Overview of the TCV tokamak experimental programme

PAPER • OPEN ACCESS Overview of the TCV tokamak experimental programme To cite this article: H. Reimerdes et al 2022 Nucl. Fusion 62 042018 View the article online for updates and enhancements. You may also like LOCALIZATION AND BROADBAND FOLLOW-UP OF THE GRAVITATIONALWAVE TRANSIENT GW150914 B. P. Abbott, R. Abbott, T. D. Abbott et al. - A new calibration method for charm jet identification validated with proton-proton collision events at s = 13 TeV The CMS collaboration, Armen Tumasyan, Wolfgang Adam et al. - SUPPLEMENT: “LOCALIZATION AND BROADBAND FOLLOW-UP OF THE GRAVITATIONAL-WAVE TRANSIENT GW150914” (2016, ApJL, 826, L13) B. P. Abbott, R. Abbott, T. D. Abbott et al. - This content was downloaded from IP address 150.214.182.235 on 22/09/2022 at 09:48 International Atomic Energy Agency Nuclear Fusion Nucl. Fusion 62 (2022) 042018 (16pp) https://doi.org/10.1088/1741-4326/ac369b Overview of the TCV tokamak experimental programme H. Reimerdes1,∗, M. Agostini2,42, E. Alessi3,S.Alberti 1, Y. Andrebe1, H. Arnichand1,J.Balbin 4, F. Bagnato1, M. Baquero-Ruiz1,M.Bernert 5, W. Bin3, P. Blanchard1, T.C. Blanken6, J.A. Boedo7,D.Brida 5, S. Brunner1, C. Bogar8, O. Bogar8, T. Bolzonella2, F. Bombarda9, F. Bouquey4,C.Bowman 10, D. Brunetti3,J.Buermans 11, H. Bufferand4, L. Calacci12, Y. Camenen13,S.Carli 14,D.Carnevale 12, F. Carpanese1, F. Causa3,J.Cavalier 8, M. Cavedon5, J.A. Cazabonne1, J. Cerovsky8, R. Chandra15, A. Chandrarajan Jayalekshmi1,O.Chellaï 1, P. Chmielewski16, D. Choi1, G. Ciraolo4, I.G.J. Classen15,S.Coda 3, C. Colandrea1,A.DalMolin 17,P.David 5, M.R. de Baar15, J. Decker1, W. Dekeyser14,H.deOliveira 1,D.Douai 4,M.Dreval 18, M.G. Dunne5, B.P. Duval1,S.Elmore 19,O.Embreus 20, F. Eriksson20,M.Faitsch 5, G. Falchetto4,M.Farnik 8, A. Fasoli1, N. Fedorczak4, F. Felici1,O.F ´ evrier1, O. Ficker8,A.Fil 10,19, M. Fontana1,E.Fransson 20, L. Frassinetti21, I. Furno1,D.S.Gahle 19,22, D. Galassi1, K. Galazka16,C.Galperti 1, S. Garavaglia3, M. Garcia-Munoz23,B.Geiger 24,M.Giacomin 1,G.Giruzzi 4, M. Gobbin2, T. Golfinopoulos25, T. Goodman1, S. Gorno1, G. Granucci3, J.P. Graves1, M. Griener5,M.Gruca 16, T. Gyergyek26, R. Haelterman11, A. Hakola27,W.Han 25, T. Happel5,G.Harrer 28, J.R. Harrison19, S. Henderson19, G.M.D. Hogeweij15,J.-P.Hogge 1, M. Hoppe20, J. Horacek8, Z. Huang25, A. Iantchenko1, P. Innocente2, K. Insulander Björk20, C. Ionita-Schrittweiser29, H. Isliker30,A.Jardin 31, R.J.E. Jaspers6,R.Karimov 1, A.N. Karpushov1,Y.Kazakov 11,M.Komm 8, M. Kong1,19, J. Kovacic26,O.Krutkin 1, O. Kudlacek5,U.Kumar 1, R. Kwiatkowski32,B.Labit 1, L. Laguardia3, J.T. Lammers6,E.Laribi 4, E. Laszynska16, A. Lazaros33, O. Linder5, B. Linehan25, B. Lipschultz10, X. Llobet1, J. Loizu1, T. Lunt5, E. Macusova8, Y. Marandet13, M. Maraschek5, G. Marceca1,C.Marchetto 3, S. Marchioni1, E.S. Marmar25,Y.Martin 1,L.Martinelli 1,F.Matos 5,R.Maurizio 1,42, M.-L. Mayoral19,D.Mazon 4, V. Menkovski6,A.Merle 1,G.Merlo 34, H. Meyer19, K. Mikszuta-Michalik16, P.A. Molina Cabrera1,25, J. Morales4, J.-M. Moret1,A.Moro 3, D. Moulton19, H. Muhammed19,O.Myatra 10, D. Mykytchuk1, F. Napoli9,R.D.Nem 35, A.H. Nielsen35, M. Nocente17, S. Nowak3,N.Offeddu 1, J. Olsen35, F.P. Orsitto36,O.Pan 5,G.Papp 5, A. Pau1,A.Perek 15, F. Pesamosca1, Y. Peysson4,L.Pigatto 2,C.Piron 2, M. Poradzinski16,L.Porte 1,T.Pütterich 5, M. Rabinski32,H.Raj 1, J.J. Rasmussen35,G.A.Rattá 37, T. Ravensbergen6,15,D.Ricci 3, P. Ricci1, ∗Author to whom any correspondence should be addressed. aSee Labit et al 2019 (https://doi.org/10.1088/1741-4326/ab2211)forthe EUROfusion MST1 Team. 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. 1741-4326/22/042018+16$33.00 Printed in the UK 1 ©2022 The Author(s). Published on behalf of IAEA by IOP Publishing Ltd Nucl. Fusion 62 (2022) 042018 H. Reimerdes et al N. Rispoli3,F.Riva 19, J.F. Rivero-Rodriguez23, M. Salewski35,O.Sauter 1, B.S. Schmidt35,R.Schrittweiser 29, S. Sharapov19, U.A. Sheikh1, B. Sieglin5,M.Silva 1,A.Smolders 1,14, A. Snicker27, C. Sozzi3, M. Spolaore2, A. Stagni1,38, L. Stipani1,G.Sun 1,T.Tala 27, P. Tamain4, K. Tanaka39,A.TemaBiwole 1,D.Terranova 2,J.L.Terry 25, D. Testa1, C. Theiler1, A. Thornton19, A. Thrysøe35,H.Torreblanca 1,C.K.Tsui 1,7, D. Vaccaro1,40,M.Vallar 1, M. van Berkel15, D. Van Eester11, R.J.R. van Kampen6,15, S. Van Mulders1, K. Verhaegh10, T. Verhaeghe11, N. Vianello2, F. Villone36,E.Viezzer 23, B. Vincent1, I. Voitsekhovitch19, N.M.T. Vu1, N. Walkden19,T.Wauters 11, H. Weisen1,N.Wendler 16, M. Wensing1,F.Widmer 13, S. Wiesen41,M.Wischmeier 5, T.A. Wijkamp6,15, D. Wünderlich5, C. Wüthrich1,V.Yanovskiy 8, J. Zebrowski27 and the EUROfusion MST1 Teama 1Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL), Swiss Plasma Center (SPC), Lausanne, Switzerland 2Consorzio RFX, Padova, Italy 3Istituto per la Scienza e Tecnologia dei Plasmi ISTP-CNR, Milano, Italy 4CEA, IRFM, Saint Paul-Lez-Durance Cedex, France 5Max Planck Institute for Plasma Physics, Garching, Germany 6Eindhoven University of Technology, Eindhoven, Netherlands 7Center for Energy Research (CER), University of California-San Diego (UCSD), La Jolla, CA, United States of America 8Institute of Plasma Physics of the CAS, Prague, Czech Republic 9Unit` a Tecnica Fusione, ENEA, Frascati, Italy 10 York Plasma Institute, University of York, Heslington, York, United Kingdom of Great Britain and Northern Ireland 11 Laboratory for Plasma Physics, LPP-ERM/KMS, Brussels, Belgium 12 University of Rome Tor Vergata, Rome, Italy 