Development of the heating scenarios to achieve high-ion temperature plasma in the Large Helical Device

High-ion temperature experiments in the Large Helical Device (LHD) are categorized in terms of the heating scenarios that are closely related to the development of neutral beam injection (NBI) systems. Although high-energy tangential negative-NBI heating has greatly contributed to extending the plasma parameter regime in LHD, the ion temperature does not increase because the electron heating is dominant with negative-NBIs. In the high-Z discharges, it was demonstrated that the ion temperature increased with an increasing ion heating power and achieved 13.5 keV with the negative-NBIs. Low-energy perpendicular positive-NBIs were installed for the ion heating, and the ion temperature was increased to more than 7 keV in hydrogen discharges. In the high-ion temperature plasmas, an ion internal transport barrier (ion ITB) was formed, and the impurity hole was observed in the core. Long-pulse ion cyclotron range of frequency heating (ICH)/electron cyclotron resonance heating (ECRH) helium discharges are effective for wall conditioning, leading to a decrease in the neutral density and a peaked density profile. Consequently, the ion heating efficiency increases in the core, and the central T_i is raised up to 7.5 keV. With the superposition of high-power ECRH, high-performance plasmas of T_i ∼ T_e ∼ 6 keV were obtained. In the planned deuterium experiment, the ion heating power will be increased with the deuterium beam injection, and T_i = 10 keV is expected. (author)