SUPPRESSION OF M=2 ISLANDS BY ELECTRON-CYCLOTRON HEATING IN THE TEXASEXPERIMENTAL TOKAMAK - EXPERIMENT AND THEORY

Electron cyclotron heating (ECH) is used to suppress m=2 magnetohydrod ynamic (MHD) oscillations in the Texas Experimental Tokamak (TEXT) [Nu cl. Technol. Fusion 1, 479 (1981)]. The location of ECH power depositi on is controlled by a movable antenna. The MHD activity is suppressed when the ECH beam is directed close to the q=2 surface. The experiment is simulated using a three-dimensional resistive MHD code in cylindri cal geometry. For fixed plasma current, the saturated m=2 island width is found to depend on the value of the safety factor at the magnetic axis (q0). The simulation suggests that the observed saturated m=2 isl and in the pre-ECH plasma, which typically occupies 25% of the minor r adius, corresponds to q0 approximately 1.3. The suppression of the isl and in the presence of ECH is attributed to current profile modificati on. In some discharges, the m=2 activity does not resume even after th e ECH pulse is turned off.