Extension of High-Ion-Temperature Regime in the Large Helical Device (LHD)

The production of high-ion-temperature hydrogen plasmas was successfully demonstrated in the LHD. The ion temperature (Ti) exceeded 5 keV (the record value in helical plasmas) at an average plasma density (n$_{e})$ of 1.2x10$^{19}$m$^{-3}$ and also achieved 3 keV at n$_{e}\sim $4x10$^{19}$m$^{-3}$. This achievement demonstrated the capability of high-ion-temperature plasma confinement in helical devices. The total injection power of neutral beams of 20 MW (3 parallel and 1 perpendicular injections) and ion cyclotron heating power of about 2 MW was applied in reduced helical-ripple magnetic configurations with Rax in the range of 3.575-3.65 m and B of 2.7-2.85 T. Here Rax is the vacuum magnetic axis position. High-Ti plasmas typically have large toroidal velocity, Vt, of order 50 km/s in the core region, accompanied by an increase of the Ti-gradient. \newline The measured core density of carbon-impurity ions strongly drops as the core-Ti increases. This implies that carbon-impurity ions are expelled from the core region. This unique feature may provide an efficient knob to avoid the impurity accumulation in reactor-relevant helical plasmas. Transport analysis has been performed, including comparison with relevant theories.~In these high-Ti plasmas, the ions are in 1/$\nu $ regime and the neoclassical ambipolar Er is expected to be negative (ion-root). This prediction indicates that the hollow impurity profile (usually anticipated from the positive Er (electron-root)) must be due to effects beyond neoclassical transport theory. The role of the large Vt in the improved ion heat confinement, with the viewpoint of plasma viscosity structure in three-dimensional magnetic configurations, will be discussed.