Physics research on the TCV tokamak facility: from conventional to alternative scenarios and beyond
Abstract
The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device’s unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with electron-cyclotron resonance heating (ECRH) was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1 MW neutral beam injector has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and nearly noninductive H-mode discharges sustained by electron cyclotron and neutral beam current drive. In the H-mode, the pedestal pressure increases modestly with nitrogen seeding while fueling moves the density pedestal outwards, but the plasma stored energy is largely uncorrelated to either seeding or fueling. High fueling at high triangularity is key to accessing the attractive small edge-localized mode (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The geodesic acoustic mode, possibly coupled with avalanche events, has Nucl. Fusion 59 (2019) 112023 S. Coda et al 4 been linked with particle flow to the wall in diverted plasmas. Detachment, scrape-off layer transport, and turbulence were studied in L- and H-modes in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power ‘starvation’ reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in the H-mode is obtained with impurity seeding and has shown little dependence on flux expansion in standard single-null geometry. In the attached L-mode phase, increasing the outer connection length reduces the in–out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variable configuration baffles and possibly divertor pumping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5 MW ECRH and 1 MW neutral beam injection heating will be added.
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PAPER • OPEN ACCESS Physics research on the TCV tokamak facility: from conventional to alternative scenarios and beyond To cite this article: S. Coda et al 2019 Nucl. Fusion 59 112023 View the article online for updates and enhancements. Recent citations Modeling of the response function of CdTe detectors for a hard X-ray synthetic diagnostic D. Choi et al - Classification of tokamak plasma confinement states with convolutional recurrent neural networks F. Matos et al - 27th IAEA Fusion Energy Conference: summary of sessions EX/C, EX/S and PPC D.J. Campbell - This content was downloaded from IP address 87.218.223.151 on 30/06/2020 at 10:54
1 © EURATOM 2019 Printed in the UK Nuclear Fusion Physics research on the TCV tokamak facility: from conventional to alternative scenarios and beyond S.Coda1, M.Agostini2, R.Albanese3, S.Alberti1, E.Alessi4, S.Allan5, J.Allcock5, R.Ambrosino6, H.Anand7, Y.Andrèbe1, H.Arnichand1, F.Auriemma2, J.M.Ayllon-Guerola8,9, F.Bagnato1, J.Ball1, M.Baquero- Ruiz1, A.A.Beletskii10, M.Bernert11, W.Bin4, P.Blanchard1, T.C.Blanken12, J.A.Boedo13, O.Bogar14, T.Bolzonella2, F.Bombarda15, N.Bonanomi16, F.Bouquey17, C.Bowman18, D.Brida11, J.Bucalossi17, J.Buermans19, H.Bufferand17, P.Buratti15, G.Calabró20, L.Calacci21, Y.Camenen22, D.Carnevale21, F.Carpanese1, M.Carr23, L.Carraro2, A.Casolari14, F.Causa4, J.Čeřovský14, O.Chellaï1, P.Chmielewski24, D.Choi1, N.Christen25, G.Ciraolo17, L.Cordaro2, S.Costea26, N.Cruz27, A.Czarnecka24, A.Dal Molin16, P.David11, J.Decker1, H.De Oliveira1, D.Douai17, M.B.Dreval10, B.Dudson18, M.Dunne11, B.P.Duval1, T.Eich11, S.Elmore23, O.Embréus28, B.Esposito15, M.Faitsch11, M.Farník14, A.Fasoli1, N.Fedorczak17, F.Felici1, S.Feng16, X.Feng5, G.Ferró21, O.Février1, O.Ficker14, A.Fil18, M.Fontana1, L.Frassinetti29, I.Furno1, D.S.Gahle30, D.Galassi1, K.Gałązka24, A.Gallo17, C.Galperti1, S.Garavaglia4, J.Garcia17, M.Garcia-Muñoz8,31, A.J.Garrido32, I.Garrido32, J.Gath33, B.Geiger34, G.Giruzzi17, M.Gobbin2, T.P.Goodman1, G.Gorini16, M.Gospodarczyk21, G.Granucci4, J.P.Graves1, M.Gruca24, T.Gyergyek35, A.Hakola36, T.Happel11, G.F.Harrer37, J.Harrison23, E.Havlíčková23, J.Hawke1, S.Henderson23, P.Hennequin38, L.Hesslow28, D.Hogeweij39, J.-Ph.Hogge1, C.Hopf11, M.Hoppe28, J.Horáček14, Z.Huang34, A.Hubbard40, A.Iantchenko1, V.Igochine11, P.Innocente2, C.Ionita Schrittwieser26, H.Isliker41, R.Jacquier1, A.Jardin42, A.Kappatou11, A.Karpushov1, P.-V.Kazantzidis43, D.Keeling23, N.Kirneva44,45, M.Komm14, M.Kong1, J.Kovacic35, N.Krawczyk24, O.Kudlacek11, T.Kurki-Suonio46, R.Kwiatkowski47, B.Labit1, E.Lazzaro4, B.Linehan40, B.Lipschultz18, X.Llobet1, R.Lombroni20, V.P.Loschiavo3, T.Lunt11, E.Macusova14, J.Madsen33, E.Maljaars12, P.Mantica4, M.Maraschek11, C.Marchetto4, A.Marco48, A.Mariani4, C.Marini49, Y.Martin1, F.Matos11, R.Maurizio1, B.Mavkov50, D.Mazon17, P.McCarthy51, R.McDermott11, V.Menkovski12, A.Merle1, H.Meyer23, D.Micheletti4, F.Militello23, K.Mitosinkova14, J.Mlynář14, V.Moiseenko10, P.A.Molina Cabrera1, J.Morales17, J.-M.Moret1, A.Moro4, R.T.Mumgaard40, V.Naulin33, R.D.Nem33, F.Nespoli22, A.H.Nielsen33, S.K.Nielsen33, M.Nocente16, S.Nowak4, N.Offeddu1, F.P.Orsitto3, R.Paccagnella2, A.Palha12, G.Papp11, A.Pau52, R.O.Pavlichenko10, A.Perek39, V.Pericoli Ridolfini24, F.Pesamosca1, S. Coda etal Printed in the UK 112023 NUFUAU © EURATOM 2019 59 Nucl. Fusion NF 10.1088/1741-4326/ab25cb Paper 11 Nuclear Fusion IOP 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. International Atomic Energy Agency 2019 1741-4326 1741-4326/19/112023+17$33.00 https://doi.org/10.1088/1741-4326/ab25cb Nucl. Fusion 59 (2019) 112023 (17pp)
