The Performance of ICRF Heated Plasmas in LHD

"An ICRF Heating experiment was conducted in the third campaign of the LHD in 1999. l .35 MW of ICRF power was injected into the plasma and 200kJ of stored energy was obtained, which was maintained for 5 sec only by ICRF power after the termination of the ECH. The Impurity problem was so completely overcome that the pulse length was easily extended to 68 sec at a power level of 0.75 MW. The utility of a liquid stub tuner in steady state plasma heating was demonstrated in this shot. The energy confinement time of the ICRF heated plasma has the same dependences on plasma parameters as the ISS95 stellarator scaling with a multiplication factor of 1.5, which is a high efficiency comparable to NBI. Such an improvement in performance was obtained by applying various measures, including 1 )scanning of the magnetic field intensity and minority concentration, 2)improvement of particle orbit due to a shift of the magnetic axis, and 3) reduction of impurities by means of Ti-gettering and the use of carbon divertor plates. In the optimized heating regime, ion heating turned out to be the dominant heating mechanism, different from that of in CHS and W7-AS. Due to the high quality of the heating and the extended parameter range far beyond that of previous experiments, the experiment can be regarded as the first complete demonstration of ICRF heating in stellarators.