A Coulomb collision model for PIC plasma simulation

A new approach to modeling partially collisional plasmas that provides a smooth transition from the fluid (Coulomb collision dominated) to the fully kinetic PIC (collisionless) limit is presented. In addition to the usual quantities of mass, charge, and velocity, each particle carries an isotropic Maxwellian velocity distribution. Higher resolution of velocity space is achieved by generating more particles using a procedure that preserves the first four velocity moments. This velocity space fragmentation is essential for capturing non-Maxwellian plasma behavior. The model developed here allows the efficient simulation of partially collisional plasmas by reducing both the number of particle pairings required per time step and the number of particles needed to retain non-Maxwellian plasma behavior. The method produces reasonable results when the time step is large relative to the collision frequencies and works in the limit of one particle per species per cell. Particle merging can be exploited to control the number of particles in a natural way. The collision process is fully three-dimensional and conserves energy and momentum exactly. Results from 3v and 1d3v simulations are presented and compared with previous multi-fluid and fully kinetic PIC simulations.