| 1. |
- 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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| 2. |
- Chellaï, O., et al.
(författare)
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Millimeter-wave beam scattering and induced broadening by plasma turbulence in the TCV tokamak
- 2021
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 61:6
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Tidskriftsartikel (refereegranskat)abstract
- The scattering of millimeter-wave beams from electron density fluctuations and the associated beam broadening are experimentally demonstrated. Using a dedicated setup, instantaneous deflection and (de-)focusing of the beam due to density blobs on the beam path are shown to agree with full-wave simulations. The detected time-averaged wave power transmitted through the turbulent plasma is reproduced by the radiative-transfer model implemented in the WKBeam code, which predicts a ∼50% turbulence-induced broadening of the beam cross-section. The role of core turbulence for the considered geometry is highlighted.
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| 3. |
- Coda, S., et al.
(författare)
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Physics research on the TCV tokamak facility: From conventional to alternative scenarios and beyond
- 2019
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:11
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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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| 4. |
- Decker, Joan, 1977, et al.
(författare)
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Numerical characterization of bump formation in the runaway electron tail
- 2016
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 58:2, s. 025016-
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Tidskriftsartikel (refereegranskat)abstract
- Runaway electrons are generated in a magnetized plasma when the parallel electric field exceeds a critical value. For such electrons with energies typically reaching tens of MeV, the Abraham–Lorentz–Dirac (ALD) radiation force, in reaction to the synchrotron emission, is significant and can be the dominant process limiting electron acceleration. The effect of the ALD force on runaway electron dynamics in a homogeneous plasma is investigated using the relativistic finite-difference Fokker–Planck codes LUKE (Decker and Peysson 2004 Report EUR-CEA-FC-1736, Euratom-CEA), and CODE (Landreman et al 2014 Comput. Phys. Commun. 185 847). The time evolution of the distribution function is analyzed as a function of the relevant parameters: parallel electric field, background magnetic field, and effective charge. Under the action of the ALD force, we find that runaway electrons are subject to an energy limit, and that the electron distribution evolves towards a steady-state. In addition, a bump is formed in the tail of the electron distribution function if the electric field is sufficiently strong. The mechanisms leading to the bump formation and energy limit involve both the parallel and perpendicular momentum dynamics; they are described in detail. An estimate for the bump location in momentum space is derived. We observe that the energy of runaway electrons in the bump increases with the electric field amplitude, while the population increases with the bulk electron temperature. The presence of the bump divides the electron distribution into a runaway beam and a bulk population. This mechanism may give rise to beam-plasma types of instabilities that could, in turn, pump energy from runaway electrons and alter their confinement.
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| 5. |
- Embréus, Ola, 1991, et al.
(författare)
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Conservative large-angle collision operator for runaway avalanches
- 2015
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Ingår i: 57th Annual Meeting of the APS Division of Plasma Physics. ; 60:19, s. PP12.00107-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Avalanche runaway generation is the phenomenon whereby runaway electrons (REs) are generated due to large-angle collisions of thermal electrons with existing REs, leading to an exponential growth of the runaway current. These large-angle collisions are not described by the Fokker-Planck operator commonly employed to model collisions in plasmas, and have previously been accounted for by the addition of a particle source term in the kinetic equation [M. Rosenbluth et al., 1997, Nucl. Fusion 37, 1355; S. C. Chiu et al. 1998, Nucl. Fusion 38, 1711]. In this contribution we describe a new large-angle collision operator, derived as the high-energy limit of the linearized relativistic Boltzmann collision integral. This operator generalizes previous models of large-angle collisions to account for the full momentum dependence of the primary distribution and conserves particle number, momentum and energy, while also avoiding double counting of small- and large-angle collisions. The new operator is implemented in the 2D Fokker-Planck solver CODE [M. Landreman et al. 2014, Comp. Phys. Comm. 185, 847], with which we investigate its effect on the evolution of the runaway distribution.
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| 6. |
- Embréus, Ola, 1991, et al.
