Analysis of θ Pumping for Tokamak Current Drive

Abstract

Analysis of θ pumping for tokamak current drive is presented. Diffusion of an externally applied oscillating electric field into a tokamak plasma is examined. When the oscillating electric field is in the same direction as the stationary toroidal plasma current and the current density induced by the applied electric field is larger than the average density of the plasma current over the plasma cross section, the radial profile of the safety factor has an extremum near the plasma boundary region and MHD instabilities are excited. It is assumed that anomalous diffusion of the current induced in the plasma boundary region takes place, so that the extremum in the radial profile of the safety factor disappears. The anomalous electric field due to this relaxation process has a net d.c. component, which is estimated, and the condition for tokamak current drive by θ pumping is derived. Some numerical results are presented for the case of a fusion grade plasma.