Impact of quasi-symmetry on stellarator equilibrium, stability and transport

Development of three-dimensional toroidal confinement devices with quasi-symmetry of the magnetic field strength in Boozer coordinates is a key component of the U.S. stellarator program. Previous work has led to the design of quasi-helically symmetric (QH), quasi-axisymmetric (QA) and quasi-poloidally symmetric (QP) stellarators but at very distinct aspect ratios, major radii, and plasma $\beta$. In this work, a computational analysis is undertaken to optimize stellarator configurations for a variety of quasi-symmetries, all at the same target aspect ratio, major radius and plasma $\beta$. We compare initial cases of QA, QP, and two QH configurations. The initial optimizations focused solely on enhancing quasi-symmetry. The QP configuration had high magnetic ripple which resulted in a relatively high ballooning stability $\beta$-limits and also relatively high estimates of ripple-induced neoclassical transport. The other three configurations had lower magnetic ripple and consequently lower ballooning stability $\beta$-limits and much lower estimates of ripple-induced neoclassical transport. Subsequent optimizations have focused on simultaneous optimization of degree of quasi-symmetry, higher $\beta$ stability, and lowering ripple transport.