal or greater than the fastest growing instability.In contrast to traditional and proposed architectures, the laser driver is constructed from 10$^{4}$-10$^{5}$ individual lasers,each delivering nominally 100J in several ns at a nominal wavelength of $\sim$ 355 nm with better than 3-5 diffraction-limited performance. The beamlets are individually narrowband but the ensemble of beamlets spans a wide frequency range.Currently available laser media enable system bandwidth $\sim$ 2{\%} at 355nm with the possibility of system bandwidths approaching 10{\%}. The many beamlets and large bandwidth of StarDriver provide optimal asymptotic smoothing for hydrodynamic instabilities (0-1{\%}), with smoothing times $\sim$ 30fs. The distribution of frequencies among the beamlets allows flexibility for fine control of the seeding of the Rayleigh-Taylor instability. The ultra-broad bandwidth is greater than the Doppler widths and natural linewidths of plasma excitations in the expanding corona and thus has the potential to suppress or eliminate the most problematic laser-plasma instabilities. [Preview Abstract] |
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NP8.00082: Modeling of Kinetically Enhanced Mixing of High Z Material into Fuel via Double Layer Electric-Field William Taitano, Dana Knoll, Luis Chacon, Anil Prinja The national ignition campaign has come to an end without achieving ignition of the fuel. An experimental observation that has been consistently made is a low fuel temperature. The low fuel temperature may be attributed to the radiative heat-loss due to kinetically enhanced high Z impurities mixing. In omega capsule experiments, a strong electric-field, consistent with charge-separation field has been detected [1]. It has been suggested that the source of this field is a strong shock propagation, but a strong ionization gradient at the interface could also be a source [2]. Such a strong field located at the fuel-pusher interface may accelerate higher Z materials into the fuel region, causing mix. To study the kinetically enhanced mixing phenomena, we develop a fully implicit, Vlasov-Fokker-Planck solver. We evolve the self-consistent, charge-separation field from the Ampere equation. We include two kinetic ion species (one for DT fuel and one for capsule material) and kinetic electron species in our model. We present results which supports the existence of the strong field at the interface and enhanced mixing of pusher material into fuel.\\[4pt] [1] C.K. Li et al. PRL, 100, 225001 (2008). [Preview Abstract] |
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NP8.00083: Non-Maxwellian Effects for ICF Seth Davidovits, Nathaniel Fisch While in collisional plasma the bulk of the distribution function is driven toward Maxwellian in a few collision times, the high velocity tails can take much longer to form. The fast ions in these tails have much larger fusion cross sections than thermal ions, and contribute substantially to fusion production. We investigate the possibilities for enhancement or depletion of these tails in regimes applicable to ICF capsule implosions, and the corresponding effects on fusion reactivity. There are a number of possible scenarios that might yield such non-Maxwellian tails, including, for example, hydrodynamic flows or Knudsen layer effects. [Preview Abstract] |
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NP8.00084: Self-Generated Magnetic Field Effect Simulations of a National Ignition Facility Capsule Joseph Koning Self-generated magnetic fields are simulated in 2D for the National Ignition Facility tritium-hydrogen-deuterium (THD) capsule design using the multiphysics code HYDRA. In HYDRA's MHD package the magnetic field is generated through currents driven by the electron pressure gradient initiated through the Rayleigh-Taylor instability and evolved due to magnetic diffusion, and advection. Magnetic field effects in the simulation include anisotropic thermal electron and ion conduction as well as effects on the alpha particles in the burn phase. Transport coefficients are calculated using the Epperlein-Haines coefficients with Lee-More degeneracy corrections. We compare results of simulations with perturbations on the CH ablator/DT ice layers and radiation source with and without magnetic fields. Initial simulations show maximum field magnitudes in excess of 70 MG and 5{\%} increase in yield for the capsule with perturbations on the CH ablator/DT ice layers and radiation source. [Preview Abstract] |
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NP8.00085: Simulation studies of plasma target compression by argon liners Lina Zhang, Hyoungkeun Kim, Roman Samulyak Simulation studies of plasma liners, formed by the merger of argon plasma jets, and the compression of plasma targets in the concept of the plasma jet driven magnetoinertial fusion have been performed using FronTier code. FromTier is a hybrid Lagrangian-Eulerian code that uses explicit tracking of material interfaces, thus enabling accurate resolution of hydro instabilities, and average ionization EOS models for high-Z materials. The jets merger process is accomplished through a cascade of oblique shock waves leading to the non-uniformity of imploding plasma liner and causing the Reyleigh-Taylor instability of target during compression. The stagnation pressure, deconfinement time, Rayleigh-Taylor instabilities of the target surface, and the production of fusion neutrons were analyzed for 2D simulations that included 8, 16, and 32 jets, 3D simulation with 90 jets, and compared with the corresponding cylindrically (2D) and spherically (3D) symmetric simulations. The liner non-uniformity induces instabilities in the plasma targets that result in the reduction of stagnation pressure and fusion energy. For example, 8 time reduction of the stagnation pressure and 31 time reduction of the fusion energy was observed when the 2D simulation involving 16 jets was compared to 1D simulation. [Preview Abstract] |
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NP8.00086: Magnetized Plasma Compression for Fusion Energy James Degnan, Christopher Grabowski, Matthew Domonkos, David Amdahl Magnetized Plasma Compression (MPC) uses magnetic inhibition of thermal conduction and enhancement of charge particle product capture to greatly reduce the temporal and spatial compression required relative to un-magnetized inertial fusion (IFE) - to microseconds, centimeters vs nanoseconds, sub-millimeter. MPC greatly reduces the required confinement time relative to MFE - to microseconds vs minutes. Proof of principle can be demonstrated or refuted using high current pulsed power driven compression of magnetized plasmas using magnetic pressure driven implosions of metal shells, known as imploding liners. This can be done at a cost of a few tens of millions of dollars. If demonstrated, it becomes worthwhile to develop repetitive implosion drivers. One approach is to use arrays of heavy ion beams for energy production, though with much less temporal and spatial compression than that envisioned for un-magnetized IFE, with larger compression targets, and with much less ambitious compression ratios. A less expensive, repetitive pulsed power driver, if feasible, would require engineering development for transient, rapidly replaceable transmission lines such as envisioned by Sandia National Laboratories. [Preview Abstract] |
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NP8.00087: Optimizing Field-Reversed Configuration Plasmas for Plasma Compression Experiments C. Grabowski, J.H. Degnan, D.J. Amdahl, M. Domonkos, E.L. Ruden, W. White, G.A. Wurden, M.H. Frese, S.D. Frese, J.F. Camacho, S.K. Coffey, M. Kostora, J. McCullough, W. Sommars, G.F. Kiuttu, A.G. Lynn, K. Yates, B.S. Bauer, S. Fuelling, R. Pahl The Field-Reversed Configuration Heating Experiment (FRCHX) is a collaborative experiment between the Air Force Research Laboratory (AFRL) and Los Alamos National Laboratory (LANL) to study high energy density plasmas and various associated phenomena. With FRCHX, a field-reversed configuration (FRC) plasma is formed via reversed-field theta pinch and then translated a short distance into a cylindrical aluminum shell (solid liner), where it is either compressed by the magnetically-driven implosion of the shell or diagnosed in preparation for such compression tests. The lifetime of the trapped magnetic flux within the FRC is an important parameter affecting the confinement of plasma during the compression and ultimately the final density, temperature, and yield of neutrons from the plasma. Processes occurring during formation, initial plasma temperature, and instabilities in turn all affect the trapped-flux lifetime and the integrity of the FRC. A discussion of FRC parameters measured on FRCHX and efforts that have been made to improve these parameters and the FRC stability will be presented in connection with results from recent FRCHX experiments. This work is supported by DOE-OFES. [Preview Abstract] |
