| 1. |
- Embréus, Ola, 1991, et al.
(author)
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Dynamics of positrons during relativistic electron runaway
- 2018
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In: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 84:5, s. 905840506-
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Journal article (peer-reviewed)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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| 2. |
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| 3. |
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| 4. |
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| 5. |
- Hesslow, Linnea, 1993, et al.
(author)
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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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In: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 85:6
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Journal article (peer-reviewed)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.
(author)
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Generalized collision operator for fast electrons interacting with partially ionized impurities
- 2018
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In: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 84:6
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Journal article (peer-reviewed)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.
(author)
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Modeling synchrotron radiation images of runaway electrons
- 2017
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In: 15th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems.
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Conference paper (other academic/artistic)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.
(author)
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Spatiotemporal analysis of the runaway distribution function from synchrotron images in an ASDEX Upgrade disruption
- 2021
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In: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 87:1
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Journal article (peer-reviewed)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.
(author)
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Tokamaks images advanced processing for diagnostics
- 2022
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In: IEEE International Symposium on Industrial Electronics. ; 2022-June, s. 612-614
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Conference paper (peer-reviewed)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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| 10. |
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| 11. |
- Hollmann, E M, et al.
(author)
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Estimate of pre-thermal quench non-thermal electron density profile during Ar pellet shutdowns of low-density target plasmas in DIII-D
- 2021
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In: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 28:7
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Journal article (peer-reviewed)abstract
- The radial density profile of pre-thermal quench (pre-TQ) early-time non-thermal (hot) electrons is estimated by combining electron cyclotron emission and soft x-ray data during the rapid shutdown of low-density (ne≲1019m−3) DIII-D target plasmas with cryogenic argon pellet injection. This technique is mostly limited in these experiments to the pre-TQ phase and quickly loses validity during the TQ. Two different cases are studied: a high (10 keV) temperature target and a low (4 keV) temperature target. The results indicate that early-time, low-energy (∼10 keV) hot electrons form ahead of the argon pellet as it enters the plasma, affecting the pellet ablation rate; it is hypothesized that this may be caused by rapid cross field transport of argon ions ahead of the pellet or by rapid cross field transport of hot electrons. Fokker-Planck modeling of the two shots suggests that the hot electron current is quite significant during the pre-TQ phase (up to 50% of the total current). Comparison between modeled pre-TQ hot electron current and post-TQ hot electron current inferred from avalanche theory suggests that hot electron current increases during the high-temperature target TQ but decreases during the low-temperature target TQ. The uncertainties in this estimate are large; however, if true, this suggests that TQ radial loss of hot electron current could be larger than previously estimated in DIII-D.
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| 12. |
- Hoppe, Mathias, 1993, et al.
(author)
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DREAM: A fluid-kinetic framework for tokamak disruption runaway electron simulations
- 2021
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In: Computer Physics Communications. - : Elsevier BV. - 0010-4655. ; 268
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Journal article (peer-reviewed)abstract
- Avoidance of the harmful effects of runaway electrons (REs) in plasma-terminating disruptions is pivotal in the design of safety systems for magnetic fusion devices. Here, we describe a computationally efficient numerical tool, that allows for self-consistent simulations of plasma cooling and associated RE dynamics during disruptions. It solves flux-surface averaged transport equations for the plasma density, temperature and poloidal flux, using a bounce-averaged kinetic equation to self-consistently provide the electron current, heat, density and RE evolution, as well as the electron distribution function. As an example, we consider disruption scenarios with material injection and compare the electron dynamics resolved with different levels of complexity, from fully kinetic to fluid modes. Program summary: Program Title: DREAM Developer's repository link: https://github.com/chalmersplasmatheory/DREAM Licensing provisions: MIT Programming language: C++, Python Nature of problem: Self-consistently simulates the plasma evolution in a tokamak disruption, with specific emphasis on runaway electron dynamics. The runaway electrons can be simulated either as a fluid, fully kinetically, or as a mix of the two. Plasma temperature, current density, electric field, ion density and charge states are all evolved self-consistently, where kinetic non-thermal contributions are captured using an orbit-averaged relativistic electron Fokker-Planck equation, which couples to the plasma evolution. In the typical use case, the electrons are represented by two distinct populations: a cold fluid population and a kinetic superthermal population. Solution method: The system of equations is solved using a standard multidimensional Newton's method. Partial differential equations—most prominently the bounce-averaged Fokker–Planck and current diffusion equations—are discretized using a high-resolution finite volume scheme that preserves density and positivity.
