| 1. |
- Berger, Esmée, 1998, et al.
(författare)
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Runaway dynamics in reactor-scale spherical tokamak disruptions
- 2022
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 88:6
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Tidskriftsartikel (refereegranskat)abstract
- Understanding generation and mitigation of runaway electrons in disruptions is important for the safe operation of future tokamaks. In this paper we investigate the runaway dynamics in reactor-scale spherical tokamaks, focusing on a compact nominal design with a plasma current of 21 megaamperes (MA), 1.8 T magnetic field on axis and major radius of approximately 3 m. We study both the severity of runaway generation during unmitigated disruptions, and the effect that typical mitigation schemes based on massive material injection have on runaway production. The study is conducted using the numerical framework DREAM (Disruption Runaway Electron Analysis Model). We find that, in many cases, mitigation strategies are necessary to prevent the runaway current from reaching multi-MA levels. Our results indicate that, with a suitably chosen deuterium–neon mixture for mitigation, it is possible to achieve a tolerable runaway current and ohmic current evolution. However, this does not account for the runaway source due to wall activation, which has been found to severely limit successful mitigation at conventional aspect ratios, but whose definition requires a more complete wall specification. Furthermore, the majority of the thermal energy loss is found to happen through radial transport rather than radiation, which poses a risk of unacceptable localised heat loads.
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| 2. |
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| 3. |
- Buller, Stefan, 1991, et al.
(författare)
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Neoclassical flows in deuterium-helium plasma density pedestals
- 2017
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 59:5, s. 055019-
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Tidskriftsartikel (refereegranskat)abstract
- In tokamak transport barriers, the radial scale of profile variations can be comparable to a typical ion orbit width, which makes the coupling of the distribution function across flux surfaces important in the collisional dynamics. We use the radially global steady-state neoclassical delta f code PERFECT [Landreman et al 2014 Plasma Phys. Control. Fusion 56 045005] to calculate poloidal and toroidal flows, and radial fluxes, in the pedestal. In particular, we have studied the changes in these quantities as the plasma composition is changed from a deuterium bulk species with a helium impurity to a helium bulk with a deuterium impurity, under specific profile similarity assumptions. In the presence of sharp profile variations, the poloidally resolved radial fluxes are important for the total fluxes to be divergence-free, which leads to the appearance of poloidal return-flows. These flows exhibit a complex radial–poloidal structure that extends several orbit widths into the core and is sensitive to abrupt radial changes in the ion temperature gradient. We find that a sizable neoclassical toroidal angular momentum transport can arise in the radially global theory, in contrast to the local.
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| 4. |
- Buller, Stefan, 1991, et al.
(författare)
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Neoclassical study of the isotope effect in density pedestals
- 2017
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Ingår i: 44th EPS Conference on Plasma Physics, EPS 2017. - 9781510849303 ; part 2, s. 841- 844
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Konferensbidrag (refereegranskat)abstract
- The isotope mass scaling of the energy confinement time in tokamak plasmas typically differs from gyro-Bohm estimates. This phenomenon – known as the isotope effect – remains an open issue in plasma physics, with important implications for the extrapolation from present day, mostly deuterium (D), fusion experiments to future deuterium-tritium (D-T) reactors. Differences in mass scaling in L-mode and various H-mode regimes suggest that the isotope effect may, in large part, originate from the pedestal. In the pedestal, sharp gradients render local diffusive estimates invalid, and global effects due to orbit-width scale profile variations have to be taken into account, potentially leading to mass scalings different from gyro-Bohm. We calculate cross-field fluxes from a radially-global linearized drift-kinetic equation using the PERFECT code, to study isotope composition effects in density pedestals. We define dimensionless parameters from the ratios of density length scale, pedestal width and orbit width, and study global effects in terms of these parameters for different pedestal profiles and bulk species. Quantifying global effects by the relative difference between peak heat-flux values in global and local simulations, we find that this quantity saturates at an isotope-dependent value, but the dimensionless parameters do not capture all the isotope dependencies. We also consider D-T and H-D mixtures, and compare the calculated heat fluxes to fluxes calculated from single species simulations with artificial "DT" and "HD" species.
