| 1. |
- Fülöp, Tünde, 1970, et al.
(author)
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Kinetic modelling of runaways in fusion plasmas
- 2016
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In: Proceedings of 26th IAEA Fusion Energy Conference, Kyoto, Japan. ; , s. TH/P4-1
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Conference paper (other academic/artistic)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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| 2. |
- Joffrin, E., et al.
(author)
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Overview of the JET preparation for deuterium-tritium operation with the ITER like-wall
- 2019
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:11
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Research review (peer-reviewed)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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| 3. |
- Pusztai, Istvan, 1983, et al.
(author)
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Can we study the transport of MeV ions without generating fusion alphas?
- 2015
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In: 57th Annual Meeting of the APS Division of Plasma Physics. ; 60:19, s. PP12.00093-
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Conference paper (other academic/artistic)abstract
- The novel ion cyclotron resonance heating method, utilizing three ion species, allows the generation of energetic trace minorities in the MeV range [Ye. O. Kazakov et al., 2015, Nucl. Fusion 55, 032001]. We survey which aspects of alpha particle transport may be accessed experimentally without D-T operation, such as during the non-activated phase of ITER, through a numerical investigation of the transport properties of RF heated 3He resonant ions in a 4He−H mixture plasma. The turbulent transport is simulated using the recently developed version of the gyrokinetic code GS2 that can handle strongly non-Maxwellian species [G. J. Wilkie et al., 2015, J. Plasma Phys. 81, 905810306], while the collisional transport is studied taking the temperature anisotropy of the 3He species into account.
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| 4. |
- Pusztai, Istvan, 1983, et al.
(author)
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Turbulent transport of MeV range cyclotron heated minorities as compared to alpha particles
- 2016
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In: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 58:10, s. 105001-
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Journal article (peer-reviewed)abstract
- We study the turbulent transport of an ion cyclotron resonance heated (ICRH), MeV range minority ion species in tokamak plasmas. Such highly energetic minorities, which can be produced in the three ion minority heating scheme (Kazakov et al (2015) Nucl. Fusion 55 032001), have been proposed to be used to experimentally study the confinement properties of fast ions without the generation of fusion alphas. We compare the turbulent transport properties of ICRH ions with that of fusion born alpha particles. Our theoretical predictions indicate that care must be taken when conclusions are drawn from experimental results: while the effect of turbulence on these particles is similar in terms of transport coefficients, differences in their distribution functions — ultimately their generation processes — make the resulting turbulent fluxes different.
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| 5. |
- Wilkie, George, 1983, et al.
(author)
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First principles of modelling the stabilization of microturbulence by fast ions
- 2018
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In: Nuclear Fusion. - : IOP PUBLISHING LTD. - 0029-5515 .- 1741-4326. ; 58:8
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Journal article (peer-reviewed)abstract
- The observation that fast ions stabilize ion-temperature-gradient-driven microturbulence has profound implications for future fusion reactors. It is also important in optimizing the performance of present-day devices. In this work, we examine in detail the phenomenology of fast ion stabilization and present a reduced model which describes this effect. This model is derived from the high-energy limit of the gyrokinetic equation and extends the existing 'dilution' model to account for nontrivial fast ion kinetics. Our model provides a physically-transparent explanation for the observed stabilization and makes several key qualitative predictions. Firstly, that different classes of fast ions, depending on their radial density or temperature variation, have different stabilizing properties. Secondly, that zonal flows are an important ingredient in this effect precisely because the fast ion zonal response is negligible. Finally, that in the limit of highly-energetic fast ions, their response approaches that of the 'dilution' model; in particular, alpha particles are expected to have little, if any, stabilizing effect on plasma turbulence. We support these conclusions through detailed linear and nonlinear gyrokinetic simulations.
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| 6. |
- Wilkie, George, 1983, et al.
(author)
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Fundamental physics of the fast ion stabilization of electromagnetic ITG turbulence
- 2017
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In: 44th EPS Conference on Plasma Physics, EPS 2017.
