Generalized collision operator for fast electrons interacting with partially ionized impurities

TL;DR

This paper develops a generalized collision operator that accounts for partial screening effects of impurities on fast electrons, improving the accuracy of tokamak disruption modeling, especially regarding runaway electron growth.

Contribution

It introduces a new collision operator including partial screening effects and calculates effective ion length-scales for fusion-relevant ions, enhancing plasma interaction models.

Findings

Secondary runaway growth rate increases at high electric fields with impurities.

Fokker-Planck formalism remains valid even with large impurity content.

Partial screening significantly affects electron-impurity interactions.

Abstract

Accurate modelling of the interaction between fast electrons and partially ionized atoms is important for evaluating tokamak disruption mitigation schemes based on material injection. This requires accounting for the effect of screening of the impurity nuclei by the cloud of bound electrons. In this paper, we detail the derivation of a generalized collision operator including the effect of partial screening. We calculate the effective ion length-scales, which are needed in the components of the collision operator, for a number of ion species commonly appearing in fusion experiments. We show that for high electric fields, the secondary runaway growth rate can be substantially larger than in a fully ionized plasma with the same effective charge, although the growth rate is significantly reduced at near-critical electric fields. Furthermore, by comparison with the Boltzmann collision operator, we show that the Fokker--Planck formalism is valid even for large impurity content.