MHD stability of low-n ideal external mode in Large-Helical-Device plasma

Y. Narushima, K.Y. Watanabe, S. Sakakibara, N. Nakajima, K. Nishimura,
H. Yamada, K. Yamazaki and LHD experimental group, W.A. Cooper1)

National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
1)Centre de Recherches en Physique des Plasmas, Association Euratom / Confederartion Suisse, EPFL, 1015 Lausanne, Switzerland

It is worthwhile to investigate the characteristics of the MHD activities in the Large Helical Device (LHD) plasma theoretically and experimentally although the serious MHD instabilities leading to the termination of the discharge have not been observed in experiment. We focus on the external mode (particularly m/n=1/1 mode) in this study. The low-n ideal MHD analysis is performed for n=1 mode family on the LHD plasma using the three-dimensional stability code TERPSICHORE [1]. The analysis with TERPSICHORE predicted that the external mode would be unstable in the high beta region [2]. This external mode, which has the m/n=1/1 mode, appears under the condition of free boundary and rotational transform without ι/2π=1.0 surface, even in Mercier stable and/or low-b region. In case the position of the perfectly conducting wall which is defined by the ratio of the wall radius to the plasma radius is at awall <1.08, the external mode is stabilized. The real LHD wall is equivalent to the awall=3.0, which is almost free boundary condition. The experiment is also carried out to investigate the stability. In the plasma without rational surface of ι/2π=1.0, the external mode does not occur, which suggests that the plasma surrounding the bulk region has a role of stabilization.

References

[1] W. A. Cooper, Plasma Phys. Control. Fusion 34 (1992) 1011
[2] Y. Narushima, SAKAKIBARA Satoru, WATANABE Kiyomasa, NISHIMURA Kiyohiko, YAMADA Hiroshi, NAKAJIMA Noriyoshi, YAMAZAKI Kozo, COOPER W. Anthony, and LHD experimental group, Journal of Plasma and Fusion Research SERIES, 5 (2002) 514


This study was supported by a Grant-in-Aid for Young Scientists(B)(No.15760627) from the Ministry of Education,Culture,Sports,Science and Technology of Japan.