| 1. |
- Decker, Joan, 1977, et al.
(author)
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Numerical characterization of bump formation in the runaway electron tail
- 2016
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In: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 58:2, s. 025016-
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Journal article (peer-reviewed)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. |
- Embréus, Ola, 1991, et al.
(author)
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Effect of bremsstrahlung radiation emission on fast electrons in plasmas
- 2016
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In: 43rd European Physical Society Conference on Plasma Physics, EPS 2016.
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Conference paper (peer-reviewed)abstract
- Bremsstrahlung radiation emission is an important energy loss mechanism for energetic electrons in plasmas. In this contribution we investigate the effect of spontaneous bremsstrahlung emission on the momentum-space structure of the electron distribution, using a Boltzmanntransport model fully accounting for the emission of finite-energy photons. We implement the model in a 2D continuum kinetic-equation solver, and study the solutions to determine the effect of bremsstrahlung on the electron distribution function. We find that electrons acceleratedby electric fields can reach significantly higher energies than predicted in previous work,which considered only the average energy loss of a test particle. We demonstrate that significantfractions of electrons reach twice the expected energy or more, due to the difference betweenthe average and Boltzmann model of bremsstrahlung radiation losses. Furthermore, we show that the emission of low-energy photons, which have previously been neglected because theydo not contribute to net energy loss, can contribute significantly to the dynamics of electrons with an anisotropic distribution by enhancing the angular-deflection rate.
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| 3. |
- Embréus, Ola, 1991, et al.
(author)
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Effect of bremsstrahlung radiation emission on fast electrons in plasmas
- 2016
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In: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 18:9, s. 093023-
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Journal article (peer-reviewed)abstract
- Bremsstrahlung radiation emission is an important energy loss mechanism for energetic electrons in plasmas. In this paper we investigate the effect of spontaneous bremsstrahlung emission on the momentum-space structure of the electron distribution, fully accounting for the emission of finite-energy photons. We find that electrons accelerated by electric fields can reach significantly higher energies than what is expected from energy-loss considerations. Furthermore, we show that the emission of soft photons can contribute significantly to the dynamics of electrons with an anisotropic distribution.
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| 4. |
- Embréus, Ola, 1991
(author)
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Kinetic modelling of runaway in plasmas
- 2016
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Licentiate thesis (other academic/artistic)abstract
- The phenomenon of runaway occurs in plasmas in the presence of a strong electric field, which overcomes the collisional friction acting on the charged particles moving through the plasma. A subpopulation of particles can then be accelerated to energies significantly higher than the thermal energy. Such events are observed in both laboratory and space plasmas, and are of great importance in fusion-energy research, where highly energetic runaway electrons can damage the plasma-facing components of the reactor.In this thesis, a series of papers are presented which investigate various aspects of runaway dynamics. The emission of synchrotron and bremsstrahlung radiation are important energy-loss mechanisms for relativistic runaway electrons. Photons emitted in bremsstrahlung radiation often have energy comparable to the energetic electrons, and we therefore use a Boltzmann transport equation in order to describe their effect on the electron motion. This treatment reveals that electrons can reach significantly higher energies than previously thought. In comparison, synchrotron radiation has lower frequency, and is well described by the classical electromagnetic radiation-reaction force. This loss mechanism, often dominant in laboratory plasmas, significantly alters the electron dynamics, and is found to produce non-monotonic features in the runaway tail.A study is also presented of the related phenomenon of ion runaway acceleration, which differs from electron runaway due to their larger mass. Renewed interest in this topic has been sparked after recent observations of fast ions in various experiments. Finally a new method is explored to treat the non-linear Fokker-Planck equation which is commonly used to describe the collisional dynamics in a plasma. The new method is appealing for its physically intuitive description and analytic simplicity.
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| 5. |
- Fülöp, Tünde, 1970, et al.
