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- Embréus, Ola, 1991, et al.
(författare)
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Relativistic Boltzmann collision operator for runaway-avalanche studies
- 2017
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Ingår i: International Sherwood Fusion Theory Conference, Annapolis, USA.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Avalanche runaway generation is a critical threat to large tokamaks such as ITER: according to the Rosenbluth-Putvinski theory[1], a small seed population of fast electrons will multiply through knock-on collisions by a factor of order R~exp(3 I[MA]), which for plasma currents of order I=10-15 MA means that a single fast electron can convert the entire ohmic current to runaway current.The exponential sensitivity of the avalanche multiplication factor R to the details of the runaway-generation dynamics shows a need for more accurate models of large-angle collisions.Existing models of large-angle collisions that have been used in magnetic-confinement fusion studies[1,2] have two main flaws: (i) they do not conserve electron momentum or energy, and (ii) they double count collisions with small-angle collisions, which are typically accounted for with a Fokker-Planck collision operator.We have developed a new large-angle collision operator based on the full relativistic Boltzmann equation which resolves these issues.We use kinetic simulations to show how the new improved collision operator modify the avalanche growth rates obtained with previous models. In particular, we compare results with the steady-state theory of Rosenbluth and Putvinski, as well as with theoretical predictions of a recent study[3] for avalanche generation in near-threshold electric fields E~E_c in the presence of synchrotron-radiation losses.References:[1] M. N. Rosenbluth and S. V. Putvinski, Nucl. Fusion 37, 1355 (1997).[2] S. C. Chiu, M. N. Rosenbluth, R. W. Harvey and V. S. Chan, Nucl. Fusion 38, 1711 (1998).[3] P. Aleynikov and B. N. Breizman, Phys. Rev. Lett. 114, 155001 (2015).
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| 4. |
- Hesslow, Linnea, 1993, et al.
(författare)
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Effect of partially-screened nuclei on fast-electron dynamics
- 2017
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 118:25, s. article no. 5501-
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Tidskriftsartikel (refereegranskat)abstract
- We analyze the dynamics of fast electrons in plasmas containing partially ionized impurity atoms, where the screening effect of bound electrons must be included. We derive analytical expressions for the deflection and slowing-down frequencies, and show that they are increased significantly compared to the results obtained with complete screening, already at sub-relativistic electron energies. Furthermore, we show that the modifications to the deflection and slowing down frequencies are of equal importance in describing the runaway current evolution. Our results greatly affect fast-electron dynamics and have important implications, e.g. for the efficacy of mitigation strategies for runaway electrons in tokamak devices, and energy loss during relativistic breakdown in atmospheric discharges.
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| 5. |
- Hesslow, Linnea, 1993, et al.
(författare)
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Fast-electron dynamics in the presence of weakly ionized impurities
- 2017
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Ingår i: 44th EPS Conference on Plasma Physics, EPS 2017.
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Konferensbidrag (refereegranskat)abstract
- Runaway electrons constitute a significant threat to tokamak experiments. To minimize the risk of damage, it is crucial to understand the runaway-electron dynamics, which during runaway mitigation can be heavily influenced by the interaction with partially ionized atoms. Experiments have shown that mitigation via heavy-impurity injection is more effective than would be expected from standard collisional theory, highlighting the need for more accurate kinetic models. To achieve this, partial screening of the nuclei by the bound electrons must be taken into account. In this contribution, we analyze the dynamics of fast electrons in plasmas containing partially ionized impurity atoms. A generalized collision operator is derived from first principles using quantum-mechanical models. We obtain analytical expressions for the deflection and slowing-down frequencies, and show that they are increased by more than an order of magnitude compared to the results obtained with complete screening, already at sub-relativistic electron energies. Moreover, we implement the generalized collision operator in the continuum kinetic equation solver CODE and demonstrate that interaction with partially ionized atoms greatly affects fast-electron dynamics by enhancing the rates of angular deflection and energy loss. This has important implications, not only for the efficacy of mitigation strategies for runaway electrons in tokamak devices, but also for example for energy loss during relativistic breakdown in lightning discharges.
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| 6. |
- Hesslow, Linnea, 1993, et al.
(författare)
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Kinetic effects of partially screened impurities in runaway-electron mitigation scenarios
- 2017
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Ingår i: International Sherwood Fusion Theory Conference, Annapolis, USA.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Runaway electrons constitute a significant threat to tokamak devices. Their mitigation by heavy-impurity injection has been experimentally shown to be more effective than would be expected from standard collisional theory [1]. In order to understand this effect and develop runaway mitigation schemes, more accurate kinetic models are needed to describe the interaction between electrons and partially ionized atoms. Such models require the partial screening of the nuclei by the bound electrons to be taken into account.In this contribution, we analyze the dynamics of fast electrons in plasmas containing partially ionized impurity atoms. A generalized collision operator is derived from first principles using quantum-mechanical models. We obtain analytical expressions for the deflection and slowing-down frequencies. Even at sub-relativistic energies, these are increased by more than an order of magnitude compared to the results obtained with complete screening. Moreover, we implement the generalized collision operator in the continuum kinetic-equation solver CODE [2, 3] and demonstrate that interaction with partially ionized atoms greatly affects fast-electron dynamics by enhancing the rates of angular deflection and energy loss. In particular, we investigate the decay of a runaway-electron current coupled to a self-consistent electric field. The effect of the interaction with partially ionized impurities has important implications for the efficacy of mitigation strategies for runaway electrons in tokamak devices.References[1] E. M. Hollmann et al., Physics of Plasmas 22, 056108 (2015).[2] M. Landreman, A. Stahl and T. Fülöp, Comp. Phys. Comm. 185, 847 (2014).[3] A. Stahl et al., Nuclear Fusion 56, 112009 (2016).
