Dynamics of positrons during relativistic electron runaway*

In plasmas, sufficiently strong electric fields can accelerate charged particles to relativistic energies via the runaway mechanism. In this contribution we describe the dynamics of positrons that are created during a runaway avalanche.

We derive a threshold electric field above which the direct pair production in collisions will exceed the pair production due to photons produced in hard X-ray emission, which is traditionally the main positron producing mechanism. This threshold field is found to be of the order of tens of avalanche threshold fields.

We present analytical and numerical solutions of the positron kinetic equation, illustrating similarities and differences between the runaway dynamics of positrons and electrons. The numerical study provides the ratio of positrons to runaway electrons, which is used to predict the amount of annihilation radiation emitted during tokamak disruptions as a function of the plasma parameters. This is compared to the HXR emission of electrons, and we calculate the signal-to-noise ratio as well as the total annihilation photon count.