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| 5. |
- Buller, Stefan, 1991, et al.
(författare)
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Optimization of flux-surface density variation in stellarator plasmas with respect to the transport of collisional impurities
- 2019
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 59:6
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Tidskriftsartikel (refereegranskat)abstract
- Avoiding impurity accumulation is a requirement for steady-state stellarator operation. The accumulation of impurities can be heavily affected by variations in their density on the flux-surface. Using recently derived semi-analytic expressions for the transport of a collisional impurity species with high-Z and flux-surface density-variation in the presence of a low-collisionality bulk ion species, we numerically optimize the impurity density-variation on the flux-surface to minimize the radial peaking factor of the impurities. These optimized density-variations can reduce the core impurity density by 0.75^Z (with Z the impurity charge number) in the large helical device case considered here, and by 0.89^Z in a Wendelstein 7-X standard configuration case. On the other hand, when the same procedure is used to find density-variations that maximize the peaking factor, it is notably increased compared to the case with no density-variation. This highlights the potential importance of measuring and controlling these variations in experiments.
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| 6. |
- Embréus, Ola, 1991, et al.
(författare)
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Numerical calculation of ion runaway distributions
- 2015
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Ingår i: 42nd European Physical Society Conference on Plasma Physics, EPS 2015.
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Konferensbidrag (refereegranskat)abstract
- Numerical solver of the 2D ion Fokker-Planck equation has been presented, and its usefulness demonstrated in investigating the conditions required for ion runaway in cold and hot tokamak plasmas. It is shown that Alfvénic activity observed in disruption experiments are unlikely to be explained by the runaway mechanism alone.
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| 7. |
- Embréus, Ola, 1991, et al.
(författare)
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Numerical calculation of ion runaway distributions
- 2015
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 22:5, s. 052122-
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Tidskriftsartikel (refereegranskat)abstract
- Ions accelerated by electric fields (so-called runaway ions) in plasmas may explain observations in solar flares and fusion experiments; however, limitations of previous analytic work have prevented definite conclusions. In this work, we describe a numerical solver of the 2D non-relativistic linearized Fokker-Planck equation for ions. It solves the initial value problem in velocity space with a spectral-Eulerian discretization scheme, allowing arbitrary plasma composition and time-varying electric fields and background plasma parameters. The numerical ion distribution function is then used to consider the conditions for runaway ion acceleration in solar flares and tokamak plasmas. Typical time scales and electric fields required for ion acceleration are determined for various plasma compositions, ion species, and temperatures, and the potential for excitation of toroidal Alfvén eigenmodes during tokamak disruptions is considered.
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| 8. |
- Fülöp, Tünde, 1970, et al.
(författare)
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Alfvénic instabilities driven by runaways in fusion plasmas
- 2014
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 21:8, s. 080702-
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Tidskriftsartikel (refereegranskat)abstract
- Runaway particles can be produced in plasmas with large electric fields. Here, we address the possibility that such runaway ions and electrons excite Alfvénic instabilities. The magnetic perturbation induced by these modes can enhance the loss of runaways. This may have important implications for the runaway electron beam formation in tokamak disruptions.
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| 9. |
- Fülöp, Tünde, 1970, et al.
(författare)
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Kinetic modelling of runaways in fusion plasmas
- 2016
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Ingår i: Proceedings of 26th IAEA Fusion Energy Conference, Kyoto, Japan. ; , s. TH/P4-1
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Mitigation of runaway electrons is one of the outstanding issues for a reliable operationof ITER and other large tokamaks. To achieve this, quantitatively accurate estimatesfor the expected runaway electron energies and current are needed. In this work we de-scribe an accurate theoretical framework for studying the effects of collisional nonlinear-ities, bremsstrahlung and synchrotron radiation emission, and knock-on collisions on therunaway electron distribution. We outline the identification of significant features of run-away electron behaviour enabled by this framework and their potential to affect the growthof a runaway population.
