Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session YO5: Computer Simulation Methods |
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Chair: Gian Luca Delzano, Los Alamos National Laboratory Room: 200 |
Friday, November 20, 2015 9:30AM - 9:42AM |
YO5.00001: The semi-implicit, adaptive Multi-Level Multi-Domain method for Particle In Cell plasma simulations Maria Elena Innocenti, Stefano Markidis, Giovanni Lapenta The Multi Level Multi Domain (MLMD) method (Innocenti (2013), Beck (2014)) is a fully kinetic, semi-implicit PIC method which simulates a domain as a collection of sub-domains where increasingly higher resolution is used. The aim is to reduce the computational costs of PIC simulations: simulations which are computationally challenging even with a traditional semi-implicit PIC code, e.g., realistic mass ratio simulations, become feasible with moderate computational resources. We present two sets of realistic mass ratio simulations: magnetic reconnection and Lower Hybrid Drift Instability (LHDI). MLMD reconnection simulations are discussed in Innocenti (2015). In the MLMD LHDI simulations, we show how the MLMD method cheaply extends the range of simulatedwavenumbers with respect to traditional simulations. We simulate the three LHDI stages (fast and slow LHDI branch, kink instability), which are well separated in wavenumber at realistic mass ratio. The coupling observed by Norgren (2012) between the magnetic field and perpendicular electric field LHDI oscillations in the magnetotail is investigated in these different stages. [Preview Abstract] |
Friday, November 20, 2015 9:42AM - 9:54AM |
YO5.00002: ABSTRACT WITHDRAWN |
Friday, November 20, 2015 9:54AM - 10:06AM |
YO5.00003: Numerical Modeling of Discrete Space Charge Affected Field Emission in Planar Emitters I.M. Rittersdorf, K.L. Jensen, D.A. Shiffler, A.S. Richardson, R.J. Allen, J.L. Lebowitz, Y.Y. Lau, J.W. Schumer, J.W. Luginsland Field emission sources generate high brightness electron beams that may meet the needs of particle accelerators, high power microwave and x-ray sources, and vacuum electronic devices. Such sources are profoundly affected by space charge near emission sites, such as surface roughness protrusions. It is often impractical to resolve the spatial scale of such emission sites in a particle-in-cell code, and it is therefore desirable to treat such emission sites within a separate model (i.e., a Unit Cell model). This paper will focus on a Monte Carlo model that investigates discrete sheets of electrons emitted from a planar surface as a first step towards the goal of modeling geometrical surfaces. Planar model results showing oscillations in the current density that arise once the first sheet of charge transits across the diode gap and its convergence to analytical results will be presented. [Preview Abstract] |
Friday, November 20, 2015 10:06AM - 10:18AM |
YO5.00004: Taking larger timesteps with speed-limited particle-in-cell simulation Gregory Werner, John Cary Particle-in-cell (PIC) simulation is often impractical because it includes too much unnecessary physics. For example, to avoid instability in many simulations the timestep must be small enough to resolve the plasma frequency, even if plasma oscillations do not play a significant role. Other methods (e.g., MHD/fluid and hybrid approaches) allow faster simulation, but often don't include enough physics. A new method, speed-limited PIC (SLPIC) simulation, offers kinetic simulation with an arbitrary-strength approximation tied to the timestep. With a small (standard PIC) timestep, SLPIC is identical to PIC, while a larger timestep (e.g., large compared to the inverse plasma frequency) results in the relaxation of fast particles over slower timescales. SLPIC is therefore useful in situations where the particle distribution functions change slowly compared to the timestep required by PIC. For example, SLPIC can simulate collisionless sheaths with a timestep hundreds of times larger than the inverse plasma frequency. SLPIC involves relatively isolated changes of a standard PIC code and poses no extra difficulties for parallelism; complexities of PIC, such as field solvers, collisions, and boundary conditions, carry over naturally to SLPIC with little change. [Preview Abstract] |
Friday, November 20, 2015 10:18AM - 10:30AM |
YO5.00005: A new conservative multispecies BGK model Jeff Haack, Michael Murillo, Cory Hauck In this talk, I will present joint work with C. Hauck (ORNL) and M. Murillo (LANL) on a new multi-species BGK model for dense plasmas. This model conserves mass, momentum, and kinetic energy and allows for a more clear connection to the underlying cross sections and inter-species collision rates. [Preview Abstract] |
Friday, November 20, 2015 10:30AM - 10:42AM |
