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Overview of new MAST physics in anticipation of first results from MAST upgrade.

Harrison, J.R. and Akers, R.J. and Allan, S.Y. and Allcock, J.S. and Allen, J.O. and Appel, L. and Barnes, M. and Ben Ayed, N. and Boeglin, W. and Bowman, C. and Bradley, J. and Browning, P. and Bryant, P. and Carr, M. and Cecconello, M. and Challis, C.D. and Chapman, S. and Chapman, I.T. and Colyer, G.J. and Conroy, S. and Conway, N.J. and Cox, M. and Cunningham, G. and Dendy, R.O. and Dorland, W. and Dudson, B.D. and Easy, L. and Elmore, S.D. and Farley, T. and Feng, X. and Field, A.R. and Fil, A. and Fishpool, G.M. and Fitzgerald, M. and Flesch, K. and Fox, M.F.J. and Frerichs, H. and Gadgil, S. and Gahle, D. and Garzotti, L. and Ghim, Y. -C. and Gibson, S. and Gibson, K.J. and Hall, S. and Ham, C. and Heiberg, N. and Henderson, S.S. and Highcock, E. and Hnat, B. and Howard, J. and Huang, J. and Irvine, S.W.A. and Jacobsen, A.S. and Jones, O. and Katramados, I. and Keeling, D. and Kirk, A. and Klimek, I. and Kogan, L. and Leland , J. and Lipschultz, B. and Lloyd, B. and Lovell, J. and Madsen, B. and Marshall, O. and Martin, R. and McArdle, G. and McClements, K. and McMillan, B. and Meakins, A. and Meyer, H.F. and Militello, F. and Milnes, J. and Mordijck, S. and Morris, A.W. and Moulton, D. and Muir, D. and Mukhi, K. and Murphy-Sugrue, S. and Myatra, O. and Naylor, G. and Naylor, P. and Newton, S.L. and O\textquoterightGorman, T. and Omotani, J. and O\textquoterightMullane, M.G. and Orchard, S. and Pamela, S.J.P. and Pangione, L. and Parra, F. and Perez, R.V. and Piron, L. and Price, M. and Reinke, M.L. and Riva, F. and Roach, C.M. and Robb, D. and Ryan, D. and Saarelma, S. and Salewski, M. and Scannell, S. and Schekochihin, A.A. and Schmitz, O. and Sharapov, S. and Sharples, R. and Silburn, S.A. and Smith, S.F. and Sperduti, A. and Stephen, R. and Thomas-Davies, N.T. and Thornton, A.J. and Turnyanskiy, M. and Valovi\vc, M. and Van Wyk, F. and Vann, R.G.L. and Walkden, N.R. and Waters, I. and Wilson, H.R. and MAST-U Team, and EUROfusion MST1 Team, (2019) 'Overview of new MAST physics in anticipation of first results from MAST upgrade.', Nuclear fusion., 59 (11). p. 112011.

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.

Item Type:Article
Full text:(AM) Accepted Manuscript
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1088/1741-4326/ab121c
Date accepted:21 March 2019
Date deposited:31 October 2019
Date of first online publication:05 June 2019
Date first made open access:05 June 2020

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