Transition from L mode to high ion temperature mode in CHS heliotron/torsatron plasmas

A high ion temperature (T-i) mode is observed for neutral beam heated plasmas in the Compact Helical System (CHS) heliotron/torsatron. The high T-i mode plasma is characterized by a high central ion temperature, T-i(0), and is associated with a peaked electron density profile produced by neutral beam fuelling with low wall recycling. Transition from L mode to high T-i mode has been studied in CHS. T-i(0) in the high T-i mode discharges reaches 1 keV, which is 2.5 times higher than that in the L mode discharges. The ion thermal diffusivity is significantly reduced by a factor of more than 2-3 in the high T-i mode plasma. The ion loss cone is observed in neutral particle flux in the energy range 1-6 keV with a narrow range of pitch angle (90 degrees +/- 20 degrees) in the high T-i mode. However, the degradation of ion energy confinement due to this loss cone is negligible.