13 CNRS, Aix-Marseille Univ., Marseille, France 14 KU Leuven, Department of Mechanical Engineering, Leuven, Belgium 15 DIFFER-Dutch Institute for Fundamental Energy Research, Eindhoven, Netherlands 16 Institute of Plasma Physics and Laser Microfusion (IPPLM), Warsaw, Poland 17 Universita’ di Milano-Bicocca, Milano, Italy 18 Institute of Plasma Physics of the NSC KIPT, Kharkov, Ukraine 19 CCFE, Culham Science Centre, Abingdon, Oxon, United Kingdom of Great Britain and Northern Ireland 20 Chalmers University of Technology, Gothenburg, Sweden 21 KTH Royal Institute of Technology, Stockholm, Sweden 22 SUPA, University of Strathclyde, Glasgow, United Kingdom of Great Britain and Northern Ireland 23 University of Seville, Spain 24 University of Wisconsin - Madison, Madison, WI, United States of America 25 Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA, United States of America 26 Jozˇef Stefan Institute, Ljubljana, Slovenia 27 VTT, Espoo, Finland 28 Institute of Applied Physics, Fusion@ÖAW, T.U. Wien, Vienna, Austria 29 Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Innsbruck, Austria 30 Aristotle University of Thessaloniki, Thessaloniki, Greece 31 Institute of Nuclear Physics Polish Academy of Sciences (IFJ PAN), Krakow, Poland 32 National Centre for Nuclear Research (NCBJ), Otwock, Poland 33 Department of Physics, National and Kapodistrian University of Athens, Athens, Greece 34 University of Texas at Austin, Austin, TX, United States of America 35 Department of Physics, Technical University of Denmark, Kgs. Lyngby, Denmark 36 University of Napoli ‘Federico II’, Consorzio CREATE, Napoli, Italy 37 Laboratorio Nacional de Fusi´ on, CIEMAT, Madrid, Spain 38 Politecnico di Milano, Milan, Italy 39 National Institute for Fusion Science, National Institutes of Natural Sciences, Toki, Gifu, Japan 40 Department of Economics, Engineering, Society and Business Organization (DEIm), University of Tuscia, Viterbo, Italy 2 Nucl. Fusion 62 (2022) 042018 H. Reimerdes et al 41 Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung—Plasmaphysik, Jülich, Germany 42 Oak Ridge Associated Universities (ORAU), Oak Ridge, TN, United States of America E-mail: [email protected] Received 24 June 2021, revised 16 October 2021 Accepted for publication 4 November 2021 Published 1 March 2022 Abstract The tokamak ` a configuration variable (TCV) continues to leverage its unique shaping capabilities, flexible heating systems and modern control system to address critical issues in preparation for ITER and a fusion power plant. For the 2019–20 campaign its configurational flexibility has been enhanced with the installation of removable divertor gas baffles, its diagnostic capabilities with an extensive set of upgrades and its heating systems with new dual frequency gyrotrons. The gas baffles reduce coupling between the divertor and the main chamber and allow for detailed investigations on the role of fuelling in general and, together with upgraded boundary diagnostics, test divertor and edge models in particular. The increased heating capabilities broaden the operational regime to include Te/Ti∼1 and have stimulated refocussing studies from L-mode to H-mode across a range of research topics. ITER baseline parameters were reached in type-I ELMy H-modes and alternative regimes with ‘small’ (or no) ELMs explored. Most prominently, negative triangularity was investigated in detail and confirmed as an attractive scenario with H-mode level core confinement but an L-mode edge. Emphasis was also placed on control, where an increased number of observers, actuators and control solutions became available and are now integrated into a generic control framework as will be needed in future devices. The quantity and quality of results of the 2019–20 TCV campaign are a testament to its successful integration within the European research effort alongside a vibrant domestic programme and international collaborations. Keywords: nuclear fusion, tokamak, TCV, EUROfusion (Some figures may appear in colour only in the online journal) 1. Introduction The tokamak ` a configuration variable (TCV) [1] is a carbon walled, medium sized (major radius R0=0.88 m, magnetic field B0⩽1.5 T), conventional aspect ratio (A≈4) tokamak with unique shaping capabilities. Together with flexible heating systems for electrons and ions, a steadily growing number of diagnostics and a modern control system, it continues to address critical issues in preparation for ITER and a fusion power plant. For the 2019–20 campaign its configurational flexibility has been enhanced with the installation of divertor baffles, its diagnostic capabilities with numerous upgrades and its heating systems with new gyrotrons. These recent enhancements are part of a greater facility upgrade that commenced in 2015 [2] in contribution to the EU strategy to solve the DEMO exhaust problem [3]. Experiments are performed in part by topical teams, under the auspices of the EUROfusion medium-size tokamak programme and local teams at the Swiss Plasma Center together with international collaborators. Auxiliary heating is provided by a neutral beam injection (NBI) system [4] and X2 and X3 electron cyclotron resonance heating (ECRH) systems. An improved acceleration grid for the NBI system, with reduced beam divergence[5] and, hence, lower power losses in the duct together with an improved duct cooling [6] permitted a 2.5×increase in injected energy with injected powers up to 1.3 MW. In addition, 1.0 MW NBI