title: Effective critical electric field for runaway-electron generation.
abstract: In this Letter we investigate factors that influence the effective critical electric field for runaway-electron generation in plasmas. We present numerical solutions of the kinetic equation and discuss the implications for the threshold electric field. We show that the effective electric field necessary for significant runaway-electron formation often is higher than previously calculated due to both (1) extremely strong dependence of primary generation on temperature and (2) synchrotron radiation losses. We also address the effective critical field in the context of a transition from runaway growth to decay. We find agreement with recent experiments, but show that the observation of an elevated effective critical field can mainly be attributed to changes in the momentum-space distribution of runaways, and only to a lesser extent to a de facto change in the critical field.
authors.affiliation: Department of Applied Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden.
authors.forename: A
authors.initials: A
authors.lastname: Stahl
authors.affiliation: Department of Applied Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden.
authors.forename: E
authors.initials: E
authors.lastname: Hirvijoki
authors.affiliation: Department of Applied Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden.|École Polytechnique Fédérale de Lausanne (EPFL), Centre de Recherches en Physique des Plasmas (CRPP), CH-1015 Lausanne, Switzerland.
authors.forename: J
authors.initials: J
authors.lastname: Decker
authors.affiliation: Department of Applied Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden.
authors.forename: O
authors.initials: O
authors.lastname: Embréus
authors.affiliation: Department of Applied Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden.
authors.forename: T
authors.initials: T
authors.lastname: Fülöp