| 1. |
- Joffrin, E., et al.
(författare)
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Overview of the JET preparation for deuterium-tritium operation with the ITER like-wall
- 2019
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:11
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Forskningsöversikt (refereegranskat)abstract
- For the past several years, the JET scientific programme (Pamela et al 2007 Fusion Eng. Des. 82 590) has been engaged in a multi-campaign effort, including experiments in D, H and T, leading up to 2020 and the first experiments with 50%/50% D-T mixtures since 1997 and the first ever D-T plasmas with the ITER mix of plasma-facing component materials. For this purpose, a concerted physics and technology programme was launched with a view to prepare the D-T campaign (DTE2). This paper addresses the key elements developed by the JET programme directly contributing to the D-T preparation. This intense preparation includes the review of the physics basis for the D-T operational scenarios, including the fusion power predictions through first principle and integrated modelling, and the impact of isotopes in the operation and physics of D-T plasmas (thermal and particle transport, high confinement mode (H-mode) access, Be and W erosion, fuel recovery, etc). This effort also requires improving several aspects of plasma operation for DTE2, such as real time control schemes, heat load control, disruption avoidance and a mitigation system (including the installation of a new shattered pellet injector), novel ion cyclotron resonance heating schemes (such as the three-ions scheme), new diagnostics (neutron camera and spectrometer, active Alfven eigenmode antennas, neutral gauges, radiation hard imaging systems...) and the calibration of the JET neutron diagnostics at 14 MeV for accurate fusion power measurement. The active preparation of JET for the 2020 D-T campaign provides an incomparable source of information and a basis for the future D-T operation of ITER, and it is also foreseen that a large number of key physics issues will be addressed in support of burning plasmas.
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| 2. |
- 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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| 3. |
- Pusztai, Istvan, 1983, et al.
(författare)
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Can we study the transport of MeV ions without generating fusion alphas?
- 2015
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Ingår i: 57th Annual Meeting of the APS Division of Plasma Physics. ; 60:19, s. PP12.00093-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)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.
(författare)
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Turbulent transport of MeV range cyclotron heated minorities as compared to alpha particles
- 2016
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 58:10, s. 105001-
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Tidskriftsartikel (refereegranskat)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.
(författare)
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First principles of modelling the stabilization of microturbulence by fast ions
- 2018
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Ingår i: Nuclear Fusion. - : IOP PUBLISHING LTD. - 0029-5515 .- 1741-4326. ; 58:8
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Fundamental physics of the fast ion stabilization of electromagnetic ITG turbulence
- 2017
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Ingår i: 44th EPS Conference on Plasma Physics, EPS 2017.
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Konferensbidrag (refereegranskat)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.
(författare)
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Global anomalous transport of ICRH and NBI-heated fast ions
- 2017
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 59:4, s. 044007-
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Tidskriftsartikel (refereegranskat)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. |
- 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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| 9. |
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| 10. |
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| 11. |
- Hesslow, Linnea, 1993, et al.
(författare)
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Effect of partially ionized impurities and radiation on the effective critical electric field for runaway generation
- 2018
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 60:7
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Tidskriftsartikel (refereegranskat)abstract
- We derive a formula for the effective critical electric field for runaway generation and decay that accounts for the presence of partially ionized impurities in combination with synchrotron and bremsstrahlung radiation losses. We show that the effective critical field is drastically larger than the classical Connor-Hastie field, and even exceeds the value obtained by replacing the free electron density by the total electron density (including both free and bound electrons). Using a kinetic equation solver with an inductive electric field, we show that the runaway current decay after an impurity injection is expected to be linear in time and proportional to the effective critical electric field in highly inductive tokamak devices. This is relevant for the efficacy of mitigation strategies for runaway electrons since it reduces the required amount of injected impurities to achieve a certain current decay rate.
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| 12. |
- Hesslow, Linnea, 1993, et al.
(författare)
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Fast-electron dynamics in the presence of weakly ionized impurities
- 2017
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Ingår i: 44th EPS Conference on Plasma Physics, EPS 2017.
