Optimizing stellarators for turbulent impurity and energy transport using linear gyrokinetic simulations

In neoclassically optimized stellarators, turbulence may be needed to avoid impurity accumulation. Comparisons between standand and turbulence reduced scenarios in Wendelstein 7-X indeed show order of magnitude higher impurity concentrations in the latter scenarios (A. Langenberg et al 2021 Nucl. Fusion 61 116018). As lower levels of turbulences generally correlate with increased energy confinement, this poses a potentially contradictory set of requirements on the optimization of stellarator geometries. It remains to be seen if it is possible to design stellarator magnetic fields that simultanously achieve high enough outward impurity transport and low enough energy transport.

In this work, we investigate the possibility of using linear gyrokinetic simulations to find stellarators that simultanously satisfy these criteria. Linear simulations are fast enough to be suitable for inclusion in stellarator optimization loops, but may not be a good proxy for the full nonlinear behaviour. However, the ratio of particle and energy fluxes from linear simulations are often more reliable than the individual fluxes, which suggests that the ratio of impurity to energy transport might be a useful proxy for optimization. We investigate this proxy by comparing linear and nonlinear simulations.