On the possibility of inducing strong plasma convection in the divertor ofMAST

In this paper, a theory is developed to describe scrape-off layer (SOL) bro adening by inducing convective cells through divertor plate biasing in a to kamak. The theory is applied to the Mega-Ampere Spherical Tokamak, where su ch experiments are planned in the near future. Criteria are derived for ach ieving strong broadening and for exciting shear-flow turbulence in the SOL, and these criteria are shown to be attainable in practice. It is also show n that the magnetic shear present in the vicinity of the X-point is likely to confine the potential perturbations to the divertor region below the X-p oint, leaving the part of the SOL that is in direct contact with the core p lasma intact. The current created in the SOL by the biasing and the associa ted heating power are also calculated and are found to be modest.