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NP8.00088: Star Mode -- The improved operating regime of a Fusor Matthew Lilley, Nicolas Niasse It was found in 1997, by G. H. Miley, that Fusors can operate in a regime where the effective transparency of the accelerating grid is greatly enhanced over the value one would traditionally expect from considering the fraction of area taken up by the grid wires. This ``Star Mode'' reduces the heating of the grid wires and so should in principle allow smaller devices to be constructed. At present there is no satisfactory explanation for this Star Mode. In this presentation we revisit some of the basic ideas and offer some new insights into the problem by considering how the discrete symmetry of the system affects the stability of the individual particle orbits. [Preview Abstract] |
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NP8.00089: Post shot analysis of plasma conditions of Gold Spheres illuminated by the URLLE Omega laser, as measured via Thomson scattering M.D. Rosen, J.S. Ross, H.A. Scott, N. Landen, D. Froula, E. Dewald, M. May, K. Widmann Recently there was a follow up to the 2006 campaign to illuminate 1 mm diameter gold spheres using the Omega laser at LLE. The 2013 campaign uses Thomson scattering to diagnose the plasma conditions as a function of time, at various radial positions in the coronal, laser heated, blow-off region. Laser irradiances were 1, 5, and 10 x 10$^{14}$ W/sqcm, usually in a 1 ns pulse duration. Depleted uranium and Ag spheres were also tested. We compare the predictions of plasma conditions using various non-LTE computational models of atomic physics and electron transport (as implemented into the rad-hydro code Lasnex) to this data. The ``high flux model (HFM)'' (DCA atomic physics and non local transport) compares well for some of experiments, while an intermediate model that radiates a bit less total x-ray fluence than the HFM, does better on other experiments. [Preview Abstract] |
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NP8.00090: Improved inline model for Raman backscatter in HYDRA M.M. Marinak, G.D. Kerbel, P. Michel, D.J. Strozzi, S.M. Sepke Backscatter of laser light due to laser plasma instabilities has a pronounced impact on the flow of energy in a National Ignition Facility (NIF) hohlraum. We have implemented a new inline model for stimulated Raman scattering (SRS) in HYDRA. The model integrates the coupled-mode equations for SRS gain and inverse bremsstrahlung absorption along the rays, including pump depletion and energetic electron production by Langmuir waves. This enables more precise resolution of the spatial variation of the backscattered intensity along the beam path. The SRS escaping power and wavelength are constrained to match the experimentally measured history for each beam cone. Run in conjunction with the inline model for energy transfer between crossed laser beams, the new model enables more accurate treatment of energy and momentum deposition in hohlraum simulations. We examine the impact of the model on an integrated HYDRA simulation of a NIF hohlraum. [Preview Abstract] |
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NP8.00091: Characterization of laser-produced miniature hohlraum XUV sources Andrew McKelvey, Thomas Batson, Calvin Zulick, Franklin Dollar, John Nees, Bixue Hou, Anatoly Maksimchuk, Victor Yanovsky, Vladmir Chvykov, Alexander Thomas, Karl Krushelnick Experiments at the National Ignition Facility (NIF) allow the radiative properties of dense, high-temperature matter to be studied at previously unreachable regimes, but are limited by cost and system availability.~ A scaled down system using ultra-short laser pulses and delivering energy to a much smaller hohlraum could be capable of reaching comparable energy densities by~depositing the energy in a significantly smaller volume before ablation of the wall material closes the cavity.~ The laser is tightly focused through the cavity and then expands to illuminate the wall with an intensity closer to that of a long pulse laser. Experiments were performed on a number of Ti:sapphire tabletop laser system all with short pulses.~ Cavities are machined in target material using either low laser powers, and then shot in situ with a single full power pulse or using nano-fabricated targets.~ The emitted radiation is analyzed with an XUV spectrometer.~ This method may allow studies such as opacity measurements using plasma and radiation with the temperatures comparable to NIF type hohlraums, but with a significantly higher repetition rate and in a university scale system. [Preview Abstract] |
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NP8.00092: The simulations of indirect-drive targets for ignition on megajoule lasers. Vladimir Lykov, Eugene Andreev, Ludmila Ardasheva, Michael Avramenko, Valerian Chernyakov, Maxim Chizhkov, Nikalai Karlykhanov, Michael Kozmanov, Serge Lebedev, George Rykovanov, Vladimir Seleznev, Lev Sokolov, Margaret Timakova, Alexander Shestakov, Aleksander Shushlebin The calculations were performed with use of radiation hydrodynamic codes developed in RFNC-VNIITF [1]. The analysis of published calculations of indirect-drive targets to obtain ignition on NIF and LMJ lasers has shown that these targets have very low margins for ignition: according to 1D- ERA code calculations it could not be ignited under decreasing of thermonuclear reaction rate less than in 2 times.The purpose of new calculations is search of indirect-drive targets with the raised margins for ignition. The calculations of compression and thermonuclear burning of targets are carried out for conditions of X-ray flux asymmetry obtained in simulations of Rugby hohlraum that were performed with 2D- SINARA code [2]. The requirements to accuracy of manufacturing and irradiation symmetry of targets were studied with use of 2D-TIGR-OMEGA-3T code. The necessity of performed researches is caused by the construction of magajoule laser in Russia [3].\\[4pt] [1] A.V. Andriyash et al, Physics-Uspekhi, 49, 1084 (2006) \\[0pt] [2] M.N. Chizhkov, et al, J. Phys.: Conf. Ser., 244, 022073 (2010)\\[0pt] [3] S.G. Garanin, Physics-Uspekhi, 181, 434 (2011) [Preview Abstract] |
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NP8.00093: Tail-ion transport and Knudsen layer formation in the presence of magnetic fields Paul Schmit, Kim Molvig The impact of magnetic fields on Knudsen layer formation [1] in ICF-relevant plasma is investigated for the first time. Magnetic fields change the energy scaling of the ion diffusivity in a way that eliminates the preferential losses of fast ions compared to thermal ions. Simple threshold criteria give conditions such that the restoration of the ion tail distribution is sufficient to recover much of the lost fusion reactivity. The tail-ion kinetic equations are solved for hot fuel bounded by a cold, nonreacting wall using a numerical stochastic differential equation solver, and the modified fusion reactivities are calculated. We find that modest magnetic fields too weak to magnetize thermal ions are still sufficient to restore much of the lost reactivity, consistent with the threshold conditions. We also find that the Maxwell-averaged fusion reactivities are recovered more fully in cylindrical targets compared to spherical targets. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.\\[4pt] [1] K. Molvig et al., PRL 109, 095001 (2012). [Preview Abstract] |
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NP8.00094: ICF Implosions, Space-Charge Electric Fields, and Their Impact on Mix and Compression Dana Knoll, Luis Chacon, Andrei Simakov The single-fluid, quasi-neutral, radiation hydrodynamics codes, used to design the NIF targets, predict thermonuclear ignition for the conditions that have been achieved experimentally. A logical conclusion is that the physics model used in these codes is missing one, or more, key phenomena. Two key model-experiment inconsistencies on NIF are: 1) a lower implosion velocity than predicted by the design codes, and 2) transport of pusher material deep into the hot spot. We hypothesize that both of these model-experiment inconsistencies may be a result of a large, space-charge, electric field residing on the distinct interfaces in a NIF target. Large space-charge fields have been experimentally observed in Omega experiments. Given our hypothesis, this presentation will: 1) Develop a more complete physics picture of initiation, sustainment, and dissipation of a current-driven plasma sheath / double-layer at the Fuel-Pusher interface of an ablating plastic shell implosion on Omega, 2) Characterize the mix that can result from a double-layer field at the Fuel-Pusher interface, prior to the onset of fluid instabilities, and 3) Quantify the impact of the double-layer induced surface tension at the Fuel-Pusher interface on the peak observed implosion velocity in Omega. [Preview Abstract] |
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NP8.00095: Modeling the effect of energetic electrons on laser driven implosions Wallace Manheimer, Denis Colombant, Andrew J. Schmitt We have developed a simple Krook model to evaluate the effect of energetic electrons, produced for instance by the two plasmon decay instability at the quarter critical surface. Our previous theory [1] has been extended to include spherical geometry. The method is simple to implement and easy to use in a fluid simulation. We follow these electrons as they travel into the target. The data on energetic electron production [2] by a 1/3 micron laser has been used. Furthermore, from this data, we can infer the results for a 1/4 micron laser. In this way one compares the prediction of the effect of target gain for these two lasers (with and without accounting for the energetic electrons), taking account of reasonable models of their production and transport.\\[4pt] [1] W. Manheimer, Laser and Particle Beams \textbf{31} 95, 2013.\\[0pt] [2] B. Yaakobi et al, Phys. Plasmas \textbf{19} 012704, 2012. [Preview Abstract] |