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| 13. |
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| 14. |
- Hoppe, Mathias, 1993, et al.
(author)
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Interpretation of runaway electron synchrotron and bremsstrahlung images
- 2018
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 58:8
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Journal article (peer-reviewed)abstract
- The crescent spot shape observed in DIII-D runaway electron synchrotron radiation images is shown to result from the high degree of anisotropy in the emitted radiation, the finite spectral range of the camera and the distribution of runaways. The finite spectral camera range is found to be particularly important, as the radiation from the high-field side can be stronger by a factor 106than the radiation from the low-field side in DIII-D. By combining a kinetic model of the runaway dynamics with a synthetic synchrotron diagnostic we see that physical processes not described by the kinetic model (such as radial transport) are likely to be limiting the energy of the runaways. We show that a population of runaways with lower dominant energies and larger pitch-angles than those predicted by the kinetic model provide a better match to the synchrotron measurements. Using a new synthetic bremsstrahlung diagnostic we also simulate the view of the gamma ray imager diagnostic used at DIII-D to resolve the spatial distribution of runaway-generated bremsstrahlung.
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| 15. |
- Hoppe, Mathias, 1993, et al.
(author)
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Runaway electron generation during tokamak start-up
- 2022
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In: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 88:3
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Journal article (peer-reviewed)abstract
- Tokamak start-up is characterized by low electron densities and strong electric fields, in order to quickly raise the plasma current and temperature, allowing the plasma to fully ionize and magnetic flux surfaces to form. Such conditions are ideal for the formation of superthermal electrons, which may reduce the efficiency of ohmic heating and prevent the formation of a healthy thermal fusion plasma. This is of particular concern in ITER where engineering limitations put restrictions on the allowable electric fields and limit the prefill densities during start-up. In this study, we present a new 0D burn-through simulation tool called STREAM (STart-up Runaway Electron Analysis Model), which self-consistently evolves the plasma density, temperature and electric field, while accounting for the generation and loss of relativistic runaway electrons. After verifying the burn-through model, we investigate conditions under which runaway electrons can form during tokamak start-up as well as their effects on the plasma initiation. We find that Dreicer generation plays a crucial role in determining whether a discharge becomes runaway-dominated or not, and that a large number of runaway electrons could limit the ohmic heating of the plasma, thus preventing successful burn-through or further ramp-up of the plasma current. The runaway generation can be suppressed by raising the density via gas fuelling, but only if done sufficiently early. Otherwise a large runaway seed may have already been built up, which can avalanche even at relatively low electric fields and high densities.