(författare)
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Dynamics of positrons during relativistic electron runaway
- 2018
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 84:5, s. 905840506-
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Tidskriftsartikel (refereegranskat)abstract
- Sufficiently strong electric fields in plasmas can accelerate charged particles to relativistic energies. In this paper we describe the dynamics of positrons accelerated in such electric fields, and calculate the fraction of created positrons that become runaway accelerated, along with the amount of radiation that they emit. We derive an analytical formula that shows the relative importance of the different positron production processes, and show that, above a certain threshold electric field, the pair production by photons is lower than that by collisions. We furthermore present analytical and numerical solutions to the positron kinetic equation; these are applied to calculate the fraction of positrons that become accelerated or thermalized, which enters into rate equations that describe the evolution of the density of the slow and fast positron populations. Finally, to indicate operational parameters required for positron detection during runaway in tokamak discharges, we give expressions for the parameter dependencies of detected annihilation radiation compared to bremsstrahlung detected at an angle perpendicular to the direction of runaway acceleration. Using the full leading-order pair-production cross-section, we demonstrate that previous related work has overestimated the collisional pair production by at least a factor of four.
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| 7. |
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| 8. |
- Embréus, Ola, 1991, et al.
(författare)
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Effect of bremsstrahlung radiation emission on fast electrons in plasmas
- 2016
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Ingår i: 43rd European Physical Society Conference on Plasma Physics, EPS 2016.
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Konferensbidrag (refereegranskat)abstract
- Bremsstrahlung radiation emission is an important energy loss mechanism for energetic electrons in plasmas. In this contribution we investigate the effect of spontaneous bremsstrahlung emission on the momentum-space structure of the electron distribution, using a Boltzmanntransport model fully accounting for the emission of finite-energy photons. We implement the model in a 2D continuum kinetic-equation solver, and study the solutions to determine the effect of bremsstrahlung on the electron distribution function. We find that electrons acceleratedby electric fields can reach significantly higher energies than predicted in previous work,which considered only the average energy loss of a test particle. We demonstrate that significantfractions of electrons reach twice the expected energy or more, due to the difference betweenthe average and Boltzmann model of bremsstrahlung radiation losses. Furthermore, we show that the emission of low-energy photons, which have previously been neglected because theydo not contribute to net energy loss, can contribute significantly to the dynamics of electrons with an anisotropic distribution by enhancing the angular-deflection rate.
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| 9. |
- Embréus, Ola, 1991, et al.
(författare)
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Effect of bremsstrahlung radiation emission on fast electrons in plasmas
- 2016
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 18:9, s. 093023-
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Tidskriftsartikel (refereegranskat)abstract
- Bremsstrahlung radiation emission is an important energy loss mechanism for energetic electrons in plasmas. In this paper we investigate the effect of spontaneous bremsstrahlung emission on the momentum-space structure of the electron distribution, fully accounting for the emission of finite-energy photons. We find that electrons accelerated by electric fields can reach significantly higher energies than what is expected from energy-loss considerations. Furthermore, we show that the emission of soft photons can contribute significantly to the dynamics of electrons with an anisotropic distribution.
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| 10. |
- Embréus, Ola, 1991
(författare)
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Kinetic modelling of runaway in plasmas
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The phenomenon of runaway occurs in plasmas in the presence of a strong electric field, which overcomes the collisional friction acting on the charged particles moving through the plasma. A subpopulation of particles can then be accelerated to energies significantly higher than the thermal energy. Such events are observed in both laboratory and space plasmas, and are of great importance in fusion-energy research, where highly energetic runaway electrons can damage the plasma-facing components of the reactor.In this thesis, a series of papers are presented which investigate various aspects of runaway dynamics. The emission of synchrotron and bremsstrahlung radiation are important energy-loss mechanisms for relativistic runaway electrons. Photons emitted in bremsstrahlung radiation often have energy comparable to the energetic electrons, and we therefore use a Boltzmann transport equation in order to describe their effect on the electron motion. This treatment reveals that electrons can reach significantly higher energies than previously thought. In comparison, synchrotron radiation has lower frequency, and is well described by the classical electromagnetic radiation-reaction force. This loss mechanism, often dominant in laboratory plasmas, significantly alters the electron dynamics, and is found to produce non-monotonic features in the runaway tail.A study is also presented of the related phenomenon of ion runaway acceleration, which differs from electron runaway due to their larger mass. Renewed interest in this topic has been sparked after recent observations of fast ions in various experiments. Finally a new method is explored to treat the non-linear Fokker-Planck equation which is commonly used to describe the collisional dynamics in a plasma. The new method is appealing for its physically intuitive description and analytic simplicity.
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