Preferential acceleration of positrons by a filamentation instability between an electron-proton beam and a pair plasma beam

TL;DR

This study uses PIC simulations to demonstrate that a filamentation instability between an unmagnetized electron-proton plasma and a pair plasma beam can lead to preferential positron acceleration, revealing new insights into plasma interactions.

Contribution

The paper shows that filamentation instability causes preferential positron acceleration in a non-relativistic pair plasma beam interacting with an electron-proton plasma, a novel finding.

Findings

Positron filaments are compressed to higher density and temperature.

Electric fields accelerate some positrons to nearly twice their initial speed.

Beam electrons lose significant kinetic energy to electric fields during filament mergers.

Abstract

Particle-in-cell (PIC) simulations of collisionless jets of electrons and positrons in an ambient electron-proton plasma have revealed an acceleration of positrons at the expense of electron kinetic energy. The dominant instability within the jet was a filamentation instability between electrons, protons and positrons. In this work we show that a filamentation instability, between an initially unmagnetized ambient electron-proton plasma at rest and a beam of pair plasma that moves through it at a non-relativistic speed, indeed results in preferential positron acceleration. Filaments form that are filled predominantly with particles with the same direction of their electric current vector. Positron filaments are separated by electromagnetic fields from beam electron filaments. Some particles can cross the field boundary and enter the filament of the other species. Positron filaments can neutralize their net charge by collecting the electrons of the ambient plasma while protons cannot easily follow the beam electron filaments. Positron filaments can thus be compressed to a higher density and temperature than the beam electron filaments. Filament mergers, which take place after the exponential growth phase of the instability has ended, lead to an expansion of the beam electron filaments, which amplifies the magnetic field they generate and induces an electric field in this filament. Beam electrons lose a substantial fraction of their kinetic energy to the electric field. Some positrons in the beam electron filament are accelerated by the induced electric field to almost twice their initial speed. The simulations show that a weaker electric field is induced in the positron filament and particles in this filament hardly change their speed.