S. Coda etal 2 V.Piergotti15, L.Pigatto2, P.Piovesan2, C.Piron2, V.Plyusnin27, E.Poli11, L.Porte1, G.Pucella15, M.E.Puiatti2, T.Pütterich11, M.Rabinski47, J.Juul Rasmussen33, T.Ravensbergen12,39, M.Reich11, H.Reimerdes1, F.Reimold53, C.Reux17, D.Ricci4, P.Ricci1, N.Rispoli4, J.Rosato22, S.Saarelma23, M.Salewski33, A.Salmi36, O.Sauter1, M.Scheffer12, Ch.Schlatter1, B.S.Schneider26, R.Schrittwieser26, S.Sharapov23, R.R.Sheeba22, U.Sheikh1, R.Shousha12, M.Silva1, J.Sinha7, C.Sozzi4, M.Spolaore2, L.Stipani1, P.Strand28, T.Tala36, A.S.Tema Biwole1, A.A.Teplukhina54, D.Testa1, C.Theiler1, A.Thornton23, G.Tomaž35, M.Tomes14, M.Q.Tran1, C.Tsironis43, C.K.Tsui1,13, J.Urban14, M.Valisa2, M.Vallar2, D.Van Vugt12, S.Vartanian17, O.Vasilovici26, K.Verhaegh18, L.Vermare38, N.Vianello2, E.Viezzer8,31, W.A.J.Vijvers39, F.Villone3, I.Voitsekhovitch23, N.M.T.Vu1, N.Walkden23, T.Wauters19, M.Weiland11, H.Weisen1, M.Wensing1, M.Wiesenberger33, G.Wilkie28, M.Wischmeier11, K.Wu20, M.Yoshida55, R.Zagorski24, P.Zanca2, J.Zebrowski47, A.Zisis56, M.Zuin2 and the EUROfusion MST1 Teama 1 Ecole Polytechnique Fédérale de Lausanne (EPFL), Swiss Plasma Center (SPC), 1015 Lausanne, Switzerland 2 Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy 3 University of Napoli ‘Federico II’, Consorzio CREATE, Via Claudio 21, 80125 Napoli, Italy 4 IFP-CNR, via R. Cozzi 53, 20125 Milano, Italy 5 Department of Physics, Durham University, Durham, DH1 3LE, United Kingdom of Great Britain and Northern Ireland 6 University of Napoli Parthenope, Consorzio CREATE, Via Claudio 21, 80125 Napoli, Italy 7 ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, 13067 St.-Paul-lez-Durance Cedex, France 8 Centro Nacional de Aceleradores (CNA), Universidad de Sevilla, Junta de Andalucia, Consejo Superior de Investigaciones Cientificas (CSIC), Parque Cientifico y Tecnologico Cartuja, c/Thomas Alva Edison, 7, 41092 Sevilla, Spain 9 Department of Mechanical Engineering and Manufacturing, University of Seville, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain 10 Institute of Plasma Physics, National Science Center, Kharkov Institute of Physics and Technology, 61108 Kharkov, Ukraine 11 Max-Planck-Institut für Plasmaphysik, 85748 Garching, Germany 12 Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, Netherlands 13 University of California, San Diego, Energy Research Center, La Jolla, CA 92093, United States of America 14 Institute of Plasma Physics AS CR, Za Slovankou 1782/3, 182 00 Praha 8, Czech Republic 15 Unità Tecnica Fusione, ENEA C. R. Frascati, via E. Fermi 45, 00044 Frascati (Roma), Italy 16 Department of Physics ‘G. Occhialini’, University of Milano-Bicocca, Piazza Della Scienza 3, 20126 Milano, Italy 17 CEA, IRFM, 13108 Saint Paul Lez Durance, France 18 York Plasma Institute, Department of Physics, University of York, Heslington, York, YO10 5DD, United Kingdom of Great Britain and Northern Ireland 19 Laboratory for Plasma Physics, Koninklijke Militaire School—Ecole Royale Militaire, Renaissancelaan 30 Avenue de la Renaissance, 1000 Brussels, Belgium 20 Department of Economics, Engineering, Society and Business Organization (DEIm), University of Tuscia, Largo dell’Università snc, 01100 Viterbo, Italy 21 University of Rome Tor Vergata, via del Politecnico 1, 00133 Rome, Italy 22 Aix-Marseille Université, CNRS, PIIM, 13013 Marseille, France 23 CCFE, Culham Science Centre, Abingdon, Oxon, OX14 3DB, United Kingdom of Great Britain and Northern Ireland 24 Institute of Plasma Physics and Laser Microfusion, Hery 23, 01-497 Warsaw, Poland 25 Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford, Oxon, OX1 3PU, United Kingdom of Great Britain and Northern Ireland 26 Institut für Ionen- und Angewandte Physik, Universität Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria 27 Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal a See the author list of Meyer et al [2]. Nucl. Fusion 59 (2019) 112023
S. Coda etal 3 28 Department of Physics, Chalmers University of Technology, 41296 Gothenburg, Sweden 29 Fusion Plasma Physics, EES, KTH, 10044 Stockholm, Sweden 30 Department of Physics SUPA, University of Strathclyde, Glasgow, G4 0NG, United Kingdom of Great Britain and Northern Ireland 31 Department of Atomic, Molecular and Nuclear Physics, University of Seville, 41012 Sevilla, Spain 32 Faculty of Engineering, University of the Basque Country (UPV/EHU), Paseo Rafael Moreno 3, 48013 Bilbao, Spain 33 Department of Physics, Technical University of Denmark, Bldg 309, 2800 Kgs Lyngby, Denmark 34 Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald, 17491 Greifswald, Germany 35 Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia 36 VTT Technical Research Centre of Finland Ltd, PO Box 1000, 02044 VTT, Finland 37 Institute of Applied Physics, T.U. Wien, Fusion@ÖAW, Wiedner Hauptstr. 8-10, 1040 Vienna, Austria 38 Laboratoire de Physique des Plasmas, CNRS UMR7648, Ecole Polytechnique, 91128 Palaiseau, France 39 FOM Institute DIFFER ‘Dutch Institute for Fundamental Energy Research’, 5600 HH Eindhoven, Netherlands 40 Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA 02139, United States of America 41 Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece 42 Institute of Nuclear Physics Polish Academy of Sciences (IFJ PAN), 31-342 Krakow, Poland 43 National Technical University of Athens, 10682 Athens, Greece 44 Institute of Physics of Tokamaks, National Research Center ‘Kurchatov Institute’, 123182 Kurchatov Sq., 1, Moscow, Russian Federation 45 National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), 115409, Kashirskoe Sh., 31, Moscow, Russian Federation 46 Department of Applied Physics, Aalto University, 00076 Aalto, Finland 47 National Centre for Nuclear Research (NCBJ), 