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Runaway Electron Beam Stability and Decay in COMPASS O Ficker DOI:10.1088/1741-4326/ab210f EX/P1-26 Runaway Electron Beam Stability and Decay in COMPASS O. Ficker1,2, J. Mlynář1, E. Macusova1, J. Cerovsky1,2, A. Casolari1, M. Farnik1,2, A. Havranek1,3, H. Ales1,3, M. Hron1, M. Imrisek1,4, P. Kulhanek1,3, T. Markovic1,2, D. Naydenkova1, R. Panek1, M. Tomes1,2, J. Urban1, M. Vlainic5, P. Vondracek1,2, V. Weinzettl1, J. Decker6, G. Papp7, Y. Peysson8, M. Gobbin9, M. Gospodarczyk10, V. V. Plyusnin11, M. Rabinski12, and C. Reux8 The COMPASS Team and EUROfusion MST1 Team 1Institute of Plasma Physics AS CR v.v.i., Prague, Czech Republic 2Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University, Prague, Czech Republic 3Faculty of Electrical Engineering, Czech Technical University, Prague, Czech Republic 4Faculty of Mathematics and Physics, Czech Technical University, Prague, Czech Republic 5Institute of Physics, University of Belgrade, Belgrade, Serbia 6Swiss Plasma Center (SPC), École polytechnique fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland 7Max-Planck-Institut für Plasmaphysik, Garching, Germany 8Institut de Recherche sur la Fusion par confinement Magnétique (IRFM), Commissariat à l’énergie atomique (CEA/Cadarache), 13108 St. Paul lez Durance, France 9Consorzio RFX, Associazione EURATOM-ENEA sulla Fusione, Padova, Italy 10Università di Tor Vergata, 00173 Rome, Italy 11Institute of Plasmas and Nuclear Fusion (IPNF), Association EURATOM/IST, Lisbon, Portugal 12National Centre for Nuclear Research (NCBJ), Świerk, Poland Corresponding Author: O. Ficker, fi[email protected] Runaway electrons (REs) as one of the yet unsolved threats for ITER and future tokamaks are a topic of intensive research at most tokamaks. The experiments performed on COMPASS are complementary to the experiments at JET and MST (Medium-Size Tokamaks), building on the flexibility of the diagnostics set-up and low safety constraints at this smaller (R0 “ 0:56 m, a “ 0:23 m) device. During the past couple of years two different scenarios with the RE beam generation triggered by gas injection have been developed and investigated. The first one is based on Ar or Ne massive gas injection (MGI) into the current ramp-up phase leading to a disruption accompanied by runaway plateau generation, while the second uses smaller amounts of gas in order to get runaway current dominated plasmas. The successful generation of the beam in the first scenario depends on various parameters, including the toroidal magnetic field. The generated beam is often radially unstable, and the stability seems to be a function of various parameters, including the value of current lost during the CQ. Surprisingly, the current decay rate of the stable beams is rather similar in most discharges. The second scenario is much more quiescent, with no observable fast current quench. This allows to better diagnose the beam phase and also to apply secondary injections or resonant magnetic perturbations (RMP) to assist the decay of the beam. In this regard, interesting results have been achieved using secondary deuterium injection into a runaway electron beam triggered by Ar or Ne injection and also using n “ 1 error field generated by top and bottom RMP coils. While D dilution is clearly able to almost stop the beam decay, RMPs help to accelerate the beam decay. The effect of RMPs seems to be very different when acting on Ar and Ne background plasmas. Very interesting effects have been observed also by the high-speed cameras, including filamentation during the application of the RMPs and a slow local variation of the light intensity similar to turbulence during the beam decay. Published as a journal article in Nuclear Fusion http://iopscience.iop.org/article/10.1088/1741-4326/ab210f Ficker et al. RUNAWAY ELECTRON BEAM STABILITY AND DECAY IN COMPASS O. Ficker1,2, E. Macusova1, J. Mlynar1, J. Cerovsky1,2, A. Casolari1, M. Farnik1,2, A. Havranek1,4, M. Hron1, M. Imrisek1,3, P. Kulhanek1, T. Markovic1,3, D. Naydenkova1, R. Panek1, M. Tomes1,3, J. Urban1, M. Vlainic5, P. Vondracek1,3, V. Weinzettl1, D. Carnevale6, J. Decker7, M. Gobbin8, M. Gospodarczyk6, G. Papp9, Y. Peysson10, V.V. Plyusnin11, M. Rabinski12, C. Reux10, COMPASS team1 and the EUROfusion MST1 Team* 1 Institute of Plasma Physics of the CAS, CZ-18200 Prague 8, Czech Republic 2 FNSPE, Czech Technical University in Prague, CZ-11519 Prague 1, Czech Republic 3 FMP, Charles University, CZ-12116 Prague 2, Czech Republic 4 FEE, Czech Technical University in