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| 16. |
- Hoppe, Mathias, 1993
(author)
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Runaway-electron model development and validation in tokamaks
- 2021
-
Doctoral thesis (other academic/artistic)abstract
- Magnetic confinement fusion (MCF), in which a hot plasma at more than 100 million kelvin is confined using magnetic fields, is the most successful fusion energy concept developed to date. After decades of research, MCF devices designed to demonstrate a positive net energy output are being constructed, completing a crucial milestone on the path to making fusion a commercially viable energy source. Several hurdles remain on this path, however, and one of the most pressing issues concerns the sudden and rapid loss of confinement of the fusion plasma, known as a disruption. An undesirable consequence of disruptions is the acceleration of a fraction of the plasma electrons to relativistic energies which---if the electrons were to strike the device wall---could deposit a significant portion of the plasma energy on a small area, causing severe and potentially irreparable damage. The aim of this thesis is to develop a robust simulation tool capable of accurately predicting the number of runaway electrons produced in different disruption scenarios. Since the evolution of the runaway electrons affects the background plasma, it is important to also allow quantities such as electron temperature, ion density, and electric field to evolve self-consistently in the simulation. This leads to a tightly coupled system of non-linear equations, and to solve it we have developed the numerical tool DREAM. The complexity of the models used to simulate runaway electrons demands that the validity of the models is carefully evaluated by comparing predictions with existing experimental data. One of the most informative techniques for studying the dynamics of runaway electrons in MCF experiments utilises synchrotron radiation, and to facilitate direct comparison of runaway electron simulations with experiments we have developed the synthetic diagnostic framework SOFT. Using SOFT, we study runaway electrons in the ASDEX Upgrade and TCV fusion devices, and develop powerful techniques for accurately extracting information about the location and momentum of runaway electrons.
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| 17. |
- Hoppe, Mathias, 1993, et al.
(author)
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Runaway electron synchrotron radiation in a vertically translated plasma
- 2020
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 60:9
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Journal article (peer-reviewed)abstract
- Synchrotron radiation observed from runaway electrons (REs) in tokamaks depends upon the position and size of the RE beam, the RE energy and pitch distributions, as well as the location of the observer. We show experimental synchrotron images of a vertically moving RE beam sweeping past the detector in the Tokamak a Configuration Variable (TCV) tokamak and compare it with predictions from the synthetic synchrotron diagnosticSoft. This experimental validation lends confidence to the theory underlying the synthetic diagnostics which are used for benchmarking theoretical models of and probing runaway dynamics. We present a comparison of synchrotron measurements in TCV with predictions of kinetic theory for runaway dynamics in uniform magnetic fields. We find that to explain the detected synchrotron emission, significant non-collisional pitch angle scattering as well as radial transport of REs would be needed. Such effects could be caused by the presence of magnetic perturbations, which should be further investigated in future TCV experiments.
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| 18. |
- Hoppe, Mathias, 1993
(author)
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Simulation and analysis of radiation from runaway electrons
- 2019
-
Licentiate thesis (other academic/artistic)abstract
- Electron runaway constitutes one of the primary threats to future tokamak fusion reactors such as ITER. Successful prevention and mitigation of runaways relies on the development of theoretical models which accurately describe the dynamics of runaway electrons, and these models must in turn be validated in experiments. Experimental validation of models is however often made difficult by the fact that the diagnostic signals obtained in experiments only depend indirectly on the particle dynamics. In this thesis, a synthetic diagnostic model is presented which has been implemented in the Synchrotron-detecting Orbit Following Toolkit (SOFT), and which bridges this divide between theory and experiment. The synthetic diagnostic calculates the bremsstrahlung and synchrotron radiation diagnostic signals corresponding to a given runaway electron population, which can be directly compared to camera images and radiation spectra obtained in experiments. Bremsstrahlung and synchrotron radiation from runaway electrons are particularly sensitive to the runaway dynamics and, as is shown in this thesis, they provide insight into the runaway electron distribution function. This thesis focuses on geometric effects observed in the detected radiation when magnetic field inhomogeneities and detector properties are taken into account, something which previous studies have neglected. The dependence of the observed radiation on magnetic field geometry, detector properties and runaway parameters is characterised, and it is explained how geometric effects limit the otherwise monotonic growth of the diagnostic response function with the runaway pitch angle. The synthetic diagnostic model is applied to experiments in the Alcator C-Mod and the DIII-D tokamaks and is used to validate kinetic theory predictions of the electron distribution function. It is found that the kinetic model agrees well in certain scenarios and fails in others. In the scenarios where it fails, the synthetic diagnostic model suggests that a mechanism causing a larger spread in pitch angle may be missing from the kinetic model.
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| 19. |
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| 20. |
- Hoppe, Mathias, 1993, et al.
