Impurity temperature screening in stellarators close to quasisymmetry

Abstract

Impurity temperature screening is a favorable neoclassical phenomenon involving an outward radial flux of impurity ions from the core of fusion devices. Quasisymmetric magnetic fields are intrinsically ambipolar and give rise to temperature screening for low enough $\eta^{-1}\equiv d\ln n/d\ln T$. In contrast, neoclassical fluxes in generic stellarators will depend on the radial electric field, which is predicted to be inward for ion-root plasmas, potentially leading to impurity accumulation. Here we examine the impurity particle flux in a number of approximately quasisymmetric stellarator configurations and parameter regimes while varying the amount of symmetry-breaking in the magnetic field. The amount of symmetry-breaking (from perfect quasisymmetry) required to lose temperature screening is dependent on the particular configuration and flux surface. Neoclassical fluxes have been obtained using the SFINCS drift-kinetic equation solver with the electrostatic potential $\Phi=\Phi(\psi)$, where $\psi$ is a flux surface label. Results indicate that achieving temperature screening is possible, but unlikely, at reactor-relevant conditions in the core. Thus, the small departures from symmetry in nominally quasisymmetric stellarators are large enough to significantly alter the neoclassical impurity transport. A further look at these fluxes when compared to a gyro-Bohm turbulence estimate suggests that neoclassical fluxes are small in optimized configurations compared to turbulent fluxes. Therefore, although neoclassical impurity accumulation is expected in most situations, the strength of the turbulence may render this irrelevant.