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(EX2/2) Transition from L Mode to High Ion Temperature Mode in CHS Heliotron/Torsatron Plasmas

K. Ida, M. Osakabe, K. Tanaka, T. Minami, S. Nishimura, S. Okamura, A. Fujisawa, K. Yamazaki, Y. Yoshimura, S. Kubo, R. Akiyama, D. S. Darrow¹, H. Idei,. H. Iguchi, M. Isobe, S. Kado, T. Kondo², S. Lee, K. Matsuoka, S. Morita, I. Nomura, S. Ohdachi, M. Sasao, A. Shimizu³, K. Tumori, S. Takayama², M. Takechi³, S. Takagi³, C. Takahashi, K. Toi, T. Watari

National Institute for Fusion Science, Oroshi-cho, Toki-shi, Gifu, 509-5292, Japan
¹ Princeton Plasma Physics Laboratory, Princeton, NJ 08543, U.S.A.
² Graduate University for Advanced Studies, Hayama, 240-01, Japan
³ Dept. Energy Engineering Science, Nagoya Univ., Nagoya, 464-01, Japan

Abstract.  A high ion temperature mode (high 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 fueling with low wall recycling. Transition from L mode to high T_i mode has been studied in CHS. The central ion temperature in the high T_i mode discharges reaches to 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 of 1-6 keV with a narrow range of pitch angle (90 ± 10 degree) in the high T_i mode. However, the degradation of ion energy confinement due to this loss cone is negligible.

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