Superthermal alpha transport and wall bombardment in tokamaks

Abstract

Fusion-product transport, from an axisymmetric tokamak plasma to the first wall, is modelled. Prompt-loss profiles (i.e. prior to slowing) are calculated as a function of poloidal angle. There is a significant flux incident at near-grazing angles, thus enhancing sputtering. Non-prompt losses are also obtained; the resulting wall bombardment is ≲ 1 10 of the prompt contribution but is localized at the outboard edge of the tokamak. To verify fusion-product transport, a detection experiment has been proposed and modelled for a device like PLT or TFTR. Blistering is the dominant wall erosion mechanism. Serious impurity contamination of the plasma can result: Δ〈Z〉 eff/s ~ 6.5 in a TFTR-size device, and 0.014 in UWMAK-I; this poses a potential limit on reaction burn time. These effects can be minimized by: 1) periodic B-field reversal, 2) special wall materials and/or rapid wall replacement designs in peak flux areas, 3) increased plasma-wall separation, and 4) increased plasma current.