SAWTOOTH STABILIZATION BY ELECTRON-CYCLOTRON HEATING AT THE Q = 1 SURFACE IN THE WT-3 TOKAMAK

Sawtooth oscillations (STO) in the Ohmically heated (OH) WT-3 tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1988 (IAEA, Vienna, 1989), Vol. 1, p. 563] are strongly modified or suppressed by localized electron cyclotron resonance heating (ECH) near the q = 1 surface, where q refers to the safety factor. The effect of ECH is much stronger when it is applied on the high-field side, as compared to the low-field side. Complete suppression of the STO is achieved for the duration of the ECH when it is applied on the high-field side, in a low-density plasma, provided the ECH power exceeds a threshold value. This threshold decreases with the increasing safety factor at the limiter q(L) and the complete stabilization of STO can be obtained even when ECH is applied on the low-field side in the high-q(L) region. It is noticeable that these complete stabilizations can be obtained only the localized ECH with the elliptic antenna. The STO stabilization is attributed to a modification of the current density profile by high-energy electrons generated by ECH, which reduces the shear in the q = 1 region.