Progress from ASDEX Upgrade experiments in preparing the physics basis of ITER operation and DEMO scenario development

PAPER • OPEN ACCESS Progress from ASDEX Upgrade experiments in preparing the physics basis of ITER operation and DEMO scenario development To cite this article: U. Stroth et al 2022 Nucl. Fusion 62 042006 View the article online for updates and enhancements. You may also like Overview of progress in European medium sized tokamaks towards an integrated plasma-edge/wall solution H. Meyer, T. Eich, M. Beurskens et al. - LOCALIZATION AND BROADBAND FOLLOW-UP OF THE GRAVITATIONALWAVE TRANSIENT GW150914 B. P. Abbott, R. Abbott, T. D. Abbott et al. - Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X Thomas Sunn Pedersen, I. Abramovic, P. Agostinetti et al. - This content was downloaded from IP address 150.214.182.235 on 18/07/2024 at 16:51 International Atomic Energy Agency Nuclear Fusion Nucl. Fusion 62 (2022) 042006 (17pp) https://doi.org/10.1088/1741-4326/ac207f Progress from ASDEX Upgrade experiments in preparing the physics basis of ITER operation and DEMO scenario development U. Stroth1,∗, D. Aguiam2, E. Alessi3, C. Angioni1,N.Arden 1, R. Arredondo Parra1, V. Artigues1, O. Asunta4,M.Balden 1, V. Bandaru1, A. Banon-Navarro1,K.Behler 1,A.Bergmann 1, M. Bergmann1, J. Bernardo2,M.Bernert 1, A. Biancalani1, R. Bielajew5,R.Bilato 1, G. Birkenmeier1,6,T.Blanken 7, V. Bobkov1,A.Bock 1,T.Body 1, T. Bolzonella8, N. Bonanomi1,A.Bortolon 9,B.Böswirth 1,C.Bottereau 10, A. Bottino1,H.vandenBrand 7,M.Brenzke 11 , S. Brezinsek11 ,D.Brida 1, F. Brochard12,C.Bruhn 1, J. Buchanan13, A. Buhler1, A. Burckhart1, Y. Camenen14,B.Cannas 15, P. Cano Megias16,D.Carlton 17,M.Carr 13, P. Carvalho2, C. Castaldo18, M. Cavedon1,C.Cazzaniga 8, C. Challis13, A. Chankin1,C.Cianfarani 18,F.Clairet 10, S. Coda19,R.Coelho 2, J.W. Coenen11 ,L.Colas 10,G.Conway 1,S.Costea 20,D.Coster 1, T. Cote21, A.J. Creely5,G.Croci 3, D.J. Cruz Zabala16,G.Cseh 22, A. Czarnecka17,I.Cziegler 23, O. D’Arcangelo24,A.DalMolin 25,P.David 1, C. Day26,M.deBaar 7,P.deMarn ´ e1,R.Delogu 8,S.Denk 1,6, P. Denner11 , A. Di Siena27, J.J. Dominguez Palacios Durán16, D. Dunai10,A.Drenik 1, M. Dreval13,R.Drube 1, M. Dunne1,B.P.Duval 19,R.Dux 1,T.Eich 1, S. Elgeti1, A. Encheva28,K.Engelhardt 1, B. Erdös22,I.Erofeev 1, B. Esposito18,E.Fable 1,M.Faitsch 1,U.Fantz 1,M.Farnik 29,H.Faugel 17, F. Felici19, O. Ficker29,S.Fietz 1, A. Figueredo2,R.Fischer 1,O.Ford 30, L. Frassinetti1,M.Fröschle 1,G.Fuchert 30, J.C. Fuchs1, H. Fünfgelder1, S. Futatani31, K. Galazka17, J. Galdon-Quiroga1,D.GallartEscol ` a31, A. Gallo10,Y.Gao 11 , S. Garavaglia3, M. Garcia Muñoz16,B.Geiger 21, L. Giannone1,S.Gibson 32,L.Gil 2,E.Giovannozzi 18, S. Glöggler1, M. Gobbin8, J. Gonzalez Martin16, T. Goodman19,G.Gorini 25,T.Görler 1, D. Gradic30, G. Granucci3,A.Gräter 1,H.Greuner 1, M. Griener1,M.Groth 4, A. Gude1,L.Guimarais 2, S. Günter1,G.Haas 1,A.H.Hakola 33,C.Ham 13, T. Happel1,N.denHarder 1,G.Harrer 34,J.Harrison 13,V.Hauer 26, T. Hayward-Schneider1,B.Heinemann 1, T. Hellsten35, S. Henderson13, P. Hennequin36, A. Herrmann1,E.Heyn 37,F.Hitzler 1,J.Hobirk 1, K. Höfler1,6,J.H.Holm 38,M.Hölzl 1, C. Hopf1,L.Horvath 23,T.Höschen 1, A. Houben11 , A. Hubbard5, A. Huber11 , K. Hunger1, V. Igochine1, M. Iliasova39,T.Ilkei 22, K. Insulander Björk40, C. Ionita-Schrittwieser20, I. Ivanova-Stanik17, W. Jacob1, N. Jaksic1,F.Janky 1, A. Jansen van Vuuren30,A.Jardin 10, F. Jaulmes29,F.Jenko 1, T. Jensen38,E.Joffrin 10, A. Kallenbach1,S.Kálvin 22,M.Kantor 7, A. Kappatou1, O. Kardaun1, J. Karhunen4, C.-P. Käsemann1, S. Kasilov37,41, A. Kendl22, W. Kernbichler34,E.Khilkevitch 39,A.Kirk 13, S. Kjer Hansen1, V. Klevarova42,G.Kocsis 22,M.Koleva 1,M.Komm 29, M. Kong13, A. Krämer-Flecken11 ,K.Krieger 1, A. Krivska43, O. Kudlacek1, T. Kurki-Suonio4,B.Kurzan 1,B.Labit 19, K. Lackner1,F.Laggner 9, A. Lahtinen4,P.T.Lang 1,P.Lauber 1, N. Leuthold1,L.Li 11 , J. Likonen1, O. Linder1, B. Lipschultz23,Y.Liu 35, A. Lohs1,Z.Lu 1, 1741-4326/22/042006+17$33.00 1 ©EURATOM 2022 Printed in the UK Nucl. Fusion 62 (2022) 042006 U. Stroth et al T. Luda di Cortemiglia1, N.C. Luhmann44, T. Lunt1, A. Lyssoivan43, T. Maceina1, J. Madsen38, A. Magnanimo1,H.Maier 1, J. Mailloux13, R. Maingi9,O.Maj 1,E.Maljaars 7,P.Manas 1,A.Mancini 3, A. Manhard1, P. Mantica3,M.Mantsinen 31,P.Manz 1, M. Maraschek1, C. Marchetto45, L. Marrelli8,P.Martin 8, A. Martitsch37,F.Matos 1,M.Mayer 1, M.-L. Mayoral13,D.Mazon 10, P.J. McCarthy46, R. McDermott1,R.Merkel 1, A. Merle19, D. Meshcheriakov1, H. Meyer13, D. Milanesio45, P. Molina Cabrera5,F.Monaco 1, M. Muraca1, F. Nabais2,V.Naulin 38, R. Nazikian9,R.D.Nem 38, A. Nemes-Czopf17,G.Neu 1,R.Neu 1,47, A.H. Nielsen38, S.K. Nielsen38,T.Nishizawa 1,M.Nocente 25, J.