Overview of physics results from MAST upgrade towards core-pedestal-exhaust integration

PAPER • OPEN ACCESS Overview of physics results from MAST upgrade towards core-pedestal-exhaust integration To cite this article: J.R. Harrison et al 2024 Nucl. Fusion 64 112017 View the article online for updates and enhancements. You may also like Physics basis for the divertor tokamak test facility F. Crisanti, R. Ambrosino, M.V. Falessi et al. - Density profiles in stellarators: an overview of particle transport, fuelling and profile shaping studies at TJ-II J.A. Alonso, D. Alegre, J. Alonso et al. - Negative triangularity scenarios: from TCV and AUG experiments to DTT predictions A. Mariani, L. Aucone, A. Balestri et al. - This content was downloaded from IP address 150.214.182.230 on 04/12/2024 at 10:37 International Atomic Energy Agency Nuclear Fusion Nucl. Fusion 64 (2024) 112017 (14pp) https://doi.org/10.1088/1741-4326/ad6011 Overview of physics results from MAST upgrade towards core-pedestal-exhaust integration J.R. Harrison1,∗, A. Aboutaleb2, S. Ahmed3, M. Aljunid1, S.Y. Allan1, H. Anand4, Y. Andrew5, L.C. Appel1, A. Ash1, J. Ashton1, O. Bachmann1, M. Barnes6, B. Barrett1, D. Baver7, D. Beckett1, J. Bennett1, J. Berkery8, M. Bernert9, W. Boeglin2, C. Bowman1, J. Bradley10, D. Brida9, P.K. Browning11, D. Brunetti1, P. Bryant10, J. Bryant10, J. Buchanan1, N. Bulmer1, A. Carruthers1, M. Cecconello12, Z.P. Chen13, J. Clark1,10, C. Cowley14, M. Coy1, N. Crocker15, G. Cunningham1, I. Cziegler14, T. Da Assuncao1, Y. Damizia10, P. Davies1, I.E. Day1, G.L. Derks16,17, S. Dixon1, R. Doyle18, M. Dreval19, M. Dunne9, B.P. Duval20, T. Eagles1, J. Edmond1, H. El-Haroun1, S.D. Elmore1, Y. Enters14, M. Faitsch9, F. Federici21, N. Fedorczak22, F. Felici20, A.R. Field1, M. Fitzgerald1, I. Fitzgerald1, R. Fitzpatrick13, L. Frassinetti23, W. Fuller24, D. Gahle25, J. Galdon-Quiroga26, L. Garzotti1, S. Gee1, T. Gheorghiu14, S. Gibson1, K.J. Gibson14, C. Giroud1, D. Greenhouse14, V.H. Hall-Chen27, C.J. Ham1, R. Harrison1, S.S. Henderson1, C. Hickling1,10, B. Hnat24, L. Howlett14, J. Hughes28, R. Hussain1, K. Imada14, P. Jacquet1, P. Jepson1, B. Kandan1, I. Katramados1, Y.O. Kazakov29, D. King1, R. King1, A. Kirk1, M. Knolker4, M. Kochan1, L. Kogan1, B. Kool16,17, M. Kotschenreuther13, M. Lees1, A.W. Leonard4, G. Liddiard1, B. Lipschultz14, Y.Q. Liu4, B.A. Lomanowski21, N. Lonigro14, J. Lore21, J. Lovell21, S. Mahajan13, F. Maiden14, C. Man-Friel1, F. Mansfield1, S. Marsden1, R. Martin1, S. Mazzi20, R. McAdams1, G. McArdle1, K.G. McClements1, J. McClenaghan4, D. McConville1, K. McKay10, C. McKnight1, P. McKnight1, A. McLean30, B.F. McMillan24, A. McShee1, J. Measures1, N. Mehay1, C.A. Michael15, F. Militello1, D. Morbey1, S. Mordijck31, D. Moulton1, O. Myatra1, A.O. Nelson32, M. Nicassio1, M.G. O’Mullane25, H.J.C. Oliver1, P. Ollus33, T. Osborne4, N. Osborne10, E. Parr1, B. Parry1, B.S. Patel1, D. Payne1, C. Paz-Soldan32, A. Phelps25, L. Piron34,35, C. Piron36, G. Prechel37, M. Price1, B. Pritchard14, R. Proudfoot1, H. Reimerdes20, T. Rhodes15, P. Richardson1, J. Riquezes32, J.F. Rivero-Rodriguez1, C.M. Roach1, M. Robson1, K. Ronald25, E. Rose1, P. Ryan1, D. Ryan1, S. Saarelma1, S. Sabbagh32, R. Sarwar1, P. Saunders1, O. Sauter20, R. Scannell1, T. Schuett14, R. Seath1, R. Sharma1, P. Shi1, B. Sieglin9, M. Simmonds1, J. Smith1, A. Smith1, V. A. Soukhanovskii30, D. Speirs25, G. Staebler4, R. Stephen1, P. Stevenson1, J. Stobbs1, M. Stott1, C. Stroud1, C. Tame1, C. Theiler20, N. Thomas-Davies1, A.J. Thornton1, M. Tobin32, M. Vallar20, R.G.L. Vann14, L. Velarde38, K. Verhaegh1, E. Viezzer26, C. Vincent1, G. Voss1, M. Warr1, W. Wehner4, S. Wiesen39, T.A. Wijkamp16,17, D. Wilkins1, T. Williams1, T. Wilson1, H.R. Wilson14,21, H. Wong15, M. Wood1and V. Zamkovska32 1UKAEA (United Kingdom Atomic Energy Authority), Culham Science Centre, Abingdon, Oxfordshire OX14 3DB, United Kingdom of Great Britain and Northern Ireland 2Department of Physics, Florida International University, 11200 SW, Miami, FL 33199, United States of America ∗Author to whom any correspondence should be addressed. