| 1. |
- Joffrin, E., et al.
(författare)
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Overview of the JET preparation for deuterium-tritium operation with the ITER like-wall
- 2019
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:11
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Forskningsöversikt (refereegranskat)abstract
- For the past several years, the JET scientific programme (Pamela et al 2007 Fusion Eng. Des. 82 590) has been engaged in a multi-campaign effort, including experiments in D, H and T, leading up to 2020 and the first experiments with 50%/50% D-T mixtures since 1997 and the first ever D-T plasmas with the ITER mix of plasma-facing component materials. For this purpose, a concerted physics and technology programme was launched with a view to prepare the D-T campaign (DTE2). This paper addresses the key elements developed by the JET programme directly contributing to the D-T preparation. This intense preparation includes the review of the physics basis for the D-T operational scenarios, including the fusion power predictions through first principle and integrated modelling, and the impact of isotopes in the operation and physics of D-T plasmas (thermal and particle transport, high confinement mode (H-mode) access, Be and W erosion, fuel recovery, etc). This effort also requires improving several aspects of plasma operation for DTE2, such as real time control schemes, heat load control, disruption avoidance and a mitigation system (including the installation of a new shattered pellet injector), novel ion cyclotron resonance heating schemes (such as the three-ions scheme), new diagnostics (neutron camera and spectrometer, active Alfven eigenmode antennas, neutral gauges, radiation hard imaging systems...) and the calibration of the JET neutron diagnostics at 14 MeV for accurate fusion power measurement. The active preparation of JET for the 2020 D-T campaign provides an incomparable source of information and a basis for the future D-T operation of ITER, and it is also foreseen that a large number of key physics issues will be addressed in support of burning plasmas.
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| 2. |
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| 3. |
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| 4. |
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| 5. |
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| 6. |
- Stahl, Adam, 1985, et al.
(författare)
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Kinetic modelling of runaway-electron dynamics
- 2015
-
Ingår i: Proceedings of the 14th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Improved understanding of runaway-electron formation and decayprocesses are of prime interest for the safe operation of large tokamaks, and theirdynamics during dynamical scenarios such as disruptions are of particular concern. Inthis contribution, we present kinetic modelling of scenarios with time-dependent plasmaparameters – in particular, we investigate hot-tail runaway generation during a rapiddrop in plasma temperature. With the goal of studying runaway-electron generationwith a self-consistent electric field-evolution, we also discuss the implementation ofa conservative collision operator and demonstrate its properties. An operator foravalanche runaway-electron generation which includes the proper energy dependenceof the runaway distribution, is investigated, and the avalanche growth rate is shownto be significantly affected in some parameter regimes. These developments all pavethe way for an improved modelling of runaway-electron dynamics during disruptionsor other dynamic events.
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| 7. |
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| 8. |
- Hesslow, Linnea, 1993, et al.
(författare)
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Effect of partially-screened nuclei on fast-electron dynamics
- 2017
-
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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| 9. |
- Hesslow, Linnea, 1993, et al.
(författare)
-
Kinetic effects of partially screened impurities in runaway-electron mitigation scenarios
- 2017
-
Ingår i: International Sherwood Fusion Theory Conference, Annapolis, USA.
-
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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| 10. |
- Labit, B., et al.
(författare)
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Dependence on plasma shape and plasma fueling for small edge-localized mode regimes in TCV and ASDEX Upgrade
- 2019
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:8
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Tidskriftsartikel (refereegranskat)abstract
- © 2019 Institute of Physics Publishing. All rights reserved. Within the EUROfusion MST1 work package, a series of experiments has been conducted on AUG and TCV devices to disentangle the role of plasma fueling and plasma shape for the onset of small ELM regimes. On both devices, small ELM regimes with high confinement are achieved if and only if two conditions are fulfilled at the same time. Firstly, the plasma density at the separatrix must be large enough (ne,sep/nG ∼ 0.3), leading to a pressure profile flattening at the separatrix, which stabilizes type-I ELMs. Secondly, the magnetic configuration has to be close to a double null (DN), leading to a reduction of the magnetic shear in the extreme vicinity of the separatrix. As a consequence, its stabilizing effect on ballooning modes is weakened.
