Major results from the first plasma campaign of the Wendelstein 7-X stellarator

PAPER • OPEN ACCESS Major results from the first plasma campaign of the Wendelstein 7-X stellarator To cite this article: R.C. Wolf et al 2017 Nucl. Fusion 57 102020 View the article online for updates and enhancements. Related content Review Article M Hirsch, J Baldzuhn, C Beidler et al. - Confinement in Wendelstein 7-X limiter plasmas M. Hirsch, A. Dinklage, A. Alonso et al. - Plans for the first plasma operation of Wendelstein 7-X T. Sunn Pedersen, T. Andreeva, H.-S. Bosch et al. - Recent citations RF Behavior of a Coaxial Interaction Structure for 0.24-THz, 2-MW Gyrotron Nitin Kumar and Anirban Bera - Operation of W7-X With an Inertially Cooled Divertor—On the Way to Steady State Operation Hans-Stephan Bosch et al - Upgrade of the Cryogenic Systems of Wendelstein 7-X Thomas Rummel et al - This content was downloaded from IP address 78.30.2.54 on 29/07/2020 at 13:01 1 © 2017 Max-Planck-Institut fur Plasmaphysik Printed in the UK Nuclear Fusion Major results from the first plasma campaign of the Wendelstein 7-X stellarator R.C.Wolf1, A.Ali1, A.Alonso2, J.Baldzuhn1, C.Beidler1, M.Beurskens1, C.Biedermann1, H.-S.Bosch1, S.Bozhenkov1, R.Brakel1, A.Dinklage1, Y.Feng1, G.Fuchert1, J.Geiger1, O.Grulke1, P.Helander1, M.Hirsch1, U.Höfel1, M.Jakubowski1, J.Knauer1, G.Kocsis3, R.König1, P.Kornejew1, A.Krämer-Flecken4, M.Krychowiak1, M.Landreman5, A.Langenberg1, H.P.Laqua1, S.Lazerson6, H.Maaßberg1, S.Marsen1, M.Marushchenko1, D.Moseev1, H.Niemann1, N.Pablant6, E.Pasch1, K.Rahbarnia1, G.Schlisio1, T.Stange1, T.Sunn Pedersen1, J.Svensson1, T.Szepesi3, H.Trimino Mora1, Y.Turkin1, T.Wauters7, G.Weir1, U.Wenzel1, T.Windisch1, G.Wurden8, D.Zhang1, I.Abramovic1, S.Äkäslompolo1, P.Aleynikov1, K.Aleynikova1, R.Alzbutas9, G.Anda3, T.Andreeva1, E.Ascasibar2, J.Assmann4, S.-G.Baek10, M.Banduch1, T.Barbui11, M.Barlak12, K.Baumann13, W.Behr4, A.Benndorf1, O.Bertuch4, W.Biel4, D.Birus1, B.Blackwell14, E.Blanco2, M.Blatzheim1, T.Bluhm1, D.Böckenhoff1, P.Bolgert6, M.Borchardt1, V.Borsuk4, J.Boscary15, L.-G.Böttger1, H.Brand16, Ch.Brandt1, T.Bräuer1, H.Braune1, S.Brezinsek4, K.- J.Brunner1, B.Brünner1, R.Burhenn1, B.Buttenschön1, V.Bykov1, I.Calvo2, B.Cannas17, A.Cappa2, A.Carls1, L.Carraro18, B.Carvalho19, F.Castejon2, A.Charl4, F.Chernyshev20, M.Cianciosa21, R.Citarella22, Ł.Ciupiński23, G.Claps24, M.Cole1, M.J.Cole21, F.Cordella24, G.Cseh3, A.Czarnecka25, A.Czermak26, K.Czerski27, M.Czerwinski15, G.Czymek4, A.da Molin28, A.da Silva19, G.Dammertz13, A.de la Pena2, S.Degenkolbe1, P.Denner4, D.P.Dhard1, M.Dostal4, M.Drevlak1, P.Drewelow1, Ph.Drews4, A.Dudek1, G.Dundulis9, F.Durodie7, P.van Eeten1, F.Effenberg11, G.Ehrke1, M.Endler1, D.Ennis29, E.Erckmann1, H.Esteban2, T.Estrada2, N.Fahrenkamp1, J.-H.Feist1, J.Fellinger1, H.Fernandes19, W.H.Fietz13, W.Figacz25, J.Fontdecaba2, O.Ford1, T.Fornal25, H.Frerichs11, A.Freund4, M.Führer30, T.Funaba31, A.Galkowski25, G.Gantenbein13, Y.Gao4, J.García Regaña2, M.Garcia-Munoz2, D.Gates6, G.Gawlik12, B.Geiger1, V.Giannella22, N.Gierse4, A.Gogoleva32, B.Goncalves19, A.Goriaev7, D.Gradic1, M.Grahl1, J.Green11, A.Grosman33, H.Grote1, M.Gruca25, C.Guerard2, L.Haiduk26, X.Han4, F.Harberts4, J.H.Harris21, H.-J.Hartfuß1, D.Hartmann1, D.Hathiramani1, B.Hein1, B.Heinemann15, P.Heitzenroeder6, S.Henneberg1, C.Hennig1, J.Hernandez Sanchez2, C.Hidalgo2, H.Hölbe1, K.P.Hollfeld4, A.Hölting1, D.Höschen4, M.Houry33, J.Howard14, X.Huang31, M.Huber13, V.Huber34, H.Hunger13, K.Ida31, R.C. Wolf etal Printed in the UK 102020 NUFUAU © 2017 Max-Planck-Institut fur Plasmaphysik 57 Nucl. Fusion NF 10.1088/1741-4326/aa770d Paper 10 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 2017 1741-4326 1741-4326/17/102020+13$33.00 https://doi.org/10.1088/1741-4326/aa770d Nucl. Fusion 57 (2017) 102020 (13pp) R.C. Wolf etal 2 T.Ilkei3, S.Illy13, B.Israeli6, A.Ivanov35, S.Jablonski25, J.Jagielski12, J.Jelonnek13, H.Jenzsch1, P.Junghans15, J.Kacmarczyk25, T.Kaliatka9, J.-P.Kallmeyer1, U.Kamionka1, R.Karalevicius9, H.Kasahara31, W.Kasparek36, N.Kenmochi31, M.Keunecke37, A.Khilchenko35, D.Kinna34, R.Kleiber1, T.Klinger1, M.Knaup4, Th.Kobarg13, F.Köchl38, Y.Kolesnichenko39, A.Könies1, M.Köppen1, J.Koshurinov40, R.Koslowski4, F.Köster41, R.Koziol12, M.Krämer36, R.Krampitz1, P.Kraszewsk12, N.Krawczyk25, T.Kremeyer11, Th.Krings4, J.Krom1, G.Krzesinski23, I.Ksiazek42, M.Kubkowska25, G.Kühner1, T.Kurki- Suonio43, S.Kwak1, R.Lang13, S.Langish6, H.Laqua1, R.Laube1, C.Lechte36, M.Lennartz4, W.Leonhardt13, L.Lewerentz1, Y.Liang4, Ch.Linsmeier4, S.Liu4, J.