Improvements to Particle Merge Algorithms for Sandia National Laboratories Plasma Physics Modeling Code, EMPIRE

Problems of frequent interest in the application of SNL's multiphysics plasma modeling code, EMPIRE, involve intense electron flows in an environment that includes a background gas. Collisional ionization of the background gas can lead to rapid increases in the particle population. This can occur on a timescale and with an intensity that can result in extreme performance degradation even in the presence of sophisticated parallization capabilities. An approach to dealing with phenomena of this sort is to replace a specific subset of macroparticles, such as that which resulted from the ionization avalanche, with a set that contains significantly fewer macroparticles, yet conserves essential aspects of the original particle distribution. Algorithms that perform this task are referred to as merge algorithms.