FINITE-GRID INSTABILITY IN QUASI-NEUTRAL HYBRID SIMULATIONS

Analysis and simulation of numerical instability caused by the spatial grid is presented for the case of quasineutral hybrid plasma simulati on. The simplest case of particle ions advanced in the ambipolar elect ric field due to an isothermal, charge neutralizing electron fluid is examined. Stationary plasma is destabilized by finite ion temperature with the threshold temperature T-i characterized by the ratio ZT(Theta )/T-i, where Z is the charge state of the ions and T-e is the electron temperature. A cold drifting plasma is found to be unstable with typi cal growth rate gamma approximate to 0.4-0.6 C-s/Delta X (Delta X is t he grid spacing and C-s is the ion acoustic speed); simulations show t hat unstable growth is saturated by particle trapping leading to heate d ions. For nearest grid point weighting, saturation, and threshold fo r growth are observed to be T-i approximate to 0.6 ZT(e) while for lin ear particle weighting, T-i approximate to 0.1 ZT(e). These results ar e relevant to hybrid particle simulations of laser generated plasmas, a regime where ZT(e)/T-i much greater than 1 is encountered. (C) 1995 Academic Press, Inc.