05-400 Otwock, Poland 48 Advanced Design & Analysis Department, IDOM, 48015 Bilbao, Spain 49 General Atomics, PO Box 85608, San Diego, CA 92186, United States of America 50 Department of Electrical and Electronic Engineering, University of Melbourne, Victoria 3010, Australia 51 Department of Physics, University College Cork, Cork, Ireland 52 Department of Electrical and Electronic Engineering, University of Cagliari, Piazza d’Armi, 09123 Cagliari, Italy 53 Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung—Plasmaphysik, 52425 Jülich, Germany 54 Princeton University, Princeton, NJ 08544, United States of America 55 National Institutes for Quantum and Radiological Science and Technology, Naka, Ibaraki 311-0193, Japan 56 Department of Physics, National and Kapodistrian University of Athens, 15784 Athens, Greece E-mail: [email protected] Received 23 November 2018, revised 13 May 2019 Accepted for publication 30 May 2019 Published 30 August 2019 Abstract The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device’s unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with electron-cyclotron resonance heating (ECRH) was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1 MW neutral beam injector has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and nearly noninductive H-mode discharges sustained by electron cyclotron and neutral beam current drive. In the H-mode, the pedestal pressure increases modestly with nitrogen seeding while fueling moves the density pedestal outwards, but the plasma stored energy is largely uncorrelated to either seeding or fueling. High fueling at high triangularity is key to accessing the attractive small edge-localized mode (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The geodesic acoustic mode, possibly coupled with avalanche events, has Nucl. Fusion 59 (2019) 112023
S. Coda etal 4 been linked with particle flow to the wall in diverted plasmas. Detachment, scrape-off layer transport, and turbulence were studied in L- and H-modes in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power ‘starvation’ reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in the H-mode is obtained with impurity seeding and has shown little dependence on flux expansion in standard single-null geometry. In the attached L-mode phase, increasing the outer connection length reduces the in–out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variableconfiguration baffles and possibly divertor pumping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5 MW ECRH and 1 MW neutral beam injection heating will be added. Keywords: nuclear fusion, tokamak, overview, TCV, MST1, EUROfusion (Some figuresmay appear in colour only in the online journal) 1. Introduction The tokamak à configuration variable (TCV) [1] is a mature European fusion facility, with numerous experiments conducted by international teams organized by the EUROfusion consortium through the medium-size tokamak (MST1) Task Force [2], in parallel with a nearly continuous, self-man- aged domestic campaign. A versatile device with unparalleled shaping capabilities and flexible heating systems (electron-cyclotron resonance heating (ECRH) and neutral beam heating (NBH)), TCV is employed in a multi-faceted research program ranging from conventional topologies and scenarios in support of ITER, to advanced tokamak scenarios and a broad palette of alternative divertor configurations with an eye to DEMO, to exploratory plasmas driven by theoretical speculation and insight. A strong link with academia and education is enforced organically by TCV’s nature as a university facility. As such, generous machine time is provided for training students, who in return provide an essential service as full members of the experimental and operating team. This environment is also naturally conducive to close and productive links with the SPC theory group, which has a strong tradition of analytical and numerical first-principles enquiry, while also managing a panoply of higher-level, interpretation-oriented codes. The main operating parameters of TCV are as follows: major radius R = 0.88 m, minor radius a = 0.25 m, vacuum toroidal field BT = 1.5 T, plasma current up to Ip = 1 MA. The polarities of both field and current can be chosen at will in any discharge. The primary wall-facing material is graphite. Three piezoelectric valves are used for injection of both the primary discharge fuel and seed impurities; an additional, fast, solenoid-based multi-valve system is available for disruption mitigation through massive gas injection (MGI) (recently upgraded from a previous version for greatly increased gas flow). The defining shaping versatility of the device is provided by a system of 16 independently-powered shaping poloidal-field (PF) coils, in addition to two coils internal to the vessel for control of axisymmetric instabilities with growth rates up to 5000 s−1. During most of the device’s lifetime, its primary auxiliary heating source has been ECRH, in a combination of second- (X2, 82.7 GHz) and third-harmonic (X3, 118 GHz) X-mode components with a maximum aggregate power of 4.1 MW, injected through up to seven independent launchers [3]. The finite life expectancy of the gyrotron sources has led to a gradual reduction of this power to a current total of 1.15 MW. We are currently in the process of procuring four additional gyrotrons, two 0.75 MW units for X2 waves and two 1 MW dual-frequency units for either X2 or X3 [4]. By the end of 2019 we thus expect to have 3.3 MW X2 and 3.1 MW X3 available at the tokamak