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

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 T e (0) of 6–10 keV and a high ion temperature of 2.2 keV were obtained at n e = 0.6 × 10 13 cm −3 . A clear increment of T i 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 T i increment was observed also in ICRF discharges at low density ranges of (0.4–0.6) × 10 13 cm −3 with appearance of a new operational range of T i (0) = 2.8 keV > T e (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 P i > P e . Higher T i (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) × 10 13 cm −3 . The highest T i (0) of 5 keV was recorded in Ne NBI discharges at n e <1 × 10 13 cm −3 with the achievement of T i (0)> T e (0), whereas the T i (0) remained at relatively low values of 2 keV in H 2 and He NBI discharges due to less P i . The main reasons for the high T i achievement in the Ne discharges are: (1) 30% increment of deposition power, (2) increase in P i / n i (five times, P i / n i ≫ P e / n e , P i < P e ) and (3) increase in τ ei (three times). The obtained T i (0) data can be plotted by a smooth function of P i / n i . This result strongly suggests that the ion temperature increases even in the H 2 discharge if the P i can be raised up.