The Influence of a Conducting Wall on Disruptions in HBT-EP.

The characteristics of (β)-limiting and low q* disruptions plus the conditions under which they occur have been studied as a function of plasma-wall separation. Coupling between the wall and the plasma is varied by systematically adjusting the radial position, b, of a conducting wall section relative to the surface of the plasma of minor radius a. MHD instabilities with kink-like helicities, mainly ( m=3, n=1 ), and other long wavelength modes (e.g. ( 2/1; 1/1 )) were observed in conjunction with both classes of disruptions. For low q* discharges, the modes developing in the pre-disruption stage were found to change from primarily external to the plasma when ( < b> /a = 1.52 ), where ( < b> ) is the average radius of the conducting shells, to essentially internal to the plasma when ( b/a <= 1.2 ). For (β)-limiting disruptions the pre cursor modes remained predominantly internal even as ( b/a <= 1.2 ) was changed to ( < b> /a = 1.52 ). The conducting wall, placed near the plasma surface, increased discharge lifetime by a factor of 3, enhanced particle heating as measured with soft x-ray diodes by 30% and improved stability for both cases of disruptions. (^*)Work supported by US DOE Grant DE--FG02--86ER--53222