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- Embréus, Ola, 1991, et al.
(författare)
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Dynamics of positrons during relativistic electron runaway
- 2018
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 84:5, s. 905840506-
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Tidskriftsartikel (refereegranskat)abstract
- Sufficiently strong electric fields in plasmas can accelerate charged particles to relativistic energies. In this paper we describe the dynamics of positrons accelerated in such electric fields, and calculate the fraction of created positrons that become runaway accelerated, along with the amount of radiation that they emit. We derive an analytical formula that shows the relative importance of the different positron production processes, and show that, above a certain threshold electric field, the pair production by photons is lower than that by collisions. We furthermore present analytical and numerical solutions to the positron kinetic equation; these are applied to calculate the fraction of positrons that become accelerated or thermalized, which enters into rate equations that describe the evolution of the density of the slow and fast positron populations. Finally, to indicate operational parameters required for positron detection during runaway in tokamak discharges, we give expressions for the parameter dependencies of detected annihilation radiation compared to bremsstrahlung detected at an angle perpendicular to the direction of runaway acceleration. Using the full leading-order pair-production cross-section, we demonstrate that previous related work has overestimated the collisional pair production by at least a factor of four.
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| 5. |
- Hesslow, Linnea, 1993, et al.
(författare)
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Evaluation of the Dreicer runaway generation rate in the presence of high-impurities using a neural network
- 2019
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 85:6
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Tidskriftsartikel (refereegranskat)abstract
- Integrated modelling of electron runaway requires computationally expensive kinetic models that are self-consistently coupled to the evolution of the background plasma parameters. The computational expense can be reduced by using parameterized runaway generation rates rather than solving the full kinetic problem. However, currently available generation rates neglect several important effects; in particular, they are not valid in the presence of partially ionized impurities. In this work, we construct a multilayer neural network for the Dreicer runaway generation rate which is trained on data obtained from kinetic simulations performed for a wide range of plasma parameters and impurities. The neural network accurately reproduces the Dreicer runaway generation rate obtained by the kinetic solver. By implementing it in a fluid runaway-electron modelling tool, we show that the improved generation rates lead to significant differences in the self-consistent runaway dynamics as compared to the results using the previously available formulas for the runaway generation rate. © Cambridge University Press 2019.
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| 6. |
- Hesslow, Linnea, 1993, et al.
(författare)
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Generalized collision operator for fast electrons interacting with partially ionized impurities
- 2018
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 84:6
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Tidskriftsartikel (refereegranskat)abstract
- Accurate modelling of the interaction between fast electrons and partially ionized atoms is important for evaluating tokamak disruption mitigation schemes based on material injection. This requires accounting for the effect of screening of the impurity nuclei by the cloud of bound electrons. In this paper, we generalize the Fokker–Planck operator in a fully ionized plasma by accounting for the effect of screening. We detail the derivation of this generalized operator, and calculate the effective ion length scales, needed in the components of the collision operator, for a number of ion species commonly appearing in fusion experiments. We show that for high electric fields, the secondary runaway growth rate can be substantially larger than in a fully ionized plasma with the same effective charge, although the growth rate is significantly reduced at near-critical electric fields. Furthermore, by comparison with the Boltzmann collision operator, we show that the Fokker–Planck formalism is accurate even for large impurity content.
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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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Spatiotemporal analysis of the runaway distribution function from synchrotron images in an ASDEX Upgrade disruption
- 2021
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 87:1
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Tidskriftsartikel (refereegranskat)abstract
- Synchrotron radiation images from runaway electrons (REs) in an ASDEX Upgrade discharge disrupted by argon injection are analysed using the synchrotron diagnostic tool Soft and coupled fluid-kinetic simulations. We show that the evolution of the runaway distribution is well described by an initial hot-tail seed population, which is accelerated to energies between 25-50 MeV during the current quench, together with an avalanche runaway tail which has an exponentially decreasing energy spectrum. We find that, although the avalanche component carries the vast majority of the current, it is the high-energy seed remnant that dominates synchrotron emission. With insights from the fluid-kinetic simulations, an analytic model for the evolution of the runaway seed component is developed and used to reconstruct the radial density profile of the RE beam. The analysis shows that the observed change of the synchrotron pattern from circular to crescent shape is caused by a rapid redistribution of the radial profile of the runaway density.
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| 9. |
- Barcellona, C., et al.
(författare)
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Tokamaks images advanced processing for diagnostics
- 2022
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Ingår i: IEEE International Symposium on Industrial Electronics. ; 2022-June, s. 612-614
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Konferensbidrag (refereegranskat)abstract
- In this work-in-progress paper, novel recent results obtained in the field of advanced image processing in nuclear fusion plants are reported. In particular, a strategy based on the reconstruction of the runaway electrons beam allows to infer plasma characteristic parameters which allows for an advanced real-time monitoring of the nuclear fusion experiment. Preliminary results obtained at the Frascati Tokamak Upgrade allow to assess the validity of the approach and pave the way for successive refinement of the diagnostics.
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