-F.Lobsien1, D.Loesser6, J.Loizu Cisquella1, J.Lore21, A.Lorenz1, M.Losert13, L.Lubyako40, A.Lücke44, A.Lumsdaine21, V.Lutsenko39, J.Majano-Brown10, O.Marchuk4, M.Mardenfeld6, P.Marek23, S.Massidda29, S.Masuzaki31, D.Maurer29, K.McCarthy2, P.McNeely1, A.Meier13, D.Mellein13, B.Mendelevitch15, Ph.Mertens4, D.Mikkelsen6, O.Mishchenko1, B.Missal1, J.Mittelstaedt6, T.Mizuuchi45, A.Mollen1, V.Moncada33, T.Mönnich1, T.Morizaki31, R.Munk36, S.Murakami45, F.Musielok42, G.Náfrádi3, M.Nagel1, D.Naujoks1, H.Neilson6, O.Neubauer4, U.Neuner1, T.Ngo33, R.Nocentini15, C.Nührenberg1, J.Nührenberg1, S.Obermayer15, G.Offermanns4, K.Ogawa31, J.Ongena7, J.W.Oosterbeek1, G.Orozco15, M.Otte1, L.Pacios Rodriguez2, W.Pan46, N.Panadero1, N.Panadero Alvarez2, A.Panin4, D.Papenfuß13, S.Paqay16, A.Pavone1, E.Pawelec42, G.Pelka25, X.Peng46, V.Perseo1, B.Peterson31, A.Pieper44, D.Pilopp1, S.Pingel1, F.Pisano17, B.Plaum36, G.Plunk1, M.Povilaitis9, J.Preinhaelter47, J.Proll16, M.-E.Puiatti48, A.Puig Sitjes1, F.Purps1, M.Rack4, S.Récsei3, A.Reiman6, D.Reiter4, F.Remppel36, S.Renard33, R.Riedl15, J.Riemann1, S.Rimkevicius9, K.Riße1, A.Rodatos1, H.Röhlinger36, M.Romé48, P.Rong1, H.-J.Roscher49, B.Roth36, L.Rudischhauser1, K.Rummel1, T.Rummel1, A.Runov1, N.Rust1, L.Ryc25, S.Ryosuke31, R.Sakamoto31, A.Samartsev13, M.Sanchez2, F.Sano45, S.Satake31, G.Satheeswaran4, J.Schacht1, F.Schauer1, T.Scherer13, A.Schlaich13, K.-H.Schlüter36, J.Schmitt29, H.Schmitz4, O.Schmitz11, S.Schmuck34, M.Schneider1, W.Schneider1, M.Scholz25, P.Scholz15, R.Schrittwieser38, M.Schröder1, T.Schröder1, R.Schroeder1, H.Schumacher44, B.Schweer4, B.Shanahan1, I.V.Shikhovtsev35, M.Sibilia6, P.Sinha1, S.Sipliä43, J.Skodzik50, C.Slaby1, H.Smith1, W.Spiess13, D.A.Spong21, A.Spring1, R.Stadler15, B.Standley15, L.Stephey11, M.Stoneking51, U.Stridde1, Z.Sulek26, Y.Suzuki31, V.Szabó3, T.Szabolics3, Z.Szökefalvi-Nagy3, N.Tamura31, A.Terra4, J.Terry10, J.Thomas4, H.Thomsen1, M.Thumm13, C.P.von Thun34, D.Timmermann50, P.Titus6, K.Toi31, J.M.Travere33, P.Traverso29, J.Tretter15, H.Tsuchiya31, T.Tsujimura31, S.Tulipán3, M.Turnyanskiy34, B.Unterberg4, J.Urban47, E.Urbonavicius9, I.Vakulchyk1, S.Valet1, B.van Millingen2, L.Vela32, J.- L.Velasco2, M.Vergote7, M.Vervier4, N.Vianello52, H.Viebke1, R.Vilbrandt1, A.Vorkörper15, S.Wadle13, F.Wagner1, E.Wang4, N.Wang4, F.Warmer1, L.Wegener1, J.Weggen13, Y.Wei4, J.Wendorf1, A.Werner1, B.Wiegel44, F.Wilde1, E.Winkler1, V.Winters11, S.Wolf36, J.Wolowski25, A.Wright14, P.Xanthopoulos1, H.Yamada31, I.Yamada31, R.Yasuhara31, M.Yokoyama31, J.Zajac47, M.Zarnstorff6, A.Zeitler36, H.Zhang6, J.Zhu1, M.Zilker15, A.Zimbal44, A.Zocco1, S.Zoletnik3 and M.Zuin18 Nucl. Fusion 57 (2017) 102020 R.C. Wolf etal 3 1 Max-Planck-Institut für Plasmaphysik Teilinstitut Greifswald, Wendelsteinstr. 1, 17491 Greifswald, Germany 2 CIEMAT, Avenida Complutense, 40, 28040 Madrid, Spain 3 Wigner Research Centre for Physiks, Konkoly Thege Miklós út 29-33, 1121 Budapest, XII., Hungary 4 Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung—Plasmaphysik, Partner of the Trilateral Euregio Cluster (TEC), Wilhelm-Johnen-Straße, 52428 Jülich, Germany 5 University of Maryland, Paitn Branch Drive, College Park, MA 20742, United States of America 6 Princeton Plasma Physics Laboratory, 100 Stellarator Rd, Princeton, NJ 08540, United States of America 7 Laboratory for Plasma Physics of the Ecole Royale Militaire/Koninklijke Militaire School (LPP-ERM/ KMS), Avenue de la Renaissance 30, 1000 Bruxelles, Belgien 8 Los Alamos National Laboratory, Los Alamos, NM 87544, United States of America 9 Lithuanian Energy Institute, Breslaujos g. 3, Kaunas 44403, Lithuania 10 Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, United States of America 11 University of Wisconsin–Madison, Engineering Drive, Madison, WI 53706, United States of America 12 National Centre for Nuclear Reserach Świerk, Narodowe Centrum Badań Jądrowych, ul. Andrzeja Sołtana 7, 05-400 Otwock, Świerk, Poland 13 Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 14 The Australian National University, Acton ACT 2601, Canberra, Australia 