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Author(s):
H. Yamada, S. Morita, K. Ida, S. Okamura, H. Iguchi, S. Sakakibara, K. Nishimura, R. Akiyama, H. Arimoto, M. Fujiwara, K. Hanatani, S. P. Hirshman, K. Ichiguchi, H. Idei, O. Kaneko, T. Kawamoto, S. Kubo, D. K. Lee, K. Matsuoka, O, Motojima, T. Ozaki, V. D. Pustovitov, A. Sagara, H. Sanuki, T. Shoji, C. Takahashi, Y. Takeiri, Y. Takita, S. Tanahashi, J. Todoroki, K. Toi, K. Tsumori, M. Ueda and I. Yamada
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Title:
MHD and Confinement Characteristics in the High-beta Regime on the CHS Low-Aspect-Ratio Heliotron / Torsatron
Date of publication:
Sep. 1992
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Key words:
Shafranov shift, resistive interchange mode
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Abstract:
The MHD and confinement characteristics associated with the finite-beta effect have been investigated in the low-aspect-ratio heliotron/torsatron Compact Helical System (CHS). < beta > values of 1.8% in the quasi-steady state and 1.9% in the transient phase have been realized and beta_0 has reached 5.8%. The experimentally observed equilibrium characteristics are in good agreement with current 3-D analysis. Significant instabilities suppressing beta value or terminating the discharges have not yet been observed in standard operation. This is supposed to be due to self-stabilization of the ideal interchange mode by spontaneously generated magnetic wells, which is a distinguishing feature of the low aspect ratio. Magnetic fluctuations with the frequency range less than 100 kHz have reached as much as 0.1 % of the poloidal equilibrium field. The predominant modes are globally coherent with the low mode number and do not affect confinement. The scaling of beta follows the prediction from LHD scaling. The thermal diffusivity depends on the local scale length of bdta, the characteristics of which is emphasized for the magnetic hill region. The present beta value is limited by saturation of confinement with increasing density and degradation of confinement with increasing power rather than by MHD characteristics.
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