Edge Momentum Transport by Neutrals: an Interpretive Numerical Framework

TL;DR

This paper develops a numerical framework to evaluate neutral-induced momentum transport in tokamak edges, revealing neutrals' significant role in intrinsic rotation comparable to neutral beam injection effects.

Contribution

It introduces an interpretive numerical method to assess neutral momentum transport across all collisionality regimes in tokamak edges.

Findings

Neutral momentum fluxes up to 1-2 N·m in ASDEX Upgrade edge.

Neutral-induced torque is comparable to neutral beam injection torque.

Framework can analyze neutral effects based on radial plasma profiles.

Abstract

Due to their high cross-field mobility, neutrals can contribute to momentum transport even at the low relative densities found inside the separatrix and they can generate intrinsic rotation. We use a charge-exchange dominated solution to the neutral kinetic equation, coupled to neoclassical ions, to evaluate the momentum transport due to neutrals. Numerical solutions to the drift-kinetic equation allow us to cover the full range of collisionality, including the intermediate levels typical of the tokamak edge. In the edge there are several processes likely to contribute to momentum transport in addition to neutrals. Therefore, we present here an interpretive framework that can evaluate the momentum transport through neutrals based on radial plasma profiles. We demonstrate its application by analysing the neutral angular momentum flux for an L-mode discharge in the ASDEX Upgrade tokamak. The magnitudes of the angular momentum fluxes we find here due to neutrals of up to $1{-}2\;\mathrm{N\, m}$ are comparable to the net torque on the plasma from neutral beam injection, indicating the importance of neutrals for rotation in the edge.