Finite-Grid Instability in Quasineutral Hybrid Simulations

ISSN： 0021-9991

Source： Journal of Computational Physics, Vol.118, Iss.1, 1995-04, pp. : 152-158

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Abstract

Analysis and simulation of numerical instability caused by the spatial grid is presented for the case of quasineutral hybrid plasma simulation. The simplest case of particle ions advanced in the ambipolar electric 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_e/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 typical growth rate γ ≈ 0.4-0.6 C_s/Δx (Δx is the grid spacing and C_s is the ion acoustic speed); simulations show that unstable growth is saturated by particle trapping leading to heated ions. For nearest grid point weighting, saturation, and threshold for growth are observed to be T_i ≈ 0.6 ZT_e while for linear particle weighting, T_i ≈ 0.1 ZT_e. These results are relevant to hybrid particle simulations of laser generated plasmas, a regime where ZT_e/T_i ≫ 1 is encountered.