Turbulence modeling of the toroidal wall heat load due to shear flows over cavities in the neutral gas blanket divertor regime

Heat loads to the target plate in reactor tokamaks are estimated to be orders of magnitude higher than those that can be withstood by known materials. In regimes of plasma detachment, there is strong evidence that plasma recombination occurs near the divertor plate, leading to a cold neutral gas blanket. Because of the strong coupling between the plasma and the neutrals within the divertor region, there is significant neutral flows along field lines up to Mach 1.2 and Reynolds numbers over 1000. Here the effects of three dimensional (3D) neutral turbulence within the gas blanket on heat deposition to the toroidal wall are examined. Both two dimensional (2D) mean shear flows over toroidal cavities as well as a fully 3D initial value problem of heat pulse propagation are considered. The results for algebraic stress model, K-epsilon and laminar flows are compared. It is found that 3D velocity shear turbulence has profound effects on the heat loads, indicating that simple (linear) Reynolds stress closure schemes are inadequate. (C) 1997 American Institute of Physics.