Temperature Screening of Impurities in Stellarators and Tokamaks Deviating From Symmetry

Quasisymmetric stellarator configurations aim to combine the stability of stellarators with the confinement of tokamaks, making them particularly interesting for optimization efforts. However, perfect quasisymmetry can only be achieved on a single flux surface at best, making it useful to study configurations with small deviations from perfect quasisymmetry, a regime in which devices will unavoidably have to operate. A particular phenomenon that occurs in tokamaks, which are naturally quasisymmetric, is a favorable outward radial flux of highly charged impurity ions, commonly referred to as impurity temperature screening. Conversely, stellarators generally display an inward impurity flux, causing an impurity accumulation in the core that can be detrimental to performance. Given realistic levels of departure from symmetry in stellarators or tokamaks, will real experiments have an outward impurity flux consistent with tokamaks, or an inward flux like non-symmetric stellarators? We use the SFINCS drift-kinetic solver to calculate neoclassical fluxes in order to investigate this property over various parameter regimes and configurations.