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Observation of Equilibria with a Double Magnetic Axis in LHD

H. Yamada, Y. Suzuki, K. Ida, M. Yoshinuma, T. Kobuchi, K. Y. Watanabe, K. Tanaka, T. Tokuzawa, LHD Experimental Group

Fusion Science and Technology / Volume 51 / Number 1 / January 2007 / Pages 138-143

Technical Paper / Stellarators

Finite-beta equilibria with a double magnetic axis have been realized in the Large Helical Device (LHD). Since the rotational transform is weak in the central region of the LHD, the effect of an externally applied quadrupole field is more pronounced in the central region than in the periphery. Consequently, the magnetic axis splits due to a moderate elongation. In the case of vertical elongation, the figure-eight structure of the magnetic surfaces has been observed in a soft X-ray image. Degradation of confinement due to the appearance of the separatrix is suggested in the case of horizontal elongation, which is closely related to the equilibrium beta limit. The three-dimensional magnetohydrodynamic equilibrium code HINT, which does not assume the existence of nested flux surfaces, provides physical pictures consistent with the experimental observations.

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