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 1741-4326/24/112017+14$33.00 Printed in the UK 1 © 2024 Crown copyright, UK Atomic Energy Authority & The Author(s). Published by IOP Publishing Ltd on behalf of the IAEA. All rights reserved. Nucl. Fusion 64 (2024) 112017 J.R. Harrison et al 3Department of Physics and Technology, UiT The Arctic University of Norway N-9037 Tromsø, Norway 4General Atomics, PO Box 85608, San Diego, CA 92186-5608, United States of America 5Blackett Laboratory, Imperial College London, London SW7 2BW, United Kingdom of Great Britain and Northern Ireland 6Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3NP, United Kingdom of Great Britain and Northern Ireland 7Astrodel LLC, Boulder, CO 80303, United States of America 8Princeton Plasma Physics Laboratory, Princeton, NJ, United States of America 9Max-Planck-Institut fur Plasmaphysik 85748 Garching, Germany 10 Department of Electrical Engineering and Electronics, University of Liverpool, Brownlow Hill, Liverpool L69 3GJ, United Kingdom of Great Britain and Northern Ireland 11 Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom of Great Britain and Northern Ireland 12 Department of Physics, Durham University, Durham DH1 3LE, United Kingdom of Great Britain and Northern Ireland 13 Institute for Fusion Studies, The University of Texas at Austin, Austin, TX, United States of America 14 York Plasma Institute, Department of Physics, University of York, Heslington, York YO10 5DD, United Kingdom of Great Britain and Northern Ireland 15 Physics and Astronomy Department, University of California, Los Angeles, CA 90098, United States of America 16 Dutch Institute for Fundamental Energy Research DIFFER, Eindhoven, Netherlands 17 Eindhoven University of Technology, Eindhoven, Netherlands 18 Dublin City University, Dublin, Ireland 19 National Science Center ‘Kharkov Institute of Physics and Technology’, Akademichna 1, Kharkiv 61108, Ukraine 20 Ecole Polytechnique Fédérale de Lausanne (EPFL), Swiss Plasma Center (SPC) CH-1015 Lausanne, Switzerland 21 Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States of America 22 CEA, IRFM F-13108 Saint-Paul-lez-Durance, France 23 Division of Fusion Plasma Physics, KTH Royal Institute of Technology, Stockholm SE-100 44, Sweden 24 Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom of Great Britain and Northern Ireland 25 Department of Physics, SUPA, University of Strathclyde, Glasgow, Scotland, United Kingdom of Great Britain and Northern Ireland 26 Dpto. de Física Atómica, Molecular y Nuclear, Universidad de Sevilla 41012 Sevilla, Spain 27 IHPC, Agency for Science, Technology and Research (A∗STAR), Singapore 138632, Singapore 28 Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA 02139, United States of America 29 Laboratory for Plasma Physics, LPP-ERM/KMS, TEC Partner, Brussels, Belgium 30 Lawrence Livermore National Laboratory, Livermore, CA, United States of America 31 Department of Computer Science, College of William & Mary, Williamsburg, VA, United States of America 32 Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, United States of America 33 Department of Applied Physics, Aalto University, PO Box 11100 00076 Aalto, Finland 34 Dipartimento di Fisica ‘G. Galilei’, Universit` a degli Studi di Padova, Padova, Italy 35 Consorzio RFX, Corso Stati Uniti 4 35127 Padova, Italy 36 ENEA, Fusion and Nuclear Safety Department, C. R. Frascati, Via E. Fermi 45 00044 Frascati, Roma, Italy 37 University of California Irvine, Irvine, CA 92697, United States of America 38 Dpto. de Ingeniería Energética, ETSI, Universidad de Sevilla 41092 Sevilla, Spain 39 Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung—Plasmaphysik 52425 Jülich, Germany E-mail: [email protected] Received 8 January 2024, revised 17 April 2024 Accepted for publication 8 July 2024 Published 29 August 2024 Abstract Recent results from MAST Upgrade are presented, emphasising understanding the capabilities of this new device and deepening understanding of key physics issues for the operation of ITER 2 Nucl. Fusion 64 (2024) 112017 J.R. Harrison et al and