OBSERVATION OF WALL STABILIZATION AND ACTIVE CONTROL OF LOW-N MAGNETOHYDRODYNAMIC INSTABILITIES IN A TOKAMAK

The High Beta Tokamak-Extended Pulse (HBT-EP) experiment [J. Fusion En ergy 12, 303 (1993)] combines an internal, movable conducting wall wit h a high-power, modular saddle coil system to provide passive and acti ve control of long wavelength magnetohydrodynamic (MHD) instabilities. Systematic adjustment of the radial position, b, of the conducting wa ll elements in relation to the surface of the plasma (minor radius a) resulted in the suppression of beta-limiting disruptions for discharge s in which b/a < 1.2 and a positive plasma current ramp was maintained . Conducting wall stabilization of kink instabilities was observed in discharges with strong current ramps and in plasmas with beta values n ear the Troyon stability boundary. The frequency of slowly growing mod es that persisted in wall-stabilized discharges was controlled by appl ying oscillating m=2, n=1 resonant magnetic perturbations. A compact, single-phase saddle coil system permitted modulation of the rotation v elocity of internal m/n=2/1 instabilities by a factor of 2. (C) 1996 A merican Institute of Physics.