YO5.00006: A New Poisson Solver PIC Simulations on Arbitrary Unstructured Tetrahedral Meshes Sergey Averkin, Nikolaos Gatsonis A new node-based algorithm is developed for the solution of Poisson's equation in PIC simulations on arbitrary unstructured tetrahedral meshes. The algorithm is derived by applying the integral form of the Gauss law to the indirect dual mesh constructed by connecting the centroids of edges to the centroids of faces and centroids of faces with the centroids of tetrahedral cells for each tetrahedron. The potential variation is assumed linear inside every cell and allows to estimate the potential gradient in each cell from the nodal values. The obtained sparse linear system is solved with the GMRES solver combined with the ILU(0) preconditioner. The new algorithm is verified with the simulation of the current collection by cylindrical Langmuire probes in the collisionless regime for a wide range of probe to Debye length ratios. The computed electron and ion number density variations as well as electric potential and collected currents compare well with the simulation results of Laframboise. [Preview Abstract] |
Friday, November 20, 2015 10:42AM - 10:54AM |
YO5.00007: Weak turbulence simulations with the Hermite-Fourier spectral method Juris Vencels, Gian Luca Delzanno, Gianmarco Manzini, Vadim Roytershteyn, Stefano Markidis Recently, a new (transform) method based on a Fourier-Hermite (FH) discretization of the Vlasov-Maxwell equations has been developed. The resulting set of moment equations is discretized implicitly in time with a Crank-Nicolson scheme and solved with a nonlinear Newton-Krylov technique. For periodic boundary conditions, this discretization delivers a scheme that conserves the total mass, momentum and energy of the system exactly. In this work, we apply the FH method to study a problem of Langmuir turbulence, where a low signal-to-noise ratio is important to follow the turbulent cascade and might require a lot of computational resources if studied with PIC. We simulate a weak (low density) electron beam moving in a Maxwellian plasma and subject to an instability that generates Langmuir waves and a weak turbulence field. We also discuss some optimization techniques to optimally select the Hermite basis in terms of its shift and scaling argument, and show that this technique improve the overall accuracy of the method. Finally, we discuss the applicability of the HF method for studying kinetic plasma turbulence. [Preview Abstract] |
Friday, November 20, 2015 10:54AM - 11:06AM |
YO5.00008: Particle-in-cell delta-f gyrokinetic simulations of the microtearing mode in NSTX Jugal Chowdhury, Yang Chen, Weigang Wan, Scott E. Parker, Walter Guttenfelder, John Canik The properties of the microtearing mode for the National Spherical Torus Experiment edge and core parameters are studied using the particle-in-cell method based nonlinear, electromagnetic gyrokinetic code GEM. We investigate the dependence of the microtearing mode on various equilibrium quantities for the two cases. Although, the mode in both regimes depends on electron temperature gradient and beta in the same way, we observe different behavior with respect to the electron-ion collision. The role of electrostatic potential is non-negligible in each of the cases. It plays opposite role in the core and edge of the NSTX and leads to substantial stabilization or destabilization of the mode. These results will then be compared to the microtearing mode in a conventional tokamak. [Preview Abstract] |
Friday, November 20, 2015 11:06AM - 11:18AM |
YO5.00009: Destabilization of trapped electron clouds by embedded and collision-generated ions: PIC-MCC simulations Meghraj Sengupta, Rajaraman Ganesh A 2d3v PIC code with facility for Monte Carlo Collisions of plasma with background neutrals has been developed, and is used to simulate ion resonance instability in cylindrical traps. Pure PIC simulations of the classical two-stream / ion resonance instability in partially neutralized electron clouds, reveal interesting dynamics, energetics, and collisionless cross-field transport of the e- and i$+$ component [1]. In another study a pure electron plasma is loaded with axial velocities that grossly mimic the axial bouncing in traps. MCC are invoked to simulate ionizing and other collisions of e- and generated i$+$ with background neutrals. The focus here is on understanding how the cloud gets destabilized by dynamically generated ions. The effects of prolonged evolution of the cloud within the same neutral background, and the influence of non ionizing charge-neutral collisions on the growth of the instability are also investigated [2]. Results of these simulations [1,2] will be presented. \\[4pt] [1] M. Sengupta and R. Ganesh (Accepted in Phys. Plasmas: 30$^{\mathrm{th}}$June 2015)\\[0pt] [2] M. Sengupta and R. Ganesh (Manuscript in preparation) [Preview Abstract] |
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