The impact of non-local parallel electron transport on plasma-impurity reaction rates in tokamak scrape-off layer plasmas

TL;DR

This study investigates how non-local parallel electron transport affects plasma-impurity reaction rates in tokamak scrape-off layer plasmas, revealing significant deviations from Maxwellian assumptions and implications for plasma modeling accuracy.

Contribution

It demonstrates the impact of non-Maxwellian electron distributions on impurity reaction rates and evaluates the validity of using Maxwellian-averaged rates in plasma simulations.

Findings

High-energy electron tails significantly alter ionization and radiative rates.

Fluid models with simplified conductivity can misestimate reaction rates.

Non-local transport effects are observable and relevant for plasma diagnostics.

Abstract

Plasma-impurity reaction rates are a crucial part of modelling tokamak scrape-off layer (SOL) plasmas. To avoid calculating the full set of rates for the large number of important processes involved, a set of effective rates are typically derived which assume Maxwellian electrons. However, non-local parallel electron transport may result in non-Maxwellian electrons, particularly close to divertor targets. Here, the validity of using Maxwellian-averaged rates in this context is investigated by computing the full set of rate equations for a fixed plasma background from kinetic and fluid SOL simulations. We consider the effect of the electron distribution as well as the impact of the electron transport model on plasma profiles. Results are presented for lithium, beryllium, carbon, nitrogen, neon and argon. It is found that electron distributions with enhanced high-energy tails can result in significant modifications to the ionisation balance and radiative power loss rates from excitation, on the order of 50-75% for the latter. Fluid electron models with Spitzer-H\"arm or flux-limited Spitzer-H\"arm thermal conductivity, combined with Maxwellian electrons for rate calculations, can increase or decrease this error, depending on the impurity species and plasma conditions. Based on these results, we also discuss some approaches to experimentally observing non-local electron transport in SOL plasmas.