| 1. |
- 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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| 2. |
- 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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| 3. |
- Cecconello, Marco, et al.
(författare)
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Conceptual design of a collimated neutron flux monitor and spectrometer for DTT
- 2021
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Ingår i: Fusion engineering and design. - : Elsevier. - 0920-3796 .- 1873-7196. ; 167
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Tidskriftsartikel (refereegranskat)abstract
- A conceptual design and performance studies for a collimated neutron flux monitor and neutron spectrometer for the Divertor Tokamak Test (DTT) facility are presented. This study is based on the single-null divertor configuration and for “Half Power” and “Full power” scenarios with 15 MW of negative-ion NBI, 29 MW of ECH and 3 MW of ICRF heating with a maximum neutron yield of 1.5 × 1017 s−1. Fast ion distributions (both from auxiliary heating systems and fusion born) have been simulated in TRANSP/NUBEAM and the corresponding neutron energy spectra have been calculated using DRESS. Synthetic diagnostics have been implemented to determine the neutron fluxes and spectra at the detector location. Neutron emissivity profiles, plasma position, core ion temperature and the ratio of thermal and non-thermal D ion populations can be obtained with good accuracy and time resolution.
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| 4. |
- Cecconello, Marco, et al.
(författare)
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First observations of confined fast ions in MAST Upgrade with an upgraded neutron camera
- 2023
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Ingår i: Plasma Physics and Controlled Fusion. - : Institute of Physics Publishing (IOPP). - 0741-3335 .- 1361-6587. ; 65:3
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Tidskriftsartikel (refereegranskat)abstract
- Spherical tokamaks are key to the successful design of operating scenarios of future fusion reactors in the areas of divertor physics, neutral beam current drive and fast ion physics. MAST Upgrade, which has successfully concluded its first experimental campaign, was specifically designed to address the role of the radial gradient of the fast ion distribution in driving the excitation of magneto-hydrodynamic (MHD) instabilities, such as toroidal Alfven eigenmodes, fish-bones and long-lived mode, thanks to its two tangential neutral beam injection systems, one on the equatorial plane and one that is vertically shifted 65 cm above the equatorial plane. To study the fast ion dynamics in the presence of such instabilities, as well as of sawteeth and neo-classical tearing modes, several fast ion diagnostics were upgraded and new ones added. Among them, the MAST prototype neutron camera (NC) has been upgraded to six, equatorial sight-lines. The first observations of the confined fast ion behavior with the upgraded NC in a wide range of plasma scenarios characterized by on-axis and/or off-axis heating and different MHD instabilities are presented here. The observations presented in this study confirm previous results on MAST but with a higher level of detail and highlight new physics observations unique to the MAST Upgrade. The results presented here confirm the improved performance of the NC Upgrade, which thus becomes one of the key elements, in combination with the rich set of fast ion diagnostics available on the MAST Upgrade, for a more constrained modeling of the fast ion dynamics in fusion reactor relevant scenarios.
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| 5. |
- Cecconello, Marco, et al.
(författare)
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Study of the effect of sawteeth on fast ions and neutron emission in MAST using a neutron camera
- 2018
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 60:5
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Tidskriftsartikel (refereegranskat)abstract
- The effect of the sawtooth instability on the confinement of fast ions on MAST, and the impact it has on the neutron emission, has been studied in detail using the TRANSP/NUBEAM codes coupled to a full orbit following code. The sawtooth models in TRANSP/NUBEAM indicate that, on MAST, passing and trapped fast ions are redistributed in approximately equal number and on a level that is consistent with the observations. It has not been possible to discriminate between the different sawtooth models since their predictions are all compatible with the neutron camera observations. Full orbit calculations of the fast ion motion have been used to estimate the characteristic time scales and energy thresholds that according to theoretical predictions govern the fast ions redistribution: no energy threshold for the redistribution for either passing and trapped fast ions was found. The characteristic times have, however, frequencies that are comparable with the frequencies of a m = 1, n = 1 perturbation and its harmonics with toroidal mode numbers n = 2, ..., 4, suggesting that on spherical tokamaks, in addition to the classical sawtooth-induced transport mechanisms of fast ions by attachment to the evolving perturbation and the associated E x B drift, a resonance mechanism between the m = 1 perturbation and the fast ions orbits might be at play.
