Divertor target heat load reduction by electrical biasing, and applicationto COMPASS-D

A toroidally asymmetric potential structure in the scrape-off layer (SOL) p lasma may be formed by toroidally distributed electrical biasing of the div ertor target tiles. The resulting E x B convective motions should increase the plasma radial transport in the SOL and thereby reduce the heat load at the divertor [R.H. Cohen, D.D. Ryutov, Nucl. Fus. 37 (1997) 621]. In this p aper, we develop theoretical modelling and describe the implementation of t his concept to the COMPASS-D divertor. We show that a strong magnetic shear near the X-point should cause significant squeezing of the convective cell s preventing convection from penetrating above the X-point. This should res ult in reduced heat load at the divertor target without increasing the radi al transport in the portion of the SOL in direct contact with the core plas ma, potentially avoiding any confinement degradation. Implementation of div ertor biasing is in hand on COMPASS-D involving insulation of, and modifica tions to, the present divertor tiles. Calculations based on measured edge p arameters suggest that modest currents similar to8 A/tile are required, at up to 150 V, to drive the convection. A technical test is preceding full bi as experiments. (C) 2001 UKAEA. Published by Elsevier Science B.V. All righ ts reserved.