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Konferensbidrag (refereegranskat)abstract
- Runaway electrons constitute a significant threat to tokamak experiments. To minimize the risk of damage, it is crucial to understand the runaway-electron dynamics, which during runaway mitigation can be heavily influenced by the interaction with partially ionized atoms. Experiments have shown that mitigation via heavy-impurity injection is more effective than would be expected from standard collisional theory, highlighting the need for more accurate kinetic models. To achieve this, partial screening of the nuclei by the bound electrons must be taken into account. In this contribution, we analyze the dynamics of fast electrons in plasmas containing partially ionized impurity atoms. A generalized collision operator is derived from first principles using quantum-mechanical models. We obtain analytical expressions for the deflection and slowing-down frequencies, and show that they are increased by more than an order of magnitude compared to the results obtained with complete screening, already at sub-relativistic electron energies. Moreover, we implement the generalized collision operator in the continuum kinetic equation solver CODE and demonstrate that interaction with partially ionized atoms greatly affects fast-electron dynamics by enhancing the rates of angular deflection and energy loss. This has important implications, not only for the efficacy of mitigation strategies for runaway electrons in tokamak devices, but also for example for energy loss during relativistic breakdown in lightning discharges.
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| 13. |
- Hesslow, Linnea, 1993, et al.
(författare)
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Kinetic effects of partially screened impurities in runaway-electron mitigation scenarios
- 2017
-
Ingår i: International Sherwood Fusion Theory Conference, Annapolis, USA.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Runaway electrons constitute a significant threat to tokamak devices. Their mitigation by heavy-impurity injection has been experimentally shown to be more effective than would be expected from standard collisional theory [1]. In order to understand this effect and develop runaway mitigation schemes, more accurate kinetic models are needed to describe the interaction between electrons and partially ionized atoms. Such models require the partial screening of the nuclei by the bound electrons to be taken into account.In this contribution, we analyze the dynamics of fast electrons in plasmas containing partially ionized impurity atoms. A generalized collision operator is derived from first principles using quantum-mechanical models. We obtain analytical expressions for the deflection and slowing-down frequencies. Even at sub-relativistic energies, these are increased by more than an order of magnitude compared to the results obtained with complete screening. Moreover, we implement the generalized collision operator in the continuum kinetic-equation solver CODE [2, 3] and demonstrate that interaction with partially ionized atoms greatly affects fast-electron dynamics by enhancing the rates of angular deflection and energy loss. In particular, we investigate the decay of a runaway-electron current coupled to a self-consistent electric field. The effect of the interaction with partially ionized impurities has important implications for the efficacy of mitigation strategies for runaway electrons in tokamak devices.References[1] E. M. Hollmann et al., Physics of Plasmas 22, 056108 (2015).[2] M. Landreman, A. Stahl and T. Fülöp, Comp. Phys. Comm. 185, 847 (2014).[3] A. Stahl et al., Nuclear Fusion 56, 112009 (2016).
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| 14. |
- 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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| 15. |
- Paz-Soldan, C, et al.
(författare)
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Synchrotron and collisional damping effects on runaway electron distributions
- 2016
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Ingår i: 58th Annual Meeting of the APS Division of Plasma Physics. ; 61:18, s. CO4.00010 -
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Validated models of runaway electron (RE) dissipation are required to confidently approve safe ITER Q=10 operation. DIII-D experiments using quiescent REs are exploring the importance of synchrotron and collisional damping terms to RE dissipation. New time and energy-resolved measurements of RE bremsstrahlung hard X-ray (HXR) emission reveal stark differences between high and low energy REs as damping terms are varied. Previously reported anomalously high RE dissipation only applies to low energy REs. At high energy (where synchrotron effects are strongest) low synchrotron damping cases reach higher peak RE energy despite weaker particle confinement. Low-energy RE decay is observed concurrently with high-energy RE growth. RE dissipation models predict bump-on-tail distributions whose properties depend on the damping terms. Measured HXR spectra are very broad, as expected for bump-on-tail distributions.
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| 16. |
- Stahl, Adam, 1985, et al.
(författare)
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Self-consistent nonlinear kinetic modeling of runaway-electron dynamics
- 2017
-
Konferensbidrag (refereegranskat)abstract
- Runaway electrons represent the greatest threat to the plasma-facing components of atokamak when they are highly energetic and constitute a significant fraction of the electronpopulation, a regime which has not been previously accessible in modelling since it requiresa nonlinear relativistic treatment. To address this problem, we present an efficient numericaltool called NORSE for the study of runaway-electron momentum-space dynamics.The kinetic equation solved in NORSE includes a fully nonlinear relativistic collision operator, making it possible to consider scenarios where the electric field is comparableto the Dreicer field (or larger), or the electron distribution function is otherwise far froma Maxwellian (which can be the case already in present-day runaway experiments). Thiscapability makes NORSE unique in the field of runaway-electron studies.Using NORSE, we investigate the transition to a regime where the entire electronpopulation experiences continuous acceleration, so-called electron slide-away. Forthe first time, we apply a nonlinear kinetic-equation solver to study the evolution ofthe electron distribution in an ITER disruption. We use an electric field calculated self consistently,and show that the runaway-electron density becomes substantial, makingthe nonlinear treatment essential. In addition, we find that Ohmic heating and the rateof heat loss play an important role in determining the electron dynamics, with the latteraffecting the average energy reached by the runaways by several orders of magnitude.
