Effect of finite beam width on current separation in beam plasma system: Particle-in-Cell simulations

TL;DR

This study uses Particle-in-Cell simulations to investigate how finite beam width influences current separation and magnetic field structures in beam-plasma systems, revealing that finite width causes these phenomena to occur at the beam scale rather than the electron skin depth.

Contribution

It demonstrates the impact of finite beam width on current separation and magnetic field formation, contrasting with the infinite beam assumption in prior analyses.

Findings

Current separation occurs at the beam width scale for finite beams.

Magnetic field structures peak at the beam width scale.

Infinite beam models predict magnetic fields at the electron skin depth.

Abstract

The electron beam propagation in a plasma medium is susceptible to several instabilities. In the relativistic regime typically the weibel instability leading to the current separation dominates. The linear instability analysis is carried out for a system wherein the transverse extent of the beam is infinite. Even in simulations, infinite transverse extent of the beam has been chosen. In real situations, however, beam width will always be finite. keeping this in view the role of finite beam width on the evolution of the beam plasma system has been studied here using Particle - in - Cell simulations. It is observed that the current separation between the forward and return shielding current for a beam with finite beam occurs at the scale length of the beam width itself. Consequently the magnetic field structures that form have maximum power at the scale length of the beam width. This behaviour is distinct from what happens with a beam with having an infinite extent represented by simulations in a periodic box, where the initial scale length at which the magnetic field appears is that of the electron skin depth.