Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session TO7: Computational and Theoretical Techniques |
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Chair: Dustin Fisher, University of New Mexico Room: 212 AB |
Thursday, November 3, 2016 9:30AM - 9:42AM |
TO7.00001: Measuring Landau damping in Particle-in-Cell simulations using particles of different charge-weights C. Ren$^{1,3}$, A. Sarkar$^{2}$, Y.-X. Cao$^{3}$, M. C. Huang$^{2}$, J. Li$^{1}$ We study whether putting more particles in “region of interest (ROI)” in phase space can efficiently increase Particle-in-Cell (PIC) simulation accuracy. We use Landau damping of a plasma wave as a figure of merit and set the ROI near the phase velocity of the wave. Improvement in Landau damping rate measurement is observed in 1D PIC simulations when employing more particles in the ROI but the effect is not monotonic. This is partly due to energy transfer from particles of large charge weights to those of smaller weights through the electric fields. Possible strategies to mitigate the energy transfer will also be discussed. [Preview Abstract] |
Thursday, November 3, 2016 9:42AM - 9:54AM |
TO7.00002: Solution of Poisson's equation in electrostatic Particle-In-Cell simulations. Daniel Kahnfeld, Ralf Schneider, Konstantin Matyash, Karl L\"uskow, Gunnar Bandelow, Oleksandr Kalentev, Julia Duras, Stefan Kemnitz For spacecrafts the concept of ion thrusters presents a very efficient method of propulsion. Optimization of thrusters is imperative, but experimental access is difficult. Plasma simulations offer means to understand the plasma physics within an ion thruster and can aid the design of new thruster concepts. In order to achieve best simulation performances, code optimizations and parallelization strategies need to be investigated. In this work the role of different solution strategies for Poisson's equation in electrostatic Particle-in-Cell simulations of the HEMP-DM3a ion thruster was studied. The direct solution method of LU decomposition is compared to a stationary iterative method, the successive over-relaxation solver. Results and runtime of solvers were compared, and an outlook on further improvements and developments is presented. [Preview Abstract] |
Thursday, November 3, 2016 9:54AM - 10:06AM |
TO7.00003: Speed-limited particle-in-cell (SLPIC) simulation Gregory Werner, John Cary, Thomas Jenkins Speed-limited particle-in-cell (SLPIC) simulation is a new method for particle-based plasma simulation that allows increased timesteps in cases where the timestep is determined (e.g., in standard PIC) not by the smallest timescale of interest, but rather by an even smaller physical timescale that affects numerical stability. For example, SLPIC need not resolve the plasma frequency if plasma oscillations do not play a significant role in the simulation; in contrast, standard PIC must usually resolve the plasma frequency to avoid instability. Unlike fluid approaches, SLPIC retains a fully-kinetic description of plasma particles and includes all the same physical phenomena as PIC; in fact, if SLPIC is run with a PIC-compatible timestep, it is identical to PIC. However, unlike PIC, SLPIC can run stably with larger timesteps. SLPIC has been shown to be effective for finding steady-state solutions for 1D collisionless sheath problems, greatly speeding up computation despite a large ion/electron mass ratio. SLPIC is a relatively small modification of standard PIC, with no complexities that might degrade parallel efficiency (compared to PIC), and is similarly compatible with PIC field solvers and boundary conditions. [Preview Abstract] |
Thursday, November 3, 2016 10:06AM - 10:18AM |
TO7.00004: Slurm: An innovative Particle-in-Cell Method for Magnetohydrodynamics Fabio Bacchini, Vyacheslav Olshevsky, Giovanni Lapenta We present a new Particle-in-Cell method for plasma simulations. This is based on the original algorithm of FLIP-MHD, which uses a Lagrangian formulation of the macroscopic equations. A finite-difference approximation of the equations of motion is solved on a fixed (non-moving) grid, while convection of the quantities is modelled with the support of Lagrangian particles. Interpolation with first-order b-splines is used to project the conserved quantities from particles to the grid and back. In this work, we introduce two modifications of the original scheme. A particle volume evolution procedure is adopted to reduce the computational error, based on the Material Point Method for solid mechanics. The additional step introduces little to none computational diffusion and efficiently suppresses the so-called ringing instability, allowing the use of explicit time differencing. Furthermore, we eliminate the need for a Poisson solver in the magnetic field computation with the use of a vector potential. The vector potential evolution is modelled with a moving grid and interpolated to the fixed grid points to obtain a solenoidal magnetic field. The results of a number of HD and MHD tests show good agreement with the reference solutions and rather fast time and space convergence. [Preview Abstract] |
Thursday, November 3, 2016 10:18AM - 10:30AM |