operation in hydrogen has been demonstrated. The legacy ECRH system of 1.4 MW from two 83 GHz gyrotrons (X2) and 0.9 MW from two 118 GHz gyrotrons (X3) was enhanced by two 1.0 MW dual frequency (84/126 GHZ) gyrotrons for X2 or X3 heating [7]. The first dual-frequency gyrotron started operation in 2019 and performed as designed, validating the numerical models used in its development [8]. A second unit has recently attained the same performance and is being integrated into the ECRH waveguide system for plasma operation. The most conspicuous upgrade was the installation of a first set of removable gas baffles for the 2019 experimental campaign. Its effect on the neutral distribution is described in section 2. The baffles had a large impact on the TCV edge and divertor programme, summarised in section 3. With more auxiliary heating, H-mode studies towards a better understanding of the ITER baseline scenario gained prominence, section 4. As type-I ELMs associated with the baseline scenario are of great concern for the safety of the ITER divertor, and certainly 3 Nucl. Fusion 62 (2022) 042018 H. Reimerdes et al unacceptable in a reactor, a significant part of the experimental programme was dedicated to research in alternative scenarios, discussed in section 5. Several specific issues of concern for ITER operation are addressed in section 6. Section 7 gives an overview of TCV’s contributions to real-time control of interest for ITER and beyond, before some conclusions in section 8. 2. Divertor upgrade The divertor upgrade is centred around the installation of removable gas baffles that partially separate the vessel into main and divertor chambers, figure 1(a). The baffles primarily seek to increase the divertor neutral pressure and thereby facilitate extrapolation to future devices, such as ITER that will rely upon a high divertor neutral pressure. The divertor closure of a first set of baffles was optimised for a large neutral compression in a conventional, single-null divertor, figure 1(b), using the edge transport code SOLPS-ITER, which employs a fluid description of the plasma coupled with kinetic neutrals [2]. The baffles consist of graphite tiles that replace standard wall protection tiles, requiring only short manned entries for installation and removal. The first set of baffles was installed for the 2019 experimentalcampaign [9]. The baffles were complemented with new port protection tiles that shadow magnetic field lines, shielding the lower lateral port openingsfrom direct contact with plasma. These are independent of the baffles and increase the configurational flexibility for both baffled and unbaffled configurations, figures 1(c)–(e). Divertor diagnostics were also upgraded with additional wall mounted Langmuir probes [10] and an additional IR thermography system for complete divertor chamber coverage.A new, long-throw reciprocating divertor probe array (RDPA) extends measurements into the divertor volume [11], where new divertor Thomson scattering chords, equipped with narrow filters, are now able to measure electron temperatures down to 1 eV [12]. Radiation diagnostics are greatly enhanced with a new tomography camera array each housing sensitive foil bolometers, and separate AXUV and soft x-ray (SXR) diode arrays to reconstruct radiation profiles in the divertor and the main chamber. The system is complemented by a tangentially viewing, multi camera spectral imaging system, MANTIS [13], and wider coverage divertor spectroscopy chords (DSS) [14]. Probe turbulence measurements are augmented with a new gas puff imaging (GPI) system in the proximity of the X-point. Experimentally, Ohmically heated diverted discharges, figure 1(b), qualitatively confirm SOLPS-ITER predictions [15] with up to a 5×increase in divertor neutral pressure [9,16], figure 2. The concurrently predicted decrease of the main chamber neutral density is, as yet, unconfirmed as the associated pressure is below the noise level of the current TCV baratron of several millipascal. A trend towards a higher ratio of separatrix density, ne,sep, and line averaged density, ne, in the baffled configuration, however, indicates a reduction in core ionisation and, hence, a lower main chamber neutral density [17]. Optional fuelling into the divertor or the main chamber can disentangle the effects of fuelling rate, divertor and/or main chamber neutral pressure, pn,div/main, and plasma density, ne. These new device capabilities provided by the baffles, were extensively used for studies of the divertor and the plasma edge, discussed in section 3, and contributed to several other aspects of TCV’s scientific programme. Scanning the plasma plugging by displacing the X- point with respect to the baffles, indicates that the installed divertor closure is close to optimal [16]. SOLEDGE2D-Eirene calculations with a more refined main chamber plasma–wall interaction (PWI) model than SOLPS-ITER suggest that increasing the closure may further increase the neutral compression [18] guiding the design of future baffles. The baffles were removed in early 2020, restoring TCV’s full shaping flexibility, but will be re-installed for dedicated future campaigns. In addition, a second baffle set with less closure, better suited for snowflake configurations, see section 3.2, is being procured and will be installed in 2021. This will be followed in 2022 by a third combination of inner and outer baffles with greater closure than the initial set reported herein. 