-M. Noterdaeme1, I. Novikau1,S.Nowak 3,M.Oberkofler 1, R. Ochoukov1, J. Olsen38,F.Orain 1, F. Palermo1,O.Pan 1,6,G.Papp 1, I. Paradela Perez4,A.Pau 19, G. Pautasso1,C.Paz-Soldan 35, P. Petersson48, P. Piovesan8,C.Piron 8,U.Plank 1,B.Plaum 49,B.Plöck 1,V.Plyusnin 2, G. Pokol43,E.Poli 1,L.Porte 19,T.Pütterich 1,M.Ramisch 49, J. Rasmussen38,G.Ratta 50, S. Ratynskaia48, G. Raupp1,D.R ´ efy22, M. Reich1,F.Reimold 30, D. Reiser11 , M. Reisner1,D.Reiter 11 ,T.Ribeiro 1, R. Riedl1, J. Riesch1,D.Rittich 1, J.F. Rivero Rodriguez16,G.Rocchi 18, P. Rodriguez-Fernandez5, M. Rodriguez-Ramos16, V. Rohde1, G. Ronchi7, A. Ross1,M.Rott 1,M.Rubel 48,D.A.Ryan 13,F.Ryter 1, S. Saarelma13, M. Salewski38,A.Salmi 4,O.Samoylov 1, L. Sanchis Sanchez16, J. Santos2,O.Sauter 19,G.Schall 1,K.Schlüter 1,K.Schmid 1, O. Schmitz21,P.A.Schneider 1, R. Schrittwieser20, M. Schubert1, C. Schuster1,6,T.Schwarz-Selinger 1,J.Schweinzer 1,E.Seliunin 2, A. Shabbir42,A.Shalpegin 19, S. Sharapov13,U.Sheikh 19,A.Shevelev 39, G. Sias15,M.Siccinio 17,B.Sieglin 1,A.Sigalov 1,A.Silva 2, C. Silva2, D. Silvagni1, J. Simpson13,S.Sipilä 33,E.Smigelskis 1, A. Snicker4, E. Solano50, C. Sommariva19,C.Sozzi 3,G.Spizzo 8, M. Spolaore8, A. Stegmeir1,M.Stejner 38, J. Stober1, E. Strumberge1, G. Suarez Lopez1, H.-J. Sun1,W.Suttrop 1,E.Sytova 1,T.Szepesi 22,B.Tál 1,T.Tala 33, G. Tardini1, M. Tardocchi3,D.Terranova 8, M. Teschke1, E. Thor´ en48, W. Tierens1,D.Told 1, W. Treutterer1, G. Trevisan8,E.Trier 1,M.Tripský 43, M. Usoltceva1,M.Valisa 8,M.Valovic 13, M. van Zeeland35, F. Vannini1, B. Vanovac1, P. Varela2, S. Varoutis26, N. Vianello8,J.Vicente 2, G. Verdoolaege42,43,T.Vierle 1,E.Viezzer 1, I. Voitsekhovitch13, U. von Toussaint1, D. Wagner1,X.Wang 1, M. Weiland1,A.E.White 5, M. Willensdorfer1, B. Wiringer1, M. Wischmeier1,R.Wolf 30,E.Wolfrum 1, Q. Yang51,Q.Yu 1,R.Zag ´ orski17,I.Zammuto 1,T.Zehetbauer 1, W. Zhang51, W. Zholobenko1,M.Zilker 1,A.Zito 1, H. Zohm1, S. Zoletnik22 and the EUROfusion MST1 Team1a 1Max Planck Institute for Plasma Physics, 85748 Garching, Germany 2Instituto de Plasmas e Fus˜ ao Nuclear, Instituto Superior T´ ecnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal 3ENEA, IFP-CNR, Milan, Italy 4Department of Applied Physics, Aalto University, Finland 5MIT Plasma Science and Fusion Center, Cambridge, MA, United States of America 6Physik-Department E28, Technische Universität München, 85748 Garching, Germany 7Eindhoven, University of Technology, Eindhoven, Netherlands 8Consorzio RFX, Padova, Italy 9Princeton Plasma Physics Laboratory, Princeton, NJ, United States of America 10 CEA/IRFM, Saint Paul Lez Durance, France 11 Forschungszentrum Jülich, Germany 12 Institut Jean Lamour, Universit´ e de Lorraine, Nancy, France 13 CCFE, Culham Science Centre, Abingdon, United Kingdom 14 Aix-Marseille University, CNRS, Marseille, France 2 Nucl. Fusion 62 (2022) 042006 U. Stroth et al 15 Department of Electrical and Electronic Engineering, University of Cagliari, Italy 16 Universidad de Sevilla, Sevilla, Spain 17 Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland 18 ENEA, Centro Ricerche Frascati, Italy 19 Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL), Swiss Plasma Center (SPC), Lausanne, Switzerland 20 ÖAW, IAP, University of Innsbruck, Innsbruck, Austria 21 University of Wisconsin, Madison, United States of America 22 Wigner Research Centre for Physics, Budapest, Hungary 23 York Plasma Institute, University of York, United Kingdom 24 ENEA Consorzio CREATE, Naples, Italy 25 ENEA, University of Milano-Bicocca, Milano, Italy 26 Karlsruhe Institut für Technology, Karlsruhe, Germany 27 Oden Institute for Computational Engineeringand Sciences, Austin, United States of America 28 ITER Organization, Saint-Paul-lez-Durance, France 29 Institute of Plasma Physics of the CAS, Praha, Czech Republic 30 Max-Planck-Institut für Plasmaphysik, 17491 Greifswald, Germany 31 Supercomputing Center-Centro Nacional de Supercomputaci´ on, Barcelona, Spain 32 Department of Physics, Durham University, United Kingdom 33 VTT Technical Research Centre of Finland, VTT, Finland 34 ÖAW, IAP, Vienna University of Technology, Austria 35 General Atomics, San Diego, California, United States of America 36 Laboratoire de Physique des Plasmas, Ecole Polytechnique, Palaiseau, France 37 ÖAW, Graz University of Technology, Graz, Austria 38 Department of Physics, Technical University of Denmark, Kgs. Lyngby, Denmark 39 Ioffe Institute, St. Petersburg, Russia 40 Department of Physics, Chalmers University of Technology, Gothenburg, Sweden 41 Institute of Plasma Physics, National Science Center Kharkov Institute of Physics and Technology, Krakov, Ukraine 42 Ghent University, Ghent, Belgium 43 ERM/KMS, Brussels, Belgium 44 Electrical and Computer Engineering, University of California, Davis, United States of America 45 ISC-CNR and Politecnico di Torino, Torino, Italy 46 Department of Physics, National University of Ireland, Cork, Ireland 47 Technische Universität München, Garching, Germany 48 KTH Royal Institute of Technology, Stockholm, Sweden 49 IGVP Universität Stuttgart, Germany 50 Laboratorio Nacional de Fusi´ on, CIEMAT, Madrid, Spain 51 Chinese Academy of Sciences, Hefei, China