Improving fast-particle confinement in quasi-axisymmetric stellarator optimization

Abstract

A method to improve fast-particle confinement during quasi-axisymmetric stellarator optimization has been identified. Quasi-axisymmetric (qa) stellarator designs have improved neoclassical transport due to their special symmetry of the magnetic field strength. Previously, it has been shown that, in general, quasi-symmetry can only be obtained on one single flux surface (Garren and Boozer 1991 Phys. Fluids B 3 2805-21). Even though quasi-symmetry can be a crucial property of stellarator design, there is no established convention for choosing the flux surface on which this should be optimized. To address this question, the flux surface on which quasi-axisymmetry is optimized has been varied in a qa configuration. The optimal location was found to lie between half radius and the plasma edge, since this allows for two beneficial features: it increases the number of flux-surfaces with improved quasi-axisymmetry and it increases the volume enclosed by the flux surface with the best qa quality.