LANGEVIN REPRESENTATION OF COULOMB COLLISIONS IN PIC SIMULATIONS

An efficient grid-based Langevin formulation is developed for treating electron-electron (e-e) and electron-ion collisions within a particle -in-cell plasma simulation code. The formulation is energy-and momentu m-conserving. If the thermal part of the electron velocity distributio n is reasonably close to isotropy in any local frame of reference, the basic scattering algorithm is quantitatively accurate for electrons w ith any value of energy. This is particularly important in calculating the approach to equilibrium of the high-energy tail, or the equilibri um under the competing influences of e-e collisions, inelastic electro n-neutral collisions, and end losses through sheaths. If the electron velocity distribution is multi-peaked or very anisotropic, accurate ca lculations can be performed by representing the electrons as a superpo sition of several beams. Computational examples are given illustrating both equilibrium energy distribution and approach to equilibrium. (C) 1997 Academic Press.