3. Edge physics Plasma exhaust, and its compatibility with high core performance, is one of the main challenges on the way to fusion energy. The challenge is set by the width of the scrape-off layer (SOL) with solutions hinging on achievable separatrix densities and the tolerable seed impurity concentrations. Since the conditionsof future high-performancedevices, with higher heat fluxes, larger dimensions and greater magnetic fields, cannot be achieved in today’s experiments, the development of predictive edge modelling capability is mandatory. TCV contributes by testing edge models, including those used in the design of the ITER divertor [19] and by exploring alternative configurations to the conventional single-null divertor to improve future reactor designs [20,21]. 3.1. Plasma exhaust in the conventional divertor The effect of the neutral pressure in a conventional singlenull divertor, figure 1(b), is investigated by comparing identically programed density ramp discharges in baffled and un-baffled configurations. These discharges are performed in reverse field, where the magnetic field is directed in the unfavourable direction for H-mode access to remain in L-mode. L-mode plasmas facilitate tests of divertor models as they are easier to diagnose and plasma parameters are generally more stationary. Measurements with target Langmuir probes and the novel RDPA diagnostic show that, as predicted, increasing pn,div in the baffled divertor leads to a cooler and denser divertor plasma for the same core density [22]. Ion flux measurements with the RDPA also reveal a stronger ionisation source in the baffled divertor. As a result, and again as predicted, detachment detected by its characteristic target particle flux roll-over, figure 3(a), and a displacement of the CIII emission front towards the X-point, figure 3(b), commences at 20%–30% lower ne[16]. Experiments using main chamber fuelling provide further evidence that the onset of detachment is primarily determined by pn,div rather than ne. 4 Nucl. Fusion 62 (2022) 042018 H. Reimerdes et al Figure 1. (a) Fisheye lens photograph of the inside of the baffled TCV vessel (Photo: Alain Herzog/EPFL) and poloidal cross sections of (b) single-null diverted, (c) snowflake-minus and (d) super-X configurations in TCV with baffles (magenta) and (e) a diverted negativetriangularity configuration employing the port protection tiles (light grey). Gas fuelling (black) and neutral pressure measurement locations (green) are indicated in (b). Reproduced courtesy of IAEA. Figure from [9]. ©2021 EURATOM. Figure 2. Baratron measurements of the neutral pressure in the divertor in Ohmic density ramps with (magenta) and without the baffles (blue). Reproduced courtesy of IAEA. Figure from [9]. ©2021 EURATOM. Efforts to test drift models in the SOLPS-ITER code predicted an electric potential well and altered drift patterns in reverse field discharges with extremely low plasma temperatures in the divertor [23]. RDPA measurements in baffled discharges at sufficiently high density and, hence, sufficiently low plasma temperature, subsequently confirmed the presence of such a well [24]. Target current measurements show that, in the cold TCV divertor, more easily accessible with baffles, SOL currents are dominated by Pfirsch–Schlüter currents [24]. These measurements employed a plasma current of 190 kA, the highest value for reliable Ohmic L-mode operation, and both field directions. Measured peaks in the target current, their directions and dependenceon field directions and density, are found in qualitative agreement with SOLPS-ITER calculations. A high-fidelity test of SOLPS-ITER was attempted for a density ramp discharge in reverse field and with a somewhat higher current of 250 kA [25]. Cross-field diffusivities were set to match the Teand neprofiles measured by Thomson scattering well above the X-point. The model Figure 3. Density dependence of (a) the particle fluxes at the outer target measured with Langmuir probes and (b) the CIII (465 nm) emission front position measured with MANTIS in Ohmic discharges with (magenta) and without the baffles (blue). Reprinted from [16], Copyright (2021), with permission from Elsevier. CC BY-NC-ND 4.0. then reasonably describes Teand nebelow the X-point, measured with the reciprocating probe (RPTCV), and near the outer target, by the new divertor Thomson chords, albeit with somewhat broader than predicted profiles. The model, however, clearly overpredicts the target plasma density and 5 Nucl. Fusion 62 (2022) 042018 H. Reimerdes et al underpredicts its temperature. The model, also, concurrently overpredicts the divertor neutral pressure (by approximately a factor 4). The identified discrepancieswill guide furtherrefinements in the model and spur diagnostic developments. One recent question, for example, is the accuracy of the description of molecular-activated recombination (MAR) that, according to a recent spectroscopic investigation, can provide a significant volumetric particle sink in TCV discharges [26,27]. Detachment was also obtained with nitrogen seeding in reverse field, Ohmic, L-mode discharges [28]. A sufficiently high seeding rate led to the detachment at both targets and the formation of a stable radiator around the X-point. Measurements with TCV’s gold-foil bolometer system, with 64 intersecting lines of sight, indicate that up to 80% of the power crossing the last closed flux surface (LCFS) into the SOL can thus be radiated. While nitrogen seeding generally reduced the energy confinement time, this reduction is only a marginal effect at higher plasma density, indicative of a lower level of nitrogen penetration into the core plasma and, yet again, consistent with SOLPS-ITER simulations [29]. 