15 Max-Planck-Institut für Plasmaphysik, Boltzmannstraße 2, 85748 Garching bei München, Germany 16 Eindhoven University of Technology, 5612 AZ Eindhoven, Netherlands 17 University of Cagliary, Via Università, 40, 09124 Cagliari, Italy 18 Consorzio RFX, Corso Stati Uniti, 4-35127 Padova, Italy 19 Instituto de Plasmas e Fusao Nuclear, Av. Rovisco Pais, 1049-001 Lisboa, Portugal 20 Ioffe Physical-Technical Institute of the Russian Academy of Sciences, 26 Politekhnicheskaya, St Petersburg 194021, Russian Federation 21 Oak Ridge National Laboratory, 1 Bethel Valley Rd, Oak Ridge, TN 37830, United States of America 22 University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano SA, Italy 23 Warsaw University of Technology, plac Politechniki 1, 00-661 Warszawa, Poland 24 ENEA—Centro Ricerche Frascati, Via Enrico Fermi, 45, 00044 Frascati RM, Italy 25 IPPLM Institute of Plasma Physics and Laser Microfusion, 23 Hery Str., 01-497 Warsaw, Poland 26 Institute of Nuclear Physics PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland 27 University of Szczecin, 70-453, aleja Papieża Jana Pawła II 22A, Szczecin, Poland 28 University of Milano-Bicocca, Piazza dell’Ateneo Nuovo, 1-20126, Milano, Italy 29 Auburn University, Auburn, AL 36849, United States of America 30 Brandenburg University of Technology Cottbus-Senftenberg, Universitätsplatz 1, 01968 Senftenberg, Germany 31 National Institute for Fusion Science (NIFS), 322-6 Oroshicho, Toki, Gifu Prefecture 509-5202, Japan 32 Universidad Carlos III de Madrid, Av. de la Universidad, 30, Madrid, Spain 33 CEA Cadarache, 13115 Saint-Paul-lez-Durance, France 34 Culham Centre for Fusion Energy, Abingdon OX14 3EB, United Kingdom 35 Budker Institute of Nuclear Physics (BINP), 11, akademika Lavrentieva prospect, Novosibirsk, 630090, Russian Federation 36 University Stuttgart, Institut für Grenzflächenverfahrenstechnik und Plasmatechnologie, Nobelstr. 12, 70569 Stuttgart, Germany 37 Fraunhofer-Institut für Schicht- und Oberflächentechnik IST, Bienroder Weg 54 E, 38108 Braunschweig, Germany 38 Austrian Academy of Science, Doktor-Ignaz-Seipel-Platz 2, 1010 Wien, Austria 39 Institute for Nuclear Research, b.47, prospekt Nauky, Kiev 03680, Ukraine 40 Institute of Applied Physics (IAP) of the Russian Academy of Science, 46 Ul’yanov Street, 603950 Nizhny Novgorod, Russian Federation 41 Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany 42 University of Opole, plac Kopernika 11a, 45-001 Opole, Poland 43 Aalto University, 02150 Espoo, Finnland 44 Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany 45 Kyoto University, Yoshidahonmachi, Sakyo Ward, Kyoto, Kyoto Prefecture 606-8501, Japan 46 Institute of Plasma Physics, Chinese Academy of Sciences, 350 Shushanhu Rd., Hefei, Anhui 230031, People’s Republic of China 47 Institute of Plasma Physics of the Czech Academy of Science, Za Slovankou 1782/3, 182 00 Prague 8—Libeň, Czechia 48 Istituto di Fisica del Plasma ‘Piero Caldirola’, Via Roberto Cozzi, 53, 20125 Milano, Italy Nucl. Fusion 57 (2017) 102020 R.C. Wolf etal 4 49 Fraunhofer-Institut für Werkzeugmaschinen und Umformtechnik IWU, Reichenhainer Straße 88, 09126 Chemnitz, Germany 50 Universität in Rostock, Albert-Einstein-Straße 2, 18059 Rostock, Germany 51 Lawrence University, 711 E Boldt Way, Appleton, WI 54911, United States of America 52 Consiglio Nazionale delle Ricerche, Piazzale Aldo Moro, 7, 00185 Roma, Italy E-mail: [email protected] Received 3 February 2017, revised 8 May 2017 Accepted for publication 5 June 2017 Published 27 July 2017 Abstract After completing the main construction phase of Wendelstein 7-X (W7-X) and successfully commissioning the device, first plasma operation started at the end of 2015. Integral commissioning of plasma start-up and operation using electron cyclotron resonance heating (ECRH) and an extensive set of plasma diagnostics have been completed, allowing initial physics studies during the first operational campaign. Both in helium and hydrogen, plasma breakdown was easily achieved. Gaining experience with plasma vessel conditioning, discharge lengths could be extended gradually. Eventually, discharges lasted up to 6 s, reaching an injected energy of 4 MJ, which is twice the limit originally agreed for the limiter configuration employed during the first operational campaign. At power