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| 11. |
- Murari, A., et al.
(författare)
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A control oriented strategy of disruption prediction to avoid the configuration collapse of tokamak reactors
- 2024
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Ingår i: Nature Communications. - 2041-1723 .- 2041-1723. ; 15:1
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Tidskriftsartikel (refereegranskat)abstract
- The objective of thermonuclear fusion consists of producing electricity from the coalescence of light nuclei in high temperature plasmas. The most promising route to fusion envisages the confinement of such plasmas with magnetic fields, whose most studied configuration is the tokamak. Disruptions are catastrophic collapses affecting all tokamak devices and one of the main potential showstoppers on the route to a commercial reactor. In this work we report how, deploying innovative analysis methods on thousands of JET experiments covering the isotopic compositions from hydrogen to full tritium and including the major D-T campaign, the nature of the various forms of collapse is investigated in all phases of the discharges. An original approach to proximity detection has been developed, which allows determining both the probability of and the time interval remaining before an incoming disruption, with adaptive, from scratch, real time compatible techniques. The results indicate that physics based prediction and control tools can be developed, to deploy realistic strategies of disruption avoidance and prevention, meeting the requirements of the next generation of devices.
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| 12. |
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| 13. |
- Stahl, Adam, 1985, et al.
(författare)
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Kinetic modelling of runaway electrons in dynamic scenarios
- 2016
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 56:11, s. 112009-
-
Tidskriftsartikel (refereegranskat)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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| 14. |
- Stahl, Adam, 1985, et al.
(författare)
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Runaway-electron formation and electron slide-away in an ITER post-disruption scenario
- 2016
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 775:1
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Konferensbidrag (refereegranskat)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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| 15. |
- Stahl, Adam, 1985, et al.
(författare)
-
Self-consistent nonlinear kinetic modeling of runaway-electron dynamics
- 2017
-
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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| 16. |
- Stahl, Adam, 1985, et al.
(författare)
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Synchrotron radiation from a runaway electron distribution in tokamaks
- 2013
-
Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 20:9, s. 093302-
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Tidskriftsartikel (refereegranskat)abstract
- The synchrotron radiation emitted by runaway electrons in a fusion plasma provides information regarding the particle momenta and pitch-angles of the runaway electron population through the strong dependence of the synchrotron spectrum on these parameters. Information about the runaway density and its spatial distribution, as well as the time evolution of the above quantities, can also be deduced. In this paper, we present the synchrotron radiation spectra for typical avalanching runaway electron distributions. Spectra obtained for a distribution of electrons are compared with the emission of mono-energetic electrons with a prescribed pitch-angle. We also examine the effects of magnetic field curvature and analyse the sensitivity of the resulting spectrum to perturbations to the runaway distribution. The implications for the deduced runaway electron parameters are discussed. We compare our calculations to experimental data from DIII-D and estimate the maximum observed runaway energy.
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| 17. |
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| 18. |
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| 19. |
- Coda, S., et al.
(författare)
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Physics research on the TCV tokamak facility: From conventional to alternative scenarios and beyond
- 2019
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:11
-
Tidskriftsartikel (refereegranskat)abstract
- The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device's unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with electron-cyclotron resonance heating (ECRH) was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1 MW neutral beam injector has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and nearly non-inductive H-mode discharges sustained by electron cyclotron and neutral beam current drive. In the H-mode, the pedestal pressure increases modestly with nitrogen seeding while fueling moves the density pedestal outwards, but the plasma stored energy is largely uncorrelated to either seeding or fueling. High fueling at high triangularity is key to accessing the attractive small edge-localized mode (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The geodesic acoustic mode, possibly coupled with avalanche events, has been linked with particle flow to the wall in diverted plasmas. Detachment, scrape-off layer transport, and turbulence were studied in L- and H-modes in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power 'starvation' reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in the H-mode is obtained with impurity seeding and has shown little dependence on flux expansion in standard single-null geometry. In the attached L-mode phase, increasing the outer connection length reduces the in-out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variable-configuration baffles and possibly divertor pumping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5 MW ECRH and 1 MW neutral beam injection heating will be added.
