Electron Weibel Instability in Relativistic Counter-Streaming Plasmas with Flow-Aligned External Magnetic Fields

TL;DR

This paper investigates how an external magnetic field aligned with plasma flows influences the Weibel instability in relativistic counter-streaming plasmas, revealing that the saturation stage remains largely unaffected while the saturation level varies with wavelength and temperature.

Contribution

It provides a detailed analysis of the saturation mechanisms of the Weibel instability under external magnetic fields using analytical models and PIC simulations, including multi-mode and temperature effects.

Findings

External magnetic fields slightly increase saturation levels at large wavelengths.

Saturation levels at small wavelengths are unaffected by the magnetic field.

Temperature influences the competition between modes and the saturation level.

Abstract

The Weibel instability driven by two symmetric counter-streaming relativistic electron plasmas, also referred to as current-filamentation instability, is studied in a constant and uniform external magnetic field aligned with the plasma flows. Both the linear and non linear stages of the instability are investigated using analytical modeling and Particle-In-Cell (PIC) simulations. While previous studies have already described the stabilizing effect of the magnetic field, we show here that the saturation stage is only weakly affected. The different mechanisms responsible for the saturation are discussed in detail in the relativistic cold fluid framework considering a single unstable mode. The application of an external field leads to a slighlt increase of the saturation level for large wavelengths, while it does not affect the small wavelengths. Multi-mode and temperature effects are then investigated. While at large temperature the saturation level is independent of the external magnetic field, at small but finite temperature the competition between different modes in the presence of an external magnetic field leads to a saturation level lower with respect to the unmagnetized case.