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Conference paper (peer-reviewed)abstract
- In recent years, it has been observed that both electromagnetic effects and fast particle populations suppress transport from ITG turbulence. This effect was discovered via detailed numerical simulations of JET discharges. Further work has investigated these effects in the context of experimental scenarios, but the underlying physics remains somewhat unresolved. However, in pursuit of increased performance, experiments will continue to push to ever-higher beta. Similarly, burning plasmas will always have self-generated fast ion populations. Thus, understanding the physics behind this suppression is key to projecting its importance for future devices. Our analysis of the physical mechanisms comprises two parts: a study of the linear physics, and targeted nonlinear simulations. Firstly, an in-depth study of the linear physics is performed to disentangle the competing effects upon the ITG mode. These effects include dilution of the main ions by fast ions, changes to the magnetic equilibrium, and changes to the pressure gradients in the plasma. To clarify these results we derive a simplified dispersion relation for electromagnetic ITG including a fast ion population, and use it to demonstrate which parameters dominate the linear physics. Guided by our linear results, we use nonlinear simulations to examine the structure of the turbulence when stabilized by fast ions. Through this study, we show which effects lead to a reduction of stiffness, and why. We also explore which effects lead to changes in the nonlinear upshift of the critical temperature gradient. We enumerate which of these physical mechanisms contribute to the experimentally-observed reduction in heat flux. Given this physical understanding, we identify which class of fast ions contribute most beneficially to this reduction and the conditions under which the electromagnetic stabilization is most effective. We conclude by extrapolating these results towards ITER and DEMO.
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| 7. |
- Wilkie, George, 1983, et al.
(author)
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Global anomalous transport of ICRH and NBI-heated fast ions
- 2017
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In: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 59:4, s. 044007-
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Journal article (peer-reviewed)abstract
- By taking advantage of the trace approximation, one can gain an enormous computational advantage when solving for the global turbulent transport of impurities.In particular, this makes feasible the study of non-Maxwellian transport coupled in radius and energy, allowing collisions and transport to be accounted for on similar time scales, as occurs for fast ions. In this work, we study the fully-nonlinear ITG-driven trace turbulent transport of locally heated and injected fast ions. Previousresults indicated the existence of MeV-range minorities heated by cyclotron resonance,and an associated density pinch e ect. Here, we build upon this result using the t3core code to solve for the distribution of these minorities, consistently including the e ects of collisions, gyrokinetic turbulence, and heating. Using the same tool to study the transport of injected fast ions, we contrast the qualitative features of their transport with that of the heated minorities. Furthermore, we move beyond the trace approximation to develop a model which allows one to easily account for the reduction of anomalous transport due to the presence of fast ions in electrostatic turbulence.
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| 8. |
- Berger, Esmée, 1998, et al.
(author)
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Runaway dynamics in reactor-scale spherical tokamak disruptions
- 2022
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In: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 88:6
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Journal article (peer-reviewed)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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| 9. |
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| 10. |
- Buller, Stefan, 1991, et al.
(author)
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Collisional transport of impurities with flux-surface varying density in stellarators
- 2018
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In: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 84:4
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Journal article (peer-reviewed)abstract
- High-Z impurities in magnetic-confinement devices are prone to develop density variations on the flux surface, which can significantly affect their transport. In this paper, we generalize earlier analytic stellarator calculations of the neoclassical radial impurity flux in the mixed-collisionality regime (collisional impurities and low-collisionality bulk ions) to include the effect of such flux-surface variations. We find that only in the homogeneous density case is the transport of highly collisional impurities (in the Pfirsch-Schlhter regime) independent of the radial electric field. We study these effects for a Wendelstein 7-X (W7-X) vacuum field, with simple analytic models for the potential perturbation, under the assumption that the impurity density is given by a Boltzmann response to a perturbed potential. In the W7-X case studied, we find that larger amplitude potential perturbations cause the radial electric field to dominate the transport of the impurities. In addition, we find that classical impurity transport can be larger than the neoclassical transport in W7-X.
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