Synchrotron-driven instabilities in relativistic plasmas of arbitrary opacity

TL;DR

This paper investigates how synchrotron-driven instabilities in relativistic plasmas evolve as the plasma transitions from optically thin to thick, revealing a gradual stabilization due to increased isotropy at higher opacity.

Contribution

It extends a Fokker-Planck model to fully relativistic plasmas and analyzes the impact of opacity on plasma stability and anisotropy.

Findings

Higher-energy electrons emit higher-frequency photons that escape more easily.

Increased opacity leads to more isotropic and stable plasma configurations.

Critical parameters for plasma opacity control are identified.

Abstract

Recent work has shown that synchrotron emission from relativistic plasmas leads the electron distribution to form an anisotropic ring in momentum space, which can be unstable to both kinetic and hydrodynamic instabilities. Fundamental to these works was the assumption that the plasma was optically thin, allowing all emitted radiation to escape. Here, we examine the behavior of these instabilities as the plasma becomes more optically thick. To do this, we extend a recently-developed Fokker-Planck operator for synchrotron emission and absorption in mildly relativistic plasmas to fully relativistic plasmas. For a given set of plasma parameters, photons emitted by higher-energy electrons tend to be higher frequency, and thus more easily escape the plasma. As a result, the ratio of the photon emission rate (radiative drag) to absorption rate (radiative diffusion) for a given electron is extremely energy-dependent. Given this behavior, we determine the critical parameters that control the opacity, and show how the plasma gradually transitions to become more isotropic and stable at higher opacity.