Edge plasma characteristics during ICRF heating on JFT-2M Tokamak

The roles of the edge plasma are investigated with respect to impurity release, antenna loading, and H-mode transition. Metal impurity release is considered to be caused by an RF electric field near the antennas, and can be reduced by the out-of-phase operation. Antenna loading with H-mode plasmas is dependent on the antenna-separatrix distance, and can be increased for sufficiently small separation. Electron temperature and electron density in the edge are the key parameters for H-mode transition, and they are the same, independent of the additional heating methods, and of the impurity level.