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| 10. |
- Harrison, J.R., et al.
(författare)
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Overview of new MAST physics in anticipation of first results from MAST Upgrade
- 2019
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:11
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Forskningsöversikt (refereegranskat)abstract
- The mega amp spherical tokamak (MAST) was a low aspect ratio device (R/a = 0.85/0.65 ∼ 1.3) with similar poloidal cross-section to other medium-size tokamaks. The physics programme concentrates on addressing key physics issues for the operation of ITER, design of DEMO and future spherical tokamaks by utilising high resolution diagnostic measurements closely coupled with theory and modelling to significantly advance our understanding. An empirical scaling of the energy confinement time that favours higher power, lower collisionality devices is consistent with gyrokinetic modelling of electron scale turbulence. Measurements of ion scale turbulence with beam emission spectroscopy and gyrokinetic modelling in up-down symmetric plasmas find that the symmetry of the turbulence is broken by flow shear. Near the non-linear stability threshold, flow shear tilts the density fluctuation correlation function and skews the fluctuation amplitude distribution. Results from fast particle physics studies include the observation that sawteeth are found to redistribute passing and trapped fast particles injected from neutral beam injectors in equal measure, suggesting that resonances between the m = 1 perturbation and the fast ion orbits may be playing a dominant role in the fast ion transport. Measured D-D fusion products from a neutron camera and a charged fusion product detector are 40% lower than predictions from TRANSP/NUBEAM, highlighting possible deficiencies in the guiding centre approximation. Modelling of fast ion losses in the presence of resonant magnetic perturbations (RMPs) can reproduce trends observed in experiments when the plasma response and charge-exchange losses are accounted for. Measurements with a neutral particle analyser during merging-compression start-up indicate the acceleration of ions and electrons. Transport at the plasma edge has been improved through reciprocating probe measurements that have characterised a geodesic acoustic mode at the edge of an ohmic L-mode plasma and particle-in-cell modelling has improved the interpretation of plasma potential estimates from ball-pen probes. The application of RMPs leads to a reduction in particle confinement in L-mode and H-mode and an increase in the core ionization source. The ejection of secondary filaments following type-I ELMs correlates with interactions with surfaces near the X-point. Simulations of the interaction between pairs of filaments in the scrape-off layer suggest this results in modest changes to their velocity, and in most cases can be treated as moving independently. A stochastic model of scrape-off layer profile formation based on the superposition of non-interacting filaments is in good agreement with measured time-average profiles. Transport in the divertor has been improved through fast camera imaging, indicating the presence of a quiescent region devoid of filament near the X-point, extending from the separatrix to ψ n ∼ 1.02. Simulations of turbulent transport in the divertor show that the angle between the divertor leg on the curvature vector strongly influences transport into the private flux region via the interchange mechanism. Coherence imaging measurements show counter-streaming flows of impurities due to gas puffing increasing the pressure on field lines where the gas is ionised. MAST Upgrade is based on the original MAST device, with substantially improved capabilities to operate with a Super-X divertor to test extended divertor leg concepts. SOLPS-ITER modelling predicts the detachment threshold will be reduced by more than a factor of 2, in terms of upstream density, in the Super-X compared with a conventional configuration and that the radiation front movement is passively stabilised before it reaches the X-point. 1D fluid modelling reveals the key role of momentum and power loss mechanisms in governing detachment onset and evolution. Analytic modelling indicates that long legs placed at large major radius, or equivalently low at the target compared with the X-point are more amenable to external control. With MAST Upgrade experiments expected in 2019, a thorough characterisation of the sources of the intrinsic error field has been carried out and a mitigation strategy developed.
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