Overview of progress in European medium sized tokamaks towards an integrated plasma-edge/wall solution
Abstract
Integrating the plasma core performance with an edge and scrape-off layer (SOL) that leads to tolerable heat and particle loads on the wall is a major challenge. The new European medium size tokamak task force (EU-MST) coordinates research on ASDEX Upgrade (AUG), MAST and TCV. This multi-machine approach within EU-MST, covering a wide parameter range, is instrumental to progress in the field, as ITER and DEMO core/pedestal and SOL parameters are not achievable simultaneously in present day devices. A two prong approach is adopted. On the one hand, scenarios with tolerable transient heat and particle loads, including active edge localised mode (ELM) control are developed. On the other hand, divertor solutions including advanced magnetic configurations are studied. Considerable progress has been made on both approaches, in particular in the fields of: ELM control with resonant magnetic perturbations (RMP), small ELM regimes, detachment onset and control, as well as filamentary scrape-off-layer transport. For example full ELM suppression has now been achieved on AUG at low collisionality with n = 2 RMP maintaining good confinement HH(98,y2) 0.95. Advances have been made with respect to detachment onset and control. Studies in advanced divertor configurations (Snowflake, Super-X and X-point target divertor) shed new light on SOL physics. Cross field filamentary transport has been characterised in a wide parameter regime on AUG, MAST and TCV progressing the theoretical and experimental understanding crucial for predicting first wall loads in ITER and DEMO. Conditions in the SOL also play a crucial role for ELM stability and access to small ELM regimes.
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PAPER • OPEN ACCESS Overview of progress in European medium sized tokamaks towards an integrated plasma-edge/wall solution To cite this article: H. Meyer et al 2017 Nucl. Fusion 57 102014 View the article online for updates and enhancements. Related content Overview of MAST results I.T. Chapman, J. Adamek, R.J. Akers et al. - Enhanced confinement scenarios without large edge localized modes in tokamaks: control, performance, and extrapolability issues for ITER R. Maingi - Effect of resonant magnetic perturbations on low collisionality discharges in MAST and a comparison with ASDEX Upgrade A. Kirk, W. Suttrop, I.T. Chapman et al. - Recent citations Experimental validation of coil phase parametrisation on ASDEX Upgrade, and extension to ITER D A Ryan et al - On the role of filaments in perpendicular heat transport at the scrape-off layer D. Carralero et al - Real-time-capable prediction of temperature and density profiles in a tokamak using RAPTOR and a firstprinciple-based transport model F. Felici et al - This content was downloaded from IP address 150.214.182.32 on 12/07/2018 at 12:43
1 © 2017 Culham Centre for Fusion Energy Printed in the UK Nuclear Fusion Overview of progress in European medium sized tokamaks towards an integrated plasma-edge/wall solutiona H.Meyer5, T.Eich37, M.Beurskens5,38, S.Coda15, A.Hakola55, P.Martin8, J.Adamek29, M.Agostini8, D.Aguiam30, J.Ahn6, L.Aho-Mantila55, R.Akers5, R.Albanese51, R.Aledda11, E.Alessi23, S.Allan5, D.Alves30, R.Ambrosino52, L.Amicucci44, H.Anand15, G.Anastassiou40, Y.Andrèbe15, C.Angioni37, G.Apruzzese44, M.Ariola52, H.Arnichand6, W.Arter5, A.Baciero35, M.Barnes42, L.Barrera18,45, R.Behn15, A.Bencze56, J.Bernardo30, M.Bernert37, P.Bettini8, P.Bilková29, W.Bin23, G.Birkenmeier37, J.P.S.Bizarro30, P.Blanchard15, T.Blanken16, M.Bluteau14, V.Bobkov37, O.Bogar29, P.Böhm29, T.Bolzonella8, L.Boncagni44, A.Botrugno44, C.Bottereau6, F.Bouquey6, C.Bourdelle6, S.Brémond6, S.Brezinsek20, D.Brida37, F.Brochard46, J.Buchanan5, H.Bufferand6, P.Buratti44, P.Cahyna29, G.Calabrò44, Y.Camenen2, R.Caniello23, B.Cannas11, A.Canton8, A.Cardinali44, D.Carnevale53, M.Carr5, D.Carralero37, P.Carvalho30, L.Casali37, C.Castaldo44, F.Castejón35, R.Castro35, F.Causa44, R.Cavazzana8, M.Cavedon37, M.Cecconello12, S.Ceccuzzi44, R.Cesario44, C.D.Challis5, I.T.Chapman5, S.Chapman7, M.Chernyshova28, D.Choi15, C.Cianfarani44, G.Ciraolo6, J.Citrin19, F.Clairet6, I.Classen19, R.Coelho30, J.W.Coenen20, L.Colas6, G.Conway37, Y.Corre6, S.Costea25, F.Crisanti44, N.Cruz30, G.Cseh56, A.Czarnecka28, O.D’Arcangelo44, M.De Angeli23, G.De Masi8, G.De Temmerman31, G.De Tommasi51, J.Decker15, R.S.Delogu8, R.Dendy5, P.Denner20, C.Di Troia44, M.Dimitrova29, R.D’Inca37, V.Dorić54, D.Douai6, A.Drenik32,37, B.Dudson57, D.Dunai56, M.Dunne37, B.P.Duval15, L.Easy5, S.Elmore5, B.Erdös27, B.Esposito44, E.Fable37, M.Faitsch37, A.Fanni11, N.Fedorczak6, F.Felici16, J.Ferreira30, O.Février6, O.Ficker29, S.Fietz37, L.Figini23, A.Figueiredo30, A.Fil6, G.Fishpool5, M.Fitzgerald5, M.Fontana15, O.Ford38, L.Frassinetti22, R.Fridström22, D.Frigione44, G.Fuchert37, C.Fuchs37, M.Furno Palumbo8, S.Futatani4, L.Gabellieri44, K.Gałązka28, J.Galdon-Quiroga45, S.Galeani53, D.Gallart4, A.Gallo6, C.Galperti15,23, Y.Gao20, S.Garavaglia23, J.Garcia6, A.Garcia-Carrasco22, J.Garcia-Lopez45, M.Garcia-Munoz45, J.