Kinetic modeling of x-ray laser-driven solid Al plasmas via particle-in-cell simulation
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Royle, R.↗
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Results for “PLASMA SIMULATION”
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Turbulent impurity transport simulations in Wendelstein 7-X plasmas
A study of turbulent impurity transport by means of quasilinear and nonlinear gyrokinetic simulations is presented for Wendelstein 7-X (W7-X). The calculations have been carried out with the recently developed gyrokinetic code stella. Different impurity species are considered in the presence of various types of background instabilities: ion temperature gradient (ITG), trapped electron mode (TEM) and electron temperature gradient (ETG) modes for the quasilinear part of the work; ITG and TEM for the nonlinear results. While the quasilinear approach allows one to draw qualitative conclusions about the sign or relative importance of the various contributions to the flux, the nonlinear simulations quantitatively determine the size of the turbulent flux and check the extent to which the quasilinear conclusions hold. Although the bulk of the nonlinear simulations are performed at trace impurity concentration, nonlinear simulations are also carried out at realistic effective charge values, in order to know to what degree the conclusions based on the simulations performed for trace impurities can be extrapolated to realistic impurity concentrations. The presented results conclude that the turbulent radial impurity transport in W7-X is mainly dominated by ordinary diffusion, which is close to that measured during the recent W7-X experimental campaigns. Finally, it is also confirmed that thermodiffusion adds a weak inward flux contribution and that, in the absence of impurity temperature and density gradients, ITG- and TEM-driven turbulence push the impurities inwards and outwards, respectively.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Direct discrete simulation of the kinetic Landau-Fokker-Planck equation with an alternating external electromagnetic field
A stochastic method of simulation of Coulomb interaction is considered. The main idea of the method is to approximate the nonlinear Landau kinetic collision integral by the Boltzmann integral. In its realization, the method can be attributed to a wide class of Monte Carlo-type methods. It is easily combined with the existing particle methods used to simulate collisionless plasmas. This is important for simulation of the dynamics of both laboratory and space plasmas when the mean free path of plasma particles is comparable with the plasma inhomogeneity scale length. Illustrative examples of relaxation of two-temperature plasma being subject to a high-frequency alternating electric field are given, and differences from their classical description are considered. The method satisfies the conservation laws for the number of particles, momentum, and energy and is simple and efficient in implementation.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
3D full wave fast wave modeling with realistic antenna geometry and SOL plasma
This paper reports the significant advancement of our ability to model and to understand how RF waves interact with the SOL plasma, by simulating the full torus 3D SOL plasma together with the antenna and core plasma. We introduce and use a recently developed and open source code, Petra-M, which was constructed on the scalable MFEM C++ finite element library, and performed 3D full wave simulations in the HHFW regime for both NSTX-U and LAPD plasmas. A first full wave simulation for a full 3D torus including a realistic antenna geometry and SOL plasma region for NSTX-U is presented. A scan of the antenna phasing shows a strong interaction between FWs and the SOL plasma for lower antenna phasing, which is consistent with previous NSTX HHFW observations. The effect of the 3D wave field on the fast ion population from NBI beams in NSTX-U is also discussed by using the 3D field obtained from the Petra-M simulations in the full-orbit following particle SPIRAL code. On LAPD, 3D full wave simulations of a new HHFW 4-strap antenna recently installed by TAE Technologies on LAPD are performed showing a qualitative agreement with experimental data.
Bertelli, Nicola↗
The Element Composition Variation in Lead Zirconate Titanate upon the Ion-Plasma Deposition: Experiment and Simulation
The element composition modification is experimentally studied in PZT thin films obtained via the RF magnetron sputtering with variable working gas pressure. The experimental data were interpreted through the statistic simulation of thermalization and diffusion of sprayed Pb, Zr, and Ti atoms during the ion-plasma deposition of PZT thin films. The simulation data are shown to adequately describe (within the limits of 5%) the change in element composition at various pressures of the gas environment. The study conducted enables one to produce PZT thin films with a set element composition, which is essential for optimizing the electric and physical properties of solid solutions at the morphotropic phase boundary.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Simulation of the dc Plasma in Carbon Nanotube Growth
A model for the dc plasma used in carbon nanotube growth is presented, and one-dimensional simulations of an acetylene/ammonia/argon system are performed. The effect of dc bias is illustrated by examining electron temperature, electron and ion densities, and neutral densities. Introducing a tungsten filament in the dc plasma, as in hot filament chemical vapor deposition with plasma assistance, shows negligible influence on the system characteristics.