Prague, CZ-166 27 Prague 6, Czech Republic 5 Institute of Physics, University of Belgrade, 11080 Belgrade, Serbia 6 Universita' di Roma Tor Vergata, 18-00133 Rome, Italy 7 Swiss Plasma Centre, EPFL, CH-1015 Lausanne, Switzerland 8 Consorzio RFX, 35127 Padova, Italy 9 Max-Planck-Institut für Plasmaphysik, Garching D-85748, Germany 10 CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France 11 Instituto de Plasmas e Fusão Nuclear, IST, Lisbon, Portugal 12 National Centre for Nuclear Research (NCBJ), Otwock-Swierk, Poland * See the author list "H. Meyer et al 2017 Nucl. Fusion 57 102014" Email: [email protected] Abstract Runaway electrons (REs) as one of the yet unsolved threats for ITER and future tokamaks are a topic of intensive research at most of the European tokamaks. The experiments performed on COMPASS are complementary to the experiments at JET and MST (Medium-Size Tokamaks), building on the flexibility of the diagnostics set-up and low safety constraints at this smaller device. During the past couple of years two different scenarios with the RE beam generation triggered by gas injection have been developed and investigated. The first one is based on Ar or Ne massive gas injection (MGI) into the current ramp-up phase leading to a disruption accompanied by runaway plateau generation [1], while the second uses smaller amounts of gas in order to get runaway current dominated plasmas [2]. The successful generation of the beam in the first scenario depends on various parameters, including the toroidal magnetic field. The generated beam is often radially unstable, and the stability seems to be a function of various parameters, including the value of current lost during the CQ. The second scenario is much more quiescent, with no observable fast current quench and it is highly reproducible. This allows to reasonably diagnose the beam phase and also to apply secondary injections or resonant magnetic perturbations (RMP) to assist the decay of the beam. In this regard, interesting results have been achieved using secondary deuterium injection into a runaway electron beam triggered by Ar or Ne. The current of the RE beam can be controlled at a fixed value, however only using relatively high loop voltage. The radial position is not fully controlled and the request for the stabilising field is changing independently on plasma current which implies that the RE energy plays a key role in correct approach to beam position control. The experiments with elongated beams were also carried out. Last but not least it seems that Ar and Ne behave differently in terms of radiated power and HXR intensity during the beam decay. 1. INTRODUCTION Runaway electrons (REs) have been extensively studied at COMPASS within the framework of EUROfusion MST1 work package as they still present an issue with respect to the safe operation of ITER [3]. Without securing mitigated disruptions with no RE being generated and/or without developing a fully reliable runaway beam mitigation technique, ITER can never succeed. It seems that that the typical timescales of disruptions in ITER will be crucial [4] for the beam generation and that the position stability in the post- disruptive phase remains the critical issue. Enough time to mitigate the RE beam can be only secured in case that the beam position is stabilized which may be extremely difficult. The shaping field will cause the vertical instability of the beam just after the disruption while correct stabilizing vertical field can be hardly optimized early enough to secure the radial position stability. Once the stability of the beam is ascertained, various mitigation techniques can be employed, including injection of large amount of noble gases, preferably based on shattered pellet injection that is selected as a mitigation technique for ITER. However, the amount of gas already present in the chamber from the disruption mitigation injection may be sufficient to slowly mitigate the beam, 1 IAEA- EX/P1-26 again in case that the beam position is reasonably stable. This challenging task has to be tackled at the currently operated machines and using complex modelling approach. COMPASS and RE experiments The COMPASS tokamak is one of the Europe’s flexible smaller machines that has been operated at IPP Prague since 2008. The vacuum chamber is D-shaped with open divertor and major radius R0 is 0.56 m and minor radius a is 0.21 m. The toroidal magnetic field Bt ranges from 0.8 to 1.6 T while plasma current up to 350 kA can be driven in the machine. The toroidal magnetic field and plasma current directions