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NP8.00096: Nonlocal electron transport: direct and Greens function solution and comparison of our model with the SNB model Denis Colombant, Wallace Manheimer, Andrew J. Schmitt At least two models, ours [1] and SNB (Schurtz-Nicolai-Busquet) [2], and two methods of solution, direct numerical solution (DS) and Greens function (GF) are being used in multi-dimensional radiation hydrodynamics codes. We present results of a laser target implosion using both methods of solution. Although our model and SNB differ in some physical content, direct comparisons have been non-existent up to now. However a paper by Marocchino \textit{et al.} [3] has recently presented the results of two nanosecond-time-scale test problems, showing that the preheat calculated by the two models are different by about three orders of magnitude. We have rerun these problems and we find much less difference between the two than they do. One can show analytically that the results should be quite similar and are about an order of magnitude less than the maximum, and two orders of magnitude more than the minimum preheating in [3]. We have been able to trace the somewhat different results back to the different physical assumptions made in each model.\\[4pt] [1] W. Manheimer, D. Colombant, and A.J. Schmitt, Phys Plasmas \textbf{19}, 056317 (2012).\\[0pt] [2] G. Schurtz, P. Nicolai, and M. Busquet, ibid, \textbf{7}, 4238 (2000).\\[0pt] [3] A. Marocchino \textit{et al.}, ibid. \textbf{20}, 022702 (2013). [Preview Abstract] |
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NP8.00097: Large amplitude electric fields in inertial confinement fusion capsules Robert Bingham, R.A. Cairns, P.A. Norreys, R.M.G.M. Trines Experiments on the interaction of high power lasers with inertial fusion capsules have shown evidence of shock like structures with very high electric fields existing over very short distances [1]. Data from proton radiography in inertial confinement fusion capsules suggest the existence of fields of more than 10 GV/m over distances of the order of 10-100 nm [2]. It has been suggested that barodiffusion (i.e. pressure-driven diffusion) may be a possible explanation, but this does not seem to produce very short length scales. Here we show that a collisionless shock structure can be produced by having a finite ion temperature so that some ions are reflected by the potential maximum at the shock. This produces the asymmetry between the upstream and downstream sides which destroys the familiar symmetrical ion sound solitary wave. The consequences for inertial fusion will be discussed.\\[4pt] [1] J.R. Rygg et al, Science 319, 1223 (2008)\\[0pt] [2] P Amendt et al. Phys. Plasmas 18, 056308 (2011). [Preview Abstract] |
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NP8.00098: Raytracing and Direct-Drive Targets Andrew J. Schmitt, Jason Bates, David Fyfe, David Eimerl Accurate simulation of the effects of laser imprinting and drive asymmetries in directly driven targets requires the ability to distinguish between raytrace noise and the intensity structure produced by the spatial and temporal incoherence of optical smoothing. We have developed and implemented a smoother raytrace algorithm for our mpi-parallel radiation hydrodynamics code, \textbf{\textit{FAST3D}}. The underlying approach is to connect the rays into either sheets (in 2D) or volume-enclosing chunks (in 3D) so that the absorbed energy distribution continuously covers the propagation area illuminated by the laser. We will describe the status and show the different scalings encountered in 2D and 3D problems as the computational size, parallelization strategy, and number of rays is varied. Finally, we show results using the method in current NIKE experimental target simulations and in proposed symmetric and polar direct-drive target designs. [Preview Abstract] |
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NP8.00099: Influence of chronometry on hydrodynamic stability: design of Direct-Drive experiments Stephane Laffite, Benoit Canaud, Laurent Masse, Olivier Larroche, Frederic Girard, Veronique Tassin, Frank Philippe, Olivier Landoas, Tony Caillaud We present here the 2D design of future Direct-Drive (DD) experiments which will be carried out in 2014 at the OMEGA facility. Hydrodynamic stability of capsule is a major concern for DD and Indirect-Drive (ID) implosions. Stability can be greatly affected by the chronometry of the drive. The objective of these experiments is to study the impact of chronometry on the stability of the target. Target will be filled with 15 bars of DT or DD-Argon. Diameter will be about 900 microns. Plastic shell thickness will be 25 microns. Target dimensions will be the same for all the shots. Pulse will be varied from a square pulse to 2-steps-pulse and 3-steps-pulses. Hydrodynamic stability decreases with the number of steps: convergence ration increases from Rc$=$14 to Rc$=$20 whereas adiabat decreases from 3.5 to 1.7. For some shots, low-mode asymmetries will be created by turning off some of the beams. [Preview Abstract] |
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NP8.00100: The National Ignition Facility (NIF) as a User Facility Christopher Keane The National Ignition Facility (NIF) has made significant progress towards operation as a user facility. Through June 2013, NIF conducted over 1200 experiments in support of ICF, HED science, and development of facility capabilities. The NIF laser has met or achieved all specifications and a wide variety of diagnostic and target fabrication capabilities are in place. A NIF User Group and associated Executive Board have been formed. Two User Group meetings have been conducted since formation of the User Group. NIF experiments in fundamental science have provided important new results. NIF ramp compression experiments have been conducted using diamond and iron, with EOS results obtained at pressures up to approximately 50 Mbar and 8 Mbar, respectively. Initial experiments in supernova hydrodynamics, the fundamental physics of the Rayleigh-Taylor instability, and equation of state in the Gbar pressure regime have also been conducted. This presentation will discuss the fundamental science program at NIF, including the proposal solicitation and scientific review processes and other aspects of user facility operation. *This work was performed under the auspices of the Lawrence Livermore National Security, LLC, (LLNS) under Contract No. DE-AC52-07NA27344. [Preview Abstract] |
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NP8.00101: Aluminum Ablator Determination of Shock Strength using a Two-Shock Drive S.G. Glendinning, K.L. Baker, P.M. Celliers, T.R. Dittrich, S.J. Felker, S.A. Maclaren, D. Martinez, H.S. Park, R.M. Seugling, V.A. Smalyuk, T.M. Guymer, S. Mcalpin, A.S. Moore We have designed and performed experiments on NIF using an aluminum ablator coupled to a spherical fused silica window to allow accurate measurements of shock breakout and shock velocity in the fused silica. Igniting capsules for intertial confinement fusion must be driven by a succession of shocks to maintain a low adiabat. However, uncertainties in laser-hohlraum coupling translate into uncertainties in shock timing, and investigation of such integrated problems is difficult. Thus, a simpler experiment using a smaller number of shocks (two in this case) and an ablator material of well-known opacity and equation of state (aluminum) might allow a more fundamental investigation of possible laser-plasma coupling issues. The drive was a double-peaked laser pulse 5.5 ns long using 280 kJ into a gold cylindrical hohlraum (vacuum interior) 3.3 mm radius X 10 mm length. This produced an x-ray drive of near-constant radiation temperature of 150 eV for 3.5 ns followed by a 1 ns wide peak at about 195 eV. The fused silica was shielded from gold M-band emission from the hohlraum using a thin Au layer. We will present simulations and experimental results and show a comparison with a four-shock drive result in an ignition hohlraum. [Preview Abstract] |
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NP8.00102: Prediction of double shock formation by exploding high gain ICF target in Xe gas filled chamber Ryan Sacks, Gregory Moses The spherical micro-explosion of a 132 MJ high gain indirect drive target radiating and expanding into a surrounding 6$\mu$g/cc Xe atmosphere is simulated in 1D using the BUCKY radiation hydrodynamics code with 121 group FAC Xe opacities and equations of state.\footnote{Private communications with Howard Scott, LLNL.} An interesting double shock is formed by the Marshak wave and exploding target debris. Explanation of this double shock formation is presented. This research is supported by Lawrence Livermore National Laboratory. [Preview Abstract] |