Hash, David↗
Ion acceleration from laser-driven electrostatic shocks
Multi-dimensional particle-in-cell simulations are used to study the generation of electrostatic shocks in plasma and the reflection of background ions to produce high-quality and high-energy ion beams. Electrostatic shocks are driven by the interaction of two plasmas with different density and/or relative drift velocity. The energy and number of ions reflected by the shock increase with increasing density ratio and relative drift velocity between the two interacting plasmas. It is shown that the interaction of intense lasers with tailored near-critical density plasmas allows for the efficient heating of the plasma electrons and steepening of the plasma profile at the critical density interface, leading to the generation of high-velocity shock structures and high-energy ion beams. Our results indicate that high-quality 200 MeV shock-accelerated ion beams required for medical applications may be obtained with current laser systems.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
High voltage spheres in an unmagnetized plasma - Fluid and PIC simulations
The basic physics involving the interaction of a high voltage sphere with a LEO-Plasma was investigated via computer experiments. Two approaches were used: the fluid description and the more general particle pushing method. Both techniques displayed qualitatively similar features. Thus, the initial time-dependent response of the plasma predicted by the fluid model, including the initial current surge and sheath formation, the formation of ion and electron density shells about the sphere with a double layer potential structure, and the subsequent propagation of the density shells away from the sphere. The PIC results also displayed an oscillatory character and slightly different time constants for the growth and decay mechanisms involved.
Thiemann, H.↗
High amplitude waves in the expanding solar wind plasma
We simulated the 1-D nonlinear time-evolution of high-amplitude Alfven, slow and fast magnetoacoustic waves in the solar wind propagating outward at different angles to the mean magnetic (spiral) field, using the expanding box model. The simulation results for Alfven waves and fast magnetoacustic waves fit the observational constraints in the solar wind best, showing decreasing trends for energies and other rms-quantities due to expansion and the appearance of inward propagating waves as minor species in the wind. Inward propagating waves are generated by reflection of Alfven waves propagating at large angles to the magnetic field or they coincide with the occurrence of compressible fluctuations. In our simulations, fast and slow magnetoacoustic waves seem to have a level in the density-fluctuations which is too high when we compare with the observations. Furthermore, the evolution of energies for slow magnetoacoustic waves differs strongly from the evolution of fluctuation energies in situ.
Schmidt, J. M.↗
Eulerian simulations of collisional effects on electrostatic plasma waves
The problem of collisions in a plasma is a wide subject with a huge historical literature. In fact, the description of realistic plasmas is a tough problem to attack, both from the theoretical and the numerical point of view. In this paper, a Eulerian time-splitting algorithm for the study of the propagation of electrostatic waves in collisional plasmas is presented. Collisions are modeled through one-dimensional operators of the Fokker-Planck type, both in linear and nonlinear forms. The accuracy of the numerical code is discussed by comparing the numerical results to the analytical predictions obtained in some limit cases when trying to evaluate the effects of collisions in the phenomenon of wave plasma echo and collisional dissipation of Bernstein-Greene-Kruskal waves. Particular attention is devoted to the study of the nonlinear Dougherty collisional operator, recently used to describe the collisional dissipation of electron plasma waves in a pure electron plasma column [M. W. Anderson and T. M. O'Neil, Phys. Plasmas 14, 112110 (2007)]. Finally, for the study of collisional plasmas, a recipe to set the simulation parameters in order to prevent the filamentation problem can be provided, by exploiting the property of velocity diffusion operators to smooth out small velocity scales.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Plasma Processing of Lunar Regolith Simulant for Diverse Applications
Versatile manufacturing technologies for extracting resources from the moon are needed to support future space missions. Of particular interest is the production of gases and metals from lunar resources for life support, propulsion, and in-space fabrication. Deposits made from lunar regolith could yield highly emissive coatings and near-net shaped parts for replacement or repair of critical components. Equally important is development of high fidelity lunar simulants for ground based validation of potential lunar surface operations. Described herein is an innovative plasma processing technique for insitu production of gases, metals, coatings, and deposits from lunar regolith, and synthesis of high fidelity lunar simulant from NASA issued lunar simulant JSC-1. Initial plasma reduction trials of JSC-1 lunar simulant have indicated production of metallic iron and magnesium. Evolution of carbon monoxide has been detected subsequent to reduction of the simulant using the plasma process. Plasma processing of the simulant has also resulted in glassy phases resembling the volcanic glass and agglutinates found in lunar regolith. Complete and partial glassy phase deposits have been obtained by varying the plasma process variables. Experimental techniques, product characterization, and process gas analysis will be discussed.
Schofield, Elizabeth C.↗
Gyrokinetic simulations of collisionless reconnection in turbulent non-uniform plasmas
We present nonlinear gyrokinetic simulations of collisionless magnetic reconnection with non-uniformities in the plasma density, the electron temperature, and the ion temperature. The density gradient can stabilize reconnection due to diamagnetic effects but destabilize driftwave modes that produce turbulence. The electron temperature gradient triggers microtearing modes that drive rapid small-scale reconnection and strong electron heat transport. The ion temperature gradient destabilizes ion temperature gradient modes that, like the driftwaves, may enhance reconnection in some cases.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Sheath effects observed on a 10 meter high voltage panel in simulated low earth orbit plasma
A large (1m x 10m) flat surface of conductive material was biased to high voltage (+ or - 3000 V) to simulate the behavior of a large solar array in low earth orbit. The model array was operated in a plasma environment of 1,000 to 1,000,000/cu cm, with sufficient free space around it for the resulting plasma sheaths to develop unimpeded for 5-10 meters into the surrounding plasma. Measurements of the resulting sheath thickness were obtained. The observed thickness varied approximately as V to the 3/4 power and N to the 1/2 power. This effect appears to limit total current leakage from the test array until sheath dimensions exceed about 1 meter. Total leakage current was also measured with the array.