(author)
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SOFT: A synthetic synchrotron diagnostic for runaway electrons
- 2018
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 58:2, s. 026032-
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Journal article (peer-reviewed)abstract
- Improved understanding of the dynamics of runaway electrons can be obtained by measurement and interpretation of their synchrotron radiation emission. Models for synchrotron radiation emitted by relativistic electrons are well established, but the question of how various geometric effects -- such as magnetic field inhomogeneity and camera placement -- influence the synchrotron measurements and their interpretation remains open. In this paper we address this issue by simulating synchrotron images and spectra using the new synthetic synchrotron diagnostic tool SOFT (Synchrotron-detecting Orbit Following Toolkit). We identify the key parameters influencing the synchrotron radiation spot and present scans in those parameters. Using a runaway electron distribution function obtained by Fokker-Planck simulations for parameters from an Alcator C-Mod discharge, we demonstrate that the corresponding synchrotron image is well-reproduced by SOFT simulations, and we explain how it can be understood in terms of the parameter scans. Geometric effects are shown to significantly influence the synchrotron spectrum, and we show that inherent inconsistencies in a simple emission model (i.e. not modeling detection) can lead to incorrect interpretation of the images.
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| 21. |
- Hoppe, Mathias, 1993, et al.
(author)
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Synthetic synchrotron diagnostics for runaway electrons
- 2017
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In: 44th EPS Conference on Plasma Physics, EPS 2017.
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Conference paper (peer-reviewed)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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| 22. |
- Olasz, S., et al.
(author)
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Runaway electron modelling in the EU-IM framework
- 2021
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In: 47th EPS Conference on Plasma Physics, EPS 2021. - : European Physical Society (EPS). ; 2021-June, s. 1156-1159
-
Conference paper (peer-reviewed)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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| 23. |
- Olasz, S., et al.
(author)
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Validity of models for Dreicer generation of runaway electrons in dynamic scenarios
- 2021
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 61:6
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Journal article (peer-reviewed)abstract
- Runaway electron modelling efforts are motivated by the risk these energetic particles pose to large fusion devices. The sophisticated kinetic models can capture most features of the runaway electron generation but have high computational costs, which can be avoided by using computationally cheaper reduced kinetic codes. This paper compares the reduced kinetic and kinetic models to determine when the former solvers, based on analytical calculations assuming quasi-stationarity, can be used. The Dreicer generation rate is calculated by two different solvers in parallel in a workflow developed in the European integrated modelling framework, and this is complemented by calculations of a third code that is not yet integrated into the framework. Runaway Fluid, a reduced kinetic code, NORSE, a kinetic code using non-linear collision operator, and DREAM, a linearized Fokker-Planck solver, are used to investigate the effect of a dynamic change in the electric field for different plasma scenarios spanning across the whole tokamak-relevant range. We find that on time scales shorter than or comparable to the electron-electron collision time at the critical velocity for runaway electron generation, kinetic effects not captured by reduced kinetic models play an important role. This characteristic time scale is easy to calculate and can reliably be used to determine whether there is a need for kinetic modelling or cheaper reduced kinetic codes are expected to deliver sufficiently accurate results. This criterion can be automated, and thus it can be of great benefit for the comprehensive self-consistent modelling frameworks that are attempting to simulate complex events such as tokamak start-up or disruptions.
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| 24. |
- Paz-Soldan, C., et al.
