Stabilization of sawteeth by lower hybrid waves in the Alcator C tokamak

Sawteeth have been suppressed by lower hybrid current drive in high density Alcator C plasmas. At a density of 1.1x1014cm-3 and q(a)=4.7, the sawteeth were eliminated by application of PRF=600 kW at 4.6 GHz, which represents an estimated 30% of the radiofrequency (RF) power required to fully sustain the discharge in the absence of Ohmic power. Strong electron heating is observed with both current drive phasing and anti-current drive phasing of the waveguide arrays, but sawtooth stabilization occurs only in the former case. With current drive phasing the sawtooth period increases with RF power until the sawteeth are stabilized, whereas with anti-current drive phasing the period remains unchanged. The internal inductance of the plasma increases in the case of current drive relative to the case of anti-current drive. The disappearance of sawteeth and the increase in internal inductance suggest that the RF current is driven near the q=1 surface. At high power (PRF>850 kW), the sawteeth return 40-60 ms into the RF pulse, indicating that an optimal power range, or RF deposition profile, exists for successful sawtooth suppression. (author). 17 refs, 10 figs