3.2. Plasma exhaust in alternative divertor configurations Inspired by the MAST-Upgrade design [30], and motivated by recent SOLPS-ITER simulations that explain the absence of advantageous effects of a larger target radius in previous TCV experiments with a varying angle between the divertor leg and the target surface and poor neutral trapping [31], detailed investigationswere conductedon TCV’s super-X divertor configuration [32], figure 1(d). Specific configurations designed to disentangle the effects of a large target radius, Rt,andthe angle between the divertor leg and the target surface, and employing the divertor baffles and a newly installed gas valve to fuel into the private flux region, figure 4(a), confirm the predicted dependence of the detachment onset on this angle and demonstrate a reduction in the detachment threshold with larger Rt, figure 4(b). This reduction, however, remains weaker than predicted [31] for a strongly baffled divertor. Target measurements with Langmuir probes reveal that, while the electron temperature decreaseswith larger Rt, the electron density does not increase as expected, reminiscent of the discrepancies identified for the single-null divertor, section 3.1. Other investigations addressed the snowflake minus configuration, figure 1(c), where a secondary X-point in the divertor splits one side of the SOL into distinct divertor legs with consequences for the power distribution to the targets and the radiation volume. In attached conditions, upstream profiles, from reciprocating probe measurements, remain independent of the divertor configuration. Probe measurements at the entrance of the low-field side snowflake minus divertor, a configuration that attracted attention due to its unexpectedlystrong broadening of the SOL at the secondary X-point [33], reveal large pressure drops that depend on the position of the secondary X-point and the drift direction and highlight the dominant role of parallel convection and E×Bdrifts in the power distribution to the targets [34]. Operating the snowflake minus in the baffled TCV divertor revealed significant particle fluxes to the top of the baffle, indicative of divertor over-closure. This hypothesis is supported by experiments on a variant of the snowflake minus, where only the outer divertor is baffled, which achieved a higher divertor neutral pressure than the fully baffled snowflake [35]. This will be further tested with the next combination of baffles foreseen for 2021. 3.3. H-mode/ELMs Power exhaust studies were extended to type-I ELMy H- modes at 170 kA (q95 =4.6) and with 700 kW of NBI heating well above the NBI power of ∼450 kW needed to access H- mode [16]. The addition of the divertor baffles decreases the peak electron temperature at the outer target between ELMs from ∼5eVto∼3 eV. Lower divertor temperatures in the baffled divertor are supported by measurements of CIII emission, which is located farther from the target in baffled discharges. Seeding of nitrogen is effective in further cooling the divertor between ELMs as seen by a three-fold decrease in the target particle flux and a CIII emission front that recedes towards the X-point with baffles, figure 5(b). There are, in particular, no indications of a bi-furcationin either case. However, even with strong seeding, ELMs are able to re-attach the plasma divertor legs, figure 5(a). 3.4. SOL width/turbulence and blobs The width of the heat carrying plasma channel of the SOL, λq, and its extrapolation to future devices, is key in determining the magnitude of the heat exhaust challenge. Its value generally decreases in H-mode, relevant for most current reactor scenarios, thus magnifying this challenge. Its L-mode scaling remains important as it determinesmitigation requirements during the start-up and landing phases of the discharge. The extensive database of SOL width measurements in TCV L-modes [36], together with a cross-machine database of λq[37] spanning more than an order of magnitude, is found to be well described by an analytical model based on turbulent cross-fieldtransportdrivenby resistive ballooningmodescompeting with convective parallel transport [38]. A larger experimental spread seen on all devices is attributed to experimental conditions that extend beyond the sheath-limited regime and geometric modifications, such as plasma triangularity, known to affect the ballooning stability, that are, as yet, ignored by the model. Shaping effects are, however, included in the GBS code [39]. A rigorous validation exercise employing reciprocating probe fluctuation measurementsin TCV discharges with circular, elongated and (negative) triangular cross sections showed that the shaping model implemented in GBS improves the description of SOL turbulence [40]. In H-mode, the TCV inter-ELM SOL is typically found 2 times smaller than in L-mode, similar to observations in other devices [41], but also 2–3 times smaller than for H-modes in other devices for the same value of poloidal field (PF) [42], suggesting that a scaling solely based on Bpol does not capture all key dependencies. The TCV measurements, however, agree with cross-machine scalings [43] that include terms for the toroidal field, q95 and heating power. 