Radiation reaction induced non-monotonic features in runaway electron distributions

TL;DR

This paper investigates how radiation reaction effects can cause non-monotonic features, like bumps, in runaway electron distributions in plasmas, potentially leading to instabilities, supported by analytical and numerical analysis.

Contribution

It derives an analytical threshold for bump formation in runaway electron distributions due to radiative losses, supported by numerical calculations.

Findings

Bumps can form in runaway electron tails due to radiation reaction effects.

An analytical threshold condition for bump appearance is established.

Numerical results support the analytical predictions.

Abstract

Runaway electrons, which are generated in a plasma where the induced electric field exceeds a certain critical value, can reach very high energies in the MeV range. For such energetic electrons, radiative losses will contribute significantly to the momentum space dynamics. Under certain conditions, due to radiative momentum losses, a non-monotonic feature - a "bump" - can form in the runaway electron tail, creating a potential for bump-on-tail-type instabilities to arise. Here we study the conditions for the existence of the bump. We derive an analytical threshold condition for bump appearance and give an approximate expression for the minimum energy at which the bump can appear. Numerical calculations are performed to support the analytical derivations.