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

Plasma current ramp-up by lower-hybrid waves in a high-density regime (n̄e≳1×1019 m−3) is investigated on the Alcator C tokamak [Nucl. Fusion 25, 1127 (1985)]. A time-averaged conversion efficiency (Ẇ−Pext)/Prf of over 15% has been obtained at a density of n̄e ≂1×1019 m−3, 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.