Quasi-axisymmetric stellarators with varying rotational transform profiles

The rotational transform ι affects almost all aspects of toroidal magnetic confinement devices, including particle orbits, turbulent transport, and more.

We optimize a set of vacuum quasi-axisymmetric (QA) stellarator configurations for varying ι-profiles. By optimizing for quasisymmetry alongside different values of mean ι or mean shear, we are able to assess the compatibility of different ι-profiles with quasisymmetry. For a class of two-field period quasisymmetric configurations based on the vacuum QA configuration in [Phys. Rev. Lett. 128 (2022) 035001], we find that good quasisymmetry is limited to mean ι ≲ 0.8. Increasing ι requires increasing the flux-surface elongation, which sets this limit. Since the fast-particle confinement improves with increasing ι, this creates a trade-off between elongation and confinement.

We also investigate the effect of ι and shear on gyrokinetic turbulence. Using linear gyrokinetic stella simulations, we find that the growth rates increase with ι, while shifting towards smaller scales. At low-order rational values of ι, growth rates calculated on a single flux-tube may deviate significantly from values calculated at nearby ι. A smooth dependence on ι is recovered by running simulations on several flux tubes.