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NP8.00103: Long Duration Multi-hohlraum X-ray Sources for Eagle Nebula Laboratory Experiments Jave Kane, Robert Heeter, David Martinez, Alexis Casner, Bruno Villette, Roberto Mancini, Marc Pound A novel foam-filled multi-hohlraum long-duration x-ray source has been demonstrated at the Omega EP laser and used to obtain L-band spectra of photoionized Ti. A larger scale version of the source will be used in the Science on NIF Eagle Nebula experiments studying dynamic evolution of distinctive pillar and cometary structures in star-forming clouds, where the long duration and directionality of photoionizing radiation from nearby stars generates new classes of flows and instabilities. At NIF, a target representing an astrophysical molecular cloud will be placed several mm from an x-ray source lasting 40--100 ns. At EP, three hohlraums were illuminated in sequence with 3.3 kJ pulses lasting 6 ns, or 4.3 kJ pulses lasting 10 ns, generating 18 or 30 ns of x-ray output at 90-100 eV color temperature. Performance of the source was validated using the $\mu$ DMX and VSG spectrometers, ASBO VISAR, and x-ray pinhole imagery. The HYDRA code suggests the EP-scale source can also be shot at NIF with at least 10 kJ per hohlraum. The multi-hohlraum source concept has potential further application to hard x-ray sources, soft x-ray backlighters, and nonlinear ablative hydrodynamics. [Preview Abstract] |
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NP8.00104: Validation and Predictive Radiation-Hydrodynamic Simulations with FLASH: Shock-Generated Magnetic Field Experiments Using the Vulcan Laser Facility at RAL D.Q. Lamb, A. Scopatz, P. Tzeferacos, C. Daley, M. Fatenejad, N. Flocke, C. Graziani, D. Lee, K. Weide, H. Doyle, G. Gregori, J. Meinecke, B. Reville, F. Miniati A promising mechanism for producing seed B fields in the universe is the Biermann battery mechanism (BBM) at asymmetric shocks that occur when galaxies form. The University of Oxford and its collaborators have conducted experiments at the Vulcan Laser Facility at RAL that represent a scaled down version of this process. In these experiments, a 1 ns laser illuminates a carbon rod target in a chamber filled with Ar gas, producing an asymmetric spherical shock wave that generates B fields via the BBM. We report the results of FLASH radiation hydrodynamic simulations of these experiments. The simulations show that the result of the laser illuminating the target is a series of complex hydrodynamic phenomena. We calibrate the fraction of the laser energy that is deposited in the target by requiring that the simulations reproduce the measured shock position $r_s$ at various times for a range of laser energies. Within experimental error, the fraction is independent of laser energy for the range of energies we use to calibrate it. Within calibration error, the simulations are able to predict $r_s$ for considerably smaller and larger energies. [Preview Abstract] |
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NP8.00105: Pressure Ionization in Partition Function Algebra for Super-Configuration codes Michel Busquet The phrase ``Pressure Ionization'' stands for the progressive disappearance, delocalization or hybridization of bound orbitals of atoms immersed in plasmas when density increases. In the ion cell framework, Pressure Ionization is already partly included as orbitals disappear above some density when the one electron energies turns positive or equivalently when the average orbital radius becomes larger than the ion cell radius. However, this simple description yields a non-physical steep variation (with density) of the average \textless Z\textgreater\ when a whole electron shell vanishes at once. To overcome this problem, several authors proposed to introduce ``PIES'' (pressure ionized effective statistical weights) g*$=$ g x $\pi $* in order to obtain a progressive disappearance of the orbitals. On the other hand, super-shells and super-configurations and partition functions algebra have been introduced by A.Bar-Shalom et al.[1] for a statistical but detailed description of multi-electron, multi-ionized atoms. We present a method that merges pressure ionization and partition functions algebra. We also try to explain why both ionization potential lowering and reduction of statistical weights have to be introduced. A couple of results are presented.\\[4pt] [1] A.-Bar-Shalom, J.Oreg, W.H.Goldstein, D.Schvarts, A.Zigler, Phys. Rev. A 40, 3183 (1989) [Preview Abstract] |
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NP8.00106: ABSTRACT WITHDRAWN |
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NP8.00107: Average ionization EOS model for high-Z plasmas and applications Hyoungkeun Kim, Lina Zhang, Roman Samulyak, Paul Parks A numerical model for average ionization EOS for high-Z plasmas undergoing multiple ionization processes has been developed based on the Zeldovich model. The corresponding software library has been implemented in FronTier, a hydrodynamic code that explicitly tracks material interfaces via the front tracking method, and verified with respect to solutions of coupled systems of Saha equations. FronTier with this EOS model has been used for simulations of the formation and implosion of plasma liners in the concept of Plasma Jet driven Magneto-Inertial Fusion (PJMIF) and the pellet ablation in tokamaks. We showed that in PJMIF simulations, atomic processes are responsible for significant pressure increases in self-imploding argon liners [1] and the overall change of dynamics of the jet merger via a cascade of oblique shock waves [2]. In pellet ablation simulations, the ablation rate of neon and argon pellets decreased due to ionization-induced energy sinks, and the ablation flow reached a double transonic state similar to that observed in deuterium pellets. \\[4pt] [1] H. Kim et. al, Phys. Plasmas, 19:082711 (2012).\\[0pt] [2] H. Kim et al., Phys. Plasmas 20, 022704 (2013). [Preview Abstract] |
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NP8.00108: Measuring shell-$\rho $R perturbations in NIF capsule implosions near peak velocity B.A. Hammel, V.A. Smalyuk, T. Doeppner, S.W. Haan, T. Ma, L. Pickworth, H.A. Scott Quantitative measurements of shell-$\rho $R perturbations in capsules near peak implosion velocity (PV) are challenging. An external backlighter samples both sides of the shell, unless a re-entrant cone is used (potentially perturbing implosion). Emission from the hot core, after shock-stagnation and prior to PV, can act as a self-backlighter, providing a means to sample one side of the capsule, if emission levels can be increased over nominal values. Adding high-Z gas ($\sim$1{\%} Ar) to the capsule fill in Symcaps ($^{4}$He or propane), should produce a continuum backlighter with significant ($\sim$30x) increase in emission at h$\upsilon $ $\sim$8 keV over nominal fills. From images of the transmitted self-emission, above and below the K-edge of an internally doped high-Z layer (Cu), we can infer the growth at PV of imposed and inherent surface roughness at mode $\sim$60 with amplitudes $\sim$50-nm. [Preview Abstract] |
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NP8.00109: The effect of magnetic fields and ablative stabilization on Rayleigh-Taylor unstable inertial confinement fusion plasmas Bhuvana Srinivasan, Xian-Zhu Tang It has long been expected that Rayleigh-Taylor instabilities (RTI) in inertial confinement fusion (ICF) can generate magnetic fields at the gas-ice interface and at the ice-ablator interface during the deceleration phase of target implosion. The focus here is on the gas-ice interface where the temperature gradient is the largest. Nonlinear evolution of RTI leads to undesirable mixing of hot and cold plasmas and enhances target energy loss. RTI is also expected to generate magnetic fields via the Biermann battery effect which mitigate energy loss by decreasing electron thermal conduction at the gas-ice interface. The Hall-magnetohydrodynamics model is used to self-consistently study the generation and growth of magnetic fields in RTI. Externally applied magnetic fields grow due to the MHD dynamo and can result in mix mitigation in addition to significant electron thermal conductivity mitigation. Electron thermal conductivity can cause ablation of the ice into the gas, leading to ablative stabilization of RTI. Mitigation of electron thermal conductivity (and energy loss) in the presence of magnetic fields has a conflicting effect with the ablative stabilization of RTI brought about due to electron thermal conductivity. A study of this conflicting effect will be presented. [Preview Abstract] |
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NP8.00110: LASER-PLASMA INSTABILITIES |
