Optimization of compact quasi-axisymmetric stellarators

Abstract We present the optimization of compact quasi-axisymmetric stellarators with aspect ratio as low as A = 2.4 and varying number of field periods. This previously unexplored optimization space has the striking feature that the symmetry-preserving perturbations remain mostly on the inboard side, and that the axisymmetric volume is large compared to previous designs. Configurations of two, three, and four field periods are presented which feature an average external rotational transform of 0.1, expected to act stabilizing, while obeying quasi-symmetry sufficiently enough to provide confinement to alpha particles at a typical reactor scale. We highlight the impact of different physics constraints on the achievable quasi-symmetry and optimized plasma boundaries, and discuss the trade-offs faced in the choice of the number of field periods.