Dynamics of positrons during relativistic electron runaway
Journal article, 2018

Sufficiently strong electric fields in plasmas can accelerate charged particles to relativistic energies. In this paper we describe the dynamics of positrons accelerated in such electric fields, and calculate the fraction of created positrons that become runaway accelerated, along with the amount of radiation that they emit. We derive an analytical formula that shows the relative importance of the different positron production processes, and show that, above a certain threshold electric field, the pair production by photons is lower than that by collisions. We furthermore present analytical and numerical solutions to the positron kinetic equation; these are applied to calculate the fraction of positrons that become accelerated or thermalized, which enters into rate equations that describe the evolution of the density of the slow and fast positron populations. Finally, to indicate operational parameters required for positron detection during runaway in tokamak discharges, we give expressions for the parameter dependencies of detected annihilation radiation compared to bremsstrahlung detected at an angle perpendicular to the direction of runaway acceleration. Using the full leading-order pair-production cross-section, we demonstrate that previous related work has overestimated the collisional pair production by at least a factor of four.

runaway electrons


Ola Embréus

Chalmers, Physics, Subatomic and Plasma Physics

Linnea Hesslow

Chalmers, Physics, Subatomic and Plasma Physics

Mathias Hoppe

Chalmers, Physics, Subatomic and Plasma Physics

Gergely Papp

Max-Planck-Institute for Plasma Physics

Katya Richards

Chalmers, Physics, Subatomic and Plasma Physics

Tünde Fülöp

Chalmers, Physics, Subatomic and Plasma Physics

Journal of Plasma Physics

0022-3778 (ISSN) 1469-7807 (eISSN)

Vol. 84 5 905840506- 905840506

Subject Categories

Accelerator Physics and Instrumentation

Subatomic Physics

Fusion, Plasma and Space Physics



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