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NP8.00111: Damping of Langmuir waves in ICF plasmas with thermal transport effects A.V. Brantov, V. Yu. Bychenkov, W. Rozmus The ignition-scale hohlraum plasmas consist of regions with strong temperature gradients giving rise to thermal fluxes that are often in the weakly collisional, nonlocal regime of the transport theory. Given the high background temperatures of the hohlraum plasmas the heat-carrying electrons have energies (20 -- 40 keV) that are close to kinetic energies of the electrons that are resonant with Langmuir waves produced by parametric instabilities, such as stimulated Raman scattering. The impact of the modified, by the strong heat flux, electron distribution function (EDF) on the Langmuir wave damping and dispersion is examined. We have employed the formalism of the transport theory to obtain solution to the Fokker-Planck equation for the EDF in a plasma with the temperature gradient in the background local equilibrium state. We used an approximation involving expansion of the EDF velocity dependence in the first three angular harmonics. This solution is used in the evaluation of the plasma dispersion function and in the calculation of the Langmuir wave frequency and damping. We have found that the thermal transport can dramatically alter the Landau damping of plasma waves propagating along or oppositely to the temperature gradient. Our calculations have exposed an important role of the second order correction in the mean free path to gradient scale length ratio to the zero order harmonic of the EDF. [Preview Abstract] |
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NP8.00112: Trapped Electron Instability of Electron Plasma Waves: Vlasov simulations and theory Richard Berger, Thomas Chapman, Stephan Brunner The growth of sidebands of a large-amplitude electron plasma wave is studied with Vlasov simulations [\textit{J. W. Banks et al, IEEE Trans. Plasma Sci {\bf 38}, 2198 (2010); R.L. Berger, et al., Phys. Plasmas 20, 032107(2013)}] for a range of amplitudes ($.001< e \phi_0 /T_e < 1$) and wavenumbers ($0.25 < k_0 \lambda_{De} < 0.45 $) for systems up to $100 \lambda_0$ in the propagation direction. Here, $k_0 = 2\pi/\lambda_0 $ and $\lambda_{De} $ is the Debye length. The low statistical noise of Vlasov simulations allows the growth rate of the unstable modes to be determined accurately and compared to theory. Despite the simplicity of the dispersion relation, growth rates found with the Kruer-Dawson-Sudan model [\textit{Kruer, et al PRL 23, 838 (1969}] agree quite well with the numerical results. The most unstable modes with frequency and wavenumber $\omega, k$ satisfy the relation, $\omega - k \cdot v_{ph} = \pm \omega_{be}$, where $v_{ph} = \omega_0/k_0$ and $\omega_{be}$ is the bounce frequency of a deeply trapped electron. In 2D simulations, we find that the instability persists and co-exists with the filamentation instability. [Preview Abstract] |
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NP8.00113: Backward Raman Amplification in the Wavebreaking Regime Zeev Toroker, Vladimir Malkin, Nathaniel Fisch Backward Raman amplification in plasma is based on a 3-wave resonant interaction, which includes two-counter propagating laser pulses (pump and seed pulses) and an electron plasma wave (Langmuir wave). In the regime of large laser to plasma frequency ratio, where the phase velocity of the Langmuir wave is small, the Raman amplification can be inefficient due to the trapping of electrons in the plasma wave, destroying the wave coherency. This process, known as wavebreaking, becomes dominant as the quiver velocity of the electrons approaches the phase velocity of the Langmuir wave. The quiver energy is proportional to the pump intensity, so the wavebreaking limits the pump intensity. However, we show that, for pump intensity up to about ten times the wavebreaking threshold and for cold enough plasma, it is still possible to have moderately efficient Raman amplification. For too large pump intensity or too hot plasma, the efficiency of the Raman amplification decreases significantly as predicted by [Malkin et al., Phys.~Rev.~Lett. {\bf $82$}, $4448$ (1999)]. [Preview Abstract] |
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NP8.00114: Cooperative stimulated Brillouin scattering driven by overlapping, large spot laser beams William Kruer, Robert Kirkwood, Pierre Michel, David Turnbull In NIF hohlraums, large regions of plasma are irradiated with intense overlapping and large spot laser beams. In this regime, cooperative excitation of stimulated scattering can become a significant effect. Indeed, the potential importance of cooperative scattering has already been illustrated in calculations [1] of cross beam energy transfer, where many crossing laser beams enhance the energy of another beam- a form of (generally nonresonant) cooperative SBS in the forward direction. Similarly, cooperative interactions are thought to play some role in scattering in the backward direction [2,3]. Here we consider an interesting special case in which all the beams in a cone resonantly drive an ion sound wave along the hohlraum axis. This results in laser light being scattered backward along the cone. The frequency of this scattered light differs from that of the light directly backscattered by each beam, although there may be cross talk if the frequency of the backscattered light is sufficiently broad. A simple theory is presented, and some experiments to isolate and characterize cooperative scattering are discussed.\\[4pt] [1] Pierre Michel \textit{et. al}. \textit{Phys. }Plasmas \textbf{17}, 056305 (2010).\\[0pt] [2] R. Kirkwood \textit{et. al.} Phys. Plasmas$,$ \textbf{18}, 056311 (2011).\\[0pt] [3] W. Seka \textit{et. al}., Phys. Rev. Lett. \textbf{89}, 175002 (2002). [Preview Abstract] |
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NP8.00115: Particle-in-Cell Simulations of the High Frequency Hybrid Instability in Inertial Confiment Fusion Plasmas Frank Tsung, W.B. Mori, B.B. Afeyan We present results on the laser-plasma interaction near the quarter critical surface under conditions relevant to inertial fusion. Under these conditions, the high frequency hybrid instability (HFHI) where one of the daughter waves have mixed polarization, is likely to be dominant. In fully nonlinear kinetic simulations with the code OSIRIS we show that the spectrum at early time is consistent with theory. We also investgiate the saturated electrostatic (and electromagnetic) spectrum for long timescales for both fixed and mobile ions. For high temperatures where the HFHI is dominant the absorption is dominated by the absolutely unstable modes and absorption levels near 40\% can occur even when $C_{mult}$ is less than 1 (where $C_{mult}$ is the which the system is above the threshold). We also investigate in detail the evolution of unstable modes. Nonlinear effects, such as the generation of hot electrons, half harmonics generations and the excitation of low frequency ion fluctuations, will also be discussed. [Preview Abstract] |
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NP8.00116: Half-harmonic radiation from turbulence driven by the two-plasmon decay instability D.F. DuBois, D.A. Russell, H.X. Vu, J.A. Myatt, W. Seka It is shown, using theory and reduced model simulations, that the dominant current source for $\omega_{\mathrm{0}}$/2 radiation from a plasma excited by the laser-driven two plasmon decay (TPD) instability is proportional to the transverse component of -e$\delta $n\underline {E}$_{\mathrm{1}}$ where \underline {E}$_{\mathrm{1}}$ is the envelope Langmuir wave (LW) electric field (relative to the reference electron plasma frequency $\omega_{\mathrm{pe}} \quad \cong \quad \omega_{\mathrm{0}}$/2, where $\omega_{\mathrm{0}}$/2 is the laser frequency) and $\delta $n is the low frequency electron density fluctuation comprised of ion acoustic waves resulting from the nonlinear saturation of TPD via the Langmuir decay instability of the primary LWs of the TPD instability. This process differs fundamentally from the physics usually invoked to explain the experiments, such as Thomson down-scatter of the laser, that involves the LWs predicted by linearized theory. The new physics avoids inconsistencies of the latter theory and compares well with observations. [Preview Abstract] |
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NP8.00117: Kinetic simulations of externally driven and instability driven nonlinear electron plasma waves relevant to stimulated Raman scattering B.J. Winjum, R.L. Berger, T. Chapman, J.W. Banks, S. Brunner, V.K. Decyk, W.B. Mori We present 2D kinetic simulations, both Vlasov and PIC, of externally-driven, nonlinear electron plasma waves (EPWs) with wavenumber $k\lambda_D \sim 1/3$, and we investigate their link with EPWs that evolve self-consistently in PIC simulations of stimulated Raman scattering (SRS). Simulating externally-driven EPWs is useful for isolating aspects of EPW evolution, while SRS modeling ultimately requires understanding the self-consistent evolution of EPWs with SRS light waves. In the externally-driven EPW simulations, the intrinsically intertwined effects of self-focusing and dissipation of field energy caused by electron trapping are studied. From various initial wave states, the width and field amplitude at focus are shown to be a function of the growth rate of the transverse modulational instability, $\gamma_{tpmi}$, divided by the loss rate of field energy, $\nu_E$, to electrons, and we find an amplitude threshold for self-focusing, $\gamma_{tpmi}/\nu_E \sim 1$. These results are compared with the EPWs that arise in SRS simulations. Similarities and differences are investigated by varying the external driver and the incident and (seeded) scattered light waves. [Preview Abstract] |
