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

High-ion-temperature (exceeding 5keV) 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, Ti (even decreasing as Ti increases). The neoclassical (NC) ripple transport is suppressed by the ambipolar radial electric field, Er (<0) predicted by NC transport fluxes. The temperature ratio, Ti∕Te, 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 Ti∕Te larger) is a plausible approach to further increase Ti. Spontaneous rotation is evaluated in these high-Ti plasmas, in which a co-directed component is recognized at the radial location with a large Ti gradient, in addition to the tokamak-like counter-directed component expected fo...