Stability of a compact three-period stellarator with quasiaxial symmetry features

Abstract

The ideal magnetohydrodynamic stability properties of a three-field period stellarator with quasiaxial symmetry features proposed by Garabedian and Ku [Phys. Plasmas 6, 645 (1999)] are investigated. The stability limit imposed by local ballooning and Mercier modes allows β∼5.3% to be achieved by tailoring the pressure profile. For this system, the global external kink modes are shown to be stable. To demonstrate this, a sequence of configurations has been developed that varies from the reference system proposed to the equivalent axisymmetric tokamak. A scan of kink stability reveals that the configuration with a plasma boundary shape that corresponds to 80% of the basic three-period stellarator and 20% of the tokamak becomes marginal to global external modes. The normal magnetic field line curvature and the local magnetic shear play crucial roles in the stability properties. The local magnetic shear develops a helical stripe with increasing three-dimensional plasma deformation that inhibits the formation of global structures. The normal magnetic field line curvature, however, becomes locally amplified by the three-dimensional plasma deformation in regions where the local magnetic shear vanishes. As a result, the susceptibility to localized ballooning mode destabilization is enhanced.