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NP8.00118: Particle-in-Cell Simulations of Laser Plasma Interactions Relevant to Shock Ignitions John Tonge, F.S. Tsung, M. Tzoufras, W.B. Mori We present simulation results on the laser-plasma interaction for density and intensity ranges relevant to shock ignitions. These simulations show the importance of instabilities near the quarter critical surface and the importance of higher dimensional simulations. The saturation mechanism (which is determined by ion dynamics) and the recurrence rate of the instability will also be presented. [Preview Abstract] |
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NP8.00119: MATH/THEORY TECHNIQUES |
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NP8.00120: A Lagrangian kinetic model for collisionless magnetic reconnection Cesare Tronci A new fully kinetic system is proposed for modeling collisionless magnetic reconnection. The formulation relies on fundamental principles in Lagrangian dynamics, in which the inertia of the electron mean flow is neglected in the expression of the Lagrangian, rather than enforcing a zero electron mass in the equations of motion. This is done upon splitting the electron velocity into its mean and fluctuating parts, so that the latter naturally produce the corresponding pressure tensor. The model exhibits a new Coriolis-force term, which emerges from a change of frame in the electron dynamics. Then, if the electron heat flux is neglected, the strong electron magnetization limit yields a hybrid model, in which the electron pressure tensor is frozen into the electron mean velocity. [Preview Abstract] |
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NP8.00121: On Hamiltonian Magnetohydrodynamics: Lagrangian, Eulerian and Dynamically Accessible Stability Tommaso Andreussi, Philip J. Morrison, Francesco Pegoraro Stability conditions of magnetized plasma flows are obtained by exploiting the Hamiltonian structure of the magnetohydrodynamics (MHD) equations by using three kinds of energy principles. First, the Lagrangian variable energy principle is described and sufficient stability conditions are presented. Next, plasma flows are described in terms of Eulerian variables and the noncanonical Hamiltonian formulation of MHD is exploited. For symmetric equilibria, the energy-Casimir principle is expanded to second order and sufficient conditions for stability to symmetric perturbation are obtained. Then, dynamically accessible variations, i.e. variations that explicitly preserve invariants of the system, are introduced and the respective energy principle is considered. General criteria for stability are obtained, along with comparisons between the three different approaches. En route to our results we describe a time-dependent relabeling transformation, which to our knowledge has not heretofore been given, that will be needed in the Lagrangian variable framework in connection with the approach considered in E. A. Frieman, M. Rotenberg, {\it Rev. Mod. Phys.}, {\bf 32}, 898 (1960). [Preview Abstract] |
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NP8.00122: The Hamiltonian structure and Euler-Poincar\'e formulation of the Vlasov-Maxwell and gyrokinetic systems Jonathan Squire, Hong Qin, William Tang, Cristel Chandre We present a new variational principle for the gyrokinetic system, similar to the Maxwell-Vlasov action presented in Ref 1. The variational principle is in the Eulerian frame and based on constrained variations of the phase space fluid velocity and particle distribution function. Using a Legendre transform, we explicitly derive the field theoretic Hamiltonian structure of the system. This is carried out with a modified Dirac theory of constraints, which is used to construct meaningful brackets from those obtained directly from Euler-Poincar\'e theory. Possible applications of these formulations include continuum geometric integration techniques, large-eddy simulation models and Casimir type stability methods.\\[4pt] [1] H. Cendra, D. D. Holm, M. J. W. Hoyle, and J. E. Marsden, Journal of Mathematical Physics 39, 3138 (1998). [Preview Abstract] |
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NP8.00123: Variational analysis of linear stability of flowing equilibria based on action-angle representation Makoto Hirota, Philip J. Morrison, Yuji Hattori Stabilizing/destabilizing effects of flows on plasma equilibria are generally difficult to understand systematically, because the energy principle (or the Rayleigh-Ritz variational method) is no longer applicable to flowing equilibria. To be precise, the energy principle gives only a sufficient condition for stability due to the presence of negative energy modes. It is therefore hard to prove instability of flowing plasmas analytically. In this work, we present an advanced variational approach which is formulated based on our recent knowledge about the action-angle variables for continuous spectra as well as discrete ones. We demonstrate that this approach gives a necessary and sufficient condition for Kelvin-Helmholtz instability of shear flow. The Vlasov-Poisson and MHD systems can be analyzed similarly while symmetries and certain conditions of equilibrium profiles need to be assumed. [Preview Abstract] |
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NP8.00124: BASIC PLASMA PHYSICS |
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NP8.00125: Plasma mass filtering techniques: applications and requirements Renaud Gueroult, Nathaniel J. Fisch Plasma mass filters differ from conventional chemical filtering techniques in that elements are dissociated, and can therefore be processed without regard to chemical form. In addition, plasma filters can be in principle operated at larger velocities compared to their gaseous and/or liquid counterparts, so that larger throughputs are possible. On the other hand, one has to pay the price of ionization, which sets a lower limit for the processing cost. Plasma mass filtering techniques are consequently foreseen as a promising solution for separation processes which are simultaneously chemically challenging and of high added value. Such separation processes can be, for example, found within the context of nuclear waste remediation, or nuclear spent fuel reprocessing. However, although plasma separation techniques appear globally attractive for these distinct needs, the plasma parameters required to fulfill a particular separation process are expected to depend strongly on the process's attributes (volume, composition, mass difference), which may vary significantly. Such operating parameters' variations are shown to be well accommodated by a particular configuration, called the Magnetic Centrifugal Mass Filter. [Preview Abstract] |
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NP8.00126: Spectral analysis of argon emission in the ALEXIS plasma column N. Ivan Arnold, Connor Ballance, Stuart Loch, Edward Thomas Performing spectroscopic measurements of emission lines in relatively cold laboratory plasmas is challenging because the plasma is often neutral-dominated and is not in thermal equilibrium. However, these types of plasma do offer a unique opportunity for benchmarking the fundamental atomic data. We report on new level-resolved calculations for the dielectronic recombination and collisional excitation of the low charge states of argon. The dielectronic recombination results are compared with existing configuration-average distorted-wave results and semi-empirical calculations. The collisional excitation rates are used to generate synthetic spectra, which are compared to experimental observations. The new dielectronic recombination and collisional excitation rate coefficients, along with existing ionization rate coefficients, are processed into metastable-resolved effective ionization and recombination rate coefficients. These are then used in non-equilibrium ionization balance modeling of an argon plasma experiment on the Auburn ALEXIS facility. We outline plans to use our atomic model to interpret the ALEXIS experiment, and overview the future direction of this project. [Preview Abstract] |
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NP8.00127: Zonal Flows from Spontaneous Symmetry Breaking of Homogeneous Turbulence Jeffrey Parker, John Krommes To study how zonal flows (ZF) arise, we examine one of the simplest possible models, the stochastically forced Hasegawa-Mima equation, which displays the bifurcation of steady ZFs from a state of homogeneous turbulence; thus a statistical treatment is required. Here an approach \footnote{K. Srinivasan \& W. R. Young, J. Atmos. Sci. 69, 1633 (2012)} is adopted in which the ZFs are treated as mean fields that spontaneously break the background symmetry. The resulting inhomogeneous ensemble is treated self-consistently without assuming weak inhomogeneity. Closed statistical equations are obtained by ignoring the drift-wave self-interactions while fully retaining the drift-wave--ZF nonlinearities. We show that from the statistical point of view ZF generation can be understood as pattern formation. This leads to the surprising result that in a saturated turbulent state the ZF wavelength is not unique; a continuous band of ZF scales is allowed. Only those within a smaller sub-band are linearly stable. That stability is analyzed and the stability diagram in parameter space is calculated and successfully compared with simulations. The stability concept provides a way of interpreting the merging of zonal jets, a phenomenon commonly observed in observations and numerical studies. [Preview Abstract] |
