Extension of the high-ion-temperature regime in the Large Helical Device

High-ion-temperature (exceeding 5 keV) hydrogen plasmas have been successfully produced in the Large Helical Device [Iiyoshi et al., Nucl. Fusion 39, 1245 (1999); Motojima et al., Nucl. Fusion 47, S668 (2007)] with the ion heat confinement improvement in the core region. The experimental ion heat diffusivity at the core region is found to be almost independent of the ion temperature, T_i (even decreasing as T_i increases). The neoclassical (NC) ripple transport is suppressed by the ambipolar radial electric field, E_r (<0) predicted by NC transport fluxes. The temperature ratio, T_i/T_e, is one of the key parameters to reduce the NC ambipolar particle and heat fluxes. Thus, it is suggested that the selective ion heating (making T_i/T_e larger) is a plausible approach to further increase T_i. Spontaneous rotation is evaluated in these high-T_i plasmas, in which a co-directed component is recognized at the radial location with a large T_i gradient, in addition to the tokamak-like counter-directed component expected for E_r<0