-L.Gardarein2, L.Garzotti5, J.Gaspar2, E.Gauthier6, P.Geelen16, B.Geiger37, P.Ghendrih6, F.Ghezzi23, L.Giacomelli23, L.Giannone37, E.Giovannozzi44, C.Giroud5, C.Gleason González33, M.Gobbin8, T.P.Goodman15, G.Gorini49, M.Gospodarczyk53, G.Granucci23, M.Gruber37, A.Gude37, L.Guimarais30, R.Guirlet6, J.Gunn6, P.Hacek29, S.Hacquin6, S.Hall5, C.Ham5, T.Happel37, J.Harrison5, D.Harting5, V.Hauer33, E.Havlickova5, T.Hellsten22, W.Helou6, S.Henderson14, P.Hennequin34, M.Heyn26, B.Hnat42, M.Hölzl36, H. Meyer etal Overview of progress in European medium sized tokamaks towards an integrated plasma-edge/wall solution Printed in the UK 102014 NUFUAU © 2017 Culham Centre for Fusion Energy 57 Nucl. Fusion NF 10.1088/1741-4326/aa6084 Paper 10 Nuclear Fusion IOP International Atomic Energy Agency a In the future we will refer to the author list of the paper as the EUROfusion MST1 Team. Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 2017 1741-4326 1741-4326/17/102014+15$33.00 https://doi.org/10.1088/1741-4326/aa6084 Nucl. Fusion 57 (2017) 102014 (15pp)
H. Meyer etal 2 D.Hogeweij19, C.Honoré34, C.Hopf37, J.Horáček29, G.Hornung9, L.Horváth27, Z.Huang15, A.Huber20, J.Igitkhanov33, V.Igochine37, M.Imrisek29, P.Innocente8, C.Ionita-Schrittwieser25, H.Isliker3, I.Ivanova- Stanik28, A.S.Jacobsen43, P.Jacquet5, M.Jakubowski38, A.Jardin6, F.Jaulmes19, F.Jenko37, T.Jensen43, O.Jeppe Miki Busk43, M.Jessen43, E.Joffrin6, O.Jones5, T.Jonsson22, A.Kallenbach37, N.Kallinikos3, S.Kálvin56, A.Kappatou19,37, J.Karhunen1, A.Karpushov15, S.Kasilov26, G.Kasprowicz28, A.Kendl25, W.Kernbichler26, D.Kim15, A.Kirk5, S.Kjer43, I.Klimek12, G.Kocsis56, D.Kogut6, M.Komm29, S.B.Korsholm43, H.R.Koslowski20, M.Koubiti2, J.Kovacic32, K.Kovarik29,42, N.Krawczyk28, J.Krbec29, K.Krieger37, A.Krivska36, R.Kube25, O.Kudlacek8,37, T.Kurki- Suonio1, B.Labit15, F.M.Laggner24, L.Laguardia23, A.Lahtinen50, P.Lalousis21, P.Lang37, P.Lauber37, N.Lazányi27, A.Lazaros40, H.B.Le15, A.Lebschy37, J.Leddy57, L.Lefévre47, M.Lehnen31, F.Leipold43, A.Lessig37, M.Leyland57, L.Li20, Y.Liang20, B.Lipschultz57, Y.Q.Liu5, T.Loarer6, A.Loarte31, T.Loewenhoff20, B.Lomanowski13, V.P.Loschiavo51, T.Lunt37, I.Lupelli5, H.Lux5, A.Lyssoivan36, J.Madsen43, P.Maget6, C.Maggi5, R.Maggiora41, M.L.Magnussen43, J.Mailloux5, B.Maljaars16, A.Malygin15, P.Mantica23, M.Mantsinen4,58, M.Maraschek37, B.Marchand50, N.Marconato8, C.Marini15, M.Marinucci44, T.Markovic29, D.Marocco44, L.Marrelli8, Y.Martin15, J.R.Martin Solis35, A.Martitsch26, S.Mastrostefano17,52, M.Mattei51, G.Matthews5, M.Mavridis3, M.- L.Mayoral5,18, D.Mazon6, P.McCarthy48, R.McAdams5, G.McArdle5, P.McCarthy48, K.McClements5, R.McDermott37, B.McMillan7, G.Meisl37, A.Merle15, O.Meyer6, D.Milanesio41, F.Militello5, I.G.Miron39, K.Mitosinkova29, J.Mlynar29, A.Mlynek37, D.Molina6, P.Molina15, I.Monakhov5, J.Morales6, D.Moreau6, P.Morel34, J.-M.Moret15, A.Moro23, D.Moulton5, H.W.Müller37, F.Nabais30, E.Nardon6, V.Naulin43, A.Nemes- Czopf27, F.Nespoli15, R.Neu37, A.H.Nielsen43, S.K.Nielsen43, V.Nikolaeva30, S.Nimb43, M.Nocente49, R.Nouailletas6, S.Nowak23, M.Oberkofler37, M.Oberparleiter10, R.Ochoukov37, T.Odstrčil37, J.Olsen43, J.Omotani5, M.G.O’Mullane14, F.Orain6,37, N.Osterman32, R.Paccagnella8, S.Pamela5, L.Pangione5, M.Panjan32, G.Papp37, R.Papřok29, V.Parail5, F.I.Parra42, A.Pau11, G.Pautasso37, S.-P.Pehkonen55, A.Pereira35, E.Perelli Cippo23, V.Pericoli Ridolfini52, M.Peterka29, P.Petersson22, V.Petrzilka29, P.Piovesan8, C.Piron8, A.Pironti51, F.Pisano11, T.Pisokas3, R.Pitts31, I.Ploumistakis21, V.Plyusnin30, G.Pokol27, D.Poljak54, P.Pölöskei27, Z.Popovic35, G.Pór27, L.Porte15, S.Potzel37, I.Predebon8, M.Preynas15, G.Primc32, G.Pucella44, M.E.Puiatti8, T.Pütterich37, M.Rack20, G.Ramogida44, C.Rapson37, J.JuulRasmussen43, J.Rasmussen43, G.A.Rattá35, S.Ratynskaia22, G.Ravera44, D.Réfy56, M.Reich37, H.Reimerdes15, F.Reimold20,37, M.Reinke57, D.Reiser20, M.Resnik32, C.Reux6, D.Ripamonti23, D.Rittich37, G.Riva23, M.Rodriguez- Ramos45, V.Rohde37, J.Rosato34, F.Ryter37, S.Saarelma5, R.Sabot6, F.Saint-Laurent6, M.Salewski43, A.Salmi55, D.Samaddar5, L.Sanchis- Sanchez45, J.Santos30, O.Sauter15, R.Scannell5, M.Scheffer16, M.Schneider6,31, B.Schneider25, P.Schneider37, M.Schneller37, R.Schrittwieser25, M.Schubert37, J.Schweinzer37, J.Seidl29, M.Sertoli37, S.