The influence of the mass-ratio on the acceleration of particles by filamentation instabilities

TL;DR

This study uses 3D particle-in-cell simulations to explore how varying the proton-to-electron mass ratio affects particle acceleration and magnetic field generation in unmagnetized electron-positron-proton plasmas, revealing a transition in instability behavior at high mass ratios.

Contribution

It provides new insights into how the mass ratio influences instability development and particle acceleration in relativistic plasma jets, a key aspect of high-energy astrophysical phenomena.

Findings

Instability development time increases with proton mass.

High mass ratios (>20) lead to two-phase particle acceleration.

Coupling between electrons/positrons and protons diminishes at high mass ratios.

Abstract

Almost all sources of high energy particles and photons are associated with jet phenomena. Prominent sources of such highly relativistic outflows are pulsar winds and Active Galactic Nuclei. The current understanding of these jets assumes diluted plasmas which are best described as kinetic phenomena. In this kinetic description particle acceleration to ultra-relativistic speeds can occur in completely unmagnetized and neutral plasmas through insetting effects of instabilities. Even though the morphology and nature of particle spectra are understood to a certain extent, the composition of the jets is not known yet. While Poynting-flux dominated jets are certainly composed of electron-positron plasmas, the understanding of the governing physics in AGN jets is mostly unclear. In this article we investigate how the constituting elements of an electron-positron-proton plasma behave differently under the variation of the fundamental mass-ratio m_p/m_e. We studied initially unmagnetized counterstreaming plasmas using fully relativistic three-dimensional particle-in-cell simulations to investigate the influence of the mass-ratio on particle acceleration and magnetic field generation in electron-positron-proton plasmas. We covered a range of mass-ratios m_p/m_e between 1 and 100 with a particle number composition of n_{p^+}/n_{e^+} of 1 in one stream, only protons are injected in the other, whereas electrons are present in both to guarantee charge neutrality in the simulation box. We find that with increasing proton mass the instability takes longer to develop and for mass-ratios > 20 the particles seem to be accelerated in two phases which can be accounted to the individual instabilities of the different species. This means that for high mass ratios the coupling between electrons/positrons and the heavier protons, which occurs in low mass-ratios, disappears.