Generation of Helical SOL Current by on-axisymmetric Divertor Biasing and its effects on SOL Plasma Transport

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Heat flux reduction onto the tokamak divertor plate by non-axisymmetric divertor biasing is investigated. ExB convective cells are generated in the scrape off layr (SOL) and divertor plasmas by this non-axisymmetric divertor biasing and SOL and divertor plasma transport is strongly modified. The local divertor biasing experiments have been done in the JFT-2M toka-mak and spatial changes of the plasma potential in the SOL due to local divertor biasing is observed. The spatial structure of the plasma potential during biasing is calculated by 1-DSOL-divertor biasing model and compared with the experimentally observed one. The experimental profile of the plasma potential, which shows the generation of ExB convective cells in the SOL,agrees qualitatively with the theoretical estimation. The particle flux onto the divertor plate is strongly modified by this ExB convective cell formation.In addition to the ExB convective cell formation by DC divertor biasing, Radio Frequency (RF) induced sheath on the RF antenna and its effects on the antenna-plasma coupling are studied in the linear divertor plasma simulator. In the conventional loop antenna large DC negative sheath voltage is induced by RF voltage on the antenna and the antenna heat flux is significantly increased by energetic ion bombardments accelerated by this DC negative sheath. The plasma potential near the antenna is strongly modified by the near RF field and plasma coupling, and the formation of ExB convective cells near the antenna is expected. The study of the RF induced ExB convective cells and their effects on the plasma transport is now in progress.