Radiation reaction cooling as a source of anisotropic momentum distributions with inverted populations

TL;DR

This paper demonstrates that radiation reaction in strong electromagnetic fields causes plasmas to develop anisotropic, inverted momentum distributions, such as rings, which are kinetically unstable and relevant for astrophysical and laboratory radiation.

Contribution

It provides analytical and simulation-based insights into the formation of ring momentum distributions due to radiation reaction in magnetized plasmas, highlighting their instability and emission potential.

Findings

Ring distributions form under strong magnetic fields due to radiation reaction.

Timescales for ring formation are analytically derived and confirmed by simulations.

Resulting distributions are kinetically unstable, leading to coherent radiation emission.

Abstract

Under the presence of strong electromagnetic fields and radiation reaction, plasmas develop anisotropic momentum distributions, characterized by a population inversion. This is a general property of collisionless plasmas when the radiation reaction force is taken into account. We study the case of a plasma in a strong magnetic field and demonstrate the development of ring momentum distributions. The timescales for ring formation are derived for this configuration. The analytical results for the ring properties and the timescales for ring formation are confirmed with particle-in-cell simulations. The resulting momentum distributions are kinetically unstable and are known to lead to coherent radiation emission in astrophysical plasmas and laboratory setups.