Plasma current ramp-up and Ohmic-heating transformer recharging experiments using lower-hybrid waves on a tokamak

Abstract
Plasma current ramp‐up by lower‐hybrid waves in a high‐density regime (n̄e≳1×1019 m3) is investigated on the Alcator C tokamak [Nucl. Fusion 2 5, 1127 (1985)]. A time‐averaged conversion efficiency (Ẇ−Pext)/Prf of over 15% has been obtained at a density of n̄e ≂1×1019 m3, where W is the poloidal field energy and Pext is the external inductive power. The maximum conversion efficiency obtainable is lower at higher densities. Ramp‐up rates of over 1 MA/sec were obtained when rf current ramp‐up is applied to an Ohmically maintained plasma. Lower‐hybrid current drive can also be used to maintain the plasma while recharging the Ohmic‐heating transformer. The conversion efficiency of 10% obtained during recharging is found to be comparable to that obtained during ramp‐up under similar conditions.