Characteristics of High-Ion-Temperature Plasmas heated by Neutral Beams in the Large Helical Device

Improvement of ion heat transport in Heliotron plasmas has been realized by upgrade of ion heating power by using low energy neutral beam (NB) in the Large Helical Device (LHD). Ion internal transport barrier (ITB) (the peaked profile of ion temperature (Ti) with steep gradient in the core region) has been formed and the central Ti of 6.8 keV has been achieved in hydrogen plasma with the line-averaged electron density of 2x10 m . The experimental ion thermal diffusivity significantly decreases to the neoclassical level, indicating significant reduction of anomalous transport. The analysis of neoclassical ambipolar diffusion showed the enhancement of the negative radial electric field (Er<0) associated with the Ti increase, indicating that the anomalous transport is suppressed by the negative Er. The ion transport inside the ITB shows negative dependence on the Ti and heating power, while that outside of ITB shows gyro-Bohm properties.