the design of future fusion power plants. The impact of MHD instabilities on fast ion confinement have been studied, including the first observation of fast ion losses correlated with Compressional and Global Alfvén Eigenmodes. High-performance plasma scenarios have been developed by tailoring the early plasma current ramp phase to avoid internal reconnection events, resulting in a more monotonic q profile with low central shear. The impact of m/n=3/2, 2/1 and 1/1 modes on thermal plasma confinement and rotation profiles has been quantified, and scenarios optimised to avoid them have transiently reached values of normalised beta approaching 4.2. In pedestal and ELM physics, a maximum pedestal top temperature of ∼350 eV has been achieved, exceeding the value achieved on MAST at similar heating power. Mitigation of type-I ELMs with n=1 RMPs has been observed. Studies of plasma exhaust have concentrated on comparing conventional and Super-X divertor configurations, while X-point target, X-divertor and snowflake configurations have been developed and studied in parallel. In L-mode discharges, the separatrix density required to detach the outer divertors is approximately a factor 2 lower in the Super-X than the conventional configuration, in agreement with simulations. Detailed analysis of spectroscopy data from studies of the Super-X configuration reveal the importance of including plasma-molecule interactions and D2Fulcher band emission to properly quantify the rates of ionisation, plasma-molecule interactions and volumetric recombination processes governing divertor detachment. In H-mode with conventional and Super-X configurations, the outer divertors are attached in the former and detached in the latter with no impact on core or pedestal confinement. Keywords: MAST upgrade, exhaust, integrated scenarios, alternative divertors, pedestal, fast ions (Some figures may appear in colour only in the online journal) 1. Introduction and MAST upgrade capabilities MAST Upgrade is a low aspect ratio tokamak (major radius (R)/minor radius (a)=0.85/0.65 ∼1.3, plasma current (Ip)⩽2.0 MA, toroidal field on axis (Bφ)⩽0.8 T, pulse length <5 s) and one of the largest spherical tokamaks worldwide, together with NSTX-U [1]. A poloidal cross-section of MAST Upgrade is shown in figure 1. It has considerable flexibility to independently vary the shape of the plasma core and divertors within tightly baffled chambers utilising 22 poloidal field coils to facilitate optimision of the shaping of the plasma core to maximise confinement and stability, whilst modifying the divertor configuration to maximise the dissipation of particles, momentum and energy. Sources of nonaxisymmetic magnetic fields are available for ELM control with Resonant Magnetic Perturbations (RMPs) and correcting for intrinsic error fields (EFs), with two rows of in-vessel coils (four equally spaced toroidally above the mid-plane, eight below) and two pairs of ex-vessel coils respectively. On— and off-axis Neutral Beam Injectors (NBI) enable studies of the confinement of super Alfvénic fast ions that more closely mimics the products of fusion reactions. An extensive suite of highly resolved diagnostics is available to support a broad and deep physics programme in these key physics issues for the operation of ITER and the design of future power plants including DEMO [2] and STEP [3]. An optimal fine-alignment of the internal poloidal field coils to shape the plasma core and divertor was performed when assembling MAST-U to reduce the n=1 EF source associated with the coil design and manufacturing, which has foreseen coil shifts and tilts, of the order of mm and mrad, respectively [4]. To assess the presence of a residual n=1 EF, dedicated EF identification studies have been carried out and the main results are reported in figure 2, which represents the compass scan tests executed in Ohmic scenarios with conventional divertor configurations at Ip=450 kA, Bφ=0.4 T and Ip=750 kA, Bφ=0.5 T. In both scenarios, EF identification studies suggest that a homeopathic level of correction currents are needed to minimise