end (with a maximum simultaneous total power of 4.5 MW), restoring the plant’s erstwhile flexibility in both localized and diffuse heating at virtually all plasma locations in virtually all configurations, with a varying mix of heating and current drive. Since 2015, NBH has also been employed on TCV, using a 15–25 keV beam of maximum 1 MW power (at the highest energy), in a tangential geometry affording a double pass through the plasma cross-section [5, 6]. A second 1 MW injector, directed in the opposite direction and featuring an energy of 50–60 keV, is currently being planned for the 2020 horizon. The experimental campaigns are assisted by a continuous program of diagnostic upgrades and development. The Thomson scattering diagnostic was upgraded with the addition of 40 new spectrometers and a redesign of the optical layout to guarantee a more complete coverage of the plasma in all configurations, without spatial gaps and with increased energy resolution particularly for edge measurements [7]. A three-radiator Cherenkov detector was deployed in support of runaway-electron experiments in a collaboration with the National Centre for Nuclear Research in Poland [8]. Runaway studies were also assisted by a runaway electron imaging and spectrometry system detecting infrared and visible synchrotron radiation, on loan from ENEA (Italy) [9]. Tangential, multi-spectral, visible-light camera arrangements have been installed on TCV by groups from MIT (USA) [10] and Eindhoven University of Technology (The Netherlands) [11]. TCV was also equipped recently with a Doppler backscattering Nucl. Fusion 59 (2019) 112023
S. Coda etal 5 apparatus and a highly novel short-pulse time-of-flight reflectometer, used alternately as they share most of the hardware, including a steerable quasi-optical antenna [12, 13]. This paper reports on scientific results from the past twoyear period, during which TCV was operated regularly without major interruptions. Several of the experiments described in this paper also had counterparts in the other operating MST facility, ASDEX Upgrade (AUG) [14]. Section 2 discusses work on abnormal discharge termination events, including disruptions and runaway-electron beam formation; section3 reports on discharge scenario development and associated real-time control; section 4 deals with core physics, particularly the related issues of transport and turbulence for both the thermal and non-thermal populations; section5 is on edge and exhaust physics and detachment, both in conventional and alternative-divertor scenarios; the first successful generation of a doublet configuration is discussed in section6; conclusions and an outlook, including the description of a significant upcoming divertor upgrade, are provided in section7. 2. Disruption physics This area had not received sustained attention in the past on TCV and its recent rise to prominence is a particularly good demonstration of successful international collaborations, particularly as catalyzed by the EUROfusion framework. 2.1. Disruptions Issues related to unwanted discharge termination, being at the forefront of reactor designers’ concerns [15], are addressed vigorously in the TCV program. A path-oriented approach [16] has been advanced to deal with the changed perspective of the reactor scale, which remains grounded in safety but has to be mindful of economics. While device integrity remains paramount, value is attached to keeping peak performance as well. This yields a prioritized hierarchy of full performance recovery, disruption avoidance, and disruption mitigation—all of which are dependent upon the specific disruption path [16]. Experiments were performed in parallel on AUG and TCV. On TCV the focus was on disruptions caused by abnormal impurity inflow precipitating a disruptive event caused by the locking of a pre-existing n = 1 neoclassical tearing mode (NTM) on the q = 2 surface. The impurity inflow was simulated by a massive, controlled injection of a noble gas such as neon. The event detector was a sharp increase in radiation, specifically soft x-ray emission. The application of ECCD on the q = 2 surface can prevent the locking altogether or, if applied with some delay, unlock and stabilize the mode, still preventing the disruption. Both full performance recovery and soft landing paths were explored and documented [17]. Higher power and better precision are required to unlock the mode than to prevent locking (figure 1). Prevention was enabled by real-time triggers based on maximum entropy and maximum likelihood techniques applied to magnetic signals [18], or on radiation thresholds. Safe discharge termination through controlled current ramp-down (to 50 kA) was also tested successfully. All these techniques were finally combined in a first prototype closed-loop feedback system including tracking of the q = 2 surface through real-time equilibrium reconstruction [17] and ECRH ray tracing [19]. A path-oriented approach is bound to be costly as the paths to disruption form a large and heterogeneous set, but the techniques developed for the specific path described above have some degree of generic applicability and hold promise for generalization. A version of the strategy described above is under development in the general architecture of supervisory real-time control, including event monitoring through plasmaposition and rotating- and locked-mode detection [20], and incorporating actuator management tools [21]. In parallel, an algorithm to detect proximity to the density limit based on changes in sawtooth characteristics was also developed successfully. An offline disruption database was also constructed using the DIS_tool package, for statistical analysis of disruption triggers and as a basis for prediction and modelling (to be performed using a machine-learning technique already applied successfully to JET [22]). By processing multiple diagnostics, DIS_tool is able to detect fast transient events characterizing the disruptive process, such as thermal quenches and current spikes, and to automatically compute characteristic times and parameters of interest. The parametrization of the algorithm renders it independent of the characteristics of the individual device [23, 24]. 