Numerical Studies for a Laboratory Astrophysics Experiment of unstable Electron-Positron Beams

Abstract

The presence or non-presence of weak primordial magnetic fields in the voids between galaxy clusters is a sensitive probe for the physics of the early universe. Currently observations have only been able to set limits for the strength and coherence length of these fields. An indirect method for setting lower limits on the strength of these fields, and thus prove their presence, is the suppression of the secondary gamma ray flux from far away VHE gamma ray sources. TeV gamma rays induce electro- magnetic cascades due to pair production interactions with photons that constitute the extragalactic background light reducing the received ux at high energies. The cascade electrons and positrons produce a secondary flux of gamma rays at lower energies. The experimental non-observation of this secondary flux component can be explained by the deflection of the charged electrons and positrons in the magnetic fields. An alternative explanation that has been controversially discussed in the literature is the influence of plasma instabilities on the overall neutral beam consisting of electrons and positrons propagating through a background plasma, the intergalactic medium. These instabilities could drain energy from the beam without producing a secondary gamma ray component or deflect particles without the presence of an external magnetic field. Modern accelerators can produce beams that allow the study of the relevant instability processes in a laboratory environment. Despite the vast difference of scales it can be possible to extrapolate using scaling relationships that have to be derived. In this work we study the instability mechanisms using particle-in-cell simulations. We find that the instability leads to the build up of a structured electric field that can drain energy from the beam by heating the background medium or deflect the beam particles by a momentum diffusion process that can be modeled using a Fokker-Planck equation. The changing of the beam momentum distribution could serve as an easily accessible experimental probe in a laboratory experiment. Lastly we find that the instability mechanism takes place in both neutral pair beams and experimentally more easily handleable pure electron beams.