High-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*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 HBI 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) is 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 =2.5-6.5*10 19 m -3 ) and plasma currents (I p =1.5-2.4 MA (q a =4.2-2.6)). 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.