TO7.00005: Variational Integrators for Ideal and Reduced Magnetohydrodynamics Michael Kraus, Omar Maj, Emanuele Tassi, Daniela Grasso Ideal and reduced magnetohydrodynamics are simplified sets of magnetohydrodynamics equations with applications to both fusion and astrophysical plasmas, possessing a noncanonical Hamiltonian structure and a number of conserved functionals. We propose a new discretisation strategy for these equations based on a discrete variational principle applied to a formal Lagrangian. Discrete exterior calculus is used for the discretisation of the field variables in order to preserve their geometrical character. The resulting integrators preserve important quantities like the total energy, magnetic helicity and cross helicity exactly (up to machine precision). As these integrators are free of numerical resistivity, the magnetic field line topology is preserved and spurious reconnection is absent in the ideal case. Only when effects of finite electron mass are added, magnetic reconnection takes place. The excellent conservation properties of the methods are exemplified with numerical examples in 2D. We conclude with an outlook towards the treatment of general geometries in 3D and full magnetohydrodynamics. [Preview Abstract] |
Thursday, November 3, 2016 10:30AM - 10:42AM |
TO7.00006: Fluid moments of the nonlinear Landau collision operator Eero Hirvijoki, Manasvi Lingam, David Pfefferl\'e, Luca Comisso, Jeff Candy, Amitava Bhattacharjee One important problem in plasma physics is the lack of an accurate and complete description of Coulomb collisions in associated fluid models. To shed light on the problem, this work introduces an integral identity involving the multivariate Hermite tensor polynomials and presents a method for computing exact expressions for the fluid moments of the nonlinear Landau collision operator. The proposed methodology provides a systematic and rigorous means of extending the validity of fluid models that have an underlying inverse-square force particle dynamics to arbitrary collisionality and flow. (For details, see arXiv:1605.07589) [Preview Abstract] |
Thursday, November 3, 2016 10:42AM - 10:54AM |
TO7.00007: Fokker-Planck equation in the presence of a uniform magnetic field Ding Li, Chao Dong, Wenlu Zhang The Fokker-Planck equation in the presence of a uniform magnetic field is derived through the transform method. It has the same form as the case of no magnetic field but the Fokker-Planck coefficients are calculated based on a different motion equation and have different physical interpretations. Within the binary collision model, the Fokker-Planck coefficients are calculated explicitly which are free from infinite sums of Bessel functions. They can be used to investigate the effects of magnetic field on velocity slowing down, diffusion, and temperature relaxation conveniently. The kinetic equation is also manipulated into the Landau form and shown to be identical to the result obtained from the BBGKY approach when the collective effects are neglected and satisfy the conservation of particles, momentum, and energy. [Preview Abstract] |
Thursday, November 3, 2016 10:54AM - 11:06AM |
TO7.00008: A multilevel local discrete convolution method for the numerical solution for Maxwell's Equations Boris Lo, Phillip Colella We present a new discrete multilevel local discrete convolution method for solving Maxwell's equations in three dimensions. We obtain an explicit real-space representation for the propagator of an auxiliary system of differential equations with initial value constraints that is equivalent to Maxwell's equations. The propagator preserves finite speed of propagation and source locality. Because the propagator involves convolution against a singular distribution, we regularize via convolution with smoothing kernels (B-splines) prior to sampling. We have shown that the ultimate discrete convolutional propagator can be constructed to attain an arbitrarily high order of accuracy by using higher-order regularizing kernels and finite difference stencils. The discretized propagator is compactly supported and can be applied using Hockney's method (1970) and parallelized using domain decomposition, leading to a method that is computationally efficient. The algorithm is extended to work for locally refined fixed hierarchy of rectangular grids. [Preview Abstract] |
Thursday, November 3, 2016 11:06AM - 11:18AM |
TO7.00009: Coupling the BGK Equation and Molecular Dynamics in a Multiscale Plasma Simulation Jacob Price, Gil Shohet The Bhatnagar-Gross-Krook (BGK) approximation is an effective kinetic model of the Boltzmann equation for hot plasma given accurate relaxation parameters. These parameters are difficult to know a priori. Molecular dynamics, on the other hand, offers a fully detailed model for ionic motion. The heterogeneous multiscale method (HMM) provides a computational and analytical link between disparate physical models. In this talk, we present a proof of concept of HMM as a modeling method for hot plasma. The unknown relaxation parameters can be inferred from data collected in short, small molecular simulations, and subsequently used in the kinetic model. Simulations using the hybrid kinetic-molecular dynamic model are both more accurate than the kinetic model alone, and orders of magnitude more efficient than the molecular dynamics model alone. We will present the theory and results, comment on the advantages and limitations of the method, discuss potential applications, and propose future avenues inquiry into multiscale plasma methods. [Preview Abstract] |
Thursday, November 3, 2016 11:18AM - 11:30AM |