levels of 4 MW central electron densities reached 3 × 1019 m−3, central electron temperatures reached values of 7 keV and ion temperatures reached just above 2 keV. Important physics studies during this first operational phase include a first assessment of power balance and energy confinement, ECRH power deposition experiments, 2nd harmonic O-mode ECRH using multi-pass absorption, and current drive experiments using electron cyclotron current drive. As in many plasma discharges the electron temperature exceeds the ion temperature significantly, these plasmas are governed by core electron root confinement showing a strong positive electric field in the plasma centre. Keywords: stellarator, magnetic confinement, plasma heating, current drive (Some figuresmay appear in colour only in the online journal) 1. Introduction The main objective of the optimized stellarator Wendelstein 7-X (W7-X) is the demonstration of steady-state plasma operation at fusion-relevant plasma parameters, thereby verifying that the stellarator is a viable fusion power plant concept. The design of W7-X is based on an elaborate optimization procedure to avoid excessive neoclassical transport losses at high plasma temperature, to provide satisfactory fast ion confinement in the centre of the plasma with a drift-optimized configuration [1], and to simultaneously achieve satisfactory equilibrium and stability properties at high β as well as reliable divertor operation [2]. The main construction phase of W7-X was completed in 2014. It was followed by the commissioning of the superconducting device [3], which was successfully concluded by assessment of a series of careful measurements of the magnetic field, confirming not only the basic magnetic field topology but also demonstrating that potential error fields are within the correction capabilities of the W7-X trim coils [4–6]. After the operating permit was granted, first plasma operation started in December 2015 and continued until March 2016, with a total of 10 weeks of plasma operation. During each of these weeks, 3 days were dedicated to plasma operation. While the magnetic field coils and their support structure inside the cryostat were kept at cryogenic temperatures for the whole campaign (⩽100 K, while with the magnetic field the operational temperature of the W7-X coils is ~4 K), the magn etic field was ramped up and down on every day of plasma operation. Plasma operation of W7-X follows a staged approach in accordance with the successive completion of the in-vessel components [7]. During the first operational phase (OP 1.1) the plasma was limited to an effective minor radius of a = 0.49 m by five inboard limiters. To avoid plasma contact with the metallic walls, a magnetic field configuration was developed which had a rotational transform (at the plasma edge) above 5/6, with a 5/6 island chain lying just inside the plasma volume [8]. Assuming that the entire heating power has to be dissipated by the limiters, the heat load handling capability of the inertially cooled limiters restricted the total energy injected into the plasma to initially ∫P dt = 2 MJ. This was doubled to 4 MJ when it became clear that the limiter temper atures achieved at 2 MJ did not reach the predefined technical limits. After the installation of a test divertor unit, first experience with the magnetic island divertor will be gained Nucl. Fusion 57 (2017) 102020 R.C. Wolf etal 5 in the subsequent operational phase, OP 1.2. During this phase the uncooled divertor targets will limit the injected energy to 80 MJ, allowing plasma pulses of 10 s duration at 8 MW of heating power. OP 1.2 will be followed by the installation of a steadystate capable high-heat flux divertor and the completion of active water cooling of all in-vessel comp onents, in preparation for high power (10 MW) steady-state plasma operation at pulse lengths of up to 30 min (operational phase OP 2). The main objective of OP 1.1 was the integral commissioning of plasma start-up and operation, using an electron cyclotron resonance heating (ECRH) system and an extensive set of plasma diagnostics [8]. These included measurements of electron temperature and density profiles (ECE and Thomson scattering [9]), line-integrated electron density (dispersion interferometry [10]), line-integrated measurements of