Abstract
The behaviour of a strongly magnetised collisional electron–positron plasma that 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 magnetisation 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.
Highlights
In a companion article (Kennedy & Helander 2020), hereinafter referred to as (I), it was shown that plasmas that are optically thin to cyclotron radiation relax to strongly anisotropic distributions and a theory of collisional scattering in such systems was developed
We show that stability of electron–positron plasmas to low-frequency waves in slab geometry still holds if the distribution function is non-Maxwellian
Radiative cooling will be an important process in upcoming pair plasma laboratory experiments and can lead to a strongly anisotropic distribution function
Summary
In a companion article (Kennedy & Helander 2020), hereinafter referred to as (I), it was shown that plasmas that are optically thin to cyclotron radiation relax to strongly anisotropic distributions and a theory of collisional scattering in such systems was developed. These results were general; in particular, little was assumed about the confining magnetic geometry. In (I), suitable candidates for systems satisfying these two conditions were proposed In this contribution, we explore one of these suggestions: the first laboratory electron–positron plasma experiment, and develop the theory of collisional scattering in strongly magnetised pair plasmas
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