Non-linear Plasma Wake Growth of Electron Holes

TL;DR

This paper investigates the non-linear growth of electron holes in plasma wakes caused by electrostatic instabilities, providing a model that explains large-amplitude perturbations and matches recent simulation results.

Contribution

It introduces a non-linear analysis of electron hole growth in plasma wakes, revealing how they can reach a stable size before disrupting ion streams, which was not explained by previous linear theories.

Findings

Electron holes grow driven by drift mechanisms in plasma wakes.

Large-amplitude perturbations disrupt ion streams before ion-ion instability occurs.

Theoretical predictions match recent particle-in-cell simulation observations.

Abstract

An object's wake in a plasma with small Debye length that drifts \emph{across} the magnetic field is subject to electrostatic electron instabilities. Such situations include, for example, the moon in the solar wind wake and probes in magnetized laboratory plasmas. The instability drive mechanism can equivalently be considered drift down the potential-energy gradient or drift up the density-gradient. The gradients arise because the plasma wake has a region of depressed density and electrostatic potential into which ions are attracted along the field. The non-linear consequences of the instability are analysed in this paper. At physical ratios of electron to ion mass, neither linear nor quasilinear treatment can explain the observation of large-amplitude perturbations that disrupt the ion streams well before they become ion-ion unstable. We show here, however, that electron holes, once formed, continue to grow, driven by the drift mechanism, and if they remain in the wake may reach a maximum non-linearly stable size, beyond which their uncontrolled growth disrupts the ions. The hole growth calculations provide a quantitative prediction of hole profile and size evolution. Hole growth appears to explain the observations of recent particle-in-cell simulations.