(author)
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Kinetic modelling of runaways in fusion plasmas
- 2016
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In: Proceedings of 26th IAEA Fusion Energy Conference, Kyoto, Japan. ; , s. TH/P4-1
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Conference paper (other academic/artistic)abstract
- Mitigation of runaway electrons is one of the outstanding issues for a reliable operationof ITER and other large tokamaks. To achieve this, quantitatively accurate estimatesfor the expected runaway electron energies and current are needed. In this work we de-scribe an accurate theoretical framework for studying the effects of collisional nonlinear-ities, bremsstrahlung and synchrotron radiation emission, and knock-on collisions on therunaway electron distribution. We outline the identification of significant features of run-away electron behaviour enabled by this framework and their potential to affect the growthof a runaway population.
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| 6. |
- Paz-Soldan, C, et al.
(author)
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Synchrotron and collisional damping effects on runaway electron distributions
- 2016
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In: 58th Annual Meeting of the APS Division of Plasma Physics. ; 61:18, s. CO4.00010 -
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Conference paper (other academic/artistic)abstract
- Validated models of runaway electron (RE) dissipation are required to confidently approve safe ITER Q=10 operation. DIII-D experiments using quiescent REs are exploring the importance of synchrotron and collisional damping terms to RE dissipation. New time and energy-resolved measurements of RE bremsstrahlung hard X-ray (HXR) emission reveal stark differences between high and low energy REs as damping terms are varied. Previously reported anomalously high RE dissipation only applies to low energy REs. At high energy (where synchrotron effects are strongest) low synchrotron damping cases reach higher peak RE energy despite weaker particle confinement. Low-energy RE decay is observed concurrently with high-energy RE growth. RE dissipation models predict bump-on-tail distributions whose properties depend on the damping terms. Measured HXR spectra are very broad, as expected for bump-on-tail distributions.
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| 7. |
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| 8. |
- Stahl, Adam, 1985, et al.
(author)
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Kinetic modelling of runaway electrons in dynamic scenarios
- 2016
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 56:11, s. 112009-
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Journal article (peer-reviewed)abstract
- Improved understanding of runaway-electron formation and decay processes are of prime interest for the safe operation of large tokamaks, and the dynamics of the runaway electrons during dynamical scenarios such as disruptions are of particular concern. In this paper, we present kinetic modelling of scenarios with time-dependent plasma parameters; in particular, we investigate hot-tail runaway generation during a rapid drop in plasma temperature. With the goal of studying runaway-electron generation with a self-consistent electric-field evolution, we also discuss the implementation of a conservative collision operator and demonstrate its properties. An operator for avalanche runaway-electron generation, which takes the energy dependence of the scattering cross section and the runaway distribution into account, is investigated. We show that the simpler avalanche model of Rosenbluth & Putvinskii [Nucl. Fusion 37, 1355 (1997)] can give very inaccurate results for the avalanche growth rate (either lower or higher) for many parameters, especially when the average runaway energy is modest, such as during the initial phase of the avalanche multiplication. The developments presented pave the way for an improved modelling of runaway-electron dynamics during disruptions or other dynamic events.
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| 9. |
- Stahl, Adam, 1985, et al.
(author)
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Runaway-electron formation and electron slide-away in an ITER post-disruption scenario
- 2016
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In: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 775:1
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Conference paper (peer-reviewed)abstract
- Mitigation of runaway electrons is one of the outstanding issues for a reliable operation of ITER and other large tokamaks, and accurate estimates for the expected runaway- electron energies and current are needed. Previously, linearized tools, assuming the runaway population to be small, have been used to study the runaway dynamics, but these tools are not valid in the cases of most interest, i.e. when the runaway population becomes substantial. We study runaway-electron formation in a post-disruption ITER plasma using the newly developed non-linear code NORSE , and nd that the entire electron population is converted to runaways in the scenario considered. A new non-linear feedback mechanism is also described, by which a transition to electron slide-away can be induced at eld strengths signi cantly lower than previously expected. We nd the exact time to the transition to be highly dependent on the details of the mechanisms removing heat from the thermal electron population.
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