Divertor bias experiment on JFT-2M

Divertor bias experiments on JFT-2M have been performed in order to modify the tokamak boundaries and understand the L-H transition physics. As a result, the threshold power for L-H transition has been reduced from 340 to 140 kW by unipolar negative divertor biasing. In this case the modifications occurring in the scrape-off layer (SOL) plasma have also been clarified: (a) a steep density gradient is produced in SOL, which means suppression of radial particle transport; (b) accordingly, a negative electric field and a shear flow are observed; (c) density fluctuations at frequencies less than 100 kHz are also reduced in the shear flow region. (author). 7 refs, 3 figs