Universal Mode with Diffusive Electrons: Linear Instability and Nonlinear Saturation

摘要

The effect of spatial electron diffusion on the stability properties of the universal drift mode in a sheared magnetic field are studied using an initial value code, TEDIT. Previous studies of this problem by Hirshman and Molvig relied on an approximation to the electron resonance function equivalent to making a Krook approximation for the spatial diffusion operator, D par. delta exp 2 /par. delta x exp 2 . The present work treats the diffusion operator precisely and also allows the treatment of a realistic parallel velocity dependence of the diffusion coefficient, D = D(v/sub parallel/). For the case of a velocity independent diffusion coefficient, the qualitative features found by Hirshman and Molvig are observed. The modes with k/sub y/rho/sub i/ > 1 destabilize at small values of the diffusion coefficient and saturate at higher values, corresponding to several orders of magnitude in D. There are quantitative discrepancies with the previous work that, near the saturation point, can be accounted for reasonably well by a simple asymptotic theory.