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 have to operate. A particular neoclassical 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~\textit{inward}~impurity flux, causing an impurity accumulation in the core that can be detrimental to performance. In this work, we use the SFINCS drift-kinetic solver to explore how the impurity particle flux is influenced as the degree of symmetry-breaking is varied between realistic levels and perfect quasisymmetry, over various reactor-relevant parameter regimes and configurations. We aim to answer the question of exactly how much symmetry-breaking a particular configuration can tolerate before impurity temperature screening is lost.