On the evolution of a large-amplitude, weakly-collisional electron plasma wave

TL;DR

This paper uses VPFP simulations to study the evolution of large-amplitude electron plasma waves, revealing three phases with distinct behaviors, especially focusing on the less understood detrapping phase and its effects on wave frequency and damping.

Contribution

It provides new insights into the detrapping phase of plasma waves, including empirical models for damping rates and frequency shifts based on collision and wave parameters.

Findings

Delineation of three distinct wave evolution phases.

Identification of increased frequency shift during detrapping.

Empirical fits for damping rates and frequency shifts.

Abstract

Vlasov-Poisson-Fokker-Planck (VPFP) simulations of large-amplitude electron plasma waves, where the bounce frequency is much larger than the collision frequency, $\omega_B \gg \nu_\text{ee}$, show that the evolution of these waves exhibits three phases; I. A short-lived trapping phase during which collisional effects are minimal. II. A long-lived detrapping phase during which collisional effects are most influential. III. A short-lived Landau damping phase where the effect of collisions becomes minimal again. While the dispersion relation during the trapping and Landau damping phase is well known, the wave behavior during the detrapping phase is not as well understood. The simulations show that during the detrapping phase, the interplay between weak electron-electron collisions and strong wave-electron interactions results in an increasing frequency shift further from the linear root, $\omega_\text{EPW}$. At the conclusion of the detrapping phase, the distribution function is nearly Maxwellian, the frequency shift rapidly diminishes, and the wave damps at a larger rate than the Landau damping rate. Empirical fits to the damping rates, frequency shift enhancement rate, and the lifetime of the plasma waves are provided as functions of collision frequency, wavenumber, and wave amplitude.