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| 20. |
- Embréus, Ola, 1991, et al.
(författare)
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Dynamics of positrons during relativistic electron runaway
- 2018
-
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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| 21. |
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| 22. |
- Fülöp, Tünde, 1970, et al.
(författare)
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Runaway Positrons in Fusion Plasmas
- 2012
-
Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 108:22, s. 225003-
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Tidskriftsartikel (refereegranskat)abstract
- Runaway positrons can be produced in the presence of runaway electron avalanches in magnetized plasmas. In this Letter, we determine the positron distribution, the fraction of runaway positrons, and the parametric dependences of their synchrotron radiation spectrum. We show that the maximum production occurs around γe≃30, where γe is the Lorentz factor of the fast electrons. For an avalanching positron distribution typical of tokamak plasmas, the maximum of the synchrotron radiation spectrum should be around a micron. The radiated power and spectrum shape are sensitive to the plasma parameters. Apart from its intrinsic interest, detection of radiation from positrons could be a diagnostic tool to understand the properties of the medium they propagate through.
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| 23. |
- Fülöp, Tünde, 1970, et al.
(författare)
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Runaway positrons in magnetized plasmas
- 2011
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Ingår i: 53rd Annual Meeting of the APS Division of Plasma Physics, Salt Lake City, Utah. ; , s. GO5.00015-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Runaway electron avalanches have been frequently observed in large tokamak disruptions. The energetic runaways produced in the avalanches may give rise to electron-positron pair production. It has been estimated that up to 10^{14} positrons may be created in collisions between runaway electrons and thermal particles in tokamak disruptions [1]. At birth, these positrons are highly relativistic, and either experience runaway acceleration or are thermalized in a few hundred milliseconds before being annihilated. In this work we calculate the distribution of positrons at birth and their subsequent fate in magnetized plasmas. The production rate is calculated by using a pair-production cross-section valid for arbitrary energies. [1] P. Helander and D. Ward, Phys. Rev. Lett., 90 135004-1 (2003).
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| 24. |
- Fülöp, Tünde, 1970, et al.
(författare)
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Runaway positrons in tokamak plasmas
- 2012
-
Ingår i: Proceedings of 39th European Physical Society meeting on Plasma Physics and 16th International Congress on Plasma Physics. ; 2, s. P4.067-
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Konferensbidrag (refereegranskat)abstract
- Runaway positrons can be produced in the presence of runaway electron avalanches in magnetized plasmas. Almost all the positrons generated by avalanching runaways will run away and are expected to have lifetimes of several seconds. For an avalanching positron distribution typical of tokamak plasmas the maximum of the synchrotron radiation spectrum should be around a micron. The radiated power is sensitive to many plasma parameters, specially the number ofimpurities, temperature and density. Apart from its intrinsic interest, detection of radiation frompositrons could be a diagnostic tool to understand the properties of the annihilation medium.
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| 25. |
- 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.
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| 26. |
- 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
-
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.
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| 27. |
- Hesslow, Linnea, 1993, et al.
(författare)
-
Fast-electron dynamics in the presence of weakly ionized impurities
- 2017
-
Ingår i: 44th EPS Conference on Plasma Physics, EPS 2017.
-
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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| 28. |
- 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.
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| 29. |
- Hollmann, E M, et al.
(författare)
-
Status of research toward the ITER disruption mitigation system
- 2015
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 22:2, s. 021802-
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Tidskriftsartikel (refereegranskat)abstract
- An overview of the present status of research toward the final design of the ITER disruption mitigation system (DMS) is given. The ITER DMS is based on massive injection of impurities, in order to radiate the plasma stored energy and mitigate the potentially damaging effects of disruptions. The design of this system will be extremely challenging due to many physics and engineering constraints such as limitations on port access and the amount and species of injected impurities. Additionally, many physics questions relevant to the design of the ITER disruption mitigation system remain unsolved such as the mechanisms for mixing and assimilation of injected impurities during the rapid shutdown and the mechanisms for the subsequent formation and dissipation of runaway electron current.
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| 30. |
- 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
-
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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| 31. |
- Insulander Björk, Klara, 1982, et al.