Mccox, J. E.↗
The Plasma Wake Downstream of Lunar Topographic Obstacles: Preliminary Results from 2D Particle Simulations
Anticipating the plasma and electrical environments in permanently shadowed regions (PSRs) of the moon is critical in understanding local processes of space weathering, surface charging, surface chemistry, volatile production and trapping, exo-ion sputtering, and charged dust transport. In the present study, we have employed the open-source XOOPIC code [I] to investigate the effects of solar wind conditions and plasma-surface interactions on the electrical environment in PSRs through fully two-dimensional pattic1e-in-cell simulations. By direct analogy with current understanding of the global lunar wake (e.g., references) deep, near-terminator, shadowed craters are expected to produce plasma "mini-wakes" just leeward of the crater wall. The present results (e.g., Figure I) are in agreement with previous claims that hot electrons rush into the crater void ahead of the heavier ions, fanning a negative cloud of charge. Charge separation along the initial plasma-vacuum interface gives rise to an ambipolar electric field that subsequently accelerates ions into the void. However, the situation is complicated by the presence of the dynamic lunar surface, which develops an electric potential in response to local plasma currents (e.g., Figure Ia). In some regimes, wake structure is clearly affected by the presence of the charged crater floor as it seeks to achieve current balance (i.e. zero net current to the surface).
Zimmerman, Michael I.↗
Multistage ion acceleration in finite overdense target with a relativistic laser pulse
“Multistage ion acceleration” has been analytically and computationally studied in the relativistic regime. For non-relativistic piston velocities, this phenomenon has been described before. But, as we go to relativistic piston velocities, the non-relativistic results hold no more. We have presented a fully relativistic calculation for second stage ion velocities and energies. To verify our calculations, we performed a fully relativistic 1D3V particle in cell simulations using the code LPIC++. It has been found that the relativistic calculations matched very well with the simulation results. Also, it has been seen that at relativistic piston velocities, the non-relativistic results differed by a significant margin. The feasibility of this process has been further established by three dimensional particle in cell simulations.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Investigation of the effects of a thin dielectric layer on low-pressure hydrogen capacitive discharges driven by combined radio frequency and pulse power sources
Negative hydrogen ion sources, for instance for fusion devices, currently attract considerable attention. To generate the precursors—highly rovibrationally excited hydrogen molecules—for negative hydrogen ions effectively by electron excitation, a thin dielectric layer is introduced to cover the surface of the electrically grounded electrode of two parallel metal plates in a low-pressure hydrogen capacitive discharge driven by combined rf and pulse power sources. To understand the characteristics of such discharges, particle-in-cell simulations are conducted to study the effects that the single dielectric layer would bring onto the discharges. The simulation results show that the dielectric layer leads to a much higher plasma density and a much larger production rate of highly vibrationally excited hydrogen molecules compared to discharges without the dielectric layer on the electrode. Further investigation indicates that the nonlinear oscillation of the electrons induced by the nanosecond-pulse continues until it is finally damped down and does not show any dependence on the pulse plateau-time, which is in stark contrast to the case without the dielectric layer present. The physical reason for this phenomenon is explored and explained.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Simulation of radial expansion of an electron beam injected into a background plasma
A 2-D electrostatic particle code was used to study the beam radial expansion of a nonrelativistic electron beam injected from an isolated equipotential conductor into a background plasma. The simulations indicate that the beam radius is generally proportional to the beam electron gyroradius when the conductor is charged to a large potential. The simulations also suggest that the charge buildup at the beam stagnation point causes the beam radial expansion. From a survey of the simulation results, it is found that the ratio of the beam radius to the beam electron gyroradius increases with the square root of beam density and decreases inversely with beam injection velocity. This dependence is explained in terms of the ratio of the beam electron Debye length to the ambient electron Debye length. These results are most applicable to the SEPAC electron beam injection experiments from Spacelab 1, where high charging potential was observed.
Koga, J.↗
Characterizing turbulent transport in ASDEX Upgrade L-mode plasmas via nonlinear gyrokinetic simulations
The nature and level of turbulent transport in the outer core of low-confinement (L-mode) discharges performed at the ASDEX Upgrade tokamak [Kallenbach et al., Nucl. Fusion 51, 094012 (2011)] are examined. Previously, it was found that for an L-mode discharge of the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] gyrokinetic simulations were unable to reproduce the experimental ion heat flux, underestimating it by almost an order of magnitude. In the present work, employing the GENE gyrokinetic turbulence code, an extensive nonlinear study is performed for L-mode discharges of ASDEX Upgrade in order to cross-check this observation. It is shown that no systematic underprediction can be found in these simulations—instead, discrepancies with respect to experimental transport levels are small enough to be resolved within the uncertainties of the experimental profiles. Moreover, it is shown that some turbulence properties resemble closely those of the underlying linear microinstabilities at least out to 90% of the minor radius, so that quasilinear transport models remain, in principle, applicable even for these parameters, provided that appropriate nonlinear saturation rules can be developed.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