TO7.00010: Generalizing Microdischarge Breakdown Scaling Laws for Pressure and Gas Amanda Loveless, Allen Garner Shrinking device dimensions for micro- and nanoelectromechanical systems necessitates accurate breakdown voltage predictions for reliable operation. Additionally, one must accurately predict breakdown voltage to optimize system geometry for applications in microplasmas and micropropulsion. Traditional approaches use Paschen's law (PL) to predict breakdown, but PL fails at small gap distances (\textasciitilde 15 $\mu $m) where field emission dominates (A. Venkattraman and A. A. Alexeenko, Phys. Plasmas \textbf{19}, 123515 (2012).). Subsequent work (A. M. Loveless and A. L. Garner, Appl. Phys. Lett. \textbf{108}, 234103 (2016).) derived scaling laws and analytic expressions for breakdown voltage in argon at atmospheric pressure. Applications at high (e.g. combustion) and low (e.g. vacuum nanoelectronics) pressures for various gases motivate the generalization of these models for pressure and gas. This work addresses these concerns by deriving scaling laws generalized for gap distance, pressure, and gas, while also specifically incorporating and exploring the impact of field enhancement and work function. We compare these analytic scaling laws to experimental data and particle-in-cell simulations. [Preview Abstract] |
Thursday, November 3, 2016 11:30AM - 11:42AM |
TO7.00011: Pair-potential approximations for many-body plasma physics. M. Marciante, L.G. Stanton, M.S. Murillo Predicting properties of dense plasmas across wide parameters regimes requires the numerical solution of a many-body dynamical system whose properties depend on various underlying quantum processes. For this reason, high fidelity physics codes (e.g. DFT (orbital-free or Kohn-Sham), classical-map HNC and path integral MC) yield accurate information about the microphysical properties of dense matter. However, their computational cost restricts the simulations to a few tens to few hundreds of ions. To simulate larger systems while retaining an accurate kinetic description of ions, classical MD simulations make use of quantum-effective pair-potentials between the ions. Such potentials involve a small set of parameters, whose values are obtained from DFT calculations, and allow to simulate multi-species systems at much lower computational cost. In these models, bound electrons are usually approximated by an effective charge and free electrons are described as a continuous density. We have undertaken a detailed comparison of our DFT-informed pair-potentials, with results from higher-fidelity physics codes, including g(r), VACF Z(t), and interdiffusion coefficients, in order to determine the physical regimes in which the simpler accurate and very large-scale simulations are possible. [Preview Abstract] |
Thursday, November 3, 2016 11:42AM - 11:54AM |
TO7.00012: A Polar Discrete Ordinate Radiation Transport Method for 2D ALE Meshes in HYDRA$^{\mathrm{\ast }}$ Britton Chang, marty marinak, chris weber, luc peterson The Polar Discrete Ordinate Radiation Transport Method in HYDRA has been extended to handle general 2D r-z meshes. Previously the method was only for orthogonal 2D meshes. The new method can be employed with the ALE methodology for managing mesh motion that is used to simulate Rayleigh-Taylor and Richtmyer-Meshkov instabilities on NIF capsule implosions. The results of an examination of this kind will be compared to those obtained by the corresponding diffusion method. *This work was performed under the auspices of the Lawrence Livermore National Security, LLC, (LLNS) under Contract No. DE-AC52-07NA27344. [Preview Abstract] |
Thursday, November 3, 2016 11:54AM - 12:06PM |
TO7.00013: FRC Separatrix inference using machine-learning techniques Jesus Romero, Thomas Roche As Field Reversed Configuration (FRC) devices approach lifetimes exceeding the characteristic time of conductive structures external to the plasma, plasma stabilization cannot be achieved solely by the flux conserving effect of the external structures, and active control systems are then necessary. An essential component of such control systems is a reconstruction method for the plasma separatrix suitable for real time. We report on a method to infer the separatrix in an FRC using the information of magnetic probes located externally to the plasma. The method uses machine learning methods, namely Bayesian inference of Gaussian Processes, to obtain the most likely plasma current density distribution given the measurements of magnetic field external to the plasma. From the current sources, flux function and in particular separatrix are easily computed. The reconstruction method is non iterative and hence suitable for deterministic real time applications. Validation results with numerical simulations and application to separatrix inference of C-2U plasma discharges will be presented. [Preview Abstract] |
Thursday, November 3, 2016 12:06PM - 12:18PM |
TO7.00014: Bayesian modelling of JET high resolution Thomson scattering system using the Minerva framework Sehyun Kwak, Jakob Svensson, Sergey Bozhenkov, Joanne Flanagan, Mark Kempenaars, Young-chul Ghim A Bayesian model for JET high resolution Thomson scattering (HRTS) system has been developed to infer electron temperature and density profiles. The model has been implemented in the Minerva framework. The HRTS system detects Thomson scattered photons from the injected \textasciitilde 20 ns long laser pulse penetrating along the midplane of the JET at 63 spatial points on the low field side (R $=$ 2.9 \textasciitilde 3.9 m) with 1\textasciitilde 1.6 cm spatial resolution and 20 Hz repetition rate. The Selden-Matoba Thomson scattering model infers scattered and stray light intensities as well as associated uncertainties taking into account of photon statistics and electrical fluctuations. The Markov Chain Monte Carlo (MCMC) method explores the posterior distribution of the electron temperature and density profiles which explain both HRTS and the interferometry data simultaneously within their uncertainties. The electron temperature and density profiles are modelled via Gaussian processes mapped onto normalised flux coordinates. The electron density profiles are automatically calibrated through the inclusion of interferometers in the model. [Preview Abstract] |
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