Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session PO5: Numerical Simulation Methods |
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Chair: Greg Moses, University of Wisconsin-Madison Room: Galerie 2 |
Wednesday, October 29, 2014 2:00PM - 2:12PM |
PO5.00001: Relativistic Modeling Capabilities in PERSEUS Extended MHD Simulation Code for HED Plasmas Nathaniel Hamlin, Charles Seyler We discuss the incorporation of relativistic modeling capabilities into the PERSEUS extended MHD simulation code for high-energy-density (HED) plasmas, and present the latest simulation results. The use of fully relativistic equations enables the model to remain self-consistent in simulations of such relativistic phenomena as hybrid X-pinches and laser-plasma interactions. A major challenge of a relativistic fluid implementation is the recovery of primitive variables (density, velocity, pressure) from conserved quantities at each time step of a simulation. This recovery, which reduces to straightforward algebra in non-relativistic simulations, becomes more complicated when the equations are made relativistic, and has thus far been a major impediment to two-fluid simulations of relativistic HED plasmas. By suitable formulation of the relativistic generalized Ohm's law as an evolution equation, we have reduced the central part of the primitive variable recovery problem to a straightforward algebraic computation, which enables efficient and accurate relativistic two-fluid simulations. Our code recovers expected non-relativistic results and reveals new physics in the relativistic regime. [Preview Abstract] |
Wednesday, October 29, 2014 2:12PM - 2:24PM |
PO5.00002: Particle-In-Cell Modeling For MJ Dense Plasma Focus with Varied Anode Shape A. Link, C. Halvorson, A. Schmidt, E.C. Hagen, D. Rose, D. Welch Megajoule scale dense plasma focus (DPF) Z-pinches with deuterium gas fill are compact devices capable of producing 10$^{12}$ neutrons per shot but past predictive models of large-scale DPF have not included kinetic effects such as ion beam formation or anomalous resistivity. We report on progress of developing a predictive DPF model by extending our 2D axisymmetric collisional kinetic particle-in-cell (PIC) simulations to the 1 MJ, 2 MA Gemini DPF using the PIC code LSP. These new simulations incorporate electrodes, an external pulsed-power driver circuit, and model the plasma from insulator lift-off through the pinch phase. The simulations were performed using a new hybrid fluid-to-kinetic model transitioning from a fluid description to a fully kinetic PIC description during the run-in phase. Simulations are advanced through the final pinch phase using an adaptive variable time-step to capture the fs and sub-mm scales of the kinetic instabilities involved in the ion beam formation and neutron production. Results will be present on the predicted effects of different anode configurations. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory (LLNL) under Contract DE-AC52-07NA27344 and supported by the Laboratory Directed Research and Development Program (11-ERD-063) and the Computing Grand Challenge program at LLNL. This work supported by Office of Defense Nuclear Nonproliferation Research and Development within U.S. Department of Energy's National Nuclear Security Administration. [Preview Abstract] |
Wednesday, October 29, 2014 2:24PM - 2:36PM |
PO5.00003: Intensity-dependent criteria for PIC simulations of relativistic electrons in a laser field Ginevra E. Cochran, Alexey V. Arefiev, Douglass W. Schumacher, A.P.L. Robinson, Guangye Chen We present a study of particle-in-cell simulation error in modeling a free electron in an ultraintense laser field, comparing the codes PSC and LSP. We find an unexpectedly small timestep is required for both codes to resolve the classical electron motion, decreasing with increasing a$_{o}$, the normalized vector potential. We consider grid dispersion, the field solver, and the particle pusher as sources of error, and find by comparing the codes with a simple particle pusher that the particle pusher error dominates the results. We derive the constraint imposed by use of the relativistic Boris particle pusher on the timestep and find that it must decrease inversely as a$_{o}$. We find the particle pusher error accumulates on the small trajectory segments where the gamma-factor is approximately unity and the laser fields are strong, and present a sub-cycled version of the simple particle pusher code which reduces error. This work was supported in part by an allocation of computing time from the Ohio Supercomputer Center. This work was supported by U.S. Department of Energy Contract Nos. DE-FC52-06NA26262 and DE-FG02-04ER54742, and National Nuclear Security Administration Contract No. DE-FC52-08NA28512. [Preview Abstract] |