6 Nucl. Fusion 62 (2022) 042018 H. Reimerdes et al Figure 4. (a) Poloidal cross sections of two discharges with similar core shaping, but small (blue) and large (brown) outer target radii and (b) density dependence of the poloidal distance of the CIII (465 nm) emission front measured with MANTIS. Reproduced with permission from [32]. Figure 5. CIII emission (465 nm) obtained from inverted MANTIS images during an ELM (a) and in-between ELMs (b) in nitrogen seeded discharges with the divertor baffles. Reprinted from [16], Copyright (2021), with permission from Elsevier. CC BY-NC-ND 4.0. At sufficiently high density, the SOL is generally observed to broaden, with a distinct second, longer, fall-off length in the plasma density emerging—a process referred to as shoulder formation. While the density shoulder does little to alleviate heat loads in the divertor, it is feared it may increase main chamber recycling and erosion, with consequences for particle control, impurity influx and lifetime of the first wall in future devices. Repetition of L-mode discharges that clearly displayed a density shoulder in the un-baffled divertor,showed no such evidence in the baffled divertor, although divertor detachment starts at lower plasma density, providing further evidence that SOL collisionality is not the sole determining criterion [44]. Since baffles are expected to operate with a decreased main chamber neutral density, this behaviour indicates an important effect of neutrals on SOL transport, for example via suppressing zonal flows that stabilise anomalous transport [45] or blob fuelling [46]. Further support for the importance of the main chamber neutral density is obtained from an inner gap scan in the un-baffled divertor [44], with a smaller gap resulting in a more pronounced density shoulder, consistent with a strong, reinforcing,role of main chamber recycling and, hence, neutral pressure. The characterisation of the SOL turbulence in TCV was extended to H-mode [47]. In the analysed high triangularity (δ=0.5), low current (IP=180 kA), H-mode scenario with 1 MW of NBI heating, the inter-ELM density profiles exhibit a broadening with sufficiently strong fuelling and, hence, divertor neutral pressure, pn,div, similar to observations on ASDEX- Upgrade and JET. This broadening is accompanied by an increased blob frequency, as detected by Langmuir probes at the outboard midplane wall. Contrary to observations in L-mode, the broadening also prevails for a baffled divertor. The broadeningof the upstream density profile is, however, not observed when a dissipative divertor is formed using nitrogen seeding rather than deuterium fuelling. 4. H-mode physics H-mode studies, as the confinement regime foreseen for ITER, are a prominent part of the TCV programme. The addition of the NBI for auxiliary heating has less constraints on electron density and temperaturethan TCV’s well-established EC heating schemes and heats ions, in addition to electrons, that was found to greatly enhance the H-mode operating range. 4.1. L–H power threshold The L–H threshold, PLH, remains a critical issue for ITER and for a reactor as it determines the minimum required installed 7 Nucl. Fusion 62 (2022) 042018 H. Reimerdes et al Figure 6. Density dependence of the H-mode threshold in NBI heated discharges at various values of the safety factor. auxiliary heating power, and, neglecting hysteresis, also sets the minimum power that must be successfully exhausted in a divertor during H-mode operation. Current extrapolations are based on empirical scaling laws with ‘hidden parameters’ that were experimentally observed to affect the measured value. The NBI system allows strong decoupling between heating and plasma current that led to the discovery of a strongly increasing current (or unfavourable q95) dependence of PLH that increases with density [48], figure 6. This is not included in the present ITPA scaling [49] where good agreement is only obtained for q95 <3.5 or at a plasma density close to the minimum required to enter H-mode that, ironically, may be the least favourable operational position to enter H-mode with regard to ELM heat loads. Revisiting the isotope and charge number dependence with experiments in D, H and He resulted in good agreement with the ITPA scaling, again provided that q95 is sufficiently low. Adding small concentrations of D to H plasmas showed, however, a rapid decrease of the L–H threshold suggesting doping of H-plasmas as an option to lower PLH in the pre-nuclear phase of ITER. 4.2. H-mode pedestal Particular attention was dedicated to the pedestal in type- I ELMy H-modes, where the baffles were used to decouple the roles of main chamber and divertor neutral pressures, pn,main and pn,div, in the pedestal structure. Fuelling and, thereby, a higher neutral pressure, generally degrades the pedestal pressure, pe,ped, through a degradation of the temperature pedestal, while the density pedestal height remains approximately constant, consistent with previous findings [50] Figure 7. Measured (a) electron temperature and (b) density pedestal in unfueled, PNBI =1 MW discharge with (magenta) and without (cyan) baffles. that linked this degradation to an outward shift of the density profile. This outward shift and the related pedestal degradation are, however, found to be considerably more benign in the baffled divertor, presumably due to a weaker increase of pn,main, leading to a significantly higher Te,ped and, hence, higher pe,ped at high divertor neutral pressure, pn,div, figure 7, [51]. In this way, high fuelling rates, together with nitrogen seeding into the baffled divertor, resulted in an up to three-fold increase in the radiated power without significant core confinement degradation, figure 8. This improved performance was also observed across a range of target radii, Rt, highlighting alternative divertors’ weak effect on pedestal and core properties. 