Electric field-induced plasma convection in tokamak divertors

Abstract

Measurements of the electric fields, E in the DIII-D tokamak divertor region [J. C. Luxon and L. G. Davis, Fusion Technology 8, Part 2A, 441 (1985)] are quantitatively consistent with recent computational modeling establishing that E×BT circulation is the main cause of changes in divertor plasmas with the direction of the toroidal magnetic field, BT. Extensive two-dimensional measurements of plasma potential in the DIII-D tokamak divertor region are reported for the first time. The resulting E×BT/B2 drift particle flux is calculated for standard (ion ∇BT drift toward divertor X-point) and reversed BT direction and for low (L) and high (H) confinement modes. Perpendicular field strengths of up to E∼5 kV/m are observed at the separatrix between the divertor private region and the scrape-off layer (SOL). The E×BT drift, which reverses with reversal of BT, creates a poloidal circulation pattern in the divertor that convects 25%–40% of the total ion flow to the divertor target. The circulation strongly couples the various regions of the divertor and SOL and fuels the X-point region. An outward shift of the profiles is seen in reversed BT.