Šesnić54, A.Shabbir9, A.Shalpegin6, B.Shanahan57, S.Sharapov5, U.Sheikh15, G.Sias11, B.Sieglin37, C.Silva30, A.Silva30, M.Silva Fuglister15, J.Simpson5, A.Snicker1, C.Sommariva6, C.Sozzi23, S.Spagnolo8, G.Spizzo8, M.Spolaore8, T.Stange38, M.Stejner Pedersen43, I.Stepanov9, J.Stober37, P.Strand10, A.Šušnjara54, W.Suttrop37, T.Szepesi56, B.Tál56, Nucl. Fusion 57 (2017) 102014
H. Meyer etal 3 T.Tala55, P.Tamain6, G.Tardini37, M.Tardocchi23, A.Teplukhina15, D.Terranova8, D.Testa15, C.Theiler15, A.Thornton5, P.Tolias22, L.Tophøj43, W.Treutterer37, G.L.Trevisan8, M.Tripsky36, C.Tsironis40, C.Tsui15, O.Tudisco44, A.Uccello23, J.Urban29, M.Valisa8, P.Vallejos22, M.Valovic5, H.Van den Brand19, B.Vanovac19, S.Varoutis33, S.Vartanian6, J.Vega35, G.Verdoolaege9, K.Verhaegh57, L.Vermare34, N.Vianello8,15, J.Vicente30, E.Viezzer37, L.Vignitchouk22, W.A.J.Vijvers15,19, F.Villone17, B.Viola44, L.Vlahos3, I.Voitsekhovitch5,18, P.Vondráček29, N.M.T.Vu6, D.Wagner37, N.Walkden5, N.Wang20, T.Wauters36, M.Weiland37, V.Weinzettl29, E.Westerhof19, M.Wiesenberger25, M.Willensdorfer37, M.Wischmeier37, I.Wodniak12, E.Wolfrum37, D.Yadykin12, R.Zagórski28, I.Zammuto37, P.Zanca8, R.Zaplotnik32, P.Zestanakis40, W.Zhang9, S.Zoletnik56, M.Zuin8 and the ASDEX Upgrade, MAST and TCV Teams 1 Department of Applied Physics, Aalto University, PO Box 14100, FI-00076 Aalto, Finland 2 Aix-Marseille Université, CNRS, PIIM, F13013 Marseille, France 3 Aristotle University of Thessaloniki, Thessaloniki, Greece 4 Barcelona Supercomputing Center, Jordi Girona 29, 08034 Barcelona, Spain 5 CCFE, Culham Science Centre, Abingdon, Oxon, OX14 3DB, United Kingdom 6 CEA IRFM, F-13108 Saint Paul Lez Durance, France 7 Department of Physics, Centre for Fusion, Space and Astrophysics, Warwick University, Coventry CV4 7AL, United Kingdom 8 Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy 9 Department of Applied Physics UG Ghent University St-Pietersnieuwstraat 41 B-9000 Ghent Belgium 10 Department of Earth and Space Sciences, Chalmers University of Technology, SE-41296 Gothenburg, Sweden 11 Department of Electrical and Electronic Engineering, University of Cagliari, Piazza d’Armi 09123 Cagliari, Italy 12 Department of Physics and Astronomy, Uppsala University, SE-75120 Uppsala, Sweden 13 Department of Physics, Durham University, Durham, DH1 3LE, United Kingdom 14 Department of Physics, University of Strathclyde, 107 Rottenrow, Glasgow G4 0NG, United Kingdom 15 Ecole Polytechnique Fédérale de Lausanne (EPFL), Swiss Plasma Center (SPC), CH-1015 Lausanne, Switzerland 16 Eindhoven, University of Technology PO Box 513 NL-5600 MB Eindhoven, The Netherlands 17 ENEA/CREATE, DIEI, Università di Cassino, Via Di Biasio 43, 03043, Cassino (FR), Italy 18 EUROfusion PMU, Boltzmannstraße 2, 85748, Garching, Germany 19 FOM Institute DIFFER - Dutch Institute for Fundamental Energy Research, The Netherlands 20 Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung—Plasmaphysik, 52425 Jülich, Germany 21 Foundation of Research and Technology, Crete, Greece 22 Fusion Plasma Physics, EES, KTH, SE-10044 Stockholm, Sweden 23 IFP-CNR, Via R. Cozzi 53, 20125 Milano, Italy 24 Institut für Angewandte Physik, Technische Universität Wien, Wiedner Hauptstraße 8-10, 1040 Wien, Austria 25 Institut für Ionen- und Angewandte Physik, Universität Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria 26 Institut für Theoretische Physik, Technische Universität Graz, 8010 Graz, Austria 27 Institute of Nuclear Techniques, Budapest University of Technology and Economics, PO Box 91, H-1521 Budapest, Hungary 28 Institute of Plasma Physics and Laser Microfusion, Hery 23, 01-497 Warsaw, Poland 29 Institute of Plasma Physics AS CR, Za Slovankou 1782/3, 182 00 Praha 8, Czechia 30 Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Portugal 31 ITER Organization, Route de Vinon, CS 90 046, 13067 Saint Paul Lez Durance, France 32 Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia 33 Karlsruhe Institute of Technology, PO Box 3640, D-76021 Karlsruhe, Germany 34 Laboratoire de Physique des Plasmas, CNRS UMR7648, Ecole Polytechnique, 91128 Palaiseau, France 35 Laboratorio Nacional de Fusión, CIEMAT, Madrid, Spain 36 Laboratory for Plasma Physics Koninklijke Militaire School—Ecole Royale Militaire Renaissancelaan 30 Avenue de la Renaissance B-1000, Brussels, Belgium 37 Max-Planck-Institut für Plasmaphysik, D-85748 Garching, Germany Nucl. Fusion 57 (2017) 102014