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NP8.00128: On improving impedance probe plasma potential measurements in low density plasma David Walker, David Blackwell, Richard Fernsler, William Amatucci We have used impedance probes of various sizes and shapes in demonstrating a method of determining plasma potential, $\varphi_{p}$, when the probe radius is much larger than the Debye length. The method\footnote{\textit{Phys. Plasmas }\textbf{17}, 113503 (2010).}$^,$\footnote{\textit{NRL Memorandum Report 6750-12-9413}(2012).} relies on applying a small amplitude ac signal to a probe in a plasma and measuring the complex reflection coefficient, $\Gamma $, as a function of varying probe bias, $V_{b}$. Re($Z_{ac})$ (the real part of the ac plasma impedance determined from $\Gamma )$ is plotted versus $V_{b}$, and a minimum predicted by theory occurs at $\varphi_{p}$ for a large range of electron density, $n_{e}$.\footnote{\textit{Phys. Plasmas }\textbf{17}} However, the frequency range of the applied signal is restricted and as $n_{e}$ decreases it becomes even more restrictive. In addition, the minimum in Re($Z_{ac})$ ($\sim$ 1/$n_{e})$ becomes more difficult to discern. Here, we suggest additional means to isolate $\varphi_{p}$. These measures (1) incorporate $\Gamma $ to search for a minimum, (2) use not only the first derivative of Re($Z_{ac})$, but also that of Im($Z_{ac})$ with respect to $V_{b}$ and, (3) use the second derivatives of both. With the additional indicators, $\varphi_{p}$ is more easily detected in low density plasma. We present data for cylinders, spheres and a disk. \textit{Phys. Plasmas }\textbf{17}, 113503 (2010). \textit{NRL Memorandum Report 6750-12-9413}(2012). [Preview Abstract] |
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NP8.00129: Advances in Impedance Probe Applications and Design in the NRL Space Physics Simulation Chamber David Blackwell, David Walker, Christopher Cothran, George Gatling, Erik Tejero, William Amatucci We will present recent progress in plasma impedance probe experiments and design at NRL's Space Physics Simulation Chamber. These include our network analyzer S-parameter methods as well as more portable self-contained diagnostics with an eye towards space vehicle applications. The experiments are performed under a variety of conditions with magnetized and unmagnetized collisionless, cold ($T_e\approx1-2$eV) plasmas in density ranges of 10$^5$-10$^8$cm$^{-3}$. Large and small spheres, disks, floating dipoles and monopoles are all in development with various electronic setups, along with traditional emissive and Langmuir probes for measurement redundancy. New computational results provide experimental predictions over a larger parameter space. [Preview Abstract] |
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NP8.00130: Experimental observations of inward particle transport in a linear machine Lang Cui, George Tynan, Christian Brandt, Saikat Chakraborty Thakur, Rongjie Hong, Min Xu Experiments in various tokamak devices (AUG, DIII-D, JET, TCV, TEXTOR) have indicated the existence of an anomalous inward particle transport. Recently, such an anomalous particle transport has been unambiguously identified in the Controlled Shear Decorrelation Experiment (CSDX) linear plasma machine. A detailed description of experimental results from both Langmuir probes measurements and fast imagining camera measurements is presented, which gives direct evidence that the fluctuation-driven particle flux changes from outward flow to inward flow across the transition from low to high confinement mode. Experimental results suggest that the radial reversal of turbulent particle flux can be externally controlled by generating regions in the plasma where there is a strong velocity shear. Studies of turbulent transport in CSDX show that the phase shift between turbulent particle flux and velocity shear plays a key role in a suppression and apparent reversal of turbulent particle flux with the confinement transition. [Preview Abstract] |
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NP8.00131: The Hamiltonian mechanics of stochastic acceleration Joshua Burby, Andrey Zhmoginov, Hong Qin We show how to find the physical Langevin equation describing the trajectories of particles undergoing collisionless stochastic acceleration. These stochastic differential equations retain not only one-, but two-particle statistics, and inherit the Hamiltonian nature of the underlying microscopic equations. This opens the door to using stochastic variational integrators to perform simulations of stochastic interactions such as Fermi acceleration. We illustrate the theory by applying it to two example problems. [Preview Abstract] |
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NP8.00132: Ion flow field of a rotating plasma in a diverging magnetic field Kenichiro Terasaka, Shinji Yoshimura, Mitsutoshi Aramaki, Yuki Sakamoto, Masayoshi Y. Tanaka We are interested in plasma flow structure in weak magnetic field, in which the ions are unmagnetized. We have measured the flow field in an ECR plasma of the HYPER-I device at the National Institute for Fusion Science. The experimental evidence of ion stream line detachment (non-adiabatic detachment) form the magnetic field was observed by measuring the ion flow velocity in a diverging magnetic field region. In the detachment region, the characteristic of ion flow field is different from that of magnetized plasma, showing that the total angular momentum given by the sum of both the plasma and the electromagnetic field contributions is important. In order to clearly understand the mechanism of flow structure formation in diverging magnetic field, further experiments in the weaker magnetic field region is needed. We have developed a new experimental device (HYPER-II) at Kyushu University. The new device allows ion flow measurement in the lower magnetic field region of the order of 10 Gauss. The experimental results of the HYPER-I device and the preliminary results of the HYPER-II device will be presented. [Preview Abstract] |
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NP8.00133: Upgrades for the TCV tokamak Basil Duval Major upgrades are being implemented on the TCV tokamak to extend its operational domain towards a burning plasma regime. The goals of obtaining high normalized plasma beta and comparable ion and electron temperatures will be achieved with the addition of a 1MW neutral heating system and 2MW additional third harmonic EC power. Spatial constraints together with beam occlusion required severe design optimization and the additional of a new large tangential port on the TCV vessel. For EC, the existing vertical launch mirror will be sufficient but new 1MW EC units will be employed with the legacy X3 systems modified for lateral launch. The modifications will not affect TCV's strong RT shaping and EC actuator ranges or the open divertor vacuum chamber that permits access to Snowflake divertor or doublet configurations although some wall protection enhancement is envisaged. TCV can then contribute to disentangling effects of electron-ion coupling, rotation, current and density profile control all as a function of shape in L and H-modes with ITER (or higher) values of plasma beta. Together with fast-ion physics, TCV will also be able to explore heat, particle and momentum transport and turbulence effects in electron-heat dominated discharges for Te/Ti in the (0.02 to 3) range. [Preview Abstract] |
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NP8.00134: Non-Maxwellian distributions and their foundation on non-extensive statistical physics J. Julio E. Herrera-Velazquez Although it is common to assume that velocity current distributions of plasma species are Maxwellian, since plasmas are usually not closed systems in equilibrium, this assumption is seldom fulfilled in practice. Both in laboratory and spacecraft observations, families of distribution functions usually include suprathermal particles, and are best parametrized by the so called $\kappa$ distributions, or variations of them. The purpose of this work is to discuss their theoretical foundations in the context of non-extensive statistical physics [1, 2], as well as some of their consequences, or lack of them, in other fundamental concepts, such as Deby shielding.\\[4pt] [1] C.Tsallis, \textit{J. Statistical Physics}, \textbf{52}, 479 (1988).\\[0pt] [2] M. P. Leubner, \textit{Astorphysics and Space Science}, \textbf{282}, 573 (2002). [Preview Abstract] |
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NP8.00135: Ultracold plasma expansion rate dependence on non-neutrality Craig Witte, Jacob Roberts Ultracold plasmas are formed by photoionizing a collection of laser cooled atoms. Once formed, these plasmas expand. This expansion is driven by both the thermal energy of the plasma electrons, as well as electrostatic energy owing to non-neutrality. Both the parameters can be experimentally controlled with a significant degree of independence. Combining previous work,\footnote{F. Robicheaux and James D. Hanson, Simulation of the Expansion of an Ultracold Neutral Plasma, Phys. Rev. Lett. {\bf 88}055002, (2002).}$^,$\footnote{D Vrinceanu, G S Balaraman, and L A Collins, The King model for electrons in a finite-size ultracold plasma, J. Phys. A: Math. Theor.{\bf 41} 425501 (2008).} we have developed a theoretical model designed to investigate the dependence of ultracold plasma expansion on the degree of non-neutrality of these plasmas in a parameter range relevant to experiments. We find that variations of the plasma neutrality produce non-negligible changes in predicted electron temperature evolution and plasma expansion rate. Such behavior needs to be taken into account for an accurate interpretation of ultracold plasma parameters relevant to experimental measurements. The results of our calculations were compared to a simple expansion model of a neutral plasma.\footnote{Robicheaux and Hanson, \textbf{88}055002} [Preview Abstract] |
