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). The positive-ion-based NBI (p-NBI) with low energy of 40 keV was newly installed for ion heating experiments and was also utilized to measure Ti profile using charge exchange spectroscopy (CXS) measurement, which consequently leads to the extension of heliotron plasmas in the high-Ti regime and significant progress of ion transport study in LHD. The peaked profile of ion temperature (Ti) with steep gradient has been formed in the core region of high-power NB heated plasmas in LHD. The Ti strongly depends on direct ion heating power of NBs and is proportional to Pi''a; a = 1.0 - 1.3, while the prediction on scaling basis gives a ∼ 0.4. The central Ti of 6.8 keV has been achieved in hydrogen plasma with the electron density of 2 x 10''1''9m-''3 . The Ti increase is a transient phenomenon, and the duration time of the high-Ti phase is about 120 ms, which is a few time longer than the energy confinement time. The electron temperature is 3.5 keV and no improvement of electron transport was observed in these discharges. The experimental ion thermal diffusivity significantly decreases to the neoclassical level, indicating reduction of anomalous transport, which is the first observation in LHD. 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 significantly suppressed by the enhanced negative Er. This improved mode is characterized by the reduction of ion heat transport associated with Ti increase, that is; ∂χ(i)/∂Ti < 0. Therefore, it is considered to be possible to extend the helical plasma in the further high- Ti regime by utilizing this feature. (author)