H. Meyer etal 4 38 Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald, D-17491 Greifswald, Germany 39 National Institute for Laser, Plasma and Radiation Physics, PO Box MG-36, Bucharest, Romania 40 National Technical University of Athens, Athens, Greece 41 Politecnico di Torino, Dipartimento di Elettronica e Telecomunicazioni (DET), Torino, Italy 42 Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford, UK Culham Centre for Fusion Energy, Abingdon, United Kingdom 43 Department of Physics, Technical University of Denmark, Bldg 309, DK-2800 Kgs Lyngby, Denmark 44 Unità Tecnica Fusione—ENEA C. R. Frascati—via E. Fermi 45, 00044 Frascati (Roma), Italy 45 Universidad de Sevilla. C/ S. Fernando, 4, C.P. 41004-Sevilla, Spain 46 Université de Lorraine, Institut Jean Lamour, Vandoeuvre-lés-Nancy, 54000 Nancy, France 47 Université Grenoble Alpes, LCIS, F26902 Valence, France 48 University College Cork (UCC), Ireland 49 University Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy 50 Department of Physics, University of Helsinki, PO Box 64, FI-00014 University of Helsinki, Finland 51 University of Napoli ‘Federico II’, Consorzio CREATE, Via Claudio 21, 80125 Napoli, Italy 52 University of Napoli Parthenope, Consorzio CREATE, Via Claudio 21, 80125 Napoli, Italy 53 University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy 54 University of Split, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, R. Boskovica 32, 21 000 Split, Croatia 55 VTT Technical Research Centre of Finland, PO Box 1000, FI-02044 VTT, Finland 56 Wigner Research Centre for Physics, PO Box 49, H—1525 Budapest, Hungary 57 York Plasma Institute, Department of Physics, University of York, Heslington, York, YO10 5DD, United Kingdom 58 ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain E-mail: [email protected] Received 18 November 2016, revised 22 December 2016 Accepted for publication 25 January 2017 Published 28 June 2017 Abstract Integrating the plasma core performance with an edge and scrape-off layer (SOL) that leads to tolerable heat and particle loads on the wall is a major challenge. The new European medium size tokamak task force (EU-MST) coordinates research on ASDEX Upgrade (AUG), MAST and TCV. This multi-machine approach within EU-MST, covering a wide parameter range, is instrumental to progress in the field, as ITER and DEMO core/pedestal and SOL parameters are not achievable simultaneously in present day devices. A two prong approach is adopted. On the one hand, scenarios with tolerable transient heat and particle loads, including active edge localised mode (ELM) control are developed. On the other hand, divertor solutions including advanced magnetic configurations are studied. Considerable progress has been made on both approaches, in particular in the fields of: ELM control with resonant magnetic perturbations (RMP), small ELM regimes, detachment onset and control, as well as filamentary scrape-off-layer transport. For example full ELM suppression has now been achieved on AUG at low collisionality with n = 2 RMP maintaining good confinement () ≈H0.95 H98,y2 . Advances have been made with respect to detachment onset and control. Studies in advanced divertor configurations (Snowflake, Super-X and X-point target divertor) shed new light on SOL physics. Cross field filamentary transport has been characterised in a wide parameter regime on AUG, MAST and TCV progressing the theoretical and experimental understanding crucial for predicting first wall loads in ITER and DEMO. Conditions in the SOL also play a crucial role for ELM stability and access to small ELM regimes. Keywords: edge localised modes, divertor, heat loads, ASDEX upgrade, MAST, TCV, alternative divertor concepts (Some figuresmay appear in colour only in the online journal) Nucl. Fusion 57 (2017) 102014
