Coulomb collisions in strongly anisotropic plasmas II. Cyclotron cooling in laboratory pair plasmas

TL;DR

This paper analyzes the behavior of strongly magnetized, optically thin electron-positron plasmas, focusing on cyclotron cooling, anisotropic distributions, and Coulomb collisions, with implications for laboratory plasma experiments.

Contribution

It develops a framework to account for Coulomb collisions in strongly magnetized plasmas without explicit collision operators, and calculates energy loss rates due to cyclotron radiation.

Findings

Energy loss rate is proportional to parallel collision frequency with logarithmic dependence on plasma parameters.

No unstable drift waves are found in straight field-line geometry despite anisotropic distributions.

Energy loss occurs rapidly, within a few collision times, indicating a self-accelerating cooling process.

Abstract

The behaviour of a strongly-magnetized collisional electron-positron plasma which is optically thin to cyclotron radiation is considered, and the distribution functions accessible to it on the various timescales in the system are calculated. Particular attention is paid to the limit in which the collision time exceeds the radiation emission time, making the electron distribution function strongly anisotropic. Indeed, these are the exact conditions likely to be attained in the first laboratory electron-positron plasma experiments currently being developed, which will typically have very low densities and be confined in very strong magnetic fields. The constraint of strong-magnetization adds an additional complication in that long-range Coulomb collisions, which are usually negligible, must now be considered. A rigorous collision operator for these long-range collisions has never been written down. Nevertheless, we show that the collisional scattering can be accounted for without knowing the explicit form of this collision operator. The rate of radiation emission is calculated and it is found that the loss of energy from the plasma is proportional to the parallel collision frequency multiplied by a factor that only depends logarithmically on plasma parameters. That is, this is a self-accelerating process, meaning that the bulk of the energy will be lost in a few collision times. We show that in a simple case, that of straight field-line geometry, there are no unstable drift waves in such plasmas, despite being far from Maxwellian.