Electric biasing of the divertor tiles in COMPASS-D

It has been suggested that by electrically biasing the divertor tiles in a tokamak, one could create a toroidally-asymmetric potential structure in th e scrape-off layer (SOL) plasma. The resulting E x B convective motion shou ld increase the plasma transport in the SOL and thereby reduce the heat loa d on the divertor plates. This paper describes plans for such experiments o n COMPASS-D and preliminary theoretical modelling. The COMPASS-D divertor t arget consists of 24 graphite tiled toroidal segments. The inner and outer strike point tiles on all these segments will be insulated from each other, and cabling will be installed to allow for biasing of individual tiles in any combination. Theoretical analysis suggests that the electrostatic poten tial perturbations caused by the biasing will be confined to the divertor r egion below the X-point, and that substantial SOL broadening can be achieve d by applying modest voltages. Aside from active control of SOL transport, it is hoped that the biasing will be useful to probe other divertor and SOL physics issues.