ICRF heating and high energy particle production in the large helical device

Significant progress has been made with Ion-Cyclotron Range-of-Frequencies (ICRF) heating in the Large Helical Device (LHD). This is mainly due to better confinement of the helically trapped particles, and less accumulation of impurities in the region of the plasma core. During the past two years, ICRF heating power has been increased from 1.35 MW to 2.7 MW. Various wave-mode tests were carried out using minority-ion heating, second-harmonic heating, slow-wave heating, and high-density fast-wave heating at the fundament cyclotron frequency. This fundamental heating mode extended the plasma-density range of effective ICRF heating to a value of 1x1020 m-3. This was the first successful result of this heating mode in large fusion devices. Using the minority-ion mode gave the best performance, and the stored energy reached 240 kJ using ICRF alone. This was obtained for the inward-shifted magnetic axis configuration. The improvement associated with the axis shift was common to both bulk plasma and highly accelerated particles. For the minority-ion mode, high-energy ions up to 500 keV were observed by concentrating the heating power near the plasma axis. The confinement properties of high-energy particles were studied for different magnetic axis configurations using the power-modulation technique. It confirmed that the confinement of high-energy particles with the inward-shifted configuration was better than that with the normal configuration. The impurity problem was not serious when the plasma boundary was sufficiently far from the chamber wall. By reducing the impurity problem, it was possible to sustain the plasma for more than two minutes using ICRF alone. (author)