ion and electron temperatures (imaging x-ray spectroscopy [11]), reflectometers [12], a diamagnetic loop [13], visible, nearinfrared and infrared cameras for plasma and limiter observation [14], impurity spectroscopy (in the visible, VUV and x-ray wavelength ranges [15]), bolometer for measuring the plasma radiation [16], and neutral pressure gauges, Langmuir probes, and electron cyclotron absorption (ECA) and microwave stray radiation detectors [17] (a summary of all these diagnostic developments has been published in [18]). By the end of the first operational phase more than 30 diagnostic systems had been commissioned and had provided data [19], some of them already using Bayesian data analysis [20]. For OP 1.1 the ECRH system provided six gyrotrons with a total power of up to 5 MW for second harmonic heating at 140 GHz, corresponding to a central deposition at 2.5 T. Six independent launch positions at the low field side, using front steering launchers with movable mirrors, allowed the deposition profile to be moved vertically for off-axis heating and toroidally for current drive [21]. This paper gives an introduction to the experience with the first plasma operation on Wendelstein 7-X. It also reports on the first major results, focusing on the power balance, the characterization of the global confinement, and heating and current drive scenarios. Papers with a different focus on OP 1.1 results have been published [22, 23]. 2. Establishing first plasmas: plasma breakdown, wall conditioning and achieved plasma parameters Plasma breakdown was easily achieved in the first plasmas in helium. Plasma densities of about 2 × 1019 m−3 and central electron temperatures in the range of several keV were obtained after a few days of plasma operation. Since ions were heated only by collisional heat transfer from the electrons and the initial discharge durations were below 100 ms, ion temperatures stayed below 1 keV. An example of a helium discharge at the beginning of the campaign is shown in figure1. A prefill of helium gas is followed by the application of ECRH power. Plasma breakdown was typically achieved after 10 ms. The reduction of both the shine-through power, measured by the ECA diagnostic, and the stray radiation level (details of the protection diagnostics are described in [17]) provides evidence for the increasing absorption of the ECR waves. An increase of the neutral gas pressure precedes a radiation collapse, as evidenced by a sudden loss of wave coupling finally leading to the termination of the discharge after about 50 ms. The combination of stray radiation and ECA measurements allowed us to distinguish between the loss of absorption (both signals increase) and reaching the cut-off, in which case the ECA signal would have remained low while the stray radiation level would have increased. At the beginning, short ECRH produced plasmas were used to condition the plasma vessel walls. The sophisticated plasma control, designed for steady-state operation [24], made it possible to apply many consecutive pre-programmed ECRH pulses, realizing a plasma pulse about every 30 s. The ECRH cleaning and, once fully functional, the glow discharge cleaning (GDC), together made the out-gassing of the plasma vessel wall decrease by more than one order of magnitude, resulting in discharge durations of ~500 ms. Figure2 shows the improvement of the wall conditioning with the accumulated discharge time, and also the effect of GDC. A detailed description of the measurements and their interpretation can be found in [25]. Although these measures led to acceptable discharge lengths and core plasma temperatures up to 10 keV for the electrons and up to 2 keV for the ions, out-gassing Figure 1. The temporal evolution of gas flow, ECRH power, neutral gas pressure, electron cyclotron absorption (ECA) signal measuring the shine-through power which is not absorbed by the plasma, and the sniffer probe signal which is an indicator of the microwave stray radiation level in the plasma vessel. Plasma breakdown was achieved typically after 10 ms of applying the ECRH power (indicated by green dashed lines). In this example, four gyrotrons with a total power of up to 3 MW were used. To prevent damage by high levels of non-absorbed power, sniffer probes were used as interlock signals. The discharge