(author)
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Recent DIII-D advances in runaway electron measurement and model validation
- 2019
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:6
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Journal article (peer-reviewed)abstract
- Novel measurements and modeling of runaway electron (RE) dynamics in DIII-D have resolved experimental discrepancies and validated predictions for ITER, improving confidence that RE avoidance and mitigation can be predictably achieved. Considering RE formation, first experimental assessments of the RE seed current demonstrates that present hot-tail theories are not yet accurate and require improved treatment of the pellet dynamics. Novel measurements of kinetic instabilities in the MHz-range have been made in the RE formation phase, with the intensity of these modes correlated with previously unexplained empirical thresholds for RE generation. Controlled RE dissipation experiments in quiescent regimes have validated RE distribution function dependencies on collisional and synchrotron damping, both in terms of distribution function shape and dissipation rates. Measurements of RE bremsstrahlung and synchrotron emission are now used in tandem to resolve energy and pitch-angle effects. A resolution to long-standing dissipation anomalies in the quiescent regime is offered by taking into account kinetic instability effects on RE phase-space dynamics. Kinetic instabilities in the 100-200 MHz range are directly observed, though modeling finds the largest dissipation arises from GHz range instabilities that are beyond the reach of existing diagnostics. Kinetic instabilities are also observed in the mature post-disruption RE plateau phase, so long as the collisional damping rate is reduced with low-Z injection. Experiments with high-Z injection find that the dissipation rate saturates with injection quantity, likely due to neutral diffusion rates being slower than vertical instability rates in DIII-D. Considering the final loss, a 0D model for first-wall Joule heating is found to be in agreement with experiment, and controlled access to RE equilibria with edge safety factor of two identifies novel dynamics brought about by large-scale kink instabilities. These dynamics are typified by fast (tens of microseconds) RE loss rates without RE beam regeneration. The above measurements and comparison with theory represent significant advances in the understanding of RE dynamics and indicate possible new opportunities for RE avoidance or mitigation via kinetic instabilities.
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| 25. |
- Pokol, Gergö, 1979, et al.
(author)
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Runaway electron modelling in the self-consistent core European Transport Simulator
- 2019
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In: Nuclear Fusion. - : Institute of Physics Publishing (IOPP). - 0029-5515 .- 1741-4326. ; 59:7
-
Journal article (peer-reviewed)abstract
- Relativistic runaway electrons are a major concern in tokamaks. Although significant theoretical development had been undertaken in recent decades, we still lack a self-consistent simulator that could simultaneously capture all aspects of this phenomenon. The European framework for Integrated Modelling (EU-IM) facilitates the integration of different plasma simulation tools by providing a standard data structure for communication that enables relatively easy integration of different physics codes. A three-level modelling approach was adopted for runaway electron simulations within the EU-IM. Recently, a number of runaway electron modelling modules have been integrated into this framework. The first level of modelling (Runaway Indicator) is limited to the indication if runaway electron generation is possible or likely. The second level (Runaway Fluid) adopts an approach similar to e.g. the GO code, using analytical formulas to estimate changes in the runaway electron current density. The third level is based on the solution of the electron kinetics. One such code is LUKE that can handle the toroidicity-induced effects by solving the bounce-averaged Fokker-Planck equation. Another approach is used in NORSE, which features a fully nonlinear collision operator that makes it capable of simulating major changes in the electron distribution, for example slide-away. Both codes handle the effect of radiation on the runaway distribution. These runaway-electron modelling codes are in different stages of integration into the EU-IM infrastructure, and into the European Transport Simulator (ETS), which is a fully capable modular 1.5D core transport simulator. The ETS with Runaway Fluid was benchmarked to the GO code implementing similar physics. Coherent integration of kinetic solvers requires more effort on the coupling, especially regarding the definition of the boundary between runaway and thermal populations, and on consistent calculation of resistivity. Some of these issues are discussed.
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| 26. |
- Pucella, G., et al.
(author)
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Overview of the FTU results
- 2022
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 62:4
-
Research review (peer-reviewed)abstract
- Since the 2018 IAEA FEC Conference, FTU operations have been devoted to several experiments covering a large range of topics, from the investigation of the behaviour of a liquid tin limiter to the runaway electrons mitigation and control and to the stabilization of tearing modes by electron cyclotron heating and by pellet injection. Other experiments have involved the spectroscopy of heavy metal ions, the electron density peaking in helium doped plasmas, the electron cyclotron assisted start-up and the electron temperature measurements in high temperature plasmas. The effectiveness of the laser induced breakdown spectroscopy system has been demonstrated and the new capabilities of the runaway electron imaging spectrometry system for in-flight runaways studies have been explored. Finally, a high resolution saddle coil array for MHD analysis and UV and SXR diamond detectors have been successfully tested on different plasma scenarios.