2.2. Runaway electrons Another facet of the discharge termination problem is the production of runaway electrons (RE), which is also a central concern of reactor operation. In ITER, RE beams of up to 12 MA can be expected, with the potential to cause deep damage to the metallic structures. Mitigation and control of RE beams are thus a must. Runaway generation both in steady state and disruptive conditions, at low density and with the aid of high-Z noble gas injection, has been documented in TCV. RE beams are generated on TCV for a broad range of edge safety factor, down to 2.1, and at elongations up to 1.5. Key data on runaway dissipation by subsequent Ne and Ar injection are being used for validation of a high-Z interaction model. The increased throughput of the new MGI valve system has allowed us to quantify the increase in RE dissipation rate with gas injection rate and its dependence on gas species, Ar being more effective than Ne. Initial studies of the effect of ECRH on the RE beam have also been performed. Secondary RE avalanching was identified and quantified for the first time after massive Ne injection; simulations of the primary RE generation and secondary avalanching dynamics in stationary discharges indicate that the RE current fraction created via avalanching could reach 70%–75% of the total plasma current. Relaxation events consistent with RE losses caused by the excitation of kinetic instabilities are also observed [25]. An extensive set of experiments were performed to study the options for controlled ramp-down in the presence of a disruptive event featuring a RE beam. Upon detection of the Nucl. Fusion 59 (2019) 112023
S. Coda etal 6 current quench and plateau onset (via current and hard x-ray observers), a dedicated controller takes over. The ‘hybrid fast controller’, initially developed for the Frascati tokamak upgrade, is empirical, lightweight, easily tunable, and portable [26]. The current ramp-down is controlled through the Ohmic transformer, with assistance from MGI to limit the RE beam’s energy, and the beam position is controlled through the PF coils. The ramp-down rate is kept below a threshold to avoid the appearance of deleterious magnetohydrodynamic (MHD) instabilities that can engender a total loss of control. This control scenario appears robust and reproducible (figure 2), and intriguingly, a total conversion of RE current into thermal plasma current has also been observed. This is speculatively attributed to loop-voltage oscillations coupled with system hysteresis, and hints at a possible new termination scenario [26]. 3. Scenario development 3.1. Main reactor scenarios In the most recent campaign a stable ELMy H-mode was obtained in an ITER-like shape permitting direct scaling comparisons with the corresponding AUG scenario [27]. Work in this area has been hampered by the empirical and unexplained elusiveness of regular edge-localized modes (ELMs) in configurations centered near the vessel midplane, where NBH is located. Of different plasma shapes attempted, the most resilient has proven to be one with low upper triangularity (δ) and high lower δ, which has been taken to q95 = 3.6 at an elongation of 1.8, with NBH alone as well as NBH+X3. The application of a more ELM-resilient vertical observer in the future could alleviate some of the difficulties with these scenarios and allow also for stable operation at q95 = 3. The I-mode [28] has also been pursued, primarily mimicking the equivalent low-δ shape of AUG. NBH not being sufficient to reach I- or H-mode in this configuration, X3 was added, unsuccessfully at BT = 1.35 T but with some promising recent developments at BT = 1.53 T. The goal of the Advanced Tokamak route was to extend to higher βN, using NBH and X3, well-known fully noninductive scenarios with internal transport barriers (ITBs) and high bootstrap current fraction achieved in the past in TCV with X2 ECCD. It has not proven possible thus far to obtain non-inductive ITBs with NBH, its diffuse or central heating and modest current drive contribution being detrimental to the establishment of negative magnetic shear in the center. Conversely, ITBs with ECCD and NBH were studied for the first time, but could not be sustained non-inductively [29]. Non-inductive discharges in L-mode were sustained at Figure 1. (a) Disruption prevention versus (b) disruption avoidance: ECRH power (top), stored energy (middle), plasma current (bottom). The disruption prevention plots collect three discharges with identical deposition location for the sources used to destabilize the mode (‘scenario gyrotrons’) and varying deposition location for the electron cyclotron current drive (ECCD) source used for stabilization, showing that ECCD is effective at stabilizing when it is applied on or just inside the q = 2 surface. In this case, one 100 kW ECCD source is sufficient to restore discharge performance when mode locking is prevented. The right-hand plots show that, once the discharge enters the disruptive chain, 500 kW for 150 ms or 800 kW for 110 ms is required for recovery (only the power used for stabilization is plotted here): the times of mode unlocking are indicated by vertical lines in the middle plot. Note that these discharges do disrupt eventually during a controlled rampdown. By contrast, the disruptive reference with no stabilizing action shown for comparison (in blue) disrupts during the flat top as a result of the locked mode. Reproduced courtesy of IAEA. Figure [17]. Copyright (2018) IAEA. Nucl. Fusion 59 (2019) 112023
