On the relativistic large-angle electron collision operator for runaway avalanches in plasmas

TL;DR

This paper introduces a fully conservative relativistic large-angle collision operator for runaway electrons in plasmas, assessing its impact on growth rates and the importance of operator choice near the avalanche threshold.

Contribution

It presents the first derivation of a conservative large-angle collision operator from the relativistic Boltzmann equation, suitable for numerical implementation.

Findings

The new operator affects the runaway-electron growth rate.

Operator choice is crucial near the avalanche threshold.

The model accurately describes the reverse avalanche effect.

Abstract

Large-angle Coulomb collisions lead to an avalanching generation of runaway electrons in a plasma. We present the first fully conservative large-angle collision operator, derived from the relativistic Boltzmann operator. The relation to previous models for large-angle collisions is investigated, and their validity assessed. We present a form of the generalized collision operator which is suitable for implementation in a numerical kinetic-equation solver, and demonstrate the effect on the runaway-electron growth rate. Finally we consider the reverse avalanche effect, where runaways are slowed down by large-angle collisions, and show that the choice of operator is important if the electric field is close to the avalanche threshold.