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| 27. |
- Reimerdes, H., et al.
(author)
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Overview of the TCV tokamak experimental programme
- 2022
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 62:4
-
Journal article (peer-reviewed)abstract
- The tokamak a configuration variable (TCV) continues to leverage its unique shaping capabilities, flexible heating systems and modern control system to address critical issues in preparation for ITER and a fusion power plant. For the 2019-20 campaign its configurational flexibility has been enhanced with the installation of removable divertor gas baffles, its diagnostic capabilities with an extensive set of upgrades and its heating systems with new dual frequency gyrotrons. The gas baffles reduce coupling between the divertor and the main chamber and allow for detailed investigations on the role of fuelling in general and, together with upgraded boundary diagnostics, test divertor and edge models in particular. The increased heating capabilities broaden the operational regime to include T (e)/T (i) similar to 1 and have stimulated refocussing studies from L-mode to H-mode across a range of research topics. ITER baseline parameters were reached in type-I ELMy H-modes and alternative regimes with 'small' (or no) ELMs explored. Most prominently, negative triangularity was investigated in detail and confirmed as an attractive scenario with H-mode level core confinement but an L-mode edge. Emphasis was also placed on control, where an increased number of observers, actuators and control solutions became available and are now integrated into a generic control framework as will be needed in future devices. The quantity and quality of results of the 2019-20 TCV campaign are a testament to its successful integration within the European research effort alongside a vibrant domestic programme and international collaborations.
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| 28. |
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| 29. |
- Svenningsson, Ida, 1996, et al.
(author)
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Hot-Tail Runaway Seed Landscape during the Thermal Quench in Tokamaks
- 2021
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In: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 127:3
-
Journal article (peer-reviewed)abstract
- Runaway electron populations seeded from the hot tail generated by the rapid cooling in plasma-terminating disruptions are a serious concern for next-step tokamak devices such as ITER. Here, we present a comprehensive treatment of the thermal quench, including the superthermal electron dynamics, heat and particle transport, atomic physics, and radial losses due to magnetic perturbations: processes that are strongly linked and essential for the evaluation of the runaway seed in disruptions mitigated by material injection. We identify limits on the injected impurity density and magnetic perturbation level for which the runaway seed current is acceptable without excessive thermal energy being lost to the wall via particle impact. The consistent modeling of generation and losses shows that runaway beams tend to form near the edge of the plasma, where they could be deconfined via external perturbations.
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| 30. |
- Tinguely, R. A., et al.
(author)
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Experimental and synthetic measurements of polarized synchrotron emission from runaway electrons in Alcator C-Mod
- 2019
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:9
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Journal article (peer-reviewed)abstract
- © 2019 IAEA, Vienna. This paper presents the first experimental analysis of polarized synchrotron emission from relativistic runaway electrons (REs) in a tokamak plasma. Importantly, we show that the polarization information of synchrotron radiation can be used to diagnose spatially-localized RE pitch angle distributions. Synchrotron-producing REs were generated during low density, Ohmic, diverted plasma discharges in the Alcator C-Mod tokamak. The ten-channel motional Stark effect diagnostic was used to measure spatial profiles of the polarization angle pol and the fraction fpol of detected light that was linearly-polarized. Spatial transitions in pol of 90-from horizontal to vertical polarization and vice versa-are observed in experimental data and are well-explained by the gyro-motion of REs and high directionality of synchrotron radiation. Polarized synchrotron emission is modeled with the synthetic diagnostic Soft; its output Green's (or detector response) functions reveal a critical RE pitch angle at which pol flips by 90° and fpol is minimal. Using Soft, we determine the dominant RE pitch angle which reproduces measured pol and fpol values. The spatiotemporal evolutions of pol and fpol are explored in detail for one C-Mod discharge. For channels viewing REs near the magnetic axis and flux surfaces q = 1 and 4/3, disagreements between synthetic and experimental signals suggest that the sawtooth instability may be influencing RE dynamics. Furthermore, other sources of pitch angle scattering, not considered in this analysis, could help explain discrepancies between simulation and experiment.