is terminated by a radiation collapse (vertical red line). Nucl. Fusion 57 (2017) 102020 R.C. Wolf etal 6 events remained an issue throughout the first plasma campaign of W7-X. Regularly, in one of the five torus modules of W7-X (module 4), an increase of the neutral gas pressure (measured with a midplane gas manometer) was observed, associated with an increase of the plasma edge emission (measured with the visible-light cameras). This was followed by a reduction of the electron temperature at constant heating power which preceded the radiation collapse. Searching the plasma vessel after the campaign revealed a number of plastic parts which had unintentionally been left in the plasma vessel after completion of assembly. Although some of the plastic parts were partially molten, they were in remote places which had no direct plasma contact. In addition, no clear correlation could be established between the location of the parts and the radiation observed by visible-light cameras and the pressure increases measured in the different modules. Hydrogen plasmas were successfully established without any issues. Generally, the discharges were characterized by high electron temperatures at the beginning of the ECRH pulse (~10 keV at ~1019 m−3 and a few MWs of ECRH), quickly becoming stationary. The plasma density was controlled to become stationary within seconds. Ion temperatures increased up to 2 keV, but over a longer time due to the slow collisional heat transfer from the ECR-heated electrons. Limiter temper atures eventually reached values of several hundreds of °C, corresponding to peak heat fluxes of 2–3 MW m−2. The longest plasma discharges achieved at moderate heating powers, starting at 1 MW and dropping to 600 kW, lasted up to 6 s, reaching an injected energy of 4 MJ with stationary central temperatures of Te = 5 keV and Ti = 1.5 keV at central densities of several times 1019 m−3. These discharges could be sustained for more than 50 energy confinement times. An example of temperature profiles and the density evolution of hydrogen plasma of 1.3 s duration is shown in figure 3. Applying independent calibration methods (e.g. Raman scattering for the Thomson scattering diagnostic), Figure 2. The normalized out-gassing, defined as the ratio of the measured peak pressure divided by the energy absorbed during the respective plasma pulse, is plotted against the accumulated discharge time. Obviously, the out-gassing decreases with discharge time. Each glow discharge cleaning (GDC) leads to a further drop. However, after each GDC period the out-gassing tends to return to the envelope curve [25]. Reproduced with permission from [25]. Figure 3. The profiles on the left show the electron temperature from ECE (high field side and low field side, both mapped onto the normalized radius, r/rLCFS) and from Thomson scattering for a low power (PECRH = 0.6 MW) plasma lasting 6 s. (The temperature profiles are also shown in [22].) Ion and electron temperature profiles have been inferred from the inverted line integrals measured by x-ray imaging spectroscopy (XICS). On the right, the line integrated density measurement using the dispersion interferometer (blue signal) is overlaid with the line integral calculated from the TS density profile measurement (red dots; the line integral extends over about 1 m). The measurements take nearly the same laser path through the plasma and typically agree within 10%. The example shown here is from a plasma with 4 MW of ECRH power. Reproduced from [22]. © IOP Publishing Ltd. CC BY 3.0. Nucl. Fusion 57 (2017) 102020 R.C. Wolf etal 7 temperature and density measurements of the different diagnostics agree in many cases within about 10%. In the case of ECE, the relativistically downshifted emission at low frequencies from hot core electrons is not reabsorbed in the plasma, as at these frequencies the plasma is optically grey. This feature is not shown in figure3. This interpretation is confirmed by microwave propagation calculations, using the TRAVIS code, and the temporal behaviour of this feature after the ECRH has been turned off. The relativistic feature decays on a much faster timescale of 5–10 ms, while the timescale of the channels corre sponding to the blackbody emission exceeds 100 ms, being more in line with the energy confinement time