the intrinsic EF amplitude, of around some hundreds of A. This suggests that the intrinsic EF amplitude is smaller than in MAST, where the EF correction currents were in the kilo Ampere regime [5]. This proves that the coil alignment when assembling the device so as to minimise the intrinsic n=1 EF has been a successful passive n=1 EF correction strategy. Recent results from MAST Upgrade are presented, including fast particle physics in section 2, MHD stability and maximising beta in section 3, pedestal and Edge Localised Mode (ELM) physics in section 4, plasma exhaust with an emphasis on the relative benefits of alternative divertor configurations in section 5and plasma control and the development of high-performance plasma scenarios in section 6. A programme of extensive hardware enhancements is underway to further develop the capabilities of MAST Upgrade. These enhancements and the envisaged future MAST Upgrade programme are presented in section 7. A summary and implications of the results presented to future devices is presented in section 8. 3 Nucl. Fusion 64 (2024) 112017 J.R. Harrison et al Figure 1. Poloidal cross-section of MAST upgrade. 2. Fast particle physics Future large burning fusion devices with a significant α particle population will require good confinement of charged fusion products to maximise plasma self-heating and minimising heat fluxes arising from losses. MAST Upgrade is well suited to studying fast particle confinement and the impact of MHD instabilities, with on-and off-axis neutral beams (with tangency positions (R, Z) of (0.71 m, 0.0 m) and (0.8 m, 0.65 m) respectively) that produce anisotropic super-Alfvénic fast ions (for example, in a typical pulse the speed of the deposited fast ions vfi∼2.5 ×106m s−1exceeds the Alfvén speed in the core vA∼1.5 ×106m s−1[7]). A comprehensive suite of highly resolved diagnostics is available, including a fission chamber [8], upgraded neutron camera [7], Solid State Neutral Particle Analyser (SSNPA) [9], Fast Ion Loss Detector (FILD) [10] and a Fast-Ion Deuterium Alpha (FIDA) system [11]. Despite the closed divertors reducing the neutral density in the main chamber, inferred from mid-plane radial profiles of ne,Teand Dαemission, up to 20% of the injected NBI power from both beams are lost due to charge-exchange interactions with edge neutrals [12], and including these interactions enables reconstruction of the passive FIDA signal. While this is unlikely to be an issue in future higher power devices as the main chamber neutral pressure is expected to be lower than in MAST-U, and neutrals will be more strongly attenuated by the pedestal [13], maximising the absorbed neutral beam heating power will enable MAST-U to reach more reactor-relevant conditions. Figure 2. Quantification of the intrinsic error field in Ip=(a) 450 kA and (b) 750 kA plasma scenarios using the compass scan technique [6]. A broad spectrum of fast ion driven instabilities are excited including toroidal (TAE), compressional (CAE) and global (GAE) Alfvén eigenmodes, fishbones, as shown in figure 3, mostly due to fast ions produced by the on-axis beam. The impact of these instabilities on the thermal plasma and fast ion confinement has been studied in detail. The largest source of fast ion losses is 2/1 tearing modes that are commonly observed in NBI heated pulses (see section 3). The tearing mode amplitude typically grows throughout the NBI heated phase of a pulse that can reduce the measured neutron rate by up to 50%. Significant changes in the fast ion population are observed following sawtooth crashes [7], that can reduce the fast ion population by 40%–50% across the plasma core, in agreement with similar findings on MAST [14]. Fast ion losses in the core due to fishbones have been observed with neutron camera and FILD diagnostics, that can reduce the fast ion density by ∼20%, up to 35% near the magnetic axis, indicating a hollow neutron profile as the fishbone grows, then in the later phase becomes peaked in the core slightly toward the inboard side. Interpretive modelling with ASCOT [15] and FILD measurements