4.3. Core confinement In a continued effort to extrapolate ELMy H-mode performance to the ITER baseline (IBL) scenario,NBI and X3 heated H-modes succeeded in matching the ITER targets of κ=1.8, δ=0.5, βN=1.8 and q95 =3.0 whilst retaining good confinement (H98y2 ∼1) [52]. The Greenwald fraction reached 0.65, which is still lower than the IBL target, but sufficient for EC absorption at the third harmonic (X3) to become unreliable. Since EC heating with X3 is effective in preventing, or 8 Nucl. Fusion 62 (2022) 042018 H. Reimerdes et al [2] Fasoli A. et al 2020 Nucl. Fusion 60 016019 [3] Zohm H., Militello F., Morgan T.W., Morris W., Reimerdes H. and Siccinio M. 2021 Fusion Eng. Des. 166 112307 [4] Karpushov A.N. et al 2017 Fusion Eng. Des. 123 468 [5] Sorokin A. et al 2020 Rev. Sci. Instrum. 91 013323 [6] Toussaint M., Coda S., Dolizy F., Duval B., Karpushov A.N., Martin Y. and Maurizio R. and the TCV Team 2020 Fusion Eng. Des. 155 111695 [7] Alberti S. et al 2019 High-efficiency, long-pulse operation of MW-level dual-frequency gyrotron, 84/126 GHz, for the TCV tokamak 44th Int. Conf. Infrared, Millimeter, and Terahertz Waves (Irmmw-Thz) (Paris, France, 1–6 September 2019) [8] Hogge J.-P. et al 2020 AIP Conf. Proc. 2254 090006 [9] Reimerdes H. et al 2021 Nucl. Fusion 61 024002 [10] De Oliveira H., Marmillod P., Theiler C., Chavan R., F´ evrier O., Labit B., Lavanchy P., Marl´ etaz B. and Pitts R.A. and the TCV Team 2019 Rev. Sci. Instrum. 90 083502 [11] De Oliveira H., Theiler C. and Elaian H. and the TCV Team 2021 Rev. Sci. Instrum. 92 043547 [12] Arnichand H. et al 2019 J. Instrum. 14 C09013 [13] Perek A. et al 2019 Rev. Sci. Instrum. 90 123514 [14] Martinelli L. et al 2021 Spectroscopic studies of TCV divertor plasma with the DSS upgrade 47th EPS Conf. Plasma Physics, Europhysics Conf. Abstract (21–25 June 2021) (virtual conference) vol 45A (http://ocs.ciemat.es/ EPS2021PAP/pdf/O5.J502.pdf) [15] Wensing M. et al 2019 Plasma Phys. Control. Fusion 61 085029 [16] F´ evrier O. et al 2021 Nucl. Mater. Energy 27 100977 [17] Wensing M. 2021 Drift-related transport and plasma–neutral interaction in the TCV divertor PhD Thesis EPFL No. 8447 (https://doi.org/10.5075/epfl-thesis-8447) [18] Galassi D. et al 2020 Plasma Phys. Control. Fusion 62 115009 [19] Pitts R.A. et al 2019 Nucl. Mater. Energy 20 100696 [20] Reimerdes H. et al 2020 Nucl. Fusion 60 066030 [21] Militello F. et al 2021 Nucl. Mater. Energy 26 100908 [22] De Oliveira H. 2021 A fast-moving Langmuir probe array for the divertor of the tokamak ` a configuration variable PhD Thesis EPFL No. 8407 (https://doi.org/10.5075/epfl-thesis- 8407) [23] Wensing M., Loizu J., Reimerdes H., Duval B.P. and Wischmeier M. and the TCV Team 2020 Nucl. Fusion 60 054005 [24] Wensing M. et al 2020 Nucl. Mater. Energy 25 100839 [25] Wensing M. et al 2021 Phys. Plasmas 28 082508 [26] Verhaegh K. et al 2021 Plasma Phys. Control. Fusion 63 035018 [27] Verhaegh K. et al 2021 Nucl. Mater. Energy 26 100922 [28] F´ evrier O. et al 2020 Plasma Phys. Control. Fusion 62 035017 [29] Smolders A. et al 2020 Plasma Phys. Control. Fusion 62 125006 [30] Harrison J. et al 2021 Overview of first physics results from MAST Upgrade 2020 IAEA Fusion Energy Conf. (virtual conference) (10–15 May 2021) p EX/P6-39 [31] Fil A. et al 2020 Plasma Phys. Control. Fusion 62 035008 [32] Theiler C. et al 2021 Advances in understanding power exhaust physics with the new, baffled TCV divertor 2020 IAEA Fusion Energy Conf. (virtual conference) (10–15 May 2021) pEX/P7-5 [33] Maurizio R. et al 2019 Nucl. Fusion 59 016014 [34] Tsui C.K. et al 2021 Nucl. Fusion 61 046004 [35] Gorno S. et al 2021 Effect of baffles on the N2-seeded snowflake minus low-field side configuration in the tokamak ` a configuration variable (TCV) 47th EPS Conf. Plasma Physics, Europhysics Conf. Abstract (virtual conference) vol 45A (21–25 June 2021) (http://ocs.ciemat.es/EPS2021PAP/pdf/P3.1024. pdf) [36] Maurizio R. et al 2018 Nucl. Fusion 58 016052 [37] Horacek J. et al 2020 Nucl. Fusion 60 066016 [38] Giacomin M., Stagni A., Ricci P., Boedo J.A., Horacek J., Reimerdes H. and Tsui C.K. 2021 Nucl. Fusion 61 076002 [39] Riva F., Lanti E., Jolliet S. and Ricci P. 2017 Plasma Phys. Control. Fusion 59 035001 [40] Riva F., Tsui C.K., Boedo J.A. and Ricci P. 2020 Phys. Plasmas 27 012301 [41] Scarabosio A., Eich T., Herrmann A. and Sieglin B. 2013 J. Nucl. Mater. 