NUMERICAL INSTABILITY IN A 2D GYROKINETIC CODE CAUSED BY DIVERGENT E-X-B FLOW

In this paper, a numerical instability first observed in a 2D electros tatic gyrokinetic code is described. The instability should also be pr esent in some form in many versons of particle-in-cell simulation code s that employ guiding center drifts. A perturbation analysis of the in stability is given and its results agree quantitatively with the obser vations from the gyrokinetic code in all respects. The basic mechanism is a false divergence of the E x B flow caused by the interpolation b etween the grid and the particles as coupled with the specific numeric al method for calculating E = del phi. Stability or instability depend s in detail on the specific choice of particle interpolation method an d field method. One common interpolation method, subtracted dipole, is stable. Other commonly used interpolation methods, linear and quadrat ic, are unstable when combined with a finite difference for the electr ic field. Linear and quadratic interpolation can be rendered stable if combined with another method for the electric field, the analytic dif ferential of the interpolated potential. (C) 1994 Academic Press, Inc.