Numerical calculation of ion runaway distributions

TL;DR

This paper introduces a numerical solver for the 2D non-relativistic linearized Fokker-Planck equation to analyze ion runaway distributions in plasmas, with applications to solar flares and tokamak disruptions.

Contribution

It provides a flexible, spectral-Eulerian numerical method to compute ion distributions under varying plasma conditions, improving upon previous analytic approaches.

Findings

Determines time scales and electric fields for ion runaway in different plasmas.

Analyzes conditions for excitation of Alfvén eigenmodes during disruptions.

Offers insights into ion acceleration mechanisms in astrophysical and fusion contexts.

Abstract

Ions accelerated by electric fields (so-called runaway ions) in plasmas may explain observations in solar flares and fusion experiments, however limitations of previous analytic work have prevented definite conclusions. In this work we describe a numerical solver of the 2D non-relativistic linearized Fokker-Planck equation for ions. It solves the initial value problem in velocity space with a spectral-Eulerian discretization scheme, allowing arbitrary plasma composition and time-varying electric fields and background plasma parameters. The numerical ion distribution function is then used to consider the conditions for runaway ion acceleration in solar flares and tokamak plasmas. Typical time scales and electric fields required for ion acceleration are determined for various plasma compositions, ion species and temperatures, and the potential for excitation of toroidal Alfv\'en eigenmodes during tokamak disruptions is considered.