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| 6. |
- Cecconello, Marco, et al.
(författare)
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The neutron camera upgrade for MAST Upgrade
- 2018
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Ingår i: Review of Scientific Instruments. - : AMER INST PHYSICS. - 0034-6748 .- 1089-7623. ; 89:10
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Tidskriftsartikel (refereegranskat)abstract
- The Neutron Camera Upgrade (NCU) is a neutron flux monitor consisting of six lines of sight (LoSs) under installation on Mega Ampere Spherical Tokamak (MAST) Upgrade. The NCU is expected to contribute to the study of the confinement of fast ions and on the efficiency of non-inductive current drive in the presence of on-axis and off-axis neutral beam injection by measuring the neutron emissivity profile along the equatorial plane. This paper discusses the NCU main design criteria, the engineering and interfacing issues, and the solutions adopted. In addition, the results from the characterization and performance studies of the neutron detectors using standard gamma-rays sources and a Cf-252 source are discussed. The proposed design has a time resolution of 1 ms with a statistical uncertainty of less than 10% for all MAST Upgrade scenarios with a spatial resolution of 10 cm: higher spatial resolution is possible by moving the LoSs in-between plasma discharges. The energy resolution of the neutron detector is better than 10% for a light output of 0.8 MeVee, and the measured pulse shape discrimination is satisfactory.
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| 7. |
- Sperduti, Andrea, 1991-
(författare)
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Experimental methods of neutron diagnostics and fast ion physics for fusion devices
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Measurements of the neutron emission resulting from nuclear fusion reactions provide an abundance of information on the underlying spatial, temporal and energetic distributions of reacting ions and how they are affected by a wide range of MagnetoHydroDynamic (MHD) instabilities.This thesis focuses on studies of the neutron emission and fast ion physics at the Mega Ampere Spherical Tokamak (MAST), its upgrade MAST-U, the Joint European Torus (JET) and the Divertor Tokamak Test (DTT). In particular, measurements and simulations of neutron emissivity and neutron rates by collimated neutron flux monitor are here discussed and applied to study the properties of the plasma and of the fast ion distribution.The first part of the thesis describes plasma measurement methods based on neutron diagnostics. In particular, the design of the neutron camera upgrade on MAST-U is here presented and possible outcomes from its future measurements are discussed. MAST and MAST-U, due to their low plasma temperature, are suitable for fast ion studies and in order to relate neutron measurements with the fast ion distributions the weight functions of the neutron camera on MAST are presented. Fast ions behaviour will be studied as well on DTT where the presence of a collimated neutron flux monitor and a Time Of Flight system is envisaged. Their conceptual designs are presented here. Finally, a novel application of neutron flux monitor for measuring the plasma position is discussed and its application on JET is described here. The second part of the thesis introduces the problem with the “neutron deficit” observed on MAST and the approaches used for its resolution such as the Influence Method and the effect of the Guiding Center and Gyro-Orbit modelling on MAST. The forward modelling based on ASCOT/BBNBI, LINE21, DRESS, NRESP has been compared against the same one but based on TRANSP/NUBEAM simulations. The first modelling was used to validate neutron camera measurements on MAST reducing the observed discrepancy to values within the estimated experimental uncertainties, while the second one was used to benchmark DRESS.
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| 8. |
- Sperduti, Andrea, et al.