(författare)
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Modelling of runaway electron dynamics during argon-induced disruptions in ASDEX Upgrade and JET
- 2021
-
Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 63:8
-
Tidskriftsartikel (refereegranskat)abstract
- Disruptions in tokamak plasmas may lead to the generation of runaway electrons that have the potential to damage plasma-facing components. Improved understanding of the runaway generation process requires interpretative modelling of experiments. In this work we simulate eight discharges in the ASDEX Upgrade and JET tokamaks, where argon gas was injected to trigger the disruption. We use a fluid modelling framework with the capability to model the generation of runaway electrons through the hot-tail, Dreicer and avalanche mechanisms, as well as runaway electron losses. Using experimentally based initial values of plasma current and electron temperature and density, we can reproduce the plasma current evolution using realistic assumptions about temperature evolution and assimilation of the injected argon in the plasma. The assumptions and results are similar for the modelled discharges in ASDEX Upgrade and JET. For the modelled discharges in ASDEX Upgrade, where the initial temperature was comparatively high, we had to assume that a large fraction of the hot-tail runaway electrons were lost in order to reproduce the measured current evolution.
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| 32. |
- Meyer, H.F., et al.
(författare)
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Overview of physics studies on ASDEX Upgrade
- 2019
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:11
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Forskningsöversikt (refereegranskat)abstract
- The ASDEX Upgrade (AUG) programme, jointly run with the EUROfusion MST1 task force, continues to significantly enhance the physics base of ITER and DEMO. Here, the full tungsten wall is a key asset for extrapolating to future devices. The high overall heating power, flexible heating mix and comprehensive diagnostic set allows studies ranging from mimicking the scrape-off-layer and divertor conditions of ITER and DEMO at high density to fully non-inductive operation (q 95 = 5.5, ) at low density. Higher installed electron cyclotron resonance heating power 6 MW, new diagnostics and improved analysis techniques have further enhanced the capabilities of AUG. Stable high-density H-modes with MW m-1 with fully detached strike-points have been demonstrated. The ballooning instability close to the separatrix has been identified as a potential cause leading to the H-mode density limit and is also found to play an important role for the access to small edge-localized modes (ELMs). Density limit disruptions have been successfully avoided using a path-oriented approach to disruption handling and progress has been made in understanding the dissipation and avoidance of runaway electron beams. ELM suppression with resonant magnetic perturbations is now routinely achieved reaching transiently . This gives new insight into the field penetration physics, in particular with respect to plasma flows. Modelling agrees well with plasma response measurements and a helically localised ballooning structure observed prior to the ELM is evidence for the changed edge stability due to the magnetic perturbations. The impact of 3D perturbations on heat load patterns and fast-ion losses have been further elaborated. Progress has also been made in understanding the ELM cycle itself. Here, new fast measurements of and E r allow for inter ELM transport analysis confirming that E r is dominated by the diamagnetic term even for fast timescales. New analysis techniques allow detailed comparison of the ELM crash and are in good agreement with nonlinear MHD modelling. The observation of accelerated ions during the ELM crash can be seen as evidence for the reconnection during the ELM. As type-I ELMs (even mitigated) are likely not a viable operational regime in DEMO studies of 'natural' no ELM regimes have been extended. Stable I-modes up to have been characterised using -feedback. Core physics has been advanced by more detailed characterisation of the turbulence with new measurements such as the eddy tilt angle - measured for the first time - or the cross-phase angle of and fluctuations. These new data put strong constraints on gyro-kinetic turbulence modelling. In addition, carefully executed studies in different main species (H, D and He) and with different heating mixes highlight the importance of the collisional energy exchange for interpreting energy confinement. A new regime with a hollow profile now gives access to regimes mimicking aspects of burning plasma conditions and lead to nonlinear interactions of energetic particle modes despite the sub-Alfvénic beam energy. This will help to validate the fast-ion codes for predicting ITER and DEMO.
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| 33. |
- Olasz, S., et al.