E-mail: [email protected] Received 8 June 2021, revised 28 July 2021 Accepted for publication 24 August 2021 Published 15 March 2022 Abstract An overview of recent results obtained at the tokamak ASDEX Upgrade (AUG) is given. A work flow for predictive profile modelling of AUG discharges was established which is able to reproduce experimental H-mode plasma profiles based on engineering parameters only. In the plasma center, theoretical predictions on plasma current redistribution by a dynamo effect were confirmed experimentally. For core transport, the stabilizing effect of fast ion distributions on turbulent transport is shown to be important to explain the core isotope effect and improves the description of hollow low-Zimpurity profiles. The L–H power threshold of hydrogen plasmas is not affected by small helium admixtures and it increases continuously from the deuterium to ∗Author to whom any correspondence should be addressed. aSee Labit et al 2019 (https://doi.org/10.1088/1741-4326/ab2211) for the 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. 3 Nucl. Fusion 62 (2022) 042006 U. Stroth et al the hydrogen level when the hydrogen concentration is raised from 0 to 100%. One focus of recent campaigns was the search for a fusion relevant integrated plasma scenario without large edge localised modes (ELMs). Results from six different ELM-free confinement regimes are compared with respect to reactor relevance: ELM suppression by magnetic perturbation coils could be attributed to toroidally asymmetric turbulent fluctuations in the vicinity of the separatrix. Stable improved confinement mode plasma phases with a detached inner divertor were obtained using a feedback control of the plasma β. The enhanced DαH-mode regime was extended to higher heating power by feedback controlled radiative cooling with argon. The quasi-coherent exhaust regime was developed into an integrated scenario at high heating power and energy confinement, with a detached divertor and without large ELMs. Small ELMs close to the separatrix lead to peeling-ballooning stability and quasi continuous power exhaust. Helium beam density fluctuation measurements confirm that transport close to the separatrix is important to achieve the different ELM-free regimes. Based on separatrix plasma parameters and interchange-drift-Alfv´ en turbulence, an analytic model was derived that reproduces the experimentally found important operational boundaries of the density limit and between L- and H-mode confinement. Feedback control for the X-point radiator (XPR) position was established as an important element for divertor detachment control. Stable and detached ELM-free phases with H-mode confinement quality were obtained when the XPR was moved 10 cm above the X-point. Investigations of the plasma in the future flexible snow-flake divertor of AUG by means of first SOLPS-ITER simulations with drifts activated predict beneficial detachment properties and the activation of an additional strike point by the drifts. Keywords: Asdex Upgrade, confinement, ELLM-free discharges (Some figures may appear in colour only in the online journal) 1. Introduction ASDEX Upgrade (AUG) is a midsize tokamak with major and minor radii of R0=1.65 m and a=0.5 m, respectively. The plasma facing surfaces are covered with tungsten and the magnetic and divertor geometries are like as for ITER. AUG plasmas can match a number of fusion relevant parameters simultaneously, such as high values of the normalized plasma pressure, βN, the normalized confinementtime H98, the Greenwald density fraction fGW =n/nGW, and the power density P/R0. In general, these values can be reached also under detached divertor conditions. Experiments on AUG are carried out to enhance the physical understanding necessary to better predict the performance of ITER or fusion reactors. Furthermore, new plasma scenarios are explored and control tools developed that can facilitate the operation of a reactor plasma. To achieve these goals, AUG is equipped with a powerful heating system that, with the present power supplies, delivers up to 27 MW of heating power. Neutral beam injection (NBI) of 20 MW power, electron cyclotron resonance heating (ECRH, 6 MW) and ion cyclotron resonance heating (ICRH, 6 MW) are routinely and flexibly combined. In particular, the ECRH system equipped with 8 gyrotrons is very versatile in locally heating the plasma at variable positions [1]. In close contact with ITER and DEMO collaborators, a discharge control system is constantly being further developed and enhanced with real-time capabilities, new realtime sensors and actuators to e.g. avoid disruptions [2]orto control detachment by regulating the vertical position of the X-point radiator (XPR) [3]. The control system is also coupled to a discharge flight simulator [4] with a comprehensive plasma model. The following paper reports on recent results obtained on AUG to challenge the physical models used for predictions, to develop them further where they are still empirical, and to extend them to a wider parameter range and to new dischargeregimes. Starting with the integratedmodellingeffort in section 2, core physics and confinement is treated in section 3, followed by a summary of the investigation of different edge localised mode (ELM)-free regimes in section 4. Section 5 presents results demonstrating the importance of the plasma parameters close to the separatrix for the plasma performance, to finish with results on power exhaust and divertor physics in section 6. The paper closes with a brief summary of the main results. 