| 1. |
- Embréus, Ola, 1991, et al.
(författare)
-
Dynamics of positrons during relativistic electron runaway
- 2018
-
Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 84:5, s. 905840506-
-
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.
|
|
| 2. |
|
|
| 3. |
|
|
| 4. |
|
|
| 5. |
- Hesslow, Linnea, 1993, et al.
(författare)
-
Evaluation of the Dreicer runaway generation rate in the presence of high-impurities using a neural network
- 2019
-
Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 85:6
-
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.
|
|
| 6. |
- Hesslow, Linnea, 1993, et al.
(författare)
-
Generalized collision operator for fast electrons interacting with partially ionized impurities
- 2018
-
Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 84:6
-
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.
|
|
| 7. |
- Hoppe, Mathias, 1993, et al.
(författare)
-
Spatiotemporal analysis of the runaway distribution function from synchrotron images in an ASDEX Upgrade disruption
- 2021
-
Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 87:1
-
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.
|
|
| 8. |
- Fülöp, Tünde, 1970, et al.
(författare)
-
Effect of plasma elongation on current dynamics during tokamak disruptions
- 2020
-
Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 86:1
-
Tidskriftsartikel (refereegranskat)abstract
- Plasma terminating disruptions in tokamaks may result in relativistic runaway electron beams with potentially serious consequences for future devices with large plasma currents. In this paper, we investigate the effect of plasma elongation on the coupled dynamics of runaway generation and resistive diffusion of the electric field. We find that elongated plasmas are less likely to produce large runaway currents, partly due to the lower induced electric fields associated with larger plasmas, and partly due to direct shaping effects, which mainly lead to a reduction in the runaway avalanche gain. © Cambridge University Press 2020.
|
|
| 9. |
- Hesslow, Linnea, 1993
(författare)
-
Effect of Partial Screening on Runaway-Electron Dynamics
- 2018
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One of the essential results of kinetic plasma physics is the runaway phenomenon: sufficiently large electric fields can accelerate a fraction of an electron population to relativistic energies. While such runaway electrons are fundamentally interesting objects of study in astrophysical settings, they are also of great practical relevance to fusion research. In the most developed fusion power production device, known as the tokamak, runaway electrons have the potential to cause severe damage to the first wall. Accordingly, runaway-electron mitigation is one of the critical issues in the design of a fusion power plant. The most promising mitigation method to date is the injection of heavy atoms which only partially ionize and collisionally dissipate the energy of the runaway beam before it can collide with the wall. When the ions are partially ionized, their bound electrons screen out a fraction of the atomic charge, which directly affects the collisional scattering rates. However, accurate expressions for these collisional scattering rates between energetic electrons and partially ionized atoms have not been available previously, compromising modeling. In this thesis, we derive collisional scattering rates using a quantum-mechanical treatment, and study their effects on the kinetic runaway-electron dynamics. Using kinetic simulations, we find that the presence of partially ionized atoms significantly increases the dissipation rate of runaway electrons, compared to when the bound electrons completely screened the atomic nuclei. Moreover, we find that the increased scattering rates elevate the threshold electric field for runaway acceleration, but also enhance the avalanche growth rate at electric fields much larger than this threshold. The results outlined in this thesis contribute to more accurate runaway-electron modeling and can lead to more effective mitigation schemes in the longer term. Experimental predictions of runaway mitigation however require that the kinetic model developed here be combined with the effect of spatial variation, which is a subject for future work.
|
|
| 10. |
- Hesslow, Linnea, 1993, et al.
(författare)
-
Effect of partially ionized impurities and radiation on the effective critical electric field for runaway generation
- 2018
-
Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 60:7
-
Tidskriftsartikel (refereegranskat)abstract
- We derive a formula for the effective critical electric field for runaway generation and decay that accounts for the presence of partially ionized impurities in combination with synchrotron and bremsstrahlung radiation losses. We show that the effective critical field is drastically larger than the classical Connor-Hastie field, and even exceeds the value obtained by replacing the free electron density by the total electron density (including both free and bound electrons). Using a kinetic equation solver with an inductive electric field, we show that the runaway current decay after an impurity injection is expected to be linear in time and proportional to the effective critical electric field in highly inductive tokamak devices. This is relevant for the efficacy of mitigation strategies for runaway electrons since it reduces the required amount of injected impurities to achieve a certain current decay rate.
|
|