Wednesday, October 29, 2014 2:36PM - 2:48PM |
PO5.00004: Numerical stability of pseudo-spectral PIC code generalizations Brendan B. Godfrey, Jean-Luc Vay Laser Plasma Accelerator (LPA) particle-in-cell (PIC) simulations are computationally demanding, because they require beam transport over times and distances long compared with the natural scales of the acceleration mechanism and because they are prone to numerical instabilities. To provide greater flexibility in LPA PIC simulations, we have generalized the Pseudo-Spectral Time Domain (PSTD) algorithm to accommodate arbitrary order spatial derivative approximations and substantially longer time steps. Here, we show that, by extending approaches developed by us for other PIC algorithms, numerical Cherenkov instabilities can be suppressed for the generalized PSTD algorithm. We also illustrate the relationships between the generalized PSTD and other PIC algorithms, such as Finite Difference Time Domain (FDTD) and Pseudo-Spectral Analytical Time Domain (PSATD) algorithms. Background information can be found at http://hifweb.lbl.gov/public/BLAST/Godfrey/. [Preview Abstract] |
Wednesday, October 29, 2014 2:48PM - 3:00PM |
PO5.00005: Macroparticle merging algorithm for PIC Marija Vranic, Thomas Grismayer, Joana L. Martins, Ricardo A. Fonseca, Luis O. Silva With the development of large supercomputers ($>$1000000 cores), the complexity of the problems we are able to simulate with particle-in-cell (PIC) codes has increased substantially. However, localized density spikes can introduce load imbalance where a small fraction of cores is occupied, while the others remain idle. An additional challenge lies in self-consistent modeling of QED effects at ultra-high laser intensities (I $>$ 10$^{23}$ W/cm$^2$), where the number of pairs produced sometimes grows exponentially and may reach beyond the maximum number of particles that each processor can handle. We can overcome this by resampling the 6D phase space: the macroparticles can be merged into fewer particles with higher particle weights. Existing merging scheme [1] preserves the total charge, but not the particle distribution. Here we present a novel particle-merging [2] algorithm that preserves the energy, momentum and charge locally and thereby minimizes the potential influence to the relevant physics. Through examples of classical plasma physics and more extreme scenarios, we show that the physics is not altered but we obtain an immense increase in performance.\\[4pt] [1] A. N. Timokhin, Mon. Not. R. Astron. Soc. 408 (2010)\\[0pt] [2] M. Vranic, T. Grismayer, et. al., to be submitted (2014) [Preview Abstract] |
Wednesday, October 29, 2014 3:00PM - 3:12PM |
PO5.00006: First PIC simulations modeling the interaction of ultra-intense lasers with sub-micron, liquid crystal targets Matthew McMahon, Patrick Poole, Christopher Willis, David Andereck, Douglass Schumacher We recently introduced liquid crystal films as on-demand, variable thickness (50 -- 5000 nanometers), low cost targets for intense laser experiments [POP 21, 063109 (2014)]. Here we present the first particle-in-cell (PIC) simulations of short pulse laser excitation of liquid crystal targets treating Scarlet (OSU) class lasers using the PIC code LSP. In order to accurately model the target evolution, a low starting temperature and field ionization model are employed. This is essential as large starting temperatures, often used to achieve large Debye lengths, lead to expansion of the target causing significant reduction of the target density before the laser pulse can interact. We also present an investigation of the modification of laser pulses by very thin targets. This work was supported by the DARPA PULSE program through a grant from ARMDEC, by the US Department of Energy under Contract No. DE-NA0001976, and allocations of computing time from the Ohio Supercomputing Center. [Preview Abstract] |
Wednesday, October 29, 2014 3:12PM - 3:24PM |