2. Integrated transport modelling An important goal of the research effort is to design tools for predicting future reactor discharges based on validated physical models. For this purpose, a workflow for integrated modelling based on engineering parameters (IMEP) was established that is capable of predicting the plasma profiles in stationary phases of AUG H-mode discharges [5,6]. IMEP is based on the ASTRA transport code, coupled with the HELENA code for high-resolution equilibrium reconstruction 4 Nucl. Fusion 62 (2022) 042006 U. Stroth et al and the MISHKA MHD stability code. With this workflow it is possible to model the kinetic profiles on the entire plasma cross section of AUG discharges when the pedestal conditions are close to the peeling-ballooning stability limit. The separatrix plasma parameters follow from an empirical model with the prescribed gas fuelling and the two-point model for the scrape-off layer (SOL), the pedestal shape is given by peeling-ballooning stability where a critical temperature gradient model is used, and the core plasma profiles from the pedestal top inwards are modeled with TGLF. Figure 1shows that the energy content evaluated from fully predictively modeled profiles of stationary H-mode dischargesagrees better with the experimentalvalues than estimates derived from the IPB98(y,2) H-mode confinement scaling [5,7]. The majority of the experimental confinement times have an confinement factorH98 <1. This aspect reflects a general property of the plasmas performed on AUG with a tungsten wall [8]. To protect the plasma from impurities, operating with a tungsten wall requires stronger fueling than with a carbon wall. This leads to a relative shift of the density pedestal with respect to the temperature pedestal resulting in a reduced peeling-ballooning limit and thus a weaker pedestal [9]. Furthermore, the increase of confinement with increasing density present in the IPB98(y,2) scaling law is not observed on AUG [8], nor in several other devices with vertically elongated plasmas [10]. In addition, to prevent tungsten accumulation, the dischargesare mostly operatedwith a substantialECRH power fraction while NBI heating dominates the data represented in the IPB98(y,2) scaling. Therefore, the IPB98(y,2) scaling cannot be fully representative for AUG and revised energy confinement scaling expressions including recent data from metal devices are needed such as the one by Verdoolaege et al [10]. However also in AUG, discharges with a confinement factor of H98 >1 are obtained particularly in triangular shapes where the edge density can be low. A detailed analysis of the confinement properties of AUG plasmas has been recently published in [8]. With theIMEP software packageit is possibleto understand the physical reasons for the observed dependencies of profiles and confinement time on engineering parameters including the negative influence of fueling, the positive impact of triangularity, and the positive impact of an increase of plasma current at constant fuelling rate [5]. In parallel to the ASTRAIMEP workflow, at AUG the first tokamak flight simulator FENIX was developed [4,11,12]. FENIX is fully based on the AUG pulse schedule and directly coupled to the AUG control system. 3. Core transport and confinement Starting with the redistribution of magnetic flux in the plasma center, this section is devoted to results on core heat and particle transport, turbulence studies and the L–H transition power threshold. Figure 1. Comparison of the thermal energy content in AUG H-mode plasmas as derived from the IMEP model and from the IPB98(y,2) energy confinement scaling with experimental values. Adapted from [6]. CC BY 4.0. 