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(THP1/09) Optimization Study of L=1 Helical Magnetic Axis Systems

K. H. Saito, M. Aizawa, K. N. Saito, I. Kawakami, S. Shiina

Atomic Energy Research Institute, College of Science and Technology, Nihon University, Tokyo, Japan 101-8308

Abstract.  A new approach to the transport improvement in helical magnetic axis stellarator is proposed. First of all, the proposal is presented for L=1 helical axis systems with roughly quasihelical symmetry, then its compatibility with equilibrium and stability would be optimized and controlled. In the optimization, three key requirements are considered; the reduction of effective toroidal curvature $ \epsilon_{T}^{}$ for localized trapped particles, the suppression of indirect magnetic island formation and the magnetic well improvement by some field modifications. The important features is to find the field modifications to satisfy the other two requirements. The reduced $ \epsilon_{T}^{}$ configuration is found to be nearly omnigenous while far from quasihelical, giving rise to the possibility of stellarator design study in a wider parameter domain than quasisymmetry approaches. Up-to-date results show both the negative coil pitch modulation and the superposition of a relatively weak L=-1 field to satisfy the compatibility within the central beta value up to 2% calculated. The compatibility study at higher beta and lower aspect ratio is progressing by optimizing configuration parameters with the combination of some field modifications proposed to improve magnetic well.

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IAEA 2001