Comparison of Turbulent Transport in Quasi-Helically Symmetric and Quasi-Axisymmetric Stellarators

Employing the gyrokinetic turbulence code GENE, a comparison of ion temperature gradient mode growth rates and saturated heat fluxes between two quasi-symmetric (QS) stellarator configurations is conducted. We focus on the quasi-axisymmetric stellarator NCSX and the quasi-helically symmetric stellarator HSX in both the adiabatic and kinetic electron limits. Here, our results show that despite higher growth rates in HSX, saturated heat fluxes are lower in HSX compared to NCSX. Study of nonlinear frequencies reveal more broadband frequency structures in HSX versus NCSX where frequencies more closely match linear results. HSX exhibits a large number of subdominant modes characterized by eigenmodes extending along a field line while NCSX exhibits only a few subdominant modes that are more localized in the bad curvature region. This highlights the necessity for including nonlinear physics in the modeling of non-axisymmetric magnetic configurations, as linear growth rates alone prove to be a poor proxy for predicting turbulent transport across QS stellarator designs.