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 = 0.6×10¹³ 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)×10¹³ 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)×10¹³ cm⁻³. The highest T_i(0) of 5 keV was recorded in Ne NBI discharges at n_e<1×10¹³ 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 H₂ 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 H₂ discharge if the P_i can be raised up.