3.1. Core plasma current diffusion First an MHD phenomenon is addressed in the plasma center, where elevated central safety factor values of qs(0) >1 togetherwith sawtooth-freephases were observed,when transport modelling would predict qs(0) <1 and sawteeth. The experiments were carried out motivated by theoretical results from nonlinear MHD simulations with the MD3D-C code, where it was found that a dynamo driven by a (1, 1) quasiinterchange mode transports poloidal magnetic flux radially outward raising qs(0) above one [13]. This occurs in a stationary process with continuous redistribution of magnetic flux. In the simulations, this mechanism is activated at high values of the plasma β, when the dynamo drive is sufficiently strong in order to counteract the plasma current diffusion which would lead to a centrally peaked current profile. Recently, the βthreshold for the flux pumping effect was investigated by means of on-axis electron-cyclotron current drive (ECCD) experiments (see figure 2). In a discharge with modest co-ECCD of 100 kA, the sawteeth disappeared when the βvalue, increased by NBI power steps, surpassed a critical value [14]. A successive increase of the driven current made the sawteeth reappear. Figure 2also shows that the amount of tolerable co-ECCD without producing sawteeth increases with β[14]. The discharges were analyzed with a combination of the imaging motional Stark effect (IMSE) diagnostic [15]and the IDE equilibrium solver [16,17]. When sawteeth are suppressed,the IMSE data indicatea flat central qsprofileclamped to values near unity whereas based on neoclassical current diffusion, the equilibrium solver predicts a monotonic qsprofile starting from qs(0) <1[14]. The effective charge profile is flat in the plasma centre with values of Zeff ⩽1.5. Therefore Zeff can only play a minor role for the current profile. The observeddynamo effect can enablestrong ECCD in the center of reactor plasmas, where the current drive efficiency is 5 Nucl. Fusion 62 (2022) 042006 U. Stroth et al Figure 2. Discharge phases with (red symbols) and without (blue) sawteeth in the parameter space defined by the normalized plasma β and the volume-averaged non-inductive central current density. Adapted with permission from [14]. high, with flat central qsprofiles as required for enhanced core plasma performance. 3.2. Core heat transport One of the unresolved issues of core heat transport is the magnitude of the isotope effect and its cause. A novel strategy was used to disentangle the isotope effect in H-mode core and pedestal transport by matching the pedestal profiles with a slight increase of the plasma cross section’s triangularity δfor the deuterium case with respect to that in the hydrogen plasmas while keeping heat and particle sources the same [18,19]. Figure 3shows the kinetic profiles of a pair of hydrogen and deuterium discharges which both were carried out with a high NBI heating power of about 10 MW and a particle fuelling rate of 7–10 ×1021 s−1. A comparison with modelling results highlights the role of fast ion (FI) populations in reducing turbulent transport in NBI heated discharges. In agreement with results from the theory-based turbulence model TGLF, the core isotope effect was found to be small as long as the fraction of FIs remained below about 30 % of the main ion density [18]. At a higher NBI power with larger fast particle populations, a stronger decrease in ion heat diffusivity was indicated in deuterium with respect to that in hydrogen plasmas leading to substantially higher core ion temperatures, Due to the longer slowing down time of injected deuterium ions compared to that of hydrogen ions, the FI energy content in deuterium plasmas was about 50 % higher. Nonlinear simulations of these discharges with the gyrokinetic turbulence code GENE [20,21] revealed the importance of turbulence stabilization by electromagnetic and FI effects. For both isotopes, the FI distribution from NBI stabilises turbulence and reduces turbulent transport. The stronger effect observed in deuterium plasmas is attributed to the higher content of non-thermal ions [18]. The energy confinement in helium plasmas with dominant electron heating is similar to that in deuterium, while a degradation with an increasing fraction of ion heating is observed. These observations can be theoretically explained by a different role of zonal flows in electron and ion dominated turbulence with different main ions [22]. Low-density L-mode plasmas with dominant electron heating were studied on AUG [23] in particular for their relevance to the first pre-fusion power operation phase of ITER. For this purpose,ECR heated hydrogenand deuterium discharges were performed and modeled with TGLF-SAT1geo, a quasi linear turbulent transport model based on TGLF that includes a turbulence saturation rule and an improvement in the description of geometrical effects [24,25]. The model reproduced both the central electron temperature and the edge ion heat flux, which is critical for the L–H transition [26,27], with high confidence. This is seen as a validation of TGLF-SAT1geo also in applications for the prediction of the absorption of X3- mode ECRH in ITER, which critically depends on the central electron temperature. The same model was also used for the interpretation of cold pulse experiments using impurity laser ablation. The dynamic responseof the electrontemperatureprofile followingthe cooling event in the plasma edge was successfully modeled with the ASTRA transport code with the local transport model TGLF-SAT1. The reason for the—sometimes called