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(OV4/5) Confinement Physics Study in a Small Low-Aspect-Ratio Helical Device CHS

S. Okamura, K. Matsuoka, R. Akiyama, D. S. Darrow¹, A. Ejiri², A. Fujisawa, M. Fujiwara, M. Goto, K. Ida, H. Idei, H. Iguchi, N. Inoue, M. Isobe, K. Itoh, S. Kado, K. Khlopenkov³, T. Kondo³, S. Kubo, A. Lazaros⁴, S. Lee, G. Matsunaga⁵, T. Minami, S. Morita, S. Murakami, N. Nakajima, N. Nikai⁵, S. Nishimura, I. Nomura, S. Ohdachi, K. Ohkuni⁵, M. Osakabe, R. Pavlichenko³, B. Peterson, R. Sakamoto, H. Sanuki, M. Sasao, A. Shimizu⁵, Y. Shirai³, S. Sudo, S. Takagi⁵, C. Takahashi, S. Takayama³, M. Takechi⁵, K. Tanaka, K. Toi, K. Yamazaki, M. Yokoyama, Y. Yoshimura, T. Watari

National Institute for Fusion Science, Toki 509-5292, Japan
¹ Princeton Plasma Physics Laboratory, Princeton, N. J., U.S.A.
² The Graduate School of Science, Tokyo Univ., Tokyo 113-0033, Japan
³ The Graduate Univ. for Advanced Studies, Toki 509-5292, Japan
⁴ National Technical University, Athens, Greece
⁵ Dep. of Energy Eng. Science, Nagoya Univ., Nagoya 464-8603, Japan

Abstract.  The configuration parameter of the plasma position relative to the center of the helical coil winding is very effective one for controlling the MHD stability and the trapped particle confinement in Heliotron/Torsatron systems. But these two characteristics are contradictory to each other in this parameter. The inward shifted configuration is favorable for the drift-orbit-optimization but it is predicted unstable with the Mercier criterion. Various physics problems, such as electric field structure, plasma rotation and MHD phenomena, have been studied in CHS with a compromising intermediate position. With this standard configuration, CHS has supplied experimental results for understanding general toroidal confinement physics and low-aspect-ratio helical systems. In the recent experiments, it was found that the wide range of inward shifted configurations gives stable plasma discharges without any restriction to the special pressure profile. Such enhanced range of operation made it possible to study experimentally the drift-orbit-optimized configuration in the Heliotron/Torsatron systems. The effect of configuration improvement was studied with plasmas in a low collisionality regime.

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