Trapped Electron Stabilization of Ballooning Modes in Low Aspect Ratio Toroidal Plasmas
Authors: C.Z. Cheng and N.N. Gorelenkov
Date of PPPL Report: March 2004
Published in: Phys. Plasmas 11:10 (October 2004) 4784-4795.
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 β (= 2P/B2) threshold for the first (second) stability of the kinetic ballooning mode (KBM) from the 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 Sc approximately equal to1 + (ne/neu)δ, where ne/neu is the ratio of the total electron density to the untrapped electron density, and δ depends on the trapped electron dynamics and finite gyroradii and magnetic drift motion of ions. If ne/neu >> 1 as in the National Spherical Torus Experiment (NSTX) with an aspect ratio approximately equal to 1.4, the KBM should be stable for β ≤ 1 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.