nonlocal—fast response of the core temperature was explained by the stabilization of trapped electron modes through an induced and also correctly predicted flattening of the electron density profile [28]. The TGLF model also proved reliable in describing the role of beam ions in reducing ion heat transport in H-mode plasmas. It predicted ion temperature gradient (ITG) turbulence propertiesobservedin experiments,where the ion stiffness was reduced simultaneously with a drop of the electron to ion temperature ratio, Te/Ti. In addition, a potential role of electron temperature gradient (ETG) driven modes in strongly electron heated H-mode plasmas in limiting the increase of electron temperature gradient was identified by means of linear and nonlinear gyrokinetic simulations with the GKW code [29]. 3.3. Core particle transport The core density profiles of ITER or reactor plasmas will be dominated by transport processes. AUG experiments reproducing reactor conditions of heat, particle and momentum sources and related integrated modelling using ASTRA and TGLF demonstrated the role and dominance of a collisionality dependent turbulent pinch in producing centrally peaked plasma density profiles at reactor relevant low collisionalities [30]. Fast-ion effects turnout to also be importantfor the description of low-Zimpurity transport, which has important consequences for the operation of reactor plasmas. Especially helium transport will determine the amount of fuel dilution in the core of a burning plasma. In order to enhance the physical understanding of low-Zimpurity transport, transport studies were carried out for helium and boron. In particular, a new modulation technique was developed and applied 6 Nucl. Fusion 62 (2022) 042006 U. Stroth et al Figure 3. Kinetic profiles of a pair of hydrogen and deuterium H-mode plasmas, where the edge profiles were matched while the source profiles are kept similar. Reproduced courtesy of IAEA. Figure adapted from [18]. ©EURATOM 2021. to boron transport studies to disentangle diffusive and convective transport coefficients [31,32]. Using this technique, a database of transport coefficients covering a wide range of plasma parameterswas assembledand compared to theoretical predictions. For steep ITGs (R0/LTi>6), which coincide with strong NBI heating, outward convection and hollow boron density profiles appear. In contrast, even low levels of electron heating increase both the diffusion and inward convection and result in peaking of the impurity density profiles. Comparisons with a combination of neoclassical and quasi-linear gyrokinetic turbulence simulations (NEO and GKW, respectively) showed good agreement for plasmas with combined NBI and ECR heating. The hollowboron density profiles, onthe other hand,are not fully reproduced by the simulations, particularly around midradius. This is demonstrated in the comparison of measured and modeled radial profiles of the boron density scale length in figure 4, where the inclusion of fast ions leads to a clear increase in the outward predicted transport at a normalised radius of ρtor =0.4. In the modeling, it is a combination of different fast ion effects that leads to the development of hollow boron profiles [33,35]. The fast ion distribution stabilises ITG driven turbulence and leads to an increase of neoclassical outward convection [35]. However, at mid-radius this effect is insufficient to reach experimental levels due, at least in part, to the relatively small fast ion population at this location. Figure 4also shows that the results of the simulations are very sensitive to the gradient scale lengths of the background profiles. Small increases in the ion temperature gradeint and rotation gradient can change the sign of the predicted convection. 3.4. Core turbulence studies A validation of the physics models of turbulence codes is also carried out directly on the microscopic level of the fluctuations [36]. For this purpose, measurements of a large set of fluctuation data is assembled on L-mode discharges at two values of the electron temperature gradient length. This implies density fluctuation wavenumber spectra [37], temperature fluctuation Figure 4. Spectroscopically measured boron density scale length profile from an NBI heated discharge (7.5 MW) compared with modeled profiles form GKW and NEO with and without the fast ion effects included. As a sensitivity test, simulations were also done with flatter ion temperature and toroidal rotation velocity profiles (scale lengths were reduced by approx. 