Localized Ballooning Modes in Compact Quasiaxially Symmetric Stellarators
Authors: M.H. Redi, J. Canik, R.L. Dewar, E.D. Fredrickson, W.A. Cooper, J.L. Johnson, and S. Klasky
Date of PPPL Report: June 2001
Presented at: the 28th EPS Conference on Controlled Fusion and Plasma Physics, Madeira, Portugal, June 18-22, 2001.
Understanding of ballooning mode stability boundaries may lead to performance improvement of toroidal devices through control of disruptions. Toroidally localized ballooning modes have been found as precursors to high beta disruptions on the Tokamak Fusion Test Reactor (TFTR) arising in conditions of n=1 kink mode asymmetry. Recent optimization has shown that magnetohydrodynamic stability as well as good particle confinement are likely to be achievable in the National Compact Stellarator Experiment (NCSX), a compact, quasiaxially symmetric stellarator (QAS) for values of the plasma near b = 4%. The configuration, with a major radius of 1.42 m, an aspect ratio of 4.4, a toroidal magnetic field 1.2-1.7 T and 6 MW of neutral-beam heating, is stable to MHD instabilities, and is expected to be limited by high-n kink and ballooning modes. This paper describes the ballooning eigenvalue isosurfaces for NCSX, the first step in an examination of the kinetic stabilization of the ballooning beta limit using a hybrid WKB approach.