suggest these losses are of trapped fast ions [16]. Conversely, TAEs are correlated with fast ion losses only when the off-axis neutral beam is applied, otherwise they tend to result in fast ion redistribution. For the first time, FILD measurements indicate that CAEs and GAEs have been correlated with fast ion losses. The 4 Nucl. Fusion 64 (2024) 112017 J.R. Harrison et al Figure 3. Spectrograms of magnetic fluctuations (a) illustrating commonly observed fast particle driven instabilities, including chirping/bursting TAEs, n=1 internal kink modes (chirping fishbones then long-lived mode) and sawteeth (based on [17]) and (b) CAE/GAE modes and whether they correlate with fast ion losses, measured with a Fast Ion Loss Detector. location of these modes has been determined with Doppler backscattering (DBS) measurements, finding that modes that are more localised to the plasma core (up to √ψN∼0.7) result in fast ion redistribution, whereas modes localised near the edge (up to √ψN∼0.9) result in fast ion losses. Evidence for ion cyclotron emission from Ohmic plasmas has been observed in density fluctuations measured by DBS with a frequency of ∼3.5 MHz [18]. Fast ion redistribution and losses due to ELMs have been measured with the FILD, SSNPA and FIDA diagnostics. Type- III ELMs are observed to have minimal impact on global fast ion confinement as the neutron rate is not strongly affected [19]. However, these ELMs result in localised fast ion losses, likely from the plasma edge, with no fast-ion acceleration correlated with ELMs. Fast ion losses due to the presence of RMPs have also been observed. 3. Core MHD stability and maximising beta The avoidance of performance limiting and disruptive MHD instabilities is highly desirable to maximise fusion performance and reduce the risk of damage to the interior surfaces of a fusion device respectively. Therefore, identification and avoidance of these instabilities is a key objective of the MAST Upgrade programme. Typical plasma scenarios have a rapid Figure 4. Impact of the plasma current ramp rate on the equilibrium q profile, comparing two shots with fast and slow Ipramps (top) and MSE constrained equilibrium q profiles 20 ms after the start of the Ipflat-top (bottom). initial plasma current ramp rate, dIp/dt∼6.5 MA s−1, prior to the flat-top phase and Internal Reconnection Events (IREs) are common, due to the presence of a qprofile with reversed shear. IREs are observed to cause a transient increase in plasma current (Ip) and loop voltage, and reduction in plasma density, resulting in current redistribution from a hollow, reverse shear qprofile to a broad, monotonic one [20]. IREs have been successfully avoided with slower Ipramp rates, ∼3.5– 4.5 MA s−1, resulting in more monotonic qprofiles with lower central shear, as shown in figure 4. The performance of MAST Upgrade plasmas with strong auxiliary heating is typically moderated by m/n=1/1, 2/1 and 3/2 modes, which dampen the rotation profile (the higher order modes mostly reduce the core rotation, whereas the lower order modes reduce the entire rotation profile) and reduce core confinement. The 1/1 mode is qualitatively similar to the long-lived mode studied on MAST [21], except that it does not always limit the pulse duration and higher order harmonics are weaker in spectrograms of magnetic field fluctuations. The 2/1 tearing mode causes a characteristic frequency of 6– 10 kHz and causes flattening of the Teprofile at the q=2 flux surface. The rotation frequency of the mode is consistent with plasma rotation profile measurements at the q=2 surface, as shown in figure 5. To avoid these instabilities, the initial Ipcurrent ramp has been optimised to avoid the IRE. The slower Ipcurrent ramp tends to reduce q0in the early Ipflat-top phase, while the off-axis neutral beam provides additional current drive to help elevate q0. Further plasma scenario optimisations are underway to vary Ip,Bφand elongation (κ) to maximise normalised beta (βN), then other metrics. The off-axis neutral beam is an effective source of non-inductive current 5 Nucl. Fusion 64 (2024) 