HIGHER-HARMONIC ICRF HEATING EXPERIMENTS IN JT-60

A review of the results from JT-60 ICRF heating experiments is presented. The JT-60 ICRF experiments (120 and 131 MHz) are characterized by higher-harmonic heating with phase control of the compact 2 x 2 loop antenna array. Second-harmonic heating is most intensively investigated with antenna phase control for various cases where the resonant species (hydrogen) are either a majority component or a minority component, and with or without hydrogen NBI heating. It is found that antenna phase control plays a key role in optimizing antenna power injection characteristics as well as heating efficiency. The out-of-phase mode ((pi, 0) mode) can avoid RF sheath and parametric decay instabilities which cause harmful effects on antenna power injection capability with the in-phase mode ((0,0) mode) in some operating conditions. The incremental energy confinement time (tau(E)inc) of the (pi, 0) mode is about 50% better than that of the (0,0) mode. Minority hydrogen second-harmonic heating with (pi, 0) phasing in helium discharges shows the best results over a wide range of electron densities (n(e)BAR = 2.5-6.5 X 10(19) m-3) and plasma currents (I(P) = 1.5-2.4 MA (q(a) = 4.22. 6)). The best confinement enhancement factor (tau(E)/tau(E)(L-mode)) is 1.3, which is obtained with ohmic target plasmas. Higher-harmonic beam ion acceleration is observed up to fourth harmonics in a combined ICRF ((pi, 0) mode) and NBI heating scenario. Strong central-electron heating associated with beam ion acceleration is observed in combined third-harmonic ICRF and NBI heating. The best results with third-harmonic heating were obtained for NBI-heated target plasmas with P(NB)/P(IC) = 4 approximately 4.5. The best confinement enhancement factor is 1.2-1.3. Enhancement of the plasma's stored energy is explained by the fast-ion stored energy, which can be quantitatively estimated by local Fokker-Planck analysis except in the low electron-density regime. Sawtooth stabilization is observed in both second- and third-harmonic heating. A parameter space for the sawtooth stabilization with second-harmonic minority heating is characterized by relatively high density and low q (n(e)BAR less-than-or-equal-to 6.5 x 10(19) m-3, q(a) greater-than-or-equal-to 2.6) and low threshold power (approximately 2 MW). In combination with NBI heating, opposite dependences of the sawtooth stabilization on n(e)BAR are observed for second- and third-harmonic heating. Stability conditions are discussed both for second- and third-harmonic heating and their experimental trends are found to be consistent with the fast particle stability theory of Porcelli.