Lower hybrid current drive and heating experiments at high plasma densities on alcator C

The authors report the results of lower hybrid current drive and heating experiments on the Alcator C tokamak at plasma densities in the 1014 cm-3 range using two 16 waveguide arrays. Net RF powers up to 1 MW have been injected into the Alcator C plasma at 4.6 GHz using both arrays, or up to 650 kW of net RF power using one; the maximum RF power density achieved at the mouth of the waveguide array without arcing was 9 kW/cm2. Current drive experiments are usually carried out by phasing a waveguide array 0, π/2, π, 3π/2, which then launches a lower hybrid wave power spectrum concentrated in the range 1 < k c/ω < 2.5. Plasma currents of 150 200 kA have been driven by the RF for times greater than 100 msec at n/sub e/ up to 5 x 1013 cm-3 using one array alone. At n/sub e/ approx. = 3.5 x 1013 cm-3 at B/sub T/ = 8 T in hydrogen discharges, constant plasma currents of 125 kA have been maintained by the RF power for more than 300 msec with zero loop voltage and constant internal inductance. During these shots after plasma initiation the current in the ohmic heating transformer is zero, so that in the absence of RF these plasmas decay with a 150 msec time constant. The best current drive efficiencies obtained were at B/sub T/ = 10 T and are n/sub e/ (1014 cm-3) I/sub P/ (A)/P(W) = .19. At n/sub e/ less than or equal to 5 x 1013 cm-3 there is no evidence of any difference in current drive efficiency between hydrogen or deuterium plasmas for a given magnetic field in the range B/sub T/ = 8-10 T