On the collisional damping of plasma velocity space instabilities

TL;DR

This paper investigates how weak collisions can significantly damp plasma velocity space instabilities driven by non-Maxwellian particle distributions, with implications confirmed by kinetic simulations.

Contribution

It reveals that collisions can strongly suppress plasma instabilities beyond their rate by relaxing the distribution toward Maxwellian, especially in collisionless transport scenarios.

Findings

Collisions can damp instabilities beyond their collisional rate.

The effect is significant when non-Maxwellian distributions are driven by collisionless transport.

Kinetic simulations confirm the strong collisional damping effect.

Abstract

For plasma velocity space instabilities driven by particle distributions significantly deviated from a Maxwellian, weak collisions can damp the instabilities by an amount that is significantly beyond the collisional rate itself. This is attributed to the dual role of collisions that tend to relax the plasma distribution toward a Maxwellian and to suppress the linearly perturbed distribution function. The former effect can dominate in cases where the unstable non-Maxwellian distribution is driven by collisionless transport on a time scale much shorter than that of collisions, and the growth rate of the ideal instability has a sensitive dependence on the distribution function. The whistler instability driven by electrostatically trapped electrons is used as an example to elucidate such a strong collisional damping effect of plasma velocity space instabilities, which is confirmed by first-principles kinetic simulations.