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Generalized collision operator for fast electrons interacting with partially ionized impurities

Published online by Cambridge University Press:  16 November 2018

L. Hesslow
Affiliation:
Department of Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden
O. Embréus
Affiliation:
Department of Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden
M. Hoppe
Affiliation:
Department of Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden
T. C. DuBois
Affiliation:
Department of Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden
G. Papp
Affiliation:
Max Planck Institute for Plasma Physics, D-85748 Garching, Germany
M. Rahm
Affiliation:
Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-41296 Gothenburg, Sweden
T. Fülöp
Affiliation:
Department of Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden
Corresponding
E-mail address:

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 generalize the Fokker–Planck operator in a fully ionized plasma by accounting for the effect of screening. We detail the derivation of this generalized operator, and calculate the effective ion length scales, 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 accurate even for large impurity content.

Type
Research Article
Copyright
© Cambridge University Press 2018 

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