Marginal stability boundaries for infinite-n ballooning modes in a quasiaxisymmetric stellarator

A method for computing the ideal magnetohydrodynamic (MHD) stability boundaries in three-dimensional equilibria is employed. Following Hegna and Nakajima [Phys. Plasmas 5, 1336 (1998)], a two-dimensional family of equilibria is constructed by perturbing the pressure and rotational-transform profiles in the vicinity of a flux surface for a given stellarator equilibrium. The perturbations are constrained to preserve the MHD equilibrium condition. For each perturbed equilibrium, the infinite-n ballooning stability is calculated. Marginal stability diagrams are thus constructed that are analogous to (s,α) diagrams for axisymmetric configurations. A quasiaxisymmetric stellarator is considered. Calculations of stability boundaries generally show regions of instability can occur for either sign of the average magnetic shear. Additionally, regions of second-stability are present.