Simulation study of the filamentation of counter-streaming beams of the electrons and positrons in plasmas

TL;DR

This study uses particle-in-cell simulations to investigate the filamentation instability in counter-streaming electron-positron beams in plasmas, revealing magnetic and electrostatic field dynamics and their effects on filament charge densities.

Contribution

First detailed simulation of electron-positron beam filamentation showing magnetic and electrostatic field interactions and their impact on filament charge densities.

Findings

Magnetic fields grow spontaneously and reorganize particles.

Electrostatic fields are not generated before saturation.

Filaments with positrons reach higher charge and current densities.

Abstract

The filamentation instability driven by two spatially uniform and counter-streaming beams of charged particles in plasmas is modelled by a particle-in-cell (PIC) simulation. Each beam consists of the electrons and positrons. The four species are equally dense and they have the same temperature. The one-dimensional simulation direction is orthogonal to the beam velocity vector. The magnetic field grows spontaneously and rearranges the particles in space, such that the distributions of the electrons of one beam and the positrons of the second beam match. The simulation demonstrates that as a result no electrostatic field is generated by the magnetic field through its magnetic pressure gradient prior to its saturation. This electrostatic field would be repulsive at the centres of the filaments and limit the maximum charge and current density. The filaments of electrons and positrons in this simulation reach higher charge and current densities than in one with no positrons. The oscillations of the magnetic field strength induced by the magnetically trapped particles result in an oscillatory magnetic pressure gradient force. The latter interplays with the statistical fluctuations in the particle density and it probably enforces a charge separation, by which electrostatic waves grow after the filamentation instability has saturated.