(EX/P1-13) Characteristic Features of Edge Transport Barrier Formed in Helical Divertor Configuration of the Large Helical Device

K. Toi1), F. Watanabe2), S. Ohdachi1), K. Narihara1), T. Morisaki1), S. Sakakibara1), X. Gao3), M. Goto1), K. Ida1), M. Kobayashi1), S. Masuzaki1), J. Miyazawa1), S. Morita1), K. Tanaka1), T. Tokuzawa1), K.W. Watanabe1), A. Weller4), L. Yan5), M. Yoshinuma1), K. Kawahata1), A. Komori1), LHD Experimental Group1)
 
1) National Institute for Fusion Science, Toki-shi, Japan
2) Department of Energy Science and Engineering, Nagoya University, Nagoya, Japan
3) Institute of Plasma Physics, Chinese Academy of Science, Hefei, China
4) Max-Planck-Institut für Plasmaphysik, D-17491 Greifswald, Germany
5) Southwestern Institute of Physics, Chengdu, China

Abstract.  Edge transport barrier (ETB) was formed through low to high confinement (L-H) transition in a helical divertor configuration of the Large Helical Device (LHD) where the last closed magnetic surface is surrounded by ergodic layer defined in the vacuum field. Relationship between ETB formation and ergodic layer, and effects of sizable magnetic island in the edge region on ETB formation were investigated in this unique magnetic configuration of LHD. Electron density and its gradient in the edge region increase noticeably across the transition, while electron temperature profile has no obvious change. Thus formed ETB extends into ergodic layer. The improvement of energy confinement time is modest so far, compared to the ISS95 international stellarator scaling. The threshold power for the L-H transition is almost the same as the ITER power threshold scaling. The width of ETB is in the range of 8 cm to 16 cm for the averaged minor plasma radius of 60 cm, and has no clear dependence on the toroidal field strength over 0.5 T to 1.5 T, where the rotational transform at ETB is fixed. The ETB width is much larger than the poloidal ion gyro-radius which is 1 cm in these H-mode plasmas at the toroidal field 0.5 T. When resonant helical field perturbations were applied to expand a magnetic island size near the edge, the L-H transition was triggered at lower electron density compared with the case without application of the field perturbations, suppressing fluctuations of H-alpha emission like ELM like activities.

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