Trapped electron stabilization of ballooning modes in low aspect ratio toroidal plasmas

The kinetic effects of trapped electron dynamics and finite gyroradii and magnetic drift motion of ions are shown to give rise to a large parallel electric field and hence a parallel current that greatly enhances the stabilizing effect of field line tension for ballooning modes in low aspect ratio toroidal plasmas. For large aspect ratio the stabilizing effect increases (reduces) the beta(=2P/B-2) threshold for the first (second) stability of the kinetic ballooning mode (KBM) from the magnetohydrodynamics (MHD) beta threshold value by a factor proportional to the trapped electron density fraction. For small aspect ratio the stabilizing effect can greatly increase the beta threshold of the first stability of KBMs from the MHD beta threshold by S(c)similar or equal to1+(n(e)/n(eu))delta, where n(e)/n(eu) is the ratio of the total electron density to the untrapped electron density, and delta depends on the trapped electron dynamics and finite gyroradii and magnetic drift motion of ions. If n(e)/n(eu)much greater than1 as in the National Spherical Torus Experiment (NSTX) [M. Ono, Nucl. Fusion 40, 557 (2000)] with an aspect ratio of similar or equal to1.4, the KBM should be stable for betaless than or equal to1 for finite magnetic shear. Therefore, unstable KBMs are expected only in the weak shear region near the radial location of the minimum of the safety factor in NSTX reverse shear discharges. (C) 2004 American Institute of Physics.