Coulomb collisions in strongly anisotropic electron-positron plasmas

The canonical pair plasma, consisting solely of electrons and positrons, is an exciting new frontier in basic plasma physics. Electron-positron plasmas are an attractive proxy for complex physics due to the ``remarkable stability properties'' they exhibit as a result of mass symmetry between the two species; a symmetric pair plasma is gyrokinetically stable in a constant magnetic field. As such, laboratory pair plasmas ought to enjoy splendid confinement and such a plasma could provide a robust benchmark against theoretical predictions. Positrons are difficult to source terrestrially and this places stringent conditions on the values of plasma density $n$ that can be attained in the laboratory at fixed volume. It is thus advantageous to keep the plasma temperature, $T$ (and thus $\lambda_{D}$), as small as possible. Fortunately, the relatively small $n$ also renders the plasma optically thin to cyclotron radiation. One plan is to exploit relatively high magnetic fields; making use of cyclotron cooling to keep the plasma cold. In this talk, 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 will be paid to the limit in which the collision time exceeds the radiation emission time, making the electron distribution function strongly anisotropic. The constraint of strong magnetisation adds an additional complication in that long-range Coulomb collisions, usually negligible, must now be considered. 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.