438 S426 [42] Maurizio R., Duval B.P., Labit B., Reimerdes H., Faitsch M., Komm M., Sheikh U. and Theiler C., the TCV Team and the EUROfusion MST1 Team 2021 Nucl. Fusion 61 024003 [43] Eich T. et al 2013 Nucl. Fusion 53 093031 [44] Tsui C. et al 2021 Density shoulder dependence on divertor closure and main-chamber neutral pressure Nucl. Fusion (submitted) [45] Zhang Y. and Krasheninnikov S.I. 2020 Plasma Phys. Control. Fusion 62 115018 [46] Thrysøe A.S. et al 2020 Phys. Plasmas 27 052302 [47] Vianello N. et al 2021 SOL profile and fluctuations in different divertor recycling conditions in H-mode plasmas 2020 IAEA Fusion Energy Conf. (virtual conference) (10–15 May 2021) [48] Labit B. et al 2021 H-mode power threshold experiments in mixed ion species plasmas on TCV 25th Joint EU-US Transport Task Force Meeting [49] Martin Y.R. and Takizuka T. (The ITPA CDBM H-mode Threshold Data Group) 2008 J. Phys.: Conf. Ser. 123 012033 [50] Sheikh U.A. et al 2019 Plasma Phys. Control. Fusion 61 014002 [51] Sheikh U.A. et al 2021 Nucl. Mater. Energy 26 100933 [52] Sauter O. et al 2021 ITER baseline scenario investigations on the TCV tokamak and comparison with AUG 2020 IAEA Fusion Energy Conf. (virtual conference) (10–15 May 2021) pEX/P4-18 [53] Bernert M. et al 2015 Plasma Phys. Control. Fusion 57 014038 [54] Pau A. et al 2020 Active disruption avoidance for H-mode density limits on TCV and ASDEX Upgrade Poster Presented at 31st Symp. Fusion Technology (SOFT 2020) (20–25 Sept. 2020) virtual edition [55] Labit B. et al 2019 Nucl. Fusion 59 086020 [56] Faitsch M., Eich T., Harrer G.F., Wolfrum E., Brida D., David P., Griener M. and Stroth U., the ASDEX Upgrade Team and the EUROfusion MST1 Team 2021 Nucl. Mater. Energy 26 100890 [57] Labit B. et al 2021 H-mode physics studies on TCV supported by the EUROfusion pedestal database 2020 IAEA Fusion Energy Conf. (virtual conference) (10–15 May 2021) pEX/P4-17 [58] Faitsch M. et al 2021 High density, high confinement, power exhaust compatible H-mode regime in TCV and ASDEX Upgrade 2020 IAEA Fusion Energy Conf. (virtual conference) (10–15 May 2021) p EX/P4-20 [59] Molina Cabrera P.A., Labit B., Coda S. and Porte L. and the TCV Team 2021 Plasma Phys. Control. Fusion 63 085019 [60] Molina Cabrera P., Coda S., Porte L. and Smolders A. and the TCV Team 2019 Rev. Sci. Instrum. 90 123501 [61] Pochelon A. et al 1999 Nucl. Fusion 39 1807 [62] Camenen Y. et al 2007 Nucl. Fusion 47 510 [63] Austin M.E. et al 2019 Phys. Rev. Lett. 122 115001 [64] Porte L. et al 2021 The route to high performance, DEMO relevant, negative triangularity tokamak operation on TCV 2020 IAEA Fusion Energy Conf. (virtual conference) (10–15 May 2021) p EX/P6-RB [65] Fontana M., Porte L., Coda S., Sauter O., Brunner S., Jayalekshmi A.C., Fasoli A. and Merlo G. and the TCV Team 2020 Nucl. Fusion 60 016006 [66] Merlo G. et al 2021 Plasma Phys. Control. Fusion 63 044001 [67] Huang Z. and Coda S. and the TCV Team 2019 Plasma Phys. Control. Fusion 61 014021 15 Nucl. Fusion 62 (2022) 042018 H. Reimerdes et al [68] Han W., Offeddu N., Golfinopoulos T., Theiler C., Tsui C.K., Boedo J.A. and Marmar E.S. and the TCV Team 2021 Nucl. Fusion 61 034003 [69] Faitsch M. et al 2018 Plasma Phys. Control. Fusion 60 045010 [70] Wauters T. et al 2020 Plasma Phys. Control. Fusion 62 105010 [71] Choi D., Coda S., Decker J. and Peysson Y. and the TCV Team 2020 Fusion Eng. Des. 159 111733 [72] Choi D., Coda S., Decker J., Cazabonne J.A. and Peysson Y. and the TCV Team 2020 Plasma Phys. Control. Fusion 62 115012 [73] Chellaï O. et al 2018 Phys. Rev. Lett. 120 105001 [74] Chellaï O. et al 2021 Nucl. Fusion 61 066011 [75] Kong M., Sauter O., Felici F., Hogeweij G.M.D., Merle A. and Nowak S. and the TCV Team 2020 Nucl. Fusion 60 026002 [76] Geiger B. et al 2020 Plasma Phys. Control. Fusion 62 095017 [77] Stipani L. 2021 A fast ion loss detector for the TCV tokamak PhD Thesis EPFL No. 8327 (https://doi.org/10.5075/epfl-thesis-8327) [78] WijkamP T.A. et al 2021 Nucl. Fusion 61 046044 [79] Hoppe M. et al 2020 Nucl. Fusion 60 094002 [80] Sheikh U.A. et al 2020 Runaway electron studies and plasma restart from a RE beam on TCV IAEA Technical Meeting on Plasma Disruptions and their Mitigation (Virtual Meeting) (20–23 July 2020) pp 20–3 [81] Felici F. 2021 Integrated plasma state reconstruction, off-normal event handling and control, with application to TCV and ASDEX Upgrade IAEA Fusion Energy Conf. (Virtual Conference) (10–15 May 2021) p EX/P421 [82] Duval B.P. et al 2018 Singlet breakdown optimization to a doublet plasma configuration on the TCV tokamak 2018 IAEA Fusion Energy Conf. (Gandhinagar, India, 22–27 Oct. 2018) pEX/P1-6 [83] Ravensbergen T. et al 2020 Nucl. Fusion 60 066017 [84] Van berkel M. et al 2020 Plasma Phys. Control. Fusion 62 094001 [85] Ravensbergen T. et al 2021 Nat. Commun. 12 1105 [86] Matos F., Menkovski V., Felici F., Pau A. and Jenko F., the TCV Team and the EUROfusion MST1 Team 2020 Nucl. Fusion 60 036022 [87] Marceca G. et al 2020 Detection of plasma confinement states in the TCV tokamak Workshop at the 34th Conf. Neural Information Processing Systems (NeurIPS) (11 December 2020) virtual event (https://ml4physicalsciences.github. io/2020/files/NeurIPS_ML4PS_2020_44.pdf) [88] Matos F., Menkovski V., Pau A., Marceca G. and Jenko F. and the TCV Team 2021 Nucl. Fusion 61 046019 [89] Carpanese F., Felici F., Galperti C., Merle A., Moret J.M. and Sauter O. and the TCV Team 2020 Nucl. Fusion 60 066020 [90] Blanken T.C., Felici F., Galperti C., Vu N.M.T., Kong M., Sauter O. and de Baar M.R., the EUROfusion MST1 Team and the TCV Team 2019 Nucl. Fusion 59 026017 [91] Vu N.M.T., Blanken T.C., Felici F., Galperti C., Kong M., Maljaars E. and Sauter O., the TCV Team and the EUROfusion MST1 Team 2019 Fusion Eng. Des. 147 111260 [92] Kong M. et al 2019 Nucl. Fusion 59 076035 [93] Vu T., Felici F., Galperti C., Maraschek M., Pau A., Rispoli N., Sauter O. and Sieglin B. 2021 IEEE Trans. Nucl. Sci. 68 1855 [94] Maraschek M. et al 2018 Plasma Phys. Control. Fusion 60 014047 [95] Giacomin M. and Ricci P. 2020 J. Plasma Phys. 86 905860502 [96] Jorge R., Ricci P. and Loureiro N.F. 2017 J. Plasma Phys. 83 905830606 [97] Frei B.J., Jorge R. and Ricci P. 2020 J. Plasma Phys. 86 905860205 16