Filamentary plasma eruptions and the heating and acceleration of electrons

TL;DR

This study uses test-particle simulations to show that eruptive plasma filaments during edge localized modes can rapidly accelerate electrons to high energies, forming non-thermal tails and exhibiting super-diffusive energy transport.

Contribution

It demonstrates the role of parallel electric fields in electron acceleration during plasma eruptions and links non-Gaussian electric field statistics to anomalous transport phenomena.

Findings

Electrons are accelerated to 90 keV during filament eruptions.

Acceleration occurs mainly along magnetic field lines with a preference in the counter-current direction.

Energy transport exhibits super-diffusive behavior with exponential energy-increment tails.

Abstract

We present test-particle simulations of electrons during a nonlinear MHD simulation of a type-I edge localized mode (ELM) to explore the effect of an eruptive plasma filament on the kinetic level. The electrons are moderately heated and accelerated during the filamentary eruption on a fast time scale of the order of 0.5 ms. A clearly non-thermal tail is formed in the distribution of the kinetic energy that is of power-law shape and reaches 90 keV for some particles. The acceleration is exclusively observed in the direction parallel to the magnetic field, i.e. with a clear preference in counter-current direction, and we show that the parallel electric field is the cause of the observed acceleration. Most particles that escape from the system leave at one distinct strike-line in the outer divertor leg at some time during their energization. The escaping high energy electrons in the tail of the energy distribution are not affected by collisions, they thus show characteristics of runaway electrons. The mean square displacement indicates that transport in energy space clearly is super-diffusive, and interpreting the acceleration process as a random walk, we find that the distributions of energy-increments exhibit exponential tails, and transport in energy space is equally important of convective (systematic) and diffusive (stochastic) nature. By analyzing the MHD simulations per se, it turns out that the histograms of the parallel electric field in the edge region exhibit power-law shapes, and this clearly non-Gaussian statistics is ultimately one of the reasons for the moderately anomalous phenomena of particle transport that we find in energy space.