Global effects on neoclassical transport in the pedestal with impurities

TL;DR

This study uses a global neoclassical solver to analyze how impurities affect particle and momentum transport in the tokamak pedestal, revealing non-ambipolar fluxes and enhanced momentum transport due to impurities.

Contribution

It provides the first detailed numerical analysis of impurity effects on neoclassical transport in the tokamak pedestal with non-trace impurities.

Findings

Impurities cause non-ambipolar particle fluxes extending outside the pedestal.

Electron transport can be comparable to ion and impurity flows in the pedestal.

Impurities significantly enhance neoclassical momentum transport.

Abstract

We present a numerical study of collisional transport in a tokamak pedestal in the presence of non-trace impurities, using the radially global $\delta f$ neoclassical solver PERFECT [M. Landreman et al. 2014 Plasma Phys. Control. Fusion 56 045005]. It is known that in a tokamak core with non-trace impurities present the radial impurity flux opposes the bulk ion flux to provide an ambipolar particle transport, with the electron transport being negligibly small. However, in a sharp density pedestal with sub-sonic ion flows the electron transport can be comparable to the ion and impurity flows. Furthermore, the neoclassical particle transport is not intrinsically ambipolar, and the non-ambipolarity of the fluxes extends outside the pedestal region by the radial coupling of the perturbations. The neoclassical momentum transport, which is finite in the presence of ion orbit-width scale profile variations, is significantly enhanced when impurities are present in non-trace quantities, even if the total parallel mass flow is dominated by the bulk ions.