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Dynamics of positrons during relativistic electron runaway

Published online by Cambridge University Press:  22 October 2018

O. Embréus Open the ORCID record for O. Embréus [Opens in a new window] ,
L. Hesslow Open the ORCID record for L. Hesslow [Opens in a new window] ,
M. Hoppe ,
G. Papp ,
K. Richards  and
T. Fülöp Open the ORCID record for T. Fülöp [Opens in a new window]
O. Embréus*
Affiliation:
Department of Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden
L. Hesslow
Affiliation:
Department of Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden
M. Hoppe
Affiliation:
Department of Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden
G. Papp
Affiliation:
Max-Planck-Institute for Plasma Physics, D-85748 Garching, Garching, Germany
K. Richards
Affiliation:
Department of Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden
T. Fülöp
Affiliation:
Department of Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden
*
†Email address for correspondence: embreus@chalmers.se
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Abstract

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.

Keywords

runaway electrons
Type
Research Article
Information
Journal of Plasma Physics , Volume 84 , Issue 5 , October 2018 , 905840506
Copyright
© Cambridge University Press 2018 

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