Numerical characterization of bump formation in the runaway electron tail
Artikel i vetenskaplig tidskrift, 2016

Runaway electrons are generated in a magnetized plasma when the parallel electric field exceeds a critical value. For such electrons with energies typically reaching tens of MeV, the Abraham–Lorentz–Dirac (ALD) radiation force, in reaction to the synchrotron emission, is significant and can be the dominant process limiting electron acceleration. The effect of the ALD force on runaway electron dynamics in a homogeneous plasma is investigated using the relativistic finite-difference Fokker–Planck codes LUKE (Decker and Peysson 2004 Report EUR-CEA-FC-1736, Euratom-CEA), and CODE (Landreman et al 2014 Comput. Phys. Commun. 185 847). The time evolution of the distribution function is analyzed as a function of the relevant parameters: parallel electric field, background magnetic field, and effective charge. Under the action of the ALD force, we find that runaway electrons are subject to an energy limit, and that the electron distribution evolves towards a steady-state. In addition, a bump is formed in the tail of the electron distribution function if the electric field is sufficiently strong. The mechanisms leading to the bump formation and energy limit involve both the parallel and perpendicular momentum dynamics; they are described in detail. An estimate for the bump location in momentum space is derived. We observe that the energy of runaway electrons in the bump increases with the electric field amplitude, while the population increases with the bulk electron temperature. The presence of the bump divides the electron distribution into a runaway beam and a bulk population. This mechanism may give rise to beam-plasma types of instabilities that could, in turn, pump energy from runaway electrons and alter their confinement.




runaway electron

synchrotron emission

kinetic instabilities


Joan Decker

Ecole Polytechnique Federale de Lausanne (EPFL)

Eero Hirvijoki

Chalmers, Fysik, Subatomär fysik och plasmafysik

Ola Embréus

Chalmers, Fysik, Subatomär fysik och plasmafysik

Y Peysson

Le Commissariat à l’Énergie Atomique et aux Énergies Alternatives (CEA)

Adam Stahl

Chalmers, Fysik, Subatomär fysik och plasmafysik

Istvan Pusztai

Chalmers, Fysik, Subatomär fysik och plasmafysik

Tünde Fülöp

Chalmers, Fysik, Subatomär fysik och plasmafysik

Plasma Physics and Controlled Fusion

0741-3335 (ISSN) 1361-6587 (eISSN)

Vol. 58 2 025016- 025016


Hållbar utveckling




Grundläggande vetenskaper


Fusion, plasma och rymdfysik



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