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

Abstract

High-ion-temperature ͑exceeding 5 keV͒ hydrogen plasmas have been successfully produced in the Large Helical Device ͓Iiyoshi et al., Nucl. Fusion 39, 1245 ͑1999͒; Motojima et al., Nucl. Fusion 47, S668 ͑2007͔͒ 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 increases͒. The neoclassical ͑NC͒ ripple transport is suppressed by the ambipolar radial electric field, E r ͑Ͻ0͒ 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 larger͒ 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 Ͻ 0.

Considerations from the Viewpoint of Neoclassical Transport Towards Higher Ion Temperature Heliotron Plasmas

“…The E r can reduce the neoclassical (NC) transport in low-collisional high-T i LHD plasmas as estimated in Ref. [3]. However, it is increasing as T i is increased even with the presence of E r , although the degree of the increase is much smaller than that predicted from the "pure (E r =0)" 1/ν ripple transport [3].…”

Dynamics of ion internal transport barrier in LHD heliotron and JT-60U tokamak plasmas

“…The inward convection in JT-60U is predicted by neoclassical impurity transport because of the large density gradient [6]. However in neoclassical theory of helical plasmas [7], the inward convection due to the negative electric field always overcomes the outward convection due to the ion temperature gradient (temperature screening effect [8]) in this plasma, because T i is larger than T e by a factor of two at the end of the formation of the ITB. The prediction by the neoclassical contradicts to that the outward convection is observed in experiment in LHD.…”

Parameter Regime of Ion Internal Transport Barrier Formation in the Large Helical Device

“…The experimentally observed temperature gradient has an antineoclassical transport property, indicating that the anomalous transport decreases with decreasing collisionality. The reduction in ion thermal diffusivity seems to saturate at the neoclassical level [1], and no disruptive phenomena were observed in the ion ITB in the LHD so far. The optimization of ripple transport of helically trapped particles may be a control knob for ion ITB behavior.…”

“…No transport barrier was observed in the electron temperature profile when the ion ITB formed. The ion thermal diffusivity in the plasma core decreased by a factor of three and reached the neoclassical level, where the negative radial electric field (ion root) is predicted by neoclassical ambipolarity [1][2][3].…”