Kinetic modelling of runaway electrons in dynamic scenarios

TL;DR

This paper advances kinetic modeling of runaway electrons in tokamaks, focusing on dynamic scenarios like disruptions, with new collision operators and insights into avalanche growth rate accuracy.

Contribution

It introduces a self-consistent electric-field evolution and improved collision operators for more accurate runaway-electron modeling during dynamic plasma events.

Findings

Self-consistent electric-field evolution implementation.

Collision operator conserving momentum and energy.

Simplified avalanche model may be inaccurate for certain parameters.

Abstract

Improved understanding of runaway-electron formation and decay processes are of prime interest for the safe operation of large tokamaks, and the dynamics of the runaway electrons during dynamical scenarios such as disruptions are of particular concern. In this paper, we present kinetic modelling of scenarios with time-dependent plasma parameters; in particular, we investigate hot-tail runaway generation during a rapid drop in plasma temperature. With the goal of studying runaway-electron generation with a self-consistent electric-field evolution, we also discuss the implementation of a collision operator that conserves momentum and energy and demonstrate its properties. An operator for avalanche runaway-electron generation, which takes the energy dependence of the scattering cross section and the runaway distribution into account, is investigated. We show that the simplified avalanche model of Rosenbluth & Putvinskii [Nucl. Fusion 1997 37 1355] can give inaccurate results for the avalanche growth rate (either lower or higher) for many parameters, especially when the average runaway energy is modest, such as during the initial phase of the avalanche multiplication. The developments presented pave the way for improved modelling of runaway-electron dynamics during disruptions or other dynamic events.