Neoclassical flows in deuterium-helium plasma density pedestals

TL;DR

This paper investigates neoclassical flows and fluxes in the plasma density pedestal of tokamaks, revealing complex flow structures and significant angular momentum transport influenced by plasma composition and radial profile variations.

Contribution

It introduces a radially global neoclassical model to analyze flows and fluxes in the pedestal, highlighting the impact of plasma composition changes and sharp profile variations.

Findings

Poloidally resolved radial fluxes are not divergence-free, leading to poloidal return-flows.

Complex radial-poloidal flow structures extend several orbit widths into the core.

Significant neoclassical toroidal angular momentum transport is observed in the global model.

Abstract

In tokamak transport barriers, the radial scale of profile variations can be comparable to a typical ion orbit width, which makes the coupling of the distribution function across flux surfaces important in the collisional dynamics. We use the radially global steady-state neoclassical {\delta}f code Perfect to calculate poloidal and toroidal flows, and radial fluxes, in the pedestal. In particular, we have studied the changes in these quantities as the plasma composition is changed from a deuterium bulk species with a helium impurity to a helium bulk with a deuterium impurity, under specific profile similarity assumptions. The poloidally resolved radial fluxes are not divergence-free in isolation in the presence of sharp radial profile variations, which leads to the appearance of poloidal return-flows. These flows exhibit a complex radial-poloidal structure that extends several orbit widths into the core and is sensitive to abrupt radial changes in the ion temperature gradient. We find that a sizable neoclassical toroidal angular momentum transport can arise in the radially global theory, in contrast to the local.