Recent ion cyclotron range of frequencies experiments in JT-60U

Recent results on the minority ion second harmonic heating on JT-60U are presented. Maximum coupled power reached 6.4 MW. Good antenna-plasma coupling capability and a small fraction (less than 10%) of an incremental radiation loss to r.f. power are confirmed. Power absorption rate increases with increasing r.f. power and is saturated around unity at r.f. powers higher than 3 MW. The sawtooth stabilization by minority ion second harmonic heating was realized over a wide parameter range, i.e. Ip = 0.9 MA−4 MA, q95 = 2.3−8.6, ne = (1.3−5) × 1019m−3 and PIC ⩾ 2.2 MW. A figure of merit VP〈ne〉Ptot for efficiency of the sawtooth stabilization is about 50% higher than those in other devices where fundamental resonance minority ion heating is employed. The longest stable period reached 2.33 s. Attainable sawtooth-free periods scale with the resistive diffusion time. It was found that the energy confinement is further improved by 25% during the reheating phase after the giant sawtooth crash. The electron temperature profile became more peaked at the improved confinement phase. Those phenomena were observed only in low q discharges (q95 ⩽ 2.9).