H. Meyer etal 5 1. Introduction One of the key challenges in the realisation of a magnetic confinement fusion power plant is to integrate the highconfinement core with the edge of the plasma, such that acceptable wall conditions are obtained whilst maintaining high performance. In particular the periodic transient heat loads due to edge localised modes (ELM) [1, 2] in the otherwise promising high confinement mode will not be tolerable in ITER and DEMO [3]. Cyclic thermo-mechanical loads may limit the number and size of ELMs to much smaller values than design criteria to prevent melt damage, especially in DEMO. For the achievement of physics understanding of the plasma edge and the ability to extrapolate the findings to future devices, a wide parameter range needs to be investigated. This is a task ideally suited to the new European task force on medium sized tokamaks (EU-MST) that has combined research on three key, complementary devices ASDEX Upgrade (AUG), TCV and MAST since 2014. Research under the EU-MST task force tackles the edge challenge from two sides. On the one hand, plasma regimes reducing the transient heat loads whilst trying to maintain high confinement are developed with active ELM control techniques (section 2) and natural small ELM scenarios (section 3). On the other hand, divertor solutions with detachment control (section 4) and advanced magnetic configurations (section 5) are studied. In this paper we will give an overview of the progress made in the last two years in these two research fields within EU-MST supported by the domestic programs. In addition, we will discuss the enhanced filamentary radial transport in the scrape-off layer (SOL) becoming more important as transient heat loads on the divertor targets are reduced (section 6). Studies have been performed in D, H and He as the main discharge species to assess the feasibility of the methods for the non-nuclear phase in ITER. 2. Reducing the ELM size with actuators Approximations to the ITER baseline scenario on AUG ( ⩽⩽q33.6 95 , /=n n 0.85 G , β=1.8 N ) show that the low q95 and high triangularity lead to low frequency ELMs with exceptionally large energy loss of up to 45% of the pedestal energy on AUG [5] (see figure 1). Whilst sometimes these large energy losses can be attributed to multiple clearly distinguishable crash events sometimes called compound ELMs, here also long ELMs with crash times of the order of up to 8 ms are observed (see below) that show some small substructure in the signal for the divertor current. The relative ELM energy losses in the AUG variant of the ITER base-line scenario lie well above the scaling from Loarte etal [4]. Gas fuelling can reduce the ELM size, but will lead to a degradation of the confinement due to the erosion of pedestal pressure, as also seen on JET [6]. A likely reason for this reduction in pped is the presence of a high density front at the high field side [7], which leads to an outward shift of the density pedestal, in turn leading to a reduction of the peeling-ballooning stability. The scenario has also proven resilient to active ELM mitigation techniques such as pellet triggering or application of resonant magnetic perturbations (RMP). Application of RMPs has led to a clear, though small density pump-out, but not an increase of ELM frequency or reduction in ELM energy loss. Interestingly, a slight vertical upshift approaching a double null configuration or reduction in divertor pumping gives access to a small ELM regime, not unlike the type-II ELM regime (see section 3) [5, 8]. Assessing the performance of the ITER base-line scenario and its variant in high purity He plasmas ( /( )++nnnn80% He He HD ) shows a similar ELM behaviour with larger ELMs ∆ ≈W50 kJ ELM ( /∆ ≈WW10% ELM pl ) at lower neutral density and small ELMs at high neutral density in the divertor. The new neutral beam injection on TCV has enabled studies of these high density regimes on TCV, and scenario development has been started. For single events, the peak parallel ELM energy fluence, () / ∥ ε εα=ssin divdiv with () ∫ ε= qs tt,d div the energy fluence to the divertor target (q heat flux, s spatial target coordinate, αdiv field line angle on the target), of natural type-I ELMs on ASDEX Upgrade and JET never exceeds a value proportional to the pedestal top pressure, pe ped , times the geometric minor radius, a [9]. This data set has now been extended to MAST and discharges with active ELM control, and will be extended to TCV in the future. In figure2 the comparison of the measured () ∥ ε max to a simple ad hoc model of a toroidally symmetric reconnected flux tube () ∥ ε πκπ=∆ +≈ap B B pRma x2 1 2 3 26 ee modeleqil. 2pedt p pedge o is shown (κ: elongation, qcyl: cylindrical safety factor, Rgeo. : geometric major radius) [9, 10], where ⩽⩽∆1.82.3 eqil. accounts for the difference of the flux tube volume between a simplified Figure 1. Relative ELM energy loss of the ITER base-line scenario at q95 = 3 (red) and its q95 = 3.6 (blue) variant in comparison to the data (open symbols) showing the ν∗ scaling from a multi-machine database [4]. The different open symbols represent different devices. Nucl. Fusion 57 (2017) 102014