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| 31. |
- Tinguely, R. A., et al.
(author)
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Measurements of runaway electron synchrotron spectra at high magnetic fields in Alcator C-Mod
- 2018
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 58:7
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Journal article (peer-reviewed)abstract
- In the Alcator C-Mod tokamak, runaway electron (RE) experiments have been performed during low density, ?attop plasma discharges at three magnetic felds: 2.7, 5.4, and 7.8 T, the last being the highest feld to-date at which REs have been generated and measured in a tokamak. Time-evolving synchrotron radiation spectra were measured in the visible wavelength range (λ∼300-1000 nm) by two absolutely-calibrated spectrometers viewing co- and counter-plasma current directions. In this paper, a test particle model is implemented to predict momentum-space and density evolutions of REs on the magnetic axis and q = 1, 3/2, and 2 surfaces. Drift orbits and subsequent loss of confnement are also incorporated into the evolution. These spatiotemporal results are input into the new synthetic diagnostic SOFT (Hoppe et al 2018 Nucl. Fusion 58 026032) which reproduces experimentally-measured spectra. For these discharges, it is inferred that synchrotron radiation dominates collisional friction as a power loss mechanism and that RE energies decrease as magnetic feld is increased. Additionally, the threshold electric feld for RE generation, as determined by hard x-ray and photo-neutron measurements, is compared to current theoretical predictions.
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| 32. |
- Tinguely, R. A., et al.
(author)
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Modeling the complete prevention of disruption-generated runaway electron beam formation with a passive 3D coil in SPARC
- 2021
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 61:12
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Journal article (peer-reviewed)abstract
- The potential formation of multi-mega-ampere beams of relativistic 'runaway' electrons (REs) during sudden terminations of tokamak plasmas poses a significant challenge to the tokamak's development as a fusion energy source. Here, we use state-of-the-art modeling of disruption magnetohydrodynamics coupled with a self-consistent evolution of RE generation and transport to show that a non-axisymmetric in-vessel coil will passively prevent RE beam formation during disruptions in the SPARC tokamak, a compact, high-field, high-current device capable of achieving a fusion gain Q > 2 in deuterium-tritium plasmas.
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| 33. |
- Tinguely, R. A., et al.
(author)
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Spatiotemporal evolution of runaway electrons from synchrotron images in Alcator C-Mod
- 2018
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In: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 60:12
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Journal article (peer-reviewed)abstract
- In the Alcator C-Mod tokamak, relativistic runaway electron (RE) generation can occur during the flattop current phase of low density, diverted plasma discharges. Due to the high toroidal magnetic field (B-0 = 5.4 T), RE synchrotron radiation is measured by a wide-view camera in the visible wavelength range (lambda approximate to 400-900 nm). In this paper, a statistical analysis of over one thousand camera images is performed to investigate the plasma conditions under which synchrotron emission is observed in C-Mod. In addition, the spatiotemporal evolution of REs during one particular discharge is explored in detail via a thorough analysis of the distortion-corrected synchrotron images. To accurately predict RE energies, the kinetic solver CODE (Landreman et al 2014 Comput. Phys. Commun. 185 847-855) is used to evolve the electron momentum-space distribution at six locations throughout the plasma: the magnetic axis and flux surfaces q = 1, 4/3, 3/2, 2, and 3. These results, along with the experimentally-measured magnetic topology and camera geometry, are input into the synthetic diagnostic SOFT (Hoppe et al 2018 Nucl. Fusion 58 026032) to simulate synchrotron emission and detection. Interesting spatial structure near the surface q = 2 is found to coincide with the onset of a locked mode and increased MHD activity. Furthermore, the RE density profile evolution is fit by comparing experimental to synthetic images, providing important insight into RE spatiotemporal dynamics.
