High power ICRF heating in the 4th experimental in LHD

The utility of ICRF (Ion Cyclotron Range of Frequency) heating in the Large Helical Device (LHD) was demonstrated in the third campaign, carried out in 1999. This paper summarizes the investigations in 2000 focusing on the optimization of ICRF heating, in which scans of the magnetic field strength and the magnetic axis position were made. In the scan of the magnetic field, the optimized ICRF heating performance was obtained when the minority cyclotron resonance was located at the saddle point of the mod-B surface. Under this condition, the highest electron density of 2.7x1019 m-3 was sustained at a power level of 1.5 MW with ICRF heating only. In the scan of the magnetic axis, better performances of ICRF heating were obtained in the inward shifted configuration than in the outward shifted configuration. The difference in performance between the two cases was studied in terms of the difference of confinement of the high-energy ions as well as that of the bulk plasma. (author)