PO5.00007: Fokker Planck theory for energetic electron deposition in laser fusion Wallace Manheimer, Denis Colombant We have developed a Fokker Planck model to calculate the transport and deposition of energetic electrons, produced for instance by the two plasmon decay instability at the quarter critical surface [1]. In steady state, the Fokker Planck equation reduces to a single universal equation in energy and space, an equation which appears to be quite simple, but which has a rather unconventional boundary condition. The equation is equally valid in planar and spherical geometry, and it depends on only a single parameter, the charge state Z. Hence one can solve for a universal solution, valid for each Z. An asymptotic solution to this equation will be presented, which allows the heating of the main plasma to be calculated from a simple analytical expression. A more accurate solution in terms of a Bessel function expansion will also be presented. From this, one obtains a heating rate which can be simply incorporated into fluid simulations.\\[4pt] [1] B. Yaakobi et al, Phys. Plasmas 19, 012704, 2012. Work supported by DoE-NNSA and ONR [Preview Abstract] |
Wednesday, October 29, 2014 3:24PM - 3:36PM |
PO5.00008: Nonlocal electron transport: comparison with the SNB model and results for implosion runs Denis Colombant, Wallace Manheimer, Andrew J. Schmitt Two main models have been proposed for nonlocal transport: the SNB model\footnote{Schurz \textit{et al.}, Phys. Plasmas \textbf{7}, 4238 (2000)} and ours, the velocity dependent Krook (VDK) model.\footnote{Manheimer \textit{et al.}, Phys. Plasmas \textbf{19}, 056317 (2012)} Although these models are based on similar basic equations, they differ in some aspects. A comparison between the two models was published last year,\footnote{Marocchino \textit{et al}, Phys. Plasmas \textbf{20}, 022702 (2013)} followed this year by implosion calculations using the SNB model only.\footnote{Marocchino \textit{et al.}, Phys. Plasmas \textbf{21}, 012701 (2014)} Our model has since then been updated numerically and runs much faster than our previous one and is now comparable to SNB in running time. We have run some of the same test problems as Marocchino et al. and we have also made some implosion runs for shock ignition targets. We have also updated our code to make it easier to change the electron collision frequencies when needed. Most of the results we have obtained are quite different from Marocchino's, in particular from his version of our model. We present these results and discuss them in some detail. [Preview Abstract] |
Wednesday, October 29, 2014 3:36PM - 3:48PM |
PO5.00009: Direct Comparison of Full-Scale Vlasov-Fokker-Planck and Classical Modeling of Megagauss Magnetic Field Generation in Plasma Near Hohlraum Walls From Nanosecond Laser Pulses Archis Joglekar, Alexander Thomas, Martin Read, Robert Kingham Here, we present 2D numerical modeling of near critical density plasma using a fully implicit Vlasov-Fokker-Planck (VFP) code, IMPACTA, with the addition of a ray tracing package. In certain situations, such as those at the critical surface at the walls of a hohlraum, magnetic fields are generated through the crossed temperature and electron density gradients. Modeling shows 0.3 MG fields and the strong heating also results in magnetization of the plasma up to $\omega\tau \sim 5$. In the case without magnetic field generation, the heat flows from the laser heating region are isotropic. Including magnetic fields causes the heat flow to form jets along the wall due to the Righi-Leduc effect. The heating of the wall region causes steeper temperature gradients. This serves as a positive feedback mechanism for the field generation rate resulting in nearly twice the amount of field generated in comparison to the case without magnetic fields over 1 ns. The heat conduction, field generation, and the calculation of other transport quantities, is performed ab-initio due to the nature of the VFP equation set. In order to determine the importance of the kinetic effects from IMPACTA, we perform direct comparison with a classical (Braginskii) transport code with hydrodynamic motion (CTC+). [Preview Abstract] |
Wednesday, October 29, 2014 3:48PM - 4:00PM |
