Submitted for publication to Physical Review Letters. THE EFFICIENCY OF RF CURRENT DRIVE IN THE PRESENCE OF FAST PARTICLE LOSSES

The effects of losses of the fast electron current carriers are included in a calculation of the efficiency of RF current drive. The analytical expressions obtained for the current drive efficiency J/PA and bulk heating rate PD/PA are explicitly dependent on the fast electron confinement time TF. In present tokamak experiments, fast particle losses are predicted to reduce the current drive efficiency from 50 % to more than an order of magnitude. However, in future fusion plasmas fast electron confinement appears to be sufficient to allow nearly ideal efficiencies to be achieved. 2In tokamak experiments in which the plasma current is sustained by lower hybrid current drive, the current drive efficiency is generally found to be less than the efficiency predicted by Fisch and Boozer in Ref. 1. The cause or causes for this apparent discrepancy are not as yet understood, and in order to provide credible extrapolation of present experiments t