CONTROL OF EDGE ELECTRIC-FIELD BY LIMITER BIASING AND ELECTRON-BEAM INJECTION IN HYBTOK-II TOKAMAK

Experimental research on the modification of the edge radial electric field by limiter biasing and electron beam injection from a hot cathode has been performed in a small research tokamak, HYBTOK-II. Control of the amplitude and/or the spatial profile of the radial electric field might enable us to improve the plasma confinement properties. In HYBTOK-II a bias voltage of negative or positive polarity with respect to the wall is applied to the poloidal circular limiters. In the positive biasing the plasma potential in the edge region rises in proportion to the applied bias voltage. Penetration of the radial electric field into the core region is about 2 cm, which corresponds roughly to the ion poloidal Larmor radius in the edge. The radial electric field of about 25 V/cm was generated at the edge region by the positive biasing of 50 V, and substantial suppression of the electrostatic fluctuations was observed in both the edge and scrape-off regions. By reducing the plasma potential and making the radial electric field weak in the case of the negative biasing, the electrostatic fluctuation increases significantly.