S. Coda etal 7 Ip = 130 kA, H-factor H98(y ,2) = 0.8, and βN = 1.4. A successful attempt at H-mode was made in nearly non-inductive conditions, by targeting a low enough density in H-mode to be compatible with X2 heating. The result is shown in figure3, with H98(y ,2) = 1 and βN = 1.7 [30]. The neutral-beam deposition and fast-ion dynamics are being modeled with the nubeam-ascot code suite for these scenarios [31]. The additional power expected for the next campaign holds promise for improving performance. Finally, TCV has long established the merits of negative triangularity, which is now being considered as a serious candidate for a test reactor. All this work was performed in limited shapes and, more recently, in diverted shapes with negative upper triangularity. Finally, stable, negative-trian- gularity single-null-diverted shapes, fully mirroring conventional diverted discharges, were developed for the first time in the last campaign but have not yet been successfully established in a NBH-compatible location. 3.2. Real-time control Many achievements in the past campaign were either made possible or at least aided by advancements in plasma control. In this subsection progress on different specific aspects of control will be presented briefly in turn, concluding with the work performed on integration and unification. In the area of MHD control, work on NTMs continues to feature prominently, with increasing refinements in characterization and understanding. It has been determined that NTM destabilization through central co-ECCD only occurs within a given density range [32]. For the first time, the application of a periodic (sinusoidal) deposition-location sweep has been shown to be effective for both pre-emption and stabilization of the (2,1) NTM, the latter requiring more than twice as much power as the former. A simple new analytical model for the time history of the magnetic Δ′ stability index, for NTMs triggered as classical tearing modes, was introduced and shown to provide accurate simulations of the island evo lution [33]. Quasiin-line ECE, nearly counter-linear with the associated ECRH actuator, was tested on TCV for monitoring the island’s position, and was demonstrated to be accurate to within less than the EC beam width [34]. Though receiving less attention than NTMs in recent times, the vertical axisymmetric instability also remains a concern; while the (magnetic) stabilization technique is well understood, its economics are strongly affected by the minimum achievable stability margin in any given device. In this perspective, experiments were carried out in TCV using elongation ramps to provide data for model validation [35]. In a unique multi-institutional collaborative effort, TCV functioned as a testbed for an eclectic ensemble of currentprofile control strategies, within a unified framework using the tokamak profile simulator raptor for offline testing. Various highly specialized controllers (model-predictive [36], Lyapunov-based [37], and interconnection and damping assignment-passivity based control [38]) were all tested and Figure 2. Demonstration of RE ramp-down with the hybrid fast controller with vertical 11 cm sweep. Plasma current (top), hard x-ray emission (middle), vertical position of magnetic axis (bottom). Reproduced from [26]. © IOP Publishing Ltd. All rights reserved. Figure 3. Nearly fully non-inductive H-mode discharge: (top) plasma current and loop voltage; (middle) auxiliary power; (bottom) βN, H98(y ,2), Dα emission. Reproduced with permission from [30]. Nucl. Fusion 59 (2019) 112023
S. Coda etal 8 validated successfully using this paradigm. A parallel activity has seen the development of alternative, exploratory current control methods—based on so-called sliding mode and supertwisting controllers—specifically for TCV, yet to be tested [39]. In a related development, a model-based detector of L–H and H–L transitions and of ELMs was used successfully to actuate a power reduction and consequently a back-transition to L-mode, thus avoiding the disruptions that typically terminate ELM-free H-modes. A shape and position controller, using boundary flux errors and based on a singular-value decomposition approach, was delivered in a complete time-varying version and applied in particular to advanced divertor configurations such as snowflakes [40]. One limitation of this control scheme is its inherent coupling with vertical stability control, making its optimization highly dependent on the particular configuration. To obviate this problem, a new, decoupled set of controllers is currently under development, with promising initial tests already performed on TCV [41]. The raptor code, updated with new time-varying terms [42], was employed in a general effort towards the optimization of the ramp-down phase of tokamak discharges, using appropriate transport models including the L–H and H–L transition