(författare)
-
Runaway electron modelling in the EU-IM framework
- 2021
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Ingår i: 47th EPS Conference on Plasma Physics, EPS 2021. - : European Physical Society (EPS). ; 2021-June, s. 1156-1159
-
Konferensbidrag (refereegranskat)abstract
- The Runaway Electron Test Workflow was used to study the behaviour of the Dreicer generation of runaway electrons in dynamic scenarios to find a parameter which can be used to determine the need of kinetic modelling in more complex simulations. It was found that for processes which vary faster than the collision time at the critical velocity for runaway electron generation, kinetic modelling is advised to capture potential kinetic effects. A more complex tool, the ETS have been used to simulate a self-consistent thermal quench induced by massive material injection with promising initial results. Development of ETS capabilities continues with introduction of kinetic modelling and moving onto the new ETS6 versions.
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| 34. |
- Papp, Gergely, 1985, et al.
(författare)
-
Energetic electron transport in the presence of magnetic perturbations in magnetically confined plasmas
- 2015
-
Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 81:3, s. 475810503-
-
Tidskriftsartikel (refereegranskat)abstract
- The transport of energetic electrons is sensitive to magnetic perturbations. By using 3D numerical simulation of test particle drift orbits we show that the transport of untrapped electrons through an open region with magnetic perturbations cannot be described by a diffusive process. Based on our test particle simulations, we propose a model that leads to an exponential loss of particles.
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| 35. |
- Papp, Gergely, 1985, et al.
(författare)
-
Low frequency sawtooth precursor activity in ASDEX Upgrade
- 2011
-
Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 53:6, s. 065007-
-
Tidskriftsartikel (refereegranskat)abstract
- This paper describes the precursor activity observed in the ASDEX Upgrade tokamak before sawtooth crashes in various neutral beam heated plasmas, utilizing the soft x-ray diagnostic. In addition to the well-known (m, n) = (1,1) internal kink mode and its harmonics, a lower frequency mode is studied in detail. Power modulation of this mode is found to correlate with the power modulation of the (1, 1) kink mode in the quasistationary intervals indicating possible nonlinear interaction. Throughout the studied sawtooth crashes, the power of the lower frequency mode rose by several orders of magnitude just before the crash. In addition to its temporal behaviour, its spatial structure was estimated and the most likely value was found to be (1, 1). A possible role of this mode in the mechanism of the sawtooth crash is discussed.
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| 36. |
- Papp, Gergely, 1985, et al.
(författare)
-
Low frequency sawtooth precursor in ASDEX Upgrade
- 2011
-
Ingår i: Proceedings of 5th IAEA Technical Meeting on the Theory of Plasma Instabilities. ; , s. B4.1-
-
Konferensbidrag (refereegranskat)abstract
- The present paper describes the precursor activity observed in the ASDEX Upgrade tokamakbefore pronounced sawtooth crashes in various neutral beam heated plasmas, utilizing the soft X-raydiagnostic. Besides the well-known (m, n) = (1, 1) internal kink mode and its harmonics, a Low FrequencySawtooth Precursor (LFSP) mode is studied in detail. Indications of a second, lower frequency sawtoothprecursor have been reported on JET and HT-7 as well. Throughout the studied sawtooth crashes, thepower of the lower frequency mode rose by several orders of magnitude just before the crash with a growthrate of 400 1/s, which is shown to be consistent with the growth rate of a resistive core mode. Besidesits temporal behaviour, its spatial structure was estimated with a wavelet based method used on SXRmeasurements, and the most likely value was found to be (m, n) = (1, 1). Power modulation of this modeis found to correlate with the power modulation of the (1,1) kink mode in the quasi-stationary intervals,and significant bicoherence was measured, both indicating non-linear interaction. The frequency ratio ofthe two modes was calculated with an instantaneous frequency following algorithm and was found to bein the 0.5-0.7 range. The LFSP is expected to play a role in the partial magnetic reconnection process,hence every sawtooth crash model involving such reconnection may be affected by the existence of theLFSP.
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| 37. |
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| 38. |
- Papp, Gergely, 1985, et al.
