Experimental study on ion temperature behaviours in ECH, ICRF and NBI H2, He and Ne discharges of the Large Helical Device

Abstract

Ion heating experiments have been carried out in the large helical device using ECH (82.5, 84.0, 168 GHz, ⩽1 MW), ICRF (38.5 MHz, ⩽2.7 MW) and NBI (H° beam: 160 keV, ⩽9 MW). The central ion temperature has been observed from the Doppler broadening of Ti XXI (2.61 Å) and Ar XVII (3.95 Å) x-ray lines, which are measured using a newly installed crystal spectrometer with a charge-coupled device. Recently, in ECH discharges, on-axis heating became possible. As a result, a high Te(0) of 6–10 keV and a high ion temperature of 2.2 keV were obtained at ne = 0.6×1013 cm−3. A clear increment of Ti was also observed with the enhancement of the electron–ion energy flow when the ECH pulse was added to the NBI discharge. These results demonstrate the feasibility towards ECH ignition. A clear Ti increment was observed also in ICRF discharges at low density ranges of (0.4–0.6)×1013 cm−3 with appearance of a new operational range of Ti(0) = 2.8 keV > Te(0) = 1.9 keV. In low power ICRF heating (1 MW), the fraction of bulk ion heating is estimated to be 60% of the total ICRF input power, which means Pi>Pe. Higher Ti(0), up to 3.5 keV, was obtained for a combined heating of NBI (<4 MW) and ICRF (1 MW) at density ranges of (0.5–1.5)×1013 cm−3. The highest Ti(0) of 5 keV was recorded in Ne NBI discharges at ne<1×1013 cm−3 with the achievement of Ti(0)>Te(0), whereas the Ti(0) remained at relatively low values of 2 keV in H2 and He NBI discharges due to less Pi. The main reasons for the high Ti achievement in the Ne discharges are: (1) 30% increment of deposition power, (2) increase in Pi/ni (five times, Pi/ni≫Pe/ne, Pi<Pe) and (3) increase in τei (three times). The obtained Ti(0) data can be plotted by a smooth function of Pi/ni. This result strongly suggests that the ion temperature increases even in the H2 discharge if the Pi can be raised up.