Impact of the electron to ion mass ratio on unstable systems in particle-in-cell simulations

TL;DR

This study investigates how varying the ion-to-electron mass ratio affects the development and saturation of Buneman and two-stream instabilities in particle-in-cell simulations, revealing impacts on turbulence and beam energy loss.

Contribution

It provides new insights into the influence of ion-to-electron mass ratio on instability growth, saturation, and resulting turbulence in plasma simulations.

Findings

Lower mass ratio leads to stronger turbulence.

Buneman instability saturates earlier than two-stream instability.

Reduced mass ratio increases ion heating and electron beam energy loss.

Abstract

The evolution of the Buneman and two-stream instabilities driven by a cold dilute mildly relativistic electron beam is studied as a function of the ion\'\s charge-to-mass ratio. The growth rates of both instabilities are comparable for the selected parameters if the charge-to-mass ratio of protons is used and the Buneman instability outgrows the two-stream instability for a larger ratio. Particle-in-cell (PIC) simulations show that both instabilities grow independently during their linear growth phase. The much lower saturation amplitude of the Buneman instability implies that it saturates first even if the linear growth rates of both instabilities are equal. The electron phase space holes it drives coalesce. Their spatial size increases in time and they start interacting with the two-stream mode, which triggers the onset of electrostatic turbulence. A reduced charge-to-mass ratio results in stronger turbulence and ion heating and in an increased energy loss of the relativistic electron beam compared to that in a simulation with the correct ratio.