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Kinetic Plasma Simulation: Meeting the Demands of Increased Complexity


This dissertation concerns the development and use of numerical simulation techniques for studying nonlinear plasma systems in which accurate representations of the electron distribution function are required. The kinetic description of the electrons is accomplished via two different simulation modalities: the code OSHUN, which directly solves the Vlasov-Fokker-Planck (VFP) partial differential equation, and the code OSIRIS, which uses the particle-in-cell (PIC) method including an option for a separate Monte Carlo collision model.

The dissertation consists of ten chapters that are based on reprints of refereed publications that describe the development and use of OSHUN and OSIRIS. The increasing complexity of today’s computers necessitates an increase in the complexity of software to take full advantage of the available computing resources. This requires that software be engineered properly to ensure correct functioning and to enable more developers to contribute. The dissertation includes examples of the creation --- that is, combining new and novel algorithms with software engineering techniques --- and novel usage of simulation software packages capable of exploiting the power of today's computers to enable new capability and discovery.

OSHUN includes relativistic corrections to the Vlasov equation but uses a non-relativistic description for the collision operator. The fields can be advanced in time using the full set of Maxwell’s equations explicitly, just the electrostatic fields, or an implicit set of equations that includes Ampere’s law without the displacement current. An arbitrary number of spherical harmonics can be included permitting efficient studies of physics when the distribution function is nearly in or far from equilibrium. This can drastically reduce the computational cost when only a few spherical harmonics are required. OSHUN was tested against a variety of problems spanning collisional and collisionless systems including Landau Damping, the two stream instability, Spitzer-Harm, and Epperlein-Haines heat flow coefficients in warm magnetized and unmagnetized plasmas. It was also used to explore how the heat flow in the laser entrance hole could modify Stimulated Raman Backscatter in Inertial Confinement Fusion relevant plasmas.

New numerical/algorithmic techniques where implemented in the PIC code OSIRIS. In particular, new software engineering techniques facilitated the addition of an algorithm which uses PIC in the r-z coordinates system with a gridless description in the azimuthal angle \phi. The fields, equations, and current are decomposed into an azimuthal mode, m, expansion. This Quasi-3D description permits 3D simulations at a drastically lower computational cost (approaching the cost of 2D simulations) in systems that exhibit nearly azimuthal (cylindrical) symmetry. This capability was used to

examine laser wakefield acceleration (LWFA). It was used to verify scaling laws for LWFA in a nonlinear, self-guide regime. The Quasi-3D algorithm was coupled to an independently developed module in OSIRIS that allows simulation of LWFA in a Lorentz-boosted frame. Doing the calculations in this frame yields a computational savings that scales as gamma^2 (where gamma is the Lorentz boost factor) which typically ranges from 100 to 100,000 in the systems under consideration. These modules required the development of novel field solvers and current deposition algorithms to eliminate a numerical instability called the Numerical Cerenkov Instability (NCI). These were added to OSIRIS using the new software engineering techniques now possible with Fortran 2003.

OSIRIS was updated to utilize the Graphics Processing Units (GPUs) present in exascale systems like the Summit supercomputer recently built at the Oak Ridge National Laboratory. A GPU version of OSIRIS was used to examine the interactions of Laser Speckles from Stimulated Raman Scattering (SRS). It was found that speckles can mutually interact via scattering light, plasma waves, or non-thermal electrons transporting from speckles above threshold from SRS. This can trigger SRS in speckles that were below threshold.

Efforts towards the ultimate (and ongoing) goal of fully integrating the Quasi-3D, Lorentz-boosted frame, and GPU modules is described. When combined, these modules have the potential speed up 3D laser-plasma simulations by immense factors of a million or more.

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