| 1. |
- Decker, Joan, 1977, et al.
(författare)
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Numerical characterization of bump formation in the runaway electron tail
- 2016
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 58:2, s. 025016-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Ficker, Ondrej, et al.
(författare)
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Long slide-away discharges in the COMPASS tokamak
- 2016
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Ingår i: Proceedings of the 58th Annual Meeting of the APS Division of Plasma Physics. ; 61:18, s. GP10.00101-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this contribution, long runaway electron (RE)dominated discharges achieved in the COMPASS tokamak are presented. The extensivelength is possible due to a low consumption of available volt-seconds of thetokamak transformer in this type of discharge. Energetic electron losses in thisregime seems to be modulated mainly by small oscillations of a radial position (controllersetting) unlike in the RE discharges at higher electron density, where variousMHD phenomena affect the evolution of the losses. The behaviour of the slide-awayplasma is studied using magnetic coils, HXR detectors, ECE system and a pair of3He proportional counters of neutrons. The plasma scenario is also modelled usingFokker-Planck codes.
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| 3. |
- Hirvijoki, Eero, 1985, et al.
(författare)
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Guiding-center transformation of the radiation-reaction force in a nonuniform magnetic field
- 2015
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 81:5, s. 475810504-
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we present the guiding-center transformation of the radiation-reaction force of a classical point charge traveling in a nonuniform magnetic field. The transformation is valid as long as the gyroradius of the charged particles is much smaller than the magnetic field nonuniformity length scale, so that the guiding-center Lie-transform method is applicable. Elimination of the gyromotion time scale from the radiation-reaction force is obtained with the Poisson bracket formalism originally introduced by [A. J. Brizard, Phys. Plasmas 11 4429 (2004)], where it was used to eliminate the fast gyromotion from the Fokker-Planck collision operator. The formalism presented here is applicable to the motion of charged particles in planetary magnetic fields as well as in magnetic confinement fusion plasmas, where the corresponding so-called synchrotron radiation can be detected. Applications of the guiding-center radiation-reaction force include tracing of charged particle orbits in complex magnetic fields as well as kinetic description of plasma when the loss of energy and momentum due to radiation plays an important role, e.g., for runaway electron dynamics in tokamaks.
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| 4. |
- Hirvijoki, Eero, 1985, et al.
(författare)
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Radiation reaction induced non-monotonic features in runaway electron distributions
- 2015
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 81:5, s. 475810502-
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Tidskriftsartikel (refereegranskat)abstract
- Runaway electrons, which are generated in a plasma where the induced electric field exceeds acertain critical value, can reach very high energies in the MeV range. For such energetic electrons,radiative losses will contribute significantly to the momentum space dynamics. Under certainconditions, due to radiative momentum losses, a non-monotonic feature – a “bump” – can formin the runaway electron tail, creating a potential for bump-on-tail-type instabilities to arise. Herewe study the conditions for the existence of the bump. We derive an analytical threshold conditionfor bump appearance and give an approximate expression for the minimum energy at which thebump can appear. Numerical calculations are performed to support the analytical derivations.
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| 7. |
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| 8. |
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| 9. |
- Stahl, Adam, 1985, et al.
(författare)
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Effective Critical Electric Field for Runaway-Electron Generation
- 2015
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 114:11, s. 115002-
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Tidskriftsartikel (refereegranskat)abstract
- In this Letter we investigate factors that influence the effective critical electric field for runaway-electrongeneration in plasmas. We present numerical solutions of the kinetic equation and discuss the implications forthe threshold electric field. We show that the effective electric field necessary for significant runaway-electronformation often is higher than previously calculated due to both (1) extremely strong dependence of primarygeneration on temperature, and (2) synchrotron radiation losses. We also address the effective critical field inthe context of a transition from runaway growth to decay. We find agreement with recent experiments, but showthat 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.
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| 10. |
- Stahl, Adam, 1985, et al.
(författare)
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Reaction of runaway electron distributions to radiative processes
- 2015
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Ingår i: 57th Annual Meeting of the APS Division of Plasma Physics. ; 60:19, s. PP12.00103 -
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The emission of electromagnetic radiation by a charged particle in accelerated motion is associated with a reduction in its energy, accounted for by the inclusion of a radiation reaction force in the kinetic equation. For runaway electrons in plasmas, the dominant radiative processes are the emission of bremsstrahlung and synchrotron radiation. In this contribution, we investigate the impact of the associated radiation reaction forces on the runaway electron distribution, using both analytical and numerical studies, and discuss the corresponding change to the runaway electron growth rate, which can be substantial. We also report on the formation of non-monotonic features in the runaway electron tail as a consequence of the more complicated momentum-space dynamics in the presence of radiation reaction.
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