112017 J.R. Harrison et al Figure 5. Overview of a typical pulse with a performance limiting tearing mode. Top: Thomson scattering profiles of Teshowing flattening at the q=2 surface, middle: core Te,Ti, bottom: spectrogram of low frequency magnetic fluctuations with the toroidal rotation at the magnetic axis and q=2 flux surface. drive that allows q0>1 to be sustained throughout a pulse, as shown in figure 5, thus avoiding sawteeth. To date, the highest achieved βNis ∼4.2 transiently in shot 48 653, which reached a maximum stored energy of ∼160 kJ prior to the locking of 2/1 and 1/1 modes that resulted in a disruption. These results built on work carried out in previous campaigns where βNwas limited to ∼3 by non-disruptive mode locking events [22]. In support of further optimising plasma performance, the highest achievable elongation for a given ℓihas been characterised. To date, κup to 2.4 has been achieved with good vertical control. Density limits have been studied [23], finding that disruptivity increases with proximity to the Greenwald density limit [24]. Of the few density limit disruptions observed to date, they cross a threshold based on the turbulent transport for a given heating power across the separatrix [25]. Investigation of disruption causes (indicated by abnormalities in the plasma current and vertical position development) with the DECAFTM [26] code revealed a year-to-year (in the first and second physics campaigns) decrease by ∼20% of the plasma disruptivity rate. These improvements in scenario robustness were enabled through better real-time control of the plasma shape and density. Trigger instances of disruptive event chains were clustered in different parts of the operation space diagrams and plasma elongation was shown to be an important factor influencing details of the chains [27]. Figure 6. Electron density and temperature pedestal height in MAST [28] in double null (DN) and single null (SN) topologies and MAST upgrade. 4. Pedestal and ELM physics MAST Upgrade routinely operates in the high confinement mode (H-mode), with auxiliary heating from the on- and offaxis neutral beams. Initial studies on H-mode access have concentrated on the impact of the divertor configuration on PL-H, indicating broadly similar values for conventional and Super- X divertor configurations. In H-mode, radial profiles of electron density (ne) and temperature (Te) at the edge of the confined plasma exhibit steep gradients emblematic of the edge pedestal. The density and temperature pedestal characteristics were diagnosed with a high-resolution Thomson scattering system in MAST [28] and MAST Upgrade, where the pedestal top parameters are shown in figure 6. There is significant overlap in the achieved pedestal top parameters in both devices, however MAST Upgrade is able to sustain hotter pedestals where Te,ped >350 eV, which was not possible in MAST with comparable auxiliary heating power. The transition from type-III to type-I ELMs is observed to occur when the power crossing the separatrix is ∼1.9 MW and Te,ped ∼130 eV [29], which is lower than comparable values on MAST of ∼2.5 MW and 150 eV respectively [30]. The MHD and gyrokinetic stability of MAST pedestals was analysed with the ELITE and CGYRO codes [31], showing that the pedestal was constrained by kinetic ballooning and medium toroidal mode number peeling-ballooning modes. Stability analysis of MAST Upgrade pedestals [32], shown in figure 7, indicates that the higher elongation and squareness of the plasma boundary, compared with MAST, due to improved plasma shaping, enabled by the larger number of poloidal field coils and higher toroidal field in MAST Upgrade and enables operation at higher normalised pedestal pressure, closer to the peeling boundary. This analysis suggests that the squareness of the plasma boundary is close to optimal, but further improvements in pedestal stability, and in turn the pedestal pressure, are possible by increasing elongation and maximising particle pumping and auxiliary heating power available 6 Nucl. Fusion 64 (2024) 112017 J.R. Harrison et al Figure 7. Pedestal