Sawtooth stabilization and electron heating by high power lower hybrid waves in Alcator C

Lower hybrid current drive and electron heating experiments were carried out in sawtoothing discharges in Alcator C (R/sub o/ = 64 cm, a = 16.5 cm) at ''moderate'' magnetic fields. Sawtooth stabilization experiments were performed at B = 6.2 T, anti n/sub e/ approx. = 1.1 x 10/sup 14/ m/sup -3/, with adjacent waveguides phased at ..delta..phi = 90/sup 0/, while electron heating experiments were carried out at B = 5.5 T, anti n/sub e/ approx. = 1.5 x 10/sup 14/ cm/sup -3/, with ..delta..phi = 180/sup 0/. Up to three sets of 4 x 4 waveguide arrays, operating at f = 4.6 GHz, were employed for injecting up to 1.5 MW of rf power in the lower hybrid frequency regime. In contrast to previous experiments on Alcator C in which SiC-coated carbon limiters were used, molybdenum limiters were employed in the present experiments. The advantage of using molybdenum limiters is that Z/sub eff/ did not increase nearly as much as in previous experiments, and the grill mouths were not contaminated by carbon deposits; hence the rf power handling of each grill was outstanding. The disadvantage of operating with molybdenum limiters is that the high-Z impurity radiation from the centermore » of the plasma rose significantly during rf injection. The radiation was monitored by spectroscopic means, and in these experiments was found to be a small fraction of the total injected power.« less