On the collisional damping of plasma velocity space instabilities

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 timescale 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.

Automated summary

For plasma velocity space instabilities driven by non-Maxwellian particle distributions, weak collisions can damp the instabilities significantly beyond the collisional rate. This is because collisions have a dual effect of relaxing the plasma distribution towards a Maxwellian and suppressing the linearly perturbed distribution function. The dominance of the former effect occurs when the non-Maxwellian distribution is driven by collisionless transport on a timescale much shorter than that of collisions, and the growth rate of the ideal instability is highly dependent on the distribution function. The strong collisional damping effect of plasma velocity space instabilities is demonstrated using the example of whistler instability driven by electrostatically trapped electrons, which is confirmed through first-principles kinetic simulations.