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| 7. |
- Hoppe, Mathias, 1993, et al.
(författare)
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Modeling synchrotron radiation images of runaway electrons
- 2017
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Ingår i: 15th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- One of the most powerful means of studying runaway electrons in tokamaks is by measuring the synchrotron radiation they emit. In many current experiments, visible light and IR cameras are used to study the synchrotron radiation spot, and spectrometers measure the synchrotron radiation spectrum. Due to the strong dependence on the particle energy, pitch angle and radial position in both the synchrotron spot and spectrum, these can be used to extract valuable information about the runaway electron distribution function. Obtaining accurate information about the runaway electron distribution function from synchrotron radiation measurements however, requires both the magnetic field, camera location and camera spectral range to be handled properly and taken into account. In this contribution we present the synthetic synchrotron diagnostic SOFT (Synchrotron-detecting Orbit Following Toolkit) which simulates the synchrotron radiation from a population of runaway electrons whose energy, pitch angle and radial location are known in the outer midplane. By following the guiding-center orbits of the population, effects arising due to the inhomogeneity of the magnetic field are incorporated, which we show have significant effects on both the synchrotron radiation spot and spectrum.As an application of SOFT, we try to reproduce asynchrotron image from one discharge in the Alcator C-Mod tokamak. By taking measured parameters of the Alcator C-Mod discharge, a distribution function is obtained with the Fokker-Planck solver CODE, for which the emitted synchrotron radiation can then be simulated in SOFT, which shows good agreement. With SOFT, an interpretation for the synchrotron radiation spot observed in experiment can be given, and the characteristic comet shape of the Alcator C-Mod synchrotron radiation spot is shown to be the result of the vertical placement of the camera, together with the narrow set of pitch angles possessed by the particles, as well as their radial distribution.
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| 8. |
- Hoppe, Mathias, 1993, et al.
(författare)
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Synthetic synchrotron diagnostics for runaway electrons
- 2017
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Ingår i: 44th EPS Conference on Plasma Physics, EPS 2017.
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Konferensbidrag (refereegranskat)abstract
- The synchrotron radiation emitted by runaway electrons is an important diagnostic for studyingtheir properties, and many tokamak experiments are equipped with cameras for detectingthis radiation. In this contribution we present the flexible synthetic-diagnostic tool SOFT(Synchrotron-detecting Orbit Following Toolkit), which allows the study of not only synchrotronspot shapes, but also intensity variations within the spot. SOFT takes the full angular andspectral distributions of radiation into account, as well as the electron distribution function,the magnetic geometry, and the limited spectral range of the camera. The additional informationgained from synthetic imaging using SOFT provides valuable insight into the runaway electrondistribution function. With Fokker-Planck simulations, using measured parameterprofiles, we show that SOFT is able to reproduce the main features of measurements performedat the Alcator C-Mod tokamak. In particular, features in the synchrotron image arelinked to variations in the radial, energy and pitch-angle distributions of electrons, as well asdetector placement.
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| 9. |
- Stahl, Adam, 1985, et al.
(författare)
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NORSE: A solver for the relativistic non-linear Fokker-Planck equation for electrons in a homogeneous plasma
- 2017
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Ingår i: Computer Physics Communications. - : Elsevier BV. - 0010-4655. ; 212, s. 269-279
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Tidskriftsartikel (refereegranskat)abstract
- Energetic electrons are of interest in many types of plasmas, howe ver previous modelling of their properties have been restricted to the use of linear Fokker-Planck collision ope rators or non-relativistic formulations. Here, we describe a fully non-linear kinetic-equation solver, capable of handling large electric-field strengths (compared to the Dreicer field) and relativistic temperatures. This tool allows modelling of the momentum- space dynamics of the electrons in cases where strong departure s from Maxwellian distributions may arise. As an example, we consider electron runaway in magnetic-confinement fusion plasmas and describe a transition to electron slide-away at field strengths significantly lower than pre viously predicted.
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| 10. |
- Stahl, Adam, 1985, et al.
(författare)
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Self-consistent nonlinear kinetic modeling of runaway-electron dynamics
- 2017
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Konferensbidrag (refereegranskat)abstract
- Runaway electrons represent the greatest threat to the plasma-facing components of atokamak when they are highly energetic and constitute a significant fraction of the electronpopulation, a regime which has not been previously accessible in modelling since it requiresa nonlinear relativistic treatment. To address this problem, we present an efficient numericaltool called NORSE for the study of runaway-electron momentum-space dynamics.The kinetic equation solved in NORSE includes a fully nonlinear relativistic collision operator, making it possible to consider scenarios where the electric field is comparableto the Dreicer field (or larger), or the electron distribution function is otherwise far froma Maxwellian (which can be the case already in present-day runaway experiments). Thiscapability makes NORSE unique in the field of runaway-electron studies.Using NORSE, we investigate the transition to a regime where the entire electronpopulation experiences continuous acceleration, so-called electron slide-away. Forthe first time, we apply a nonlinear kinetic-equation solver to study the evolution ofthe electron distribution in an ITER disruption. We use an electric field calculated self consistently,and show that the runaway-electron density becomes substantial, makingthe nonlinear treatment essential. In addition, we find that Ohmic heating and the rateof heat loss play an important role in determining the electron dynamics, with the latteraffecting the average energy reached by the runaways by several orders of magnitude.
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