(författare)
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Neutron rate estimates in MAST based on gyro-orbit modelling of fast ions
- 2021
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Ingår i: Nuclear Fusion. - : Institute of Physics Publishing (IOPP). - 0029-5515 .- 1741-4326. ; 61:1
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Tidskriftsartikel (refereegranskat)abstract
- A discrepancy between predicted and measured neutron rates on MAST using TRANSP/NUBEAM has previously been observed and a correction factor of about 0.6 was needed to match the two: this correction factor could not be accounted for by the experimental uncertainties in the plasma kinetic profiles nor in the NBI energy and power (Cecconello et al 2019 Nucl. Fusion 59 016006). Further causes of this discrepancy are here studied by means of TRANSP/NUBEAM and ASCOT/BBNBI simulations. Different equilibria, toroidal field ripples, uncertainties on the NBI divergence value and gyro-orbit effects were studied and simulations were performed with both transport codes. It was found that the first three effects accounted for only a 5% variation in the fast ion density. On the other hand, full gyro-orbit simulations of the fast ions dynamics carried out in ASCOT/BBNBI resulted in an approximately 20% reduction of the fast ion population compared to TRANSP/NUBEAM. A detailed analysis of the fast ion distributions showed how the drop occurred regardless of the energy at pitch values <=-0.4. The DRESS code was then used to calculate the neutron rate at the neutron camera detector's location showing that the discrepancy is considerably reduced when the full gyro-orbit fast ion distribution is used, with now the correction factor, used to match experimental and predicted neutron rates, being around 0.9.
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| 9. |
- Sperduti, Andrea, et al.
(författare)
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On the limits of the Influence Method in the determination of the intrinsic efficiency of liquid scintillators
- 2019
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Ingår i: Journal of Instrumentation. - : IOP PUBLISHING LTD. - 1748-0221 .- 1748-0221. ; 14
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Tidskriftsartikel (refereegranskat)abstract
- The limitations of the Influence Method for the determination of the intrinsic efficiency of a EJ-301 liquid scintillator to gamma-rays and neutrons in the MeV range is presented. Discrepancies between the intrinsic efficiencies, measured according to the Influence Method, and their accepted values are reported and investigated using MCNP6. It was found that this discrepancy is caused by multiple scattering of both gamma-rays and neutrons in the detector, effects which are not properly accounted for in the Influence Method. Correction factors have been calculated in MCNP6 for different detector geometries and different gamma-ray and neutron energies. A simple correction factor is derived, which can be used to calculate the correct intrinsic efficiency for similar detectors using the Influence Method without having to recur to Monte Carlo simulation.
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| 10. |
- Sperduti, Andrea, et al.
(författare)
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Plasma position measurement with collimated neutron flux monitor diagnostics on JET
- 2021
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Ingår i: Fusion engineering and design. - : Elsevier. - 0920-3796 .- 1873-7196. ; 168
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Tidskriftsartikel (refereegranskat)abstract
- In future burning plasma devices measuring the plasma position and its shape with great spatial and temporal resolution is a crucial task. Nowadays these information are obtained by means of magnetic coils installed inside the vacuum vessel that in the future devices (like ITER and DEMO), due to the harsh environment caused by the high plasma temperature, will experience degradation over the time. Furthermore, the long plasma discharges will result in large integration drift causing a lack of accuracy in the measured signal. In order to assist the magnetic diagnostics and at the same time provide a novel tool to benchmark them, here the measurement of the plasma magnetic axis position by means of a collimated neutron flux monitor is proposed. Three different methods are here described and applied on JET by means of the neutron camera: a weighted average, the asymmetry method and a neural network. The methods are calibrated on a large database of plasma discharges including NBI and ICRH heated ones, and then compared with the magnetic axis position reconstructed by EFIT. The neural network outperforms the two other methods. In particular, the asymmetry method results to be sensitive to MHD activity, NBI power variation and to neutron emissivity profiles presenting a strong asymmetry, such as in case of peripheral NBI deposition due to high density plasmas or ICRH resonance layer. A possible application to vertical displacement events and disruptions is discussed and envisaged for future applications on DEMO. Finally, the performances of the neural network and of the asymmetry methods are studied in the case of one or two missing channels in the neutron flux monitor, showing how in general the reconstruction of the radial magnetic axis in both methods is more sensitive to the lack of channels than the vertical one. The methods here proposed can be applied and benchmarked on DTT and ITER neutron cameras as part of a future real-time control system.
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