expected for thermal electrons. With increasing temperature, the discrepancy between the different temperature measurements tends to increase [26]. Possible reasons for this discrepancy are deficiencies of the calibrations at higher temperatures, an inaccurate flux surface mapping which increases with temperatures and pressure, and suprathermal electrons. As for the presence of a significant fraction of suprathermal electrons, no clear indications exist. A more probable candidate is equilibrium effects, which were not considered in the analysis presented. As usual, the validation of this data is being further refined. For instance, the improvement of the calibration of the ECE measurements using a hot-cold-source is continuing. The details of the profile shapes are also still under investigation, which is essential for a refined transport analysis. 3. First attempt at a global power balance The global power balance provides a test of the consistency between heating power and the different contributions to the power loss from the plasma [26, 27]. For non-stationary conditions the change of the plasma energy also has to be considered: −− −=PP P W t d d0. heat rad lim During OP 1.1 only ECRH was applied for heating the plasma. The power from the gyrotrons, Pheat, has been measured, including the transmission efficiency of the quasi-optical transmission line from the gyrotrons to the plasma vessel, which is about 40 m long and consists of 16 mirrors and two polarizers [21]. By directing the power from a gyrotron on a direct path to a dummy load which incorporates a calorimeter, the gyrotron power can be determined. By using the same setup, but this time directing the gyrotron beam through most of the transmission line and using a retro-reflector back again to the dummy load, the transmission efficiency is given by the ratio of the two power measurements (see figure 4) [28]. The overall efficiency up to the vacuum windows of the ECRH launchers is estimated to be 94%. Vacuum windows and launchers are estimated to cause an additional 2% loss. This estimate is based on transmission measurements of the (diamond) vacuum windows and calculated losses caused by the finite apertures in the launchers, scraping off power in the wings of the Gaussian microwave beams. The radiated power, Prad, was measured using two bolometer systems covering the plasma cross-section [16] and assuming toroidal symmetry of the plasma radiation. In addition, it was assumed that all conductive or convective losses from the plasma are picked up by the five inboard limiters. The limiter loads were measured using two infrared camera systems [14], looking at different parts of two limiters in different modules. Inferring the total power to the limiters, Plim, initial observations of an asymmetric heat flux distribution have been taken into account. Indications for toroidal asymmetries have been experimentally observed [29] and thus constitute a systematic error in this analysis [30]. To a large degree, particle and heat fluxes followed predictions from vacuum modelling of magnetic field lines [26, 29, 31]. Qualitatively, the heat flux patterns on the limiters correspond to the magnetic field line connection lengths, whereby longer connections produce higher heat fluxes. Two distinct stripes on each limiter were measured by infrared and visible light cameras. Additionally, some asymmetries amongst the W7-X modules were detected by several diagnostics, e.g. neutral gas manometers working in four out of five modules. It was expected that such asymmetries would Figure 4. The left hand side shows the measurement of the gyrotron power, comparing the power level of the direct way to the load with the level of the indirect way via a retro-reflector. The inferred transmission efficiency is 94%. Repeating the measurements at different power levels results in a linear dependence (right hand side figure) allowing a robust determination of the power loss. Nucl. Fusion 57 (2017) 102020 R.C. Wolf etal 8 arise either from field errors or from deviations from the ideal limiter positions. A small n = 1 field was added during several experiments, utilizing a set of five trim coils [32]. By changing the phase and amplitude of the n = 1 field, it was possible to find a configuration that reduced the asymmetries in the