H. Meyer etal 6 approximation and the accurate value derived from the equilibrium reconstruction. Over two orders of magnitude the data is within a factor of three of this simple prediction, which gives a lower bound to the data. The data set includes data from the inner and outer divertor of AUG, as well as shots with ELM control using pellets and vertical kicks in the case of JET and RMPs in case of AUG. The data presented in figure2 are favourable for ITER, suggesting that even unmitigated type I ELMs in the baseline =Q10 DT scenario at =I15 MA p may lead to target energy densities which are close to, or only slightly exceed (by factors of 2–3) the currently specified ELM energy density limit ( ε=− 0.5MJm tar2 [11]). This is a significant gain compared with the mitigation factors (>20) which were previously thought to be imposed by this limit. The limit was, however, established according to specifications which must now be modified in the light of improved studies of tungsten material response to high transient numbers [3] and refinements in the ITER divertor target design and associated calculations of energy loading. In particular, the presence of target monoblock surface shaping, the inevitable gaps between the monoblocks and the requirement to operate below recrystallization temperature will all lower the previously specified allowed energy density [12]. Work is ongoing at the ITER Organization in collaboration with the EUROFusion Programme (as well as other ITER Partners) to establish a new material limit. In figure2 data of an AUG discharge with RMPs at low collisionality are shown as well. As for the cases of ELM control in JET the RMP data fits into the overall trend. In detail, the application of RMPs increases the ELM frequency on AUG, but also reduces the pedestal pressure due to the density pump out. It should be noted that this trend is only true for cases where the ELMs are still of type I. It is unclear if ELM control/ mitigation techniques for type-I ELMs will be able to move the ELM heat loads from the upper boundary to or below the lower boundary whilst maintaining the pedestal pressure in a robust way. In any case these data suggest that even mitigated type-I ELMs may not be acceptable for the =Q10 DT scenario in ITER and are most likely unacceptable for DEMO. In certain ELM cycles on MAST the density lost due to the application of RMPs could be replaced by gas fuelling whilst maintaining a reduced ELM energy loss [13]. Similar studies have been performed on AUG using pellets fuelling under ITER-like conditions [14]. In these experiments the full density pedestal could be recovered. To refuel a density pump-out of 30% the fuelling rate had to be increased by a factor of two. However, it was not possible to fully recover the loss of confinement as the increase of the density lead to a decrease of temperature and the pedestal pressure could only be partly restored. As increased gas fuelling also affects ELM stability [7, 15, 16], degrades confinement [6, 7] and can be used to control the ELM frequency [17], the increased recycling due to the increased fuelling rate may also play a role in the loss of confinement. Pellet refuelling during RMPs did not trigger further ELMs and a mitigation of the ELM energy loss, albeit at a compromised level, was maintained [14]. Within the scatter of the data the average peak heat load ≈− q4MWm peak 2 during the RMP phase with and without pellets did not change. Comparison to the pre RMP phase, Figure 3. Normalised ELM energy loss as a function of electron density at the pedestal top before the ELM, with and without RMP and two different edge safety factors. Reproduced from [19]. © IOP Publishing Ltd. All rights reserved. Figure 4. Operating space in temperature and density for discharges with and without RMP for different ELM energy losses. Reproduced from [18]. © 2017 Max-Planck-Institut für Plasmaphysik. Figure 2. Measured type-I peak ELM energy fluence against model prediction for a multi-machine database including AUG, JET and MAST [10]. Reproduced with permission from [10]. Nucl. Fusion 57 (2017) 102014