(författare)
-
Runaway electron drift orbits in magnetostatic perturbed fields
- 2011
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 51:4, s. 043004-
-
Tidskriftsartikel (refereegranskat)abstract
- Disruptions in large tokamaks can lead to the generation of a relativistic runaway electron beam that may cause serious damage to the first wall. To mitigate the disruption and suppress the runaway beam the application of resonant magnetic perturbations has been suggested. In this work we investigate the effect of resonant magnetic perturbations on the confinement of runaway electrons by simulating their drift orbits in magnetostatic perturbed fields and calculating the orbit losses for various initial energies and magnetic perturbation magnitudes. In the simulations we use a TEXTOR-like configuration and solve the relativistic, gyro-averaged drift equations for the runaway electrons including synchrotron radiation and collisions. The results indicate that runaway electrons are well confined in the core of the device, but the onset time of runaway losses closer to the edge is dependent on the magnetic perturbation level and thereby can affect the maximum runaway current. However, the runaway current damping rate is not sensitive to the magnetic perturbation level, in agreement with experimental observations.
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| 39. |
- Papp, Gergely, 1985, et al.
(författare)
-
Runaway electron drift orbits in magnetostatic perturbed fields
- 2010
-
Ingår i: Proceedings of 23rd IAEA Fusion Energy Conference, Korea 2010.
-
Konferensbidrag (refereegranskat)abstract
- Disruptions in large tokamaks can lead to the generation of a relativistic runaway electron beam that may cause serious damage to the first wall. To mitigate the disruption and suppress the runaway beam the application of resonant magnetic perturbations has been suggested. In this work we investigate the effect of resonant magnetic perturbations on the confinement of runaway electrons by simulating their drift orbits in magnetostatic perturbed fields and calculating the orbit losses for various initial energies and magnetic perturbation magnitudes. In the simulations we use a TEXTOR-like configuration and solve the relativistic, gyro-averaged drift equations for the runaway electrons including synchrotron radiation and collisions. The results indicate that runaway electrons are well confined in the core of the device, but the onset time of runaway losses closer to the edge is dependent on the magnetic perturbation level and thereby can affect the maximum runaway current. However, the runaway current damping rate is not sensitive to the magnetic perturbation level, in agreement with experimental observations.
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| 40. |
- Papp, Gergely, 1985, et al.
(författare)
-
Runaway electron dynamics and transport anisotropy due to resonant magnetic perturbations in ITER
- 2012
-
Ingår i: Proceedings of the 24th IAEA Fusion Energy Conference. ; , s. TH/P4-05
-
Konferensbidrag (refereegranskat)abstract
- In this paper the effect of resonant magnetic perturbations (RMP) on the net radial transport of runaway electrons (RE) is calculated by simulating the RE drift orbits in magnetostatic perturbed fields. Through the transport, RMP influences the time dynamics andpreferred loss directions of the REs. The distribution of the field mesh exit points of therunaway electrons become more localised compared to the unperturbed case, since the losspattern depends on the geometric properties of the RMP configuration such as periodicityor helicity. On the other hand, the loss patterns do not depend on the particle energiesand starting positions. The particle radial steps are correlated to the local radial magneticperturbation component, which makes the transport chaotic, but deterministic.
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| 41. |
- Papp, Gergely, 1985, et al.
(författare)
-
Runaway electron losses caused by resonant magnetic perturbations in ITER
- 2011
-
Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 53:9, s. 095004-
-
Tidskriftsartikel (refereegranskat)abstract
- Disruptions in large tokamaks can lead to the generation of a relativistic runaway electron beam that may cause serious damage to the first wall. To suppress the runaway beam the application of resonant magnetic perturbations (RMPs) has been suggested. In this work we investigate the effect of RMPs on the confinement of runaway electrons by simulating their drift orbits in magnetostatic perturbed fields and calculating the transport and orbit losses for various initial energies and different magnetic perturbation configurations. In the simulations we model the ITER RMP configuration and solve the relativistic, gyro-averaged drift equations for the runaway electrons including a time-dependent electric field, radiation losses and collisions. The results indicate that runaway electrons are rapidly lost from regions where the normalized perturbation amplitude δB/B is larger than 0.1% in a properly chosen perturbation geometry. This applies to the region outside the radius corresponding to the normalized toroidal flux ψ = 0.5.
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| 42. |
- Papp, Gergely, 1985, et al.