pressure distribution between the modules. Such experiments indicate that the phase of a small n = 1 error field, previously measured by flux surface mapping, agrees with the phase required to minimize the asymmetries [34]. Further increasing the n = 1 field, however, led to even stronger asymmetries in the limiter power loads, thus demonstrating that error field compensation is well within the capabilities of the trim coil system. The plasma energy and its temporal change are derived from the profiles of ion and electron temperatures and electron density. Apart from possible uncertainties with the calibration of these diagnostics, a systematic error is introduced by the unknown impurity content. A further uncertainty stems from the neglect of charge-exchange losses of the ion energy. Comparing the kinetic energy derived from these profile measurements with the diamagnetic energy, Wkin exceeds Wdia by about 30%, which could be due to impurities in the plasma or the use of the vacuum field for the profile analysis. Since the ion density, ni, is not measured, Wkin can be calculated only approximately. It is clear that the measured electron energy We < Wkin < Wkin(ni = ne), the latter representing the theoretical maximum energy value, neglecting all impurities. Given Zeff values between 1.5 and 5.5, which have been estimated for OP 1.1, and assuming flat Zeff profiles, a range of possible reductions of ni with respect to ne can be estimated. This analysis indicates that up to 20% of the deviation of Wkin from Wdia could be attributed to impurities. Accordingly, Wkin is reduced by this amount in the analysis presented here. The remaining 10% deviation may indicate mapping errors during the integration of the temperature and density profiles or systematic errors of the corresponding measurements. Figure5 shows the temporal evolution of the different power terms of an example discharge, with fairly good agreement in the power balance. In this example the overall power loss, consisting of limiter load and radiated power, is on average underestimated by approximately 25%. This is a very reasonable result, considering the uncertainties involved and the assumptions made. Altogether, the analysed discharges indicate that the deviation between heating power and loss power increases with increasing heating power, from values of 10% at 1 MW up to 40% at 4 MW. 4. Confinement and plasma transport From the knowledge of the plasma energy and the heating power coupled to the plasma, the energy confinement time can be readily derived. As described in the previous sectionthe plasma energy can be inferred from the flux change measured by the diamagnetic loop or the kinetic plasma profiles. Although the validation of this data is still ongoing and only a limited dataset exists, a first attempt to compare the measured confinement times with the scaling law is illustrated in figure6. The evaluated energy confinement times lie in a range of 0.4 to 1.1 times the confinement time given by the ISS04 scaling [34], corresponding to absolute values of τE in a range of about 100 to 150 ms. Looking at the variation of power and density in the OP 1.1 data set, within the error bars the exponent for the density is similar to the value of the scaling (n0.61±0.06 compared to n0.54 as given by the ISS04 scaling), while the power degradation seems to be weaker (P−0.35±0.03 instead of P−0.61 as given by the ISS04 scaling). However, since the variation of the density in particular was limited, these findings are still very preliminary. The dependence of τE/τISS04 on the heating power reveals that for ECRH power levels below 1 MW the confinement time consistently lies Figure 5. The temporal evolution of the global power balance of discharge 2016-03-10.034. The total loss power, Ploss = Prad + Plim + dW/dt, agrees fairly well with the heating power, Pheat. Figure 6. The figureshows the dataset which forms the basis of the ISS04 energy confinement time scaling [34] (the coloured points correspond to the different stellarator and heliotron devices). The grey triangles correspond to the L- and H-mode data from tokamaks. Also plotted are the data from the W7-X OP 1.1 campaign (yellow–green), two selected W7-X CERC discharges (dark blue), and three W7-AS CERC plasma (yellow–red). Nucl. Fusion 57 (2017) 102020