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| 34. |
- Tinguely, RA, et al.
(author)
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Synchrotron emission in Alcator C-Mod: spectra at three magnetic fields, visible camera images, and polarization data
- 2017
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In: 59th Annual Meeting of the APS Division of Plasma Physics. ; , s. JP11.00096-
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Conference paper (other academic/artistic)abstract
- Alcator C-Mod's high magnetic field allows runaway electron synchrotron emission to be observed in the visible wavelength range. Visible spectrometers were used to measure synchrotron spectra at three magnetic fields: 2.7, 5.4, and 7.8 T. Both a test-particle model and kinetic solver CODE (COllisional Distribution of Electrons) explore the energy evolution of the runaway population and the impact of magnetic-field-dependent synchrotron radiation as a power loss mechanism. Additionally, distortion-corrected visible camera images capture the spatial distribution and evolution of synchrotron emission in C-Mod. Initial results show good agreement between experiment and the new synthetic diagnostic SOFT (Synchrotron-detecting Orbit-Following Toolkit). Finally, a first look at synchrotron polarization data is presented.
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| 35. |
- Vallhagen, Oskar, 1997, et al.
(author)
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Effect of two-stage shattered pellet injection on tokamak disruptions
- 2022
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 62:11
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Journal article (peer-reviewed)abstract
- An effective disruption mitigation system in a tokamak reactor should limit the exposure of the wall to localized heat losses and to the impact of high current runaway electron beams, and avoid excessive forces on the structure. We evaluate with respect to these aspects a two-stage deuterium-neon shattered pellet injection in an ITER-like plasma, using simulations with the DREAM framework (Hoppe et al 2021 Comput. Phys. Commun. 268 108098). To minimize the obtained runaway currents an optimal range of injected deuterium quantities is found. This range is sensitive to the opacity of the plasma to Lyman radiation, which affects the ionization degree of deuterium, and thus avalanche runaway generation. The two-stage injection scheme, where dilution cooling is produced by deuterium before a radiative thermal quench caused by neon, reduces both the hot-tail seed and the localized transported heat load on the wall. However, during nuclear operation, additional runaway seed sources from the activated wall and tritium make it difficult to reach tolerably low runaway currents.
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| 36. |
- Wijkamp, T. A., et al.
(author)
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Tomographic reconstruction of the runaway distribution function in TCV using multispectral synchrotron images
- 2021
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 61:4
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Journal article (peer-reviewed)abstract
- Synchrotron radiation observed in a quiescent Tokamak Configuration Variable (TCV) runaway discharge is studied using filtered camera images targeting three distinct wavelength intervals. Through the tomographic simultaneous algebraic reconstruction technique (SART) procedure the high momentum, high pitch angle part of the spatial and momentum distribution of these relativistic particles is reconstructed. Experimental estimates of the distribution are important for verification and refinement of formation-, decay- and transport-models underlying runaway avoidance and mitigation strategy design. Using a test distribution it is demonstrated that the inversion procedure provides estimates accurate to within a few tens of percent in the region of phase-space contributing most to the synchrotron image. We find that combining images filtered around different parts of the emission spectrum widens the probed part of momentum-space and reduces reconstruction errors. Next, the SART algorithm is used to obtain information on the spatiotemporal runaway momentum distribution in a selected TCV discharge. The momentum distribution is found to relax towards an avalanche-like exponentially decaying profile. Anomalously high pitch angles and a radial profile increasing towards the edge are found for the most strongly emitting particles in the distribution.Pitch angle scattering by toroidal magnetic field ripple is consistent with this picture. An alternative explanation is the presence of high frequency instabilities in combination with the formation of a runaway shell at the edge of the plasma.
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