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| 17. |
- Wilkie, George, 1983
(författare)
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Analytic slowing-down distributions as modified by turbulent transport
- 2018
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 84:6
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Tidskriftsartikel (refereegranskat)abstract
- The effect of electrostatic microturbulence on fast particles rapidly decreases at high energy, but can be significant at moderate energy. Previous studies found that, in addition to changes in the energetic particle density, this results in non-trivial changes to the equilibrium velocity distribution. These effects have implications for plasma heating and the stability of Alfven eigenmodes, but make multiscale simulations much more difficult without further approximations. Here, several related analytic model distribution functions are derived from first principles. A single dimensionless parameter characterizes the relative strength of turbulence relative to collisions, and this parameter appears as an exponent in the model distribution functions. Even the most simple of these models reproduces key features of the numerical phase-space transport solution and provides a useful a priori heuristic for determining how strong the effect of turbulence is on the redistribution of energetic particles in toroidal plasmas.
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| 18. |
- Wilkie, George, 1983, et al.
(författare)
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Fundamental form of the electrostatic δf-PIC algorithm and discovery of a converged numerical instability
- 2016
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 23:5, s. 052111-
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Tidskriftsartikel (refereegranskat)abstract
- The δf particle-in-cell algorithm has been a useful tool in studying the physics of plasmas, particularly turbulent magnetized plasmas in the context of gyrokinetics. The reduction in noise due to not having to resolve the full distribution function indicates an efficiency advantage over the standard (“full-f”) particle-in-cell. Despite its successes, the algorithm behaves strangely in some circumstances. In this work, we document a fully resolved numerical instability that occurs in the simplest of multiple-species test cases: the electrostatic ΩH mode. There is also a poorly understood numerical instability that occurs when one is under-resolved in particle number, which may require a prohibitively large number of particles to stabilize. Both of these are independent of the time-stepping scheme, and we conclude that they exist if the time advancement were exact. The exact analytic form of the algorithm is presented, and several schemes for mitigating these instabilities are also presented.
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| 19. |
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| 20. |
- Wilkie, George, 1983, et al.
(författare)
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Transport and deceleration of fusion products in microturbulence
- 2016
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 23:6, s. 060703-
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Tidskriftsartikel (refereegranskat)abstract
- The velocity-space distribution of alpha particles born in fusion devices is subject to modification at moderateenergies due to turbulent transport. Therefore, one must calculate the evolution of an equilibrium distributionwhose functional form is not known a priori. Using a novel technique, applicable to any trace impurity, wehave made this calculation for fully nonlinear gyrokinetic simulations not only possible, but particularlyefficient. We demonstrate a microturbulence-induced departure from the local slowing-down distribution, aninversion of the energy distribution, and associated modifications to the alpha heating and pressure profilesin an ITER-like scenario.
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| 21. |
- Wilkie, George, 1983, et al.
(författare)
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Validating modeling assumptions of alpha particles in electrostatic turbulence
- 2015
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 81:3
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Tidskriftsartikel (refereegranskat)abstract
- To rigorously model fast ions in fusion plasmas, a non-Maxwellian equilibrium distribution must be used. In this work, the response of high-energy alpha particles to electrostatic turbulence has been analyzed for several different tokamak parameters. Our results are consistent with known scalings and experimental evidence that alpha particles are generally well confined: on the order of several seconds. It is also confirmed that the effect of alphas on the turbulence is negligible at realistically low concentrations, consistent with linear theory. It is demonstrated that the usual practice of using a high-temperature Maxwellian, while previously shown to give an adequate order-of-magnitude estimate of the diffusion coefficient, gives incorrect estimates for the radial alpha particle flux, and a method of correcting it in general is provided. Furthermore, we see that the timescales associated with collisions and transport compete at moderate energies, calling into question the assumption that alpha particles remain confined to a flux surface that is used in the derivation of the slowing-down distribution.
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