dynamics. The optimization is found to include in particular an early H–L transition and a sharp elongation reduction to reduce the internal inductance. Promising initial tests were performed on TCV, pointing the way to possible automation of the optimization procedure [43]. Real-time equilibrium reconstruction, now routinely available on TCV with sub-ms resolution (rtliuqe), is at the crux of modern tokamak control. From this consideration stems the need to improve over simple magnetic reconstruction, by using kinetic constraints available in real time: a kinetic equilibrium reconstruction suite compatible with real-time needs has accordingly been developed for TCV [44]. In parallel, efforts towards a unified European reconstruction code have turned an eye to TCV as a particularly challenging reconstruction problem, and first reconstructions with the equal/ equinox codes have been obtained and benchmarked with rtliuqe [45]. Controller integration is steadily progressing: NTM, βN, safety factor (estimated by raptor), density, and shape were shown to be controlled simultaneously. Key to this is the constant development and addition of new elements as need dictates: NBH power control and readback, ECRH power and launcher readback were recently incorporated in the digital control system; the torbeam (real-time ECRH beam-tracing) [46] module was also added and rabbit (NBH deposition) [47] is currently being integrated. A shift from controllerbased to task-based control is underway. The architecture for task-based integrated control separates state estimation and event detection from decisions related to actuators. A superviso ry controller coordinates the execution of multiple control tasks by assigning priorities based on the plasma state and on the discharge [21]. Crucially, this entire layer is tokamak-agnostic by construction, providing a level of abstraction to discharge planning [48]. The tokamak-specific interfaces are also standardized to minimize exceptions. New controllers may be tested and integrated continuously using a unified controller test environment comprising raptor and several common algorithms. 3.3. Wall cleaning and start-up assist with ECRH in support of JT-60SA operation TCV has been used to test techniques anticipated for the successful operation of JT-60SA [49], which will also feature a carbon wall. Characterization of wall cleaning with ECRH, as a substitute for glow discharge cleaning (GDC), has continued from the previous campaign [50]. Additionally, experiments were performed on ECRH-assisted start-up at reduced loop voltage (electric field 0.7 V m−1, consistent with JT-60SA) with residual gas and/or impurities, such as would be expected after a disruption or generally with a shortened shot cycle. The question addressed by this work is of equal importance for DEMO. The minimum ECRH power required for breakdown and successful burn-through was determined by controlling the power from a plasma-current observer. Experimental tests included variations in deuterium prefill, reductions in inter-shot pumping speed (down to 25%), and puffing of Ar impurities, always without the GDC customarily performed between TCV discharges. It was found that 0.4 MW ECRH at the reduced 0.7 V m−1 electric field was sufficient to start the plasma and sustain the plasma current, except with Ar injection, which increased the threshold. The results of this experiment are used to validate the 0D breakdown code bkd0, which is coupled with the beam tracing code gray to model the ECRH propagation [51]. 4. Pedestal and core physics 4.1. Transport and confinement The physics mechanisms underlying the different scenarios are explored through systematic parametric studies making use of all available diagnostics. The H-mode pedestal is under particular scrutiny, as it can play a formidable role in determining the global confinement in conditions of stiff core profiles. As both fueling and impurity seeding will likely be necessary in a reactor, the latter for heat load control, it is especially important to determine their effect on the pedestal and on confinement. Such a systematic study was performed on TCV in the latest campaign, in a type-I ELMy H-mode, with the addition of a shaping variable, i.e. triangularity. Specifically, a deuterium gas fueling scan and two nitrogen puffing scans—one with no fueling and one with constant fueling—were performed at two different values of the triangularity [52]. It is found that D2 fueling increases the density pedestal height and moves it outwards; interestingly, the shift is in the opposite direction to AUG [53], a metal-wall machine where it is speculated that the high-field-side high-density front observed there could play a role. The pressure pedestal height displays a generally decreasing trend with increased gas injection (figure 4). The sensitivity to fueling and seeding increases with triangularity. However, the total stored energy is largely unaffected by these changes, indicating that the core profiles do Nucl. Fusion 59 (2019) 112023