PO5.00010: A Numerical Study of the Two and Three Dimensional Richtmyer Meshkov Instability Ye Zhou, Ben Thornber The Richtmyer-Meshkov instability occurs as shock waves pass through a perturbed material interface. This paper presents a series of large-eddy-simulations of the two dimensional turbulent RM instability and compares the results to the fully three dimensional simulations conducted by Thornber et al. There are two aims to this paper, the first is to explore the number of independent realisations which are required to give a statistically converged solution for a two dimensional flow field, in a similar vein to that undertaken by Clark. The second aim is to elucidate the key differences in flow physics between the two dimensional and three dimensional Richtmyer-Meshkov instabilities, particularly their asymptotic self-similar regime. Earlier publications on the Rayleigh Taylor instability imply that lower mixing, larger structures, and more rapid late time growth are expected. The full paper will detail the statistical convergence of the 2D simulations a function of ensemble number and grid resolution, and ensemble averaged growth rates, mixing parameters, turbulent kinetic energy and spectra compared to the equivalent parameters from 3D mixing simulations. [Preview Abstract] |
Wednesday, October 29, 2014 4:00PM - 4:12PM |
PO5.00011: Thermodynamic modeling of uncertainties in NIF ICF implosions due to underlying microphysics models Jim Gaffney, Paul Springer, Gilbert Collins Design and analysis calculations for ICF implosions rely on a large number of physics models, which are extremely difficult to test in isolation. As a result, models are often run in regimes where physical models contain significant uncertainties. While efforts have been made to design ignition targets that are robust to physics uncertainties, the use of full-scale hydrodynamic simulations limit these studies to sparse, low dimensional grids. More lightweight models, while much simpler, have proven very useful in analyzing and understand experimental ICF data and play an essential role in moving the field forward. We will describe a thermodynamic hot spot model that includes all physical models, along with variations that are consistent with the expected uncertainties, that is fast and lightweight enough to perform studies consisting of a million simulation points or more. We will present results from a large number of calculations and discuss the use of these data in understanding experimental results, with particular emphasis on underlying microphysics models. [Preview Abstract] |
Wednesday, October 29, 2014 4:12PM - 4:24PM |
PO5.00012: Enhanced NLTE Atomic Kinetics Modeling Capabilities in HYDRA Mehul V. Patel, Howard A. Scott, Michael M. Marinak In radiation hydrodynamics modeling of ICF targets, an NLTE treatment of atomic kinetics is necessary for modeling high-Z hohlraum wall materials, high-Z dopants mixed in the central gas hotspot, and is potentially needed for accurate modeling of outer layers of the capsule ablator. Over the past several years, the NLTE DCA atomic physics capabilities in the 3D ICF radiation hydrodynamics code HYDRA have been significantly enhanced. The underlying atomic models have been improved, additional kinetics options including the ability to run DCA in cells with dynamic mixing of species has been added, and the computational costs have been significantly reduced using OpenMP threading. To illustrate the improved capabilities, we will show higher fidelity results from simulations of ICF hohlraum energetics, laser irradiated sphere experiments, and ICF capsule implosions. [Preview Abstract] |
Wednesday, October 29, 2014 4:24PM - 4:36PM |
PO5.00013: Variational integration for ideal MHD with built-in advection equations Yao Zhou, Hong Qin, Joshua Burby, Amitava Bhattacharjee Newcomb's Lagrangian for ideal MHD in Lagrangian labeling is discretized using discrete exterior calculus. Variational integrators for ideal MHD are derived thereafter. Besides being symplectic and momentum-preserving, the schemes inherit built-in advection equations from Newcomb's formulation, and therefore mitigate numerical resistivity significantly. We implement the method in 2D and show that it does not suffer from numerical reconnection when singular current sheets are present. We then apply it to studying the dynamics of the ideal coalescence instability with multiple islands. The relaxed equilibrium state with embedded current sheets is obtained numerically. [Preview Abstract] |
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