10%). Based on results from references [33,34]. frequency spectra [38], the correlation lengths of both, cross phases between density and temperature fluctuations [38], turbulent flow and phase velocities including zonal flow structures. The experiments are completedand the analysis is ongoing. Comparison of these data with local and global GENE simulations will provide a comprehensive test of the physics models used in the code. As a first example, the poloidal symmetry of the turbulent propagation velocity was tested using a Doppler reflectometer which probed a radial and poloidal region around the outer midplane. It turned out that the interpretation sensitively depends on the background density profile. From careful analysis, it was concluded, that the flows on the outboard side are poloidally symmetric, where on each flux surface fluctuations propagate with the same velocity at all wavenumbers. 7 Nucl. Fusion 62 (2022) 042006 U. Stroth et al Figure 5. Dependence of the L–H power threshold values in hydrogen discharges as function of the helium content (a) and for a transition from a pure deuterium to a hydrogen plasma (b). Symbols indicate different heating beams (H or D NBI) and ECRH plasmas. Reproduced courtesy of IAEA. Figure adapted from [41]. ©EURATOM 2020. A comparison with spectroscopic measurements showed that this velocity is close to the E×Bdrift velocity [39]. 3.5. L–H power threshold Due to the absence of reliable physical models for the L-mode plasma edge, predictions for the power threshold for L–H confinement transitions, PLH, rely on experimentalscaling laws. In particular, the dependence of PLH on the isotopic mass must be known to predict the performance of ITER in the pre-nuclear phase in hydrogen or helium and for the D–T phase. An earlier study on JET indicated a beneficial 40 % reduction of PLH when small concentrations of helium were added to NBI heated hydrogen plasmas. Furthermore PLH increased to the hydrogen level when even a small amount of hydrogen was admixed to deuterium plasmas [40]. Figure 5shows comparable experiments performed on AUG. The left figure shows that the threshold power did not change and remained at the hydrogen level, when up to 20 % of helium was added to the hydrogen plasmas. The data was obtained from both ECRH and hydrogen NBI heated discharges. For the right figure, a continuous transition from a pure deuterium plasma to a pure hydrogen plasma was performed. The data from the different heating schemes consistently shows a continuous increase of PLH with hydrogen concentration from the D level to the H level. A stronger increase starts only above a relative high hydrogen content of about 60 %. These differences from the JET results need further investigations. Power balance analyses show that the ion heat flux through the separatrix, which is known to be a key quantity for the L–H transition on AUG [26], is independent of the helium concentration and heating scheme [42]. In hydrogen and deuterium plasmas, the L–H transitions happen at similar values for the neoclassical E×Bshearing rate [27]. Spectroscopic measurements of the edge radial electric field at the L–H transition of hydrogen and deuterium plasmas were also consistent with the observation that the neoclassical E×Bshear, given by the ion pressure gradient, is the key physical parameter for turbulence suppression. The transition in hydrogen and deuterium plasmas is found to occur at the same E×Bvelocity [43]. 4. Plasma scenarios without large ELMs Large (type-I) ELMs are considered critical for fusion reactors to the extent that they must be avoided altogether. Therefore, it is important to search for robust small or no ELM regimes, that simultaneously have H-mode-like confinement and can be operated at high density where the divertor can be in a detached state. This section summarizes recent studies of six such regimes. An earlier overview article is reference [44]. It turns out, that the most promising candidates for regimes without large ELMs have in common, that turbulent fluctuations and more or less coherent modes near the separatrix cause transport that changes the pressure gradient of the H- mode pedestal in such a way that the peeling-ballooning limit is not reached and type-I ELMs do not occur. 4.1. ELM suppression by magnetic perturbations On AUG, the edge density was identified to be the leading parameter for ELM mitigation with magnetic perturbation fields [45]. At low densities the smallest relative pedestal energy losses are observed when a magnetic perturbation field with a toroidal mode number of n=2 is applied, inducing a so-called density pump-out. In recent experiments, the physical origin of the density pump-out was investigated [46]. By toroidal rotating of the n=2 perturbation field, a reflectometer observed toroidally asymmetric broad-band fluctuations with frequencies up to 150 kHz at the outer midplane. The fluctuation spectrum is locally quenched at a certain phase angle of the field perturbation. The fluctuations appear simultaneously with a drop in edge density associated with a density pump-out and are held responsible for it. To identify the transport losses caused by the fluctuations, their dynamicswas compared to that observed with a poloidal array of Langmuir probes on the outer divertor target. It was found that the toroidal asymmetry of the turbulence at the outboard midplane maps along the magnetic field lines to the divertor. High and low fluctuation amplitudes at the plasma edge are magnetically connected to high and low particle transport measured in the divertor [46]. 