Avoiding low-order rationals in optimization of compact quasi-axisymmetric stellarators*

Quasi-axisymmetric (QA) stellarators can be made more compact than other optimized stellarators, but generally have larger bootstrap currents for a given pressure gradient.

This makes QA stellarators more sensitive to the choice of pressure profile.

In particular, pressure profiles with more radial variation lead to more variation in the bootstrap current, which leads to higher magnetic shear.

Shear can be beneficial for MHD and gyrokinetic stability, but it also makes it harder to avoid crossing low-order rationals. Using self-consistent bootstrap current calculations based on the Redl formula [Phys. Plasmas 29, 082501 (2022)], we investigate which pressure profiles can be made compatible with avoiding low-order rationals while satisfying various proxies for magnetohydrodynamic (MHD) stability.

In order for these configurations to be of practical relevance, it must be possible to achieve the targeted pressure profiles in practice.

We will use the transport code T3D [https://t3d.readthedocs.io] coupled to the gyrokinetic code GX [https://gx.readthedocs.io] to evolve our equilibrium and profiles, using heating power as a control input.