(författare)
-
Runaway electron losses enhanced by resonant magnetic perturbations
- 2011
-
Ingår i: Proceedings of 12th Technical Meeting on Energetic Particles in Magnetic Confinement Systems. ; , s. O.26-
-
Konferensbidrag (refereegranskat)abstract
- Disruptions in large tokamaks can lead to the generation of a relativistic runaway electronbeam that may cause serious damage to the first wall. To suppress the runaway beam the applicationof resonant magnetic perturbations (RMP) has been suggested. In this work we investigate the effect ofresonant magnetic perturbations on the confinement of runaway electrons by simulating their drift orbitsin magnetostatic perturbed fields and calculating the transport and orbit losses for various initial energiesand different magnetic perturbation configurations. In the simulations we use model configurations withexisting (TEXTOR) and planned (ITER) RMP systems, and solve the relativistic, gyro-averaged driftequations for the runaway electrons including the electric field, radiation losses and collisions. The resultsindicate that runaway electrons are well confined in the core of the device, but the onset time of runawaylosses closer to the edge is dependent on the magnetic perturbation level, which can thereby affectthe maximum runaway current. Runaway electrons are rapidly lost from regions where the normalisedperturbation amplitude \delta B/B is larger than 0.1% in a properly chosen perturbation geometry. Thisapplies to the region outside the radius corresponding to the normalised flux \psi = 0.5 in ITER, when theELM mitigation coils are used at maximum current in their most favourable configuration.
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| 43. |
|
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| 44. |
- Papp, Gergely, 1985, et al.
(författare)
-
The effect of magnetic perturbations on runaway dynamics
- 2013
-
Ingår i: Proceeding of the13th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems. ; , s. IAEA-F1-TM-44766 / I12-
-
Konferensbidrag (refereegranskat)abstract
- Disruptions in large tokamaks can lead to the generation of a relativistic runaway (RE) electron beam that may cause serious damage to the first wall. To suppress the RE beamthe application of resonant magnetic perturbations (RMP) has been suggested. We investigate the effect of resonant magnetic perturbations by simulating the RE drift orbits inmagnetostatic perturbed fields and calculating the transport and orbit losses for various particle energies and different magnetic perturbation configurations. In the simulations we use model configurations with the planned ITER RMP system and solve the relativistic, gyro-averaged drift equations for the runaway electrons including radiation losses and collisions. The results indicate that runaway electrons are rapidly lost from regions where thenormalised perturbation amplitude δB/B is larger than ∼0.1% in a properly chosen perturbation geometry. This corresponds to the outer half of the confinement volume in ITER. We show that despite the chaotic magnetic topology the ensemble behaviour can only be approximated by a diffusion process.
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| 45. |
- Papp, Gergely, 1985, et al.
(författare)
-
The effect of resonant magnetic perturbations on runaway electron transport in ITER
- 2012
-
Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 54:12, s. 125008-
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper the effect of resonant magnetic perturbations (RMPs) on the net radial transport of runaway electrons (REs) is calculated by simulating the RE drift orbits in magnetostatic perturbed fields. Through the transport, RMP influences the time dynamics and preferred loss directions of the REs, which are determined for different magnetic perturbation configurations. The distribution of the field mesh exit points of the REs become more localized compared with the unperturbed case, since the loss pattern depends on the geometric properties of the RMP configuration such as periodicity or helicity. On the other hand, the loss patterns do not depend on the particle energies and starting positions. The particle radial steps are correlated with the local radial magnetic perturbation component, which makes the transport chaotic, but deterministic.
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| 46. |
|
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| 47. |
- Papp, Gergely, 1985, et al.
