RF SHEATH FORMATION AND PLASMA-DENSITY RISE DURING THE ION BERNSTEIN WAVE HEATING EXPERIMENT OF TOKAMAK PLASMAS

The effect of RF sheaths during ion Bernstein wave heating of tokamak plasma has been considered. Owing to the presence of a strong component of the RF wave electric field parallel to the toroidal electric field, an RF sheath can occur near a metallic wall, where this is cut by a line of static magnetic field. Consequently, the plasma ions in the sheath can be accelerated and produce gas desorption by impact on the metallic walls. This phenomenon is important for operating frequencies less than 100 MHz owing to the fairly strong acceleration of the ions in the sheaths and to the wider extension of the propagation region of the RF wave in the scrape-off plasma in the poloidal and toroidal directions. With this condition, the recycling can be affected, producing an increase of the plasma density during RF injection.