stability boundaries in terms of normalised pedestal pressure gradient on the x-axis and current density on the y-axis on MAST (black) and MAST-U (blue, red). The inset figure shows MAST (black) and MAST-U (red) equilibria analysed. in upcoming improvements to the device to reduce the pedestal collisionality. Mitigation of type-I ELMs with RMPs with toroidal mode number, n=1 has been achieved. The application of n=1 RMPs leads to an increase in the ELM frequency and deceleration and locking of a 2/1 tearing mode that is common to typical type-I ELMy H-mode scenarios (as discussed in section 3) and then density pump-out. Removal of the RMPs results in the mode rotation accelerating and the ELM frequency decreasing to their original values. 5. Plasma exhaust and alternative divertor configurations MAST Upgrade has significant flexibility to develop and study conventional and alternative divertor configurations, with and without up-down symmetry, including the X-divertor [33,34], Super-X [35], Snowflake [36] and X-point target [37], as shown in figure 8. The early MAST Upgrade programme prioritised comparing the Super-X with a conventional divertor configuration, including the role of total flux expansion tightly baffled divertor chambers on plasma exhaust. In spherical tokamaks, the benefits of the Super-X configuration are amplified, such as higher total flux expansion (∼2.5 in MAST- U, compared with <1.7 in TCV [38]), and consequently strong gradients in the total magnetic field from the X-point to the divertor target that is predicted to passively stabilise the movement of the detachment front [39]. Consequently, an extensive suite of diagnostics was deployed and optimised to study the Super-X divertor configuration, including 850 Langmuir probes [40], a divertor Thomson scattering system Figure 8. Equilibrium reconstructions of conventional and alternative divertor configurations developed on MAST-U. [41], a multi-wavelength imaging system [42], resistive [43] and imaging [44] bolometer diagnostics, UV–visible spectrometers, neutral pressure gauges and IR thermography. Other divertor configurations are being developed and studied in parallel, particularly the snowflake [45] and X-point target. Initial comparisons of conventional and Super-X divertor configurations were performed in Ohmic and NBI heated L- mode pulses. Experiments were performed with density ramps and repeat pulses at different (constant) flat-top density, using gas puffing to control the plasma density, to quantify the onset of divertor detachment. The detachment threshold, in terms of the estimated separatrix density, was approximately a factor of two lower in the Super-X configuration than a conventional divertor, as shown in figure 9. Detailed comparison of the detachment threshold was complicated by the onset of locked modes causing toroidal asymmetries in the divertor heat and particle flux profiles (termed ‘strike point splitting’ in figure 9), however the reduced detachment threshold in the Super-X configuration is in broad agreement with predictions from analytic models (e.g. [39]). The divertor surface power load was reduced by at least an order of magnitude in the Super-X configuration. The mid-plane neand Teprofiles at a given line-average density were not significantly affected by either the divertor configuration or whether the outer divertors 7 Nucl. Fusion 64 (2024) 112017 J.R. Harrison et al Figure 9. Measured total divertor ion flux in conventional (red) and Super-X (blue) divertor configurations. Estimated detachment thresholds are given by vertical dashed lines. were detached. This detachment behaviour in the Super-X divertor configuration is well reproduced in predictive [46] and interpretive SOLPS-ITER simulations [47], however the simulations of the conventional configuration do not exhibit the characteristic roll-over in the total divertor ion flux, which is thought to be due to a reduction in the upstream separatrix pressure in experiments from strong power losses originating from the strong gas fuelling applied to reach the required core line-average