Theory Issues for Induced Plasma Convection Experiments in the Divertor of the MAST Spherical Tokamak

Abstract

This paper surveys theory issues associated with inducing convective cells through divertor tile biasing in a tokamak to broaden the scrape-off layer (SOL). The theory is applied to the Mega-Ampere Spherical Tokamak (MAST), where such experiments are planned. Criteria are presented for achieving strong broadening and for exciting shear-flow turbulence in the SOL; these are shown to be attainable in practice. It is also shown 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-point, with little impact on the main SOL. The current created by the biasing and the associated heating power are found to be modest.