Generalized collision operator for fast electrons interacting with partially ionized impurities

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.

arXiv commentPeer-reviewed version

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• Effect of Two-Stage Shattered Pellet Injection on Tokamak Disruptions
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• The hot-tail runaway seed landscape during the thermal quench in tokamaks
• Impact of a minority relativistic electron tail interacting with a thermal plasma containing high-atomic-number impurities
• Spatially-dependent modeling and simulation of runaway electron mitigation in DIII-D
• Electron runaway in ASDEX Upgrade experiments of varying core temperature
• Kinetic modelling of runaway electron generation in argon-induced disruptions in ASDEX Upgrade
• Runaway dynamics in reactor-scale spherical tokamak disruptions
• Runaway electron modelling in the self-consistent core European Transport Simulator, ETS