INITIAL HIGH-BETA OPERATION OF THE HBT-EP TOKAMAK

HBT-EP is a new research tokamak designed and built to investigate pas sive and active feedback techniques to control MHD instabilities. In p articular, HBT-EP will be able to test techniques to control fast MHD instabilities occurring at high Troyon-normalized beta, beta(N) = beta Ba/I-p [Tm/MA], since it is equipped with a thick, close-fitting, and adjustable conducting shell. The major goals of the initial operation of HBT-EP have been the achievement of high beta operation (beta(N) s imilar to 3) using only ohmic heating and the observation of MHD insta bilities. By using a unique fast startup technique, we have successful ly achieved these goals. A variety of MHD phenomena were observed duri ng the high beta operation of HBT-EP. At modest beta (beta(N) less tha n or equal to 2), discharges have been maintained for more than 10 mse c, and these discharges exhibit saturated resistive instabilities. Whe n beta(N) approaches 3, major disruptions occur preceded by oscillatin g, growing precursors. During start-up, one or more minor disruptions are usually observed. A 1-D transport code has been used to simulate t he evolution of the current profile, and these early minor instabiliti es are predicted to be double tearing modes. The simulation also repro duces the observed high beta operation when saturated neo-Alcator ener gy confinement scaling is assumed.