(författare)
-
The effect of the ITER-like wall on runaway electron generation in JET
- 2013
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 53:12
-
Tidskriftsartikel (refereegranskat)abstract
- This paper investigates the effect of the ITER-like wall (ILW) on runaway electron (RE) generation through a comparative study of similar slow argon injection JET disruptions, performed with different wall materials. In the carbon wall case, a RE plateau is observed, while in the ITER-like wall case, the current quench is slower and the runaway current is negligibly small. The aim of the paper is to shed light on the reason for these differences by detailed numerical modelling to study which factors affected the RE formation. The post-disruption current profile is calculated by a one-dimensional model of electric field, temperature and runaway current taking into account the impurity injection. Scans of various impurity contents are performed and agreement with the experimental scenarios is obtained for reasonable argon and wall impurity contents. Our modelling shows that the reason for the changed RE dynamics is a complex, combined effect of the differences in plasma parameter profiles, the radiation characteristics of beryllium and carbon, and the difference of the injected argon amount. These together lead to a significantly higher Dreicer generation rate in the carbon wall case, which is less prone to being suppressed by RE loss mechanisms. The results indicate that the differences are greatly reduced above ~50% argon content, suggesting that significant RE current is expected in future massive gas injection experiments on both JET and ITER.
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| 48. |
- Papp, Gergely, 1985
(författare)
-
The role of magnetic perturbations in runaway electron and sawtooth dynamics
- 2011
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nuclear fusion in tokamaks is a possible future energy production mechanism. Some of the still unsolved problems are connected to the effect of magnetic perturbations on the radial plasma transport. Two such problems are addressed in this thesis.The sudden loss of plasma confinement in large tokamaks can lead to the generation of a relativistic runaway electron beam that may cause serious structural damage. To suppress the runaway beam the application of resonant magnetic perturbations (RMP) has been suggested. The numerical analysis of the RMP is based on the relativistic, gyro-averaged drift equations for the runaway electrons in the 3D perturbed equilibria of the TEXTOR and ITER tokamaks. The simulations include a time-dependent electric field, radiation losses and collisions. The simulated runaway current damping rate agrees with experimental observations at TEXTOR. The results indicate that, in a properly chosen perturbation geometry, runaway electrons are rapidly lost from regions where the normalised magnetic perturbation amplitude is larger than 0.1%. This applies in both machines roughly to the outer third - outer half of the confinement value.The second part of the thesis describes the low frequency precursor activity observed in the ASDEX Upgrade tokamak before sawtooth crashes. Sawtooth crashes are periodic density and temperature redistributions of the plasma core. Besides the well-known internal kink mode, the low frequency sawtooth precursor (LFSP) mode is studied in detail. Power modulation of the LFSP is found to correlate with the power modulation of the kink mode in the quasi-stationary intervals indicating possible non-linear interaction. In all the studied sawtooth crashes, the power of the lower frequency mode rose by several orders of magnitude just before the crash. The spatial structure of the LFSP was estimated and found to equal that of the kink mode. A possible role of this mode in the mechanism of the sawtooth crash is discussed.
|
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| 49. |
- Papp, Gergely, 1985
(författare)
-
The role of magnetic perturbations in runaway electron and sawtooth dynamics
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- As the world's fusion energy program is increasingly focused towards burning plasma experiments, it is important to address the remaining theoretical issues. In this thesis we focus on the effect of magnetic perturbations on the radial plasma transport.The sudden loss of plasma confinement in large tokamaks can lead to the generation of a relativistic runaway electron beam that may cause serious structural damage. To suppress the runaway beam the application of resonant magnetic perturbations (RMP) has been suggested. In this thesis, the numerical analysis of the RMP is based on the relativistic, gyro-averaged drift equations for the runaway electrons in the 3D perturbed equilibria of the TEXTOR and ITER tokamaks. The results indicate that, in a properly chosen perturbation geometry, runaway electrons are rapidly lost from approximately the outer half of the confinement volume. Simulation studies of runaway evolution with self-consistent electric field in the presence of impurities have been carried out for the JET tokamak with a 1D tool, where we have demonstrated the runaway suppression effect of magnetic perturbation induced radial transport. We also show that runaway electrons can generate high energy positrons, and that their synchrotron radiation may be used for diagnostic purposes.The last part of the thesis describes the low frequency precursor activity observed in the ASDEX Upgrade tokamak before sawtooth crashes, which are periodic density and temperature redistributions of the plasma core. Besides the well-known internal kink mode, the low frequency sawtooth precursor (LFSP) mode is studied in detail. Time-frequency analysis indicates non-linear interaction and a similar spatial structure for the two modes. A possible role of this mode in the evolution of the sawtooth crash is discussed in the context of magnetic perturbations.
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| 50. |
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