density. Characterisation of detachment onset in other alternative divertor configurations is underway. Multi-wavelength imaging of the D2Fulcher band emission from the lower divertor chamber was used to estimate the position of the ionisation front, where the dominant form of plasma-neutral interactions transitions from electron impact ionisation to plasma-molecule interactions [48]. As the plasma conditions in the divertor chambers trend toward deeper detachment, the ionisation front moves from the outer strike point towards the divertor entrance. The observed sensitivity of the ionisation front movement with increasing midplane separatrix density agrees well with interpretive SOLPSITER simulations [49]. There is a clear reduction in the sensitivity of the front position to changes in the separatrix density as the emission front moves through regions exhibiting strong gradients in the total magnetic field, in agreement with analytic models [39,50]. Studies of NBI heated L-mode pulses with conventional, Super-X and an intermediate, elongated, divertor configuration elucidate the benefits of increased divertor volume on power and particle exhaust [51]. In the elongated and Super- X divertor configurations after the onset of detachment, the ionisation region in the divertor chamber extends to a fixed major radius, insensitive to increases in the major radius of the outer strike points. Downstream of the ionisation region, any additional divertor volume afforded by a larger outer strike point major radius increases ion sinks, including Molecular Activated Recombination (MAR) and Electron-Ion Recombination (EIR), and power losses due to plasma-neutral Figure 10. Profiles of the electron temperature (red) and power losses due to plasma-atom interactions (blue) along the separatrix flux surface from the divertor entrance to the target in conventional and Super-X divertor configurations simulated with SOLPS-ITER. interactions that reduce divertor target power and particle fluxes. Experiments performed in type-I ELMy H-mode scenarios with conventional and Super-X divertor configurations and similar core shaping are in broad agreement with the L-mode studies, in terms of ease of access to divertor detachment in the Super-X configuration and effective decoupling of the mid-plane pedestal profiles and the divertor configuration and detachment state. In 750 kA scenarios, which maximise the ratio of the separatrix density at the midplane and the parallel heat flux entering the divertor chambers to ease access to attached divertor conditions, with ∼3 MW of NBI heating power, the conventional divertor configuration was attached and the Super-X detached. In common with L-mode experiments, global confinement parameters, including HIPB98y,2 and β, and core and pedestal neand Teprofiles were also unaffected by whether the divertor was in a conventional or Super- X configuration and whether the outer divertors are attached or detached. These observations are in good qualitative agreement with SOLPS-ITER simulations, shown in figure 10, that the plasma conditions in conventional and Super-X configurations are similar for otherwise similar boundary conditions, but Teat the divertor entrance is lower in the Super-X configuration due to higher power losses due to plasma-neutral interactions in the additional divertor volume where Te<5 eV. The divertor neutral compression, the ratio of the lower divertor and main chamber neutral pressures, is typically 100–300, with the Super-X having higher compression despite the outer divertors being detached. The fundamental mechanisms governing detachment in the Super-X divertor configuration have been studied in experiments via spectroscopic measurements interpreted using a sophisticated Bayesian framework BaSPMI [48]. To correctly interpret these measurements, it is necessary to account for plasma-molecule interactions and measurements of the D2 Fulcher band emission profile are needed to discern between emission due to electron impact excitation, which in turn can be used to estimate where ionisation of neutrals occurs, and 8