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NP8.00136: Direct Measurement of the Phase Space Ion Fluctuation Spectrum of a Laboratory Plasma Using Two Independently Tunable Lasers Sean Mattingly, Jorge Berumen, Feng Chu, Ryan Hood, Fred Skiff A novel technique for probing velocity space correlations has been developed using laser-induced fluorescence. The experiment consists of a 3m cylindrical plasma column of singly-charged Argon ions (Ar II) with density $\sim 109 cm^{-3}$, $T_e \sim 5eV$, $T_i \sim .06eV$, and a 1kG axial magnetic field. Separate metastable lines of the Ar II ions are excited using two separate narrow bandwidth lasers. The LIF response from each laser is measured through an independently moveable periscope. These periscopes may be focused on the same localized region ( $\sim 0.1 cm^3$) or separated to view different parts of the plasma simultaneously. By adjusting these lasers independently, one may measure a correlation function as a function of the difference in measured velocities. This measurement may be repeated for different periscope positions in the plasma to obtain a two-dimensional correlation function in space and velocity difference. This correlation is directly related to the fluctuation spectrum through a Fourier transform. Measurements of these correlations are reported and discussed. [Preview Abstract] |
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NP8.00137: Formation of phase space structures by sculpted initial conditions in Vlasov simulations P.J. Morrison, F. Valentini, F. Pegoraro, T.M. O'Neil Usually Vlasov-Poisson (VP) simulations are initiated close to equilibria; alternatively, dynamically accessible initial conditions are generated by applying external drive fields. We study a variety of states in VP simulations with drive fields launched into a plasma with fixed ions. Time evolution of the response electric field strongly depends on the form of the drive. In particular, complex non-sinusoidal oscillations are obtained in the case of an abrupt turning off of the drive, and phase space can be sculpted by the drives of multiple sinusoidal oscillations that open `resonances'. Examination of the phase space contours provides understanding of the resulting electric field behavior. For a single sinusoidal drive, a main BGK mode (hole) with a secondary structure of two counter-propagating smaller holes (period-2 resonance) is observed. Physical and mathematical arguments that explain this structure are given. For longitudinal propagation in a plasma with properties that change on a short time scale, pseudo-Fresnel relations between the incident, transmitted, and reflected wave amplitudes can be derived. Mathematically, the states that occur can be interpreted as periodic orbits in the VP system, akin to those that occur in finite-dimensional Hamiltonian systems. [Preview Abstract] |
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NP8.00138: Disrupting high-pressure, current-carrying filaments using cross-field injection of laser ablation plasmas S. Vincena, W. Gekelman, J. Bonde In a magnetized plasma, laser-irradiated targets may be used to produce localized, expanding plasmas. Such laser-produced plasmas (LPP's) share characteristics with injected fuel pellets in tokamak plasmas: localized high pressure, can become polarized and move via ExB motion. Pellet injection has recently been demonstrated to mitigate the intensity of edge-localized modes [1]. We present results of a basic plasma physics experiment to study the disruption of a high-pressure, current-carrying filament. The experiments are performed on UCLA's Large Plasma Device (LAPD). This is a linear device with $L$=17m, $d$=60cm, $B_{0}=$750G, $n_{e}$=$2\times10^{12}$cm$^{-3}$, $T_{e}$=6eV,$T_{i}\approx$1eV, H$^{+}$). The LPP is produced by a pulsed (8ns, 1J) Nd:YAG laser ablation of a carbon target. The current is produced using a $LaB_{6}$ cathode, with $T_{e}=20$eV, $n\approx 4\times 10^{12}$cm$^{-3}$, yielding cross-field dimensions $h=0.9c/\omega_{pi}$ and $w=3.8c/\omega_{pe}$ for a H plasma, and a Lundquist number $S=8\times 10^{3}$ Using probes and a 1Hz experiment repetition, maps of the plasma potential, electron temperature, magnetic fields (and derived currents), and induced current-sheet oscillations are presented as the current is disrupted. [Preview Abstract] |
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NP8.00139: Design study for a diverging supernova explosion experiment on NIF Markus Flaig, Tomasz Plewa, Paul Keiter, Michael Grosskopf, Carolyn Kuranz, Paul Drake, Hye-Sook Park We report on design simulations of a spherically-diverging, multi-interface, supernova-relevant Rayleigh-Taylor experiment (DivSNRT) to be carried out at the National Ignition Facility (NIF). The simulations are performed in two and three dimensions using the block-adaptive, multi-group radiative diffusion hydrodynamics code CRASH and the FLASH-based MHD code Proteus. In the present study, we concentrate mainly on a planar variant of the experiment. We assess the sensitivity of the Rayleigh-Taylor instability growth on numerical discretization, variations in the laser drive energy and the manufacturing noise at the material interfaces. We find that a simple buoyancy-drag model accurately predicts the mixed-layer width obtained in the simulations. We use synthetic radiographs to optimize the diagnostic system and the experimental setup. Finally, we perform a series of exploratory MHD simulations and investigate the self-generation of magnetic fields and their role in the system evolution. [Preview Abstract] |
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NP8.00140: Transport properties of initially neutral gas disturbed by intense electron beam Justin Angus, Steve Swanekamp, Joseph Schumer, Dave Mosher, Paul Ottinger The behavior of intense electron beams (those with current densities on the order of hundreds of kA/cm$^{\mathrm{2}}$ and beam rise times on the order of 100 ns) traveling through gaseous mediums depends strongly on the transport properties of the medium. For example, the conductivity of the medium, which is very sensitive to the ionization state and temperature of the gas, has a strong influence on the beam behavior through the plasma return current. Since the beam is responsible for ionizing and heating the gas, self-consistently solving for the gas transport properties and the beam propagation is essential for an accurate description of the system. An advanced gas chemistry model to describe the transport properties of a strongly disturbed gaseous system is presented in this work. A focal point of this work is an accurate description of the medium's conductivity as the gas progresses from its weakly ionized state, where swarm models are valid, to a strongly ionized state where the Spitzer-Harm model applies. [Preview Abstract] |
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NP8.00141: Focused Laser Initiated, RF Sustained, High Pressure Air Plasmas Yan Li, John Scharer Measurements and analysis of air breakdown processes and plasma production were carried out by focusing 193 nm, 300 mJ, 15 MW high power laser radiation inside a Pyrex chamber surrounded by a 6 cm diameter helical RF coil. We observe quantum resonant multi-photon (REMPI) and collisional cascade laser ionization processes that produce high density (n$_{\mathrm{e}}$ $\sim$ 5*10$^{15}$/cm$^{3})$ cylindrical seed plasmas. The focused laser and associated shock wave produces a plasma seed for sustainment by the RF (1-10 kW, 0.5-1.8 s) pulse. The laser seed plasma increases the air RF breakdown pressure from 60 torr to 85 torr with 5 kW incident RF power and in lower pressure conditions the laser decreases the time between firing the RF pulse and formation of plasma. We also observed that the two capacitor settings in the matching system are important in determining the breakdown pressure and plasma parameters. To diagnose the inductive ($\sim$ 10$^{12}$/cc) and capacitive ($\sim$ 10$^{11}$/cc) plasmas with different properties, we use our 105 GHz (mm wave) interferometer to measure plasma density, collision frequency and electron temperature. Spectroscopic measurements of the plasma and comparison with the SPECAIR code are made to determine rotational, vibrational and neutral gas temperatures. A directional coupler in the RF system is applied to obtain the incident and reflection RF signals, with which we can calculate both magnitude and phase of the reflection coefficient and determine via FFT methods the time dependent plasma impedance. [Preview Abstract] |
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