A GRID-BASED COULOMB COLLISION MODEL FOR PIC CODES

A new method is presented to model the intermediate regime between col lisionless and Coulomb collision dominated plasmas in particle-in-cell codes. Collisional processes between particles of different species a re treated through the concept of a grid-based ''collision field,'' wh ich can be particularly efficient for multi-dimensional applications. In this method, particles are scattered using a force which is determi ned from the moments of the distribution functions accumulated on the grid. The form of the force is such to reproduce the multi fluid trans port equations through the second (energy) moment. Collisions between particles of the same species require a separate treatment. For this, a Monte Carte-like scattering method based on the Langevin equation is used. The details of both methods are presented, and their implementa tion in a new hybrid (particle ion, massless fluid electron) algorithm is described. Aspects of the collision model are illustrated through several one- and two-dimensional test problems as well as examples inv olving laser produced colliding plasmas. (C) 1996 Academic Press, Inc.