S. Coda etal 15 fraction. Unlike on AUG [95], no clear correlation is found between the shoulder appearance and either filament size or divertor collisionality. As filaments in both devices originate primarily from resistive ballooning instabilities, the different behavior must be associated with other mechanisms—arising, presumably, from the radically different divertor geometry (closed in AUG, open in TCV) [95]. An extensive reciprocating-probe database was constructed for TCV and mined with novel analysis techniques to study the scalings of the radial velocity of filaments, motivated by analytical theory predictions [96] and with the goal of refining models that are crucial for the understanding of SOL transport. In absolute terms, filament diameters lie typically in the 3–11 mm range and their radial velocities are between 0.5–2 km s−1 (outward); however, significant tails exist in the distribution and, in particular, inward velocities are observed for the first time, only in reversed field (ion ∇B drift pointing away from the X-point) and in conditions of high poloidalvelocity shear. The maximum velocity is a function of filament size and of divertor collisionality as predicted by theory, but the velocity of most filaments is in fact independent of collisionality owing to their resistive-ballooning character, which explains the insensitivity of velocity to density and connection length [97]. A study of flows and fluctuations in the low-poloidal-field region of a LFS SF-plasma was conducted with a fast framing visible-light camera. As the normalized distance σ between X-points decreases, the flow in the outer SOL is unchanged, whereas it increases in the inner SOL; at the same time, the fluctuations between the X-points become uncorrelated from those above the primary X-point, suggesting the formation of filaments in the low-Bp region. In addition, the dominant motion of these filaments turns from poloidal to radial as σ decreases, consistent with an enhancement of cross-field transport [98]. 6. Doublets Beyond all current ‘alternative’ scenarios lies a long-dormant topological concept, the doublet [99, 100] (with a figure- of-eight flux-surface featuring an internal X-point), which is believed to afford the benefits of high elongation with increased vertical stability [99] and promises tantalizing new physics associated with its internal X-point. The primary difficulty associated with this configuration is the inherent tendency of the two lobes to collapse into one, owing either to the magnetic attraction between the two current channels or to a thermal instability favoring one lobe over the other. With a uniquely suited coil set, TCV was the natural device on which to revisit this possibility using modern control technology. In preparation for this attempt, extensive work went into tuning the plasma control system to improve and optimize the plasma breakdown and burn-through, which had a non-negligible failure rate. Proper consideration of the large currents circulating in the conducting vessel during this phase was required for this task. With these tools in hand, a double breakdown and ramp-up was attempted, with only partial success in that the top lobe always coalesced rapidly into the lower one. The next step was to apply ECRH power separately to the two lobes, each controlled from its own lobe current observer, in an attempt to equalize the currents. A successful doublet was maintained in this manner for ~30 ms, with a current up to 270 kA and peak electron temperature 1.3 keV in both lobes (figure 11). These initial data suggest the appearance of a transport barrier in the negative-shear mantle just outside the internal separatrix [101]. Power and deposition scans also showed that the scenario was surprisingly robust against coalescence, suggesting that the transport barrier common to both lobes effectively sets the boundary condition for both and results in similar pressure profiles irrespective of the input power apportionment. The reasons for the disruption at ~30 ms are not currently understood, and additional research is planned to be conducted towards achieving steady state. 7. Conclusions and outlook TCV is documenting the physics basis for ITER and exploring avenues for solving its most pressing concerns, while also casting a wide net in configuration space to identify viable alternatives for DEMO and an eventual fusion reactor. The experimental campaigns of the past two years have brought significant advances on all these fronts. Looking to the near future, a substantial upgrade program is now in full swing [102]. Additional auxiliary heating sources are being added as discussed in the introduction. Even more momentously, in-vessel baffles will be added to equip TCV with a partially closed divertor for the first time, allowing it to reach reactor-relevant neutral density and Figure 12. TCV divertor upgrade, showing the baffles (cyan) and the cryo-pumps under consideration (light blue). Reprinted from [103], Copyright (2017), with permission from Elsevier. Nucl. Fusion 59 (2019) 112023
S. Coda etal 16 impurity compression [103]. We plan to fabricate baffles of different sizes, to be swapped in relatively short interventions, in order to vary the divertor closure and investigate how this affects plasma performance. The first set to be installed comprises 32 baffles on the HFS and 64 on the LFS [104] (figure 12). Simulations with solps-iter and emc3-Eirene were performed to guide the design [103]. For enhanced control of the plasma and of the divertor region, dedicated pumps (e.g. cryo-pumps) are also under consideration in addition to toroidally distributed fuel and impurity injection valves. A phased program of diagnostic additions and upgrades is also in place. The first new diagnostics to be associated with the vessel upgrade will be ~50 new Langmuir probes, baratron gauges, infrared thermography, bolometry, divertor spectroscopy, divertor Thomson scattering, and additional Mirnov coils [102]. 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