H. Meyer etal 7 however, showed that the application of RMPs in this case only reduced qpeak by a factor of two. This is consistent with the reduction in average normalised ELM energy loss as inferred from the inverse frequency behaviour, implicitly assuming that the energy loss rate by ELMs is the same as the long term energy loss rate. The interplay between increased particle transport (e.g. density pump-out) and the efficiency of ELM control at low collisionality is a key question in extrapolating to future devices that will have low collisionality at high Greenwald fraction [18]. Analysis of the AUG data base of low col lisionality discharges using n = 2 RMP to affect the ELMs has shown that the ELM energy loss correlates best with the edge density (see figure3) and less well with pedestal col lisionality [18, 19]. However, a threshold of ν∗0.4 ped has been found, above which ELMs are not affected (figure 4). At the lowest edge density the loss of stored energy during an ELM can be reduced by 85%, but as can be seen from figure4 not at an isobar corresponding to type I ELMy H-mode [13, 19]. At this parameter regime the low density branch of type III ELMs (also called type IV) is found on DIII-D [20] and MAST [13]. On DIII-D and MAST type- IV regimes can also be accessed without RMPs. Experiments with coil waveforms optimised to achieve a fast switch-off, proved that on AUG it is not only the reduction in density that gives access to the small ELMs. More recently, ELM suppression was observed at low collisionality ⩽ ν0.25 e,ped on AUG (see figure5) [18] in a higher triangularity / / δ=0.23 0.43 ul DIII-D/AUG identity shape. The experiments on DIII-D revealed the crucial role of the triangularity for accessing full ELM suppression motivating the shape change on AUG. The suppression phase, starting at =t2.75 s , is initiated by a reduction in gas fuelling leading to a drop in density between t = 2.30– 2.75s and is accompanied by a further, faster drop in density. Consequently, this leads to a drop in confinement by 25% with respect to the mitigated phase. However, the confinement soon recovers, reaching () H0.95 H98y,2 stably from =t3.45 s onwards. This is comparable to the ELM mitigation phase (t = 2.5– 2.75s ), which in this shape has considerably higher confinement than in the low / / δ=0.10.43 ul shape. A key part of the RMP experiments under EU-MST was directed towards the understanding of the plasma response. Comparing the experimental data with plasma response calculations using the resistive MHD code MARS-F [21–23] confirmed the findings from MAST that the edge kink response needs to be maximised to affect the ELMs and is in good agreement with differential phase scans performed on AUG and MAST. The optimal phase angle for the applied perturbation depends roughly linearly on q95 [23], but also on β [24]. An analytical model based on dedicated scans of MARS-F starting Figure 5. Typical time traces for a low collisionality discharge on AUG where ELM suppression was achieved. Reproduced from [18]. © 2017 Max-Planck-Institut für Plasmaphysik. Nucl. Fusion 57 (2017) 102014
H. Meyer etal 8 from an example equilibrium has been developed to predict the phase angle prior to experiments to within ± 20 [24] by optimising the plasma response with respect to the displacement close to the X-point (kink response) and the perturbation at the outermost resonance. Measurements of the 3D perturbation of the plasma around the mid-plane using several diagnostics by rotating the perturbation field at constant phase angle between the upper and lower coils show good agreement with plasma response modelling using the 3D equilibrium code VMEC as well as MARS-F [25]. The magnetic perturbation was found to be amplified by the plasma and to be primarily non-resonant ||>| |mnq , whilst the displacement is dominated by resonant components ||=| |mnq as predicted by the codes [25]. Resonant modes arise via toroidal and elongation mode coupling [22]. The plasma response calculations have now been extended to the non-ideal MHD code JOREK [26], showing good agreement with VMEC and MARS-F calculations, together with qualitative agreement of the observed change in magnetic mode spectrum during the ELM [27] as well as the filamentary dynamics measured with ECE imaging [28]. ELM energy loss mitigation was also achieved in He discharges at low collisionality. The phase angle of the applied perturbation is similar to that measured in D. Small differences may be explained by the lower β achieved in He. This shows that it should be possible to transfer the experience gained during a potential non-activation He phase on ITER to D and DT. The ELMs are affected by the RMP in He at similar density as in D. However, the