A Family of Quasi-Axisymmetric Stellarators with Varied Rotational Transform

We apply a continuation method to recently optimized stellarator equilibria with excellent quasi-axisymmetry (QA) to generate new equilibria with a wide range of rotational transform profiles.  Using these equilibria, we investigate how the rotational transform affects fast-particle confinement, the maximum coil-plasma distance, the maximum growth rate in linear gyrokinetic ion-temperature gradient (ITG) simulations, and the ion heat flux in corresponding nonlinear simulations.  We find values of two-term quasisymmetry error comparable to or lower than the similar Landreman-Paul (Phys. Rev. Lett. 128, 035001) configuration for values of the mean rotational transform τ between 0.12 and 0.75. The fast-particle confinement improves with τ until τ=0.73, at which point the degradation in quasisymmetry outweighs the benefits of further increasing τ.  The required coil-plasma distance only varies by about ±10% for the configurations under consideration, and is between 2.8m to 3.3m when the configuration is scaled up to reactor size.  The maximum growth rate from linear gyrokinetic simulations increases with τ, but also shifts towards higher k_y values.  The maximum linear growth rate is sensitive to the choice of flux tube at rational τ, but this can be compensated for by taking the maximum over several flux tubes.  The corresponding ion heat fluxes from nonlinear simulations display a non-monotonic relation to τ.  Sufficiently large positive shear is destabilizing.  This is reflected both in linear growth rates and nonlinear heat fluxes.