8 Nucl. Fusion 62 (2022) 042006 U. Stroth et al Since the AUG will be equipped with both an ITER-like divertor and a flexible snowflake divertor starting in 2023, the evaluation of the best divertor solution for a fusion reactor will be possible on a single device. 7. Summary A summary of selected results from recent experiments performed at the AUG tokamak was presented. The aim of the work is to further develop the physical understandingof fusion plasmas and to improve models for reactor plasma predictions as they are integrated in the ASTRA-IMEP code package developed on AUG, which already successfully predicts confinement of AUG H-mode plasmas. The main results are as follows. In the plasma center, theoretical predictions on magnetic flux pumping were confirmed experimentally. The plasma current redistribution at high βfacilitates the operation of advanced plasma scenarios in a reactor with central current drive. For a quantitative modelling of the core heat and particle transport, the effects of fast particle distribution functions on the turbulence play an important role. They are found to be mainly responsible for the core isotope effect, where in deuterium the longer slowing-down time of NBI ions produces a larger fast particle content and thus a stronger reduction of turbulent transport. The inclusion of fast particles also improves the theoretical description of experimentally observed hollow low-Zimpurity profiles. On AUG the fast particles originate from NBI or ICR heating. For reactor predictions it will be necessary to transfer this effect to non-thermal αparticle distributions. Important for ITER operation in the pre-nuclear and in the D–T phases is the investigation of the L–H power threshold. It is shown that the threshold power remains on the hydrogen level when up to 20 % of helium is added to a hydrogen plasma and that the threshold power continuously increases from the deuterium to the hydrogen level when the hydrogen concentration is raised from 0 to 100 %. An integrated naturally ELM-freehigh confinementplasma scenario would be of great advantage for a safe operation of a fusion reactor. Experimental results from six different confinement regimes without large ELMs were compared on the same tokamak. Transport by turbulenceor coherent modes that flatten the pressure gradient of the edge plasma was identified as a common cause of peeling-ballooning stability and ELM suppression. The integration of high edge plasma density and divertor detachment is another obstacle to be overcome on the way to a reactor scenario. This applies in particular to ELM suppression by magnetic perturbation coils, negative δplasmas and to the QH-mode, while in I-mode plasmas detachment of the inner divertor and reduced power loadings on the outer divertor where achieved by nitrogen seeding. Furthermore, a relevant regime must be developed towards higher heating powers, which remains difficult for I-mode and EDA H-mode plasmas. For the latter, radiative cooling of the plasma edge by argon seeding was demonstrated to be a promising resource. Plasma shaping and strong gas fuelling appear to be important elements to achieve ELM suppression in the QCE regime, which is the candidate regime that currently operates closest to a reactor relevant integrated scenario. High-nballooning modes near the separatrix are made responsible for a local flattening of the pressure gradient which enhances peelingballooning stability. A QCE demonstration discharge without any large ELM at high heating power and close to the density limit was presented. The dischargecombineshigh confinement with a partially detached divertor. For detachment control of this and other discharges, a feedback controlled X-point radiator is an important element. Such a control scheme was established and used to create stable and detached ELM-free phases at relevant parameters. The plasma parameters near separatrix, which in AUG are not too far from those in a reactor, proveto be not relevantonly for ELM suppression but also for describing important operating limits of tokamak plasmas. Analytic expressions derived from gyro-fluid turbulence equations reproduce the experimentally found boundaries for the L-mode density limit, the L–H transition and the H-mode density limit, which is set by the H–L back transition at high densities. The separatrix parameters of the QCE discharges extend to both high densities and high temperatures inside the H–L transition boundary into the region of expected ITER separatrix parameters. To evaluate the best divertor solution for a fusion reactor AUG will be equipped with an additional flexible divertor geometryfrom 2023 on. 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