lower pedestal temperature in He leads to a much higher collisionality, at which in D ELM control with RMPs is not possible on AUG. This suggests that the collisionality is not the only factor determining the effect of RMPs. It should be noted, though, that ELM mitigation can be achieved on MAST with almost all edge collisionalities [13]. For the higher collisionality ITER base-line scenarios pump-out as in D could be observed, but ELMs became larger at the lower density. The physics of ELM energy loss mitigation at low and high collisionality on AUG is different. At high collisionality the RMP spectrum or alignment does not play a role [29]. Experiments at different plasma current and heating power showed that these mitigated ELMs are also likely a different ELM regime [30]. In contrast to the low collisionality regime, however, this regime also persists without magnetic perturbation (see section3). The potential for ELM control with pellets in metal walled devices is greatly reduced due to a dead time after the previous ELM [31]. Injection of N recovers the trigger potential. Analysis of the inter ELM pedestal evolution on AUG shows that with and without N seeding ELMs can be triggered after the fast density recovery phase [32]. This may be related to the existence of long and short ELMs in all-metal devices [33]. A long ELM, having an extra 2nd phase expelling filaments into the SOL [33], will degrade the pedestal more than a short ELM. Comparisons on AUG, JET and TCV of ELMs with and without N seeding seem to point at the crucial role of the SOL temperature [34] for the existence of the 2nd phase. It should be noted that the estimated () ∥ ε max of N seeded ELMs is higher than of unseeded ELMs due to the faster deposition of the energy onto the target and the smaller wetted area. However, N also leads to earlier detachment of the divertor (see section4). 3. Small ELM regimes Not only type-I ELMs are affected by the SOL; the onset of type-II ELMs is also likely related to the SOL conditions. Comparison of the filamentary structure of type-II ELMs between AUG and MAST suggested the origin of the type-II ELM filaments to be at the foot of the pedestal [35]. Type-II and type-I ELMs can coexist, giving further evidence for their different origin. Recently, the proximity of the ITER base-line scenario on AUG to small (type-II like) ELMs was discovered [5]. Replacing the gas fuelling in these discharges with pellet fuelling from the high field side ( =l1.9mm , =f35 Hz , =− v552 ms 1 , Γ=× − 1.310Ds pellet 22 1 ) reduced the SOL density, as can be seen in figure6 (right). The pedestal pressure was kept constant, and the gradients of temperature and density are also the same within the error bars in both phases. However, the pellet fuelled phase has type-I ELMs, whilst the gas fuelled phase exhibits small ELMs. The pellets penetrate roughly to ρ≈0.8 and fuel the plasma about three times more efficiently than the gas puff ( Γ=× − 3.310Ds gas 22 1 ). Experiments using n = 2 magnetic perturbations (MP) during strong fuelling ramps revealed the importance of the increasing intermittent transport for the occurence of small ELMs [30]. Discharges with three different plasma currents ( =I0.6, 0.8and 1.0MA p) and two different heating levels ( =P6.3and 8.7MW heat ) were performed in order to try to separate collisionality from density. Here, type-I ELMs and small filaments coexist at lower fuelling levels and are fully replaced by small ELMs as the fuelling is increased. At the highest fuelling levels small ELMs persist even without MP. Hence, the MP is not necessary for the small ELMs to occur. In figure7 the electron density (left) and temperature (right) for three time points for the =I0.6 p MA case are shown. With increasing fuelling ne sep and ne ped rise. With increasing ν the filaments become larger and form the ne shoulder in the SOL. The larger filaments occur together with a wider near SOL mid-plane Te decay length, the pedestal becomes wider and its gradient is shallower, due to the lower Teped . The pedestal is (filamentary) transport limited and not peeling–ballooning limited as for type-I ELMs. The perturbation of the equilibrium by the MP leads to lobe structures, Figure 6. Comparison of the electron temperature (left) and density (right) profiles in a gas fuelled phase (red) and dominantly pellet fuelled phase (blue) from Thomson scattering at otherwise constant discharge parameters. Nucl. Fusion 57 (2017) 102014
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