Pegasus: A New Hybrid-Kinetic Particle-in-Cell Code for Astrophysical Plasma Dynamics

We describe Pegasus, a new hybrid-kinetic particle-in-cell code tailored for the study of astrophysical plasma dynamics. The code incorporates an energy-conserving particle integrator into a stable, second-order--accurate, three-stage predictor-predictor-corrector integration algorithm. The constrained transport method is used to enforce the divergence-free constraint on the magnetic field. A delta-f scheme is included to facilitate a reduced-noise study of systems in which only small departures from an initial distribution function are anticipated. The effects of rotation and shear are implemented through the shearing-sheet formalism with orbital advection. These algorithms are embedded within an architecture similar to that used in the popular astrophysical magnetohydrodynamics code Athena, one that is modular, well-documented, easy to use, and efficiently parallelized for use on thousands of processors. We present a series of tests in one, two, and three spatial dimensions that demonstrate the fidelity and versatility of the code.Comment: 27 pages, 12 figures, accepted for publication in Journal of Computational Physic

Accurate particle time integration for solving Vlasov-Fokker-Planck equations with specified electromagnetic fields

The Vlasov-Fokker-Planck equation (together with Maxwell's equations) provides the basis for plasma flow calculations. While the terms accounting for long range forces are established, different drift and diffusion terms are used to describe Coulomb collisions. Here, linear drift and a constant diffusion coefficient are considered and the electromagnetic fields are imposed, i.e., plasma frequency is not addressed. The solution algorithm is based on evolving computational particles of a large ensemble according to a Langevin equation, whereas the time step size is typically limited by plasma frequency, Coulomb collision frequency and cyclotron frequency. To overcome the latter two time step size constraints, a novel time integration scheme for the particle evolution is presented. It only requires that gradients of mean velocity, bath temperature, magnetic field and electric field have to be resolved along the trajectories. In fact, if these gradients are zero, then the new integration scheme is statistically exact; no matter how large the time step is chosen. Obviously, this is a computational advantage compared to classical integration schemes, which is demonstrated with numerical experiments of isolated charged particle trajectories under the influence of constant magnetic- and electric fields. Besides single ion trajectories, also plasma flow in spatially varying electromagnetic fields was investigated, that is, the influence of time step size and grid resolution on the final solution was studied

Fully kinetic simulations of magnetic reconnction in semi-collisional plasmas

The influence of Coulomb collisions on the dynamics of magnetic reconnection is examined using fully kinetic simulations with a Monte-Carlo treatment of the Fokker-Planck collision operator. This powerful first-principles approach offers a bridge between kinetic and fluid regimes, which may prove useful for understanding the applicability of various fluid models. In order to lay the necessary groundwork, the collision algorithm is first carefully bench marked for a homogeneous plasma against theoretical predictions for beam-plasma interactions and electrical resistivity. Next, the collisional decay of a current layer is examined as a function of guide field, allowing direct comparisons with transport theory for the parallel and perpendicular resistivity as well as the thermoelectric force. Finally, the transition between collisional and collision less reconnection is examined in neutral sheet geometry. For modest Lundquist numbers S {approx}< 1000, a distinct transition is observed when the thickness of the Sweet-Parker layers falls below the ion inertia length {delta}{sub sp} {approx}< d,. At higher Lundquist number, deviations from the Sweet-Parker scaling are observed due to the growth of plasmoids (secondary-islands) within the elongated resistive layer. In certain cases, this instability leads to the onset of fast reconnection sooner than expected from {delta}{sub sp} {approx} d, condition. After the transition to fast reconnection, elongated electron current layers are formed which are unstable to the formation of new plasmoids. The structure and time-dependence of the electron diffusion region in these semi-collisional regimes is profoundly different than reported in two-fluid simulations

Short-pulse laser interactions with high density plasma

The constraints on particle-in-cell (PIC) simulations of short-pulse laser interactions with solid density targets severely limit the spatial and temporal scales which can be modelled routinely. Although recent advances in high performance computing (HPC) capabilities have rendered collisionless simulations at a scale and density directly applicable to experiments tractable, detailed modelling of the fast electron transport resulting from the laser interaction is often only possible by sampling the fast electron populations and passing this information to a separate, dedicated transport code. However, this approach potentially neglects phenomena which take place or are seeded near the transition between the two codes. Consequently there is a need to develop techniques capable of efficiently modelling fast electron transport in high density plasma without being subject to the usual grid-scale and time-step constraints. The approach employed must also be compatible with retaining the standard PIC model in the laser interaction regions in order to model laser absorption and charged particle acceleration processes. Such an approach, proposed by Cohen, Kemp and Divol [J. Comput. Phys., 229:4591, 2010], has been identified, adapted and implemented in EPOCH. The final algorithm, as implemented, is presented here. To demonstrate the ability of the adapted code to model high intensity laser-plasma interactions with peak densities at, and above, solid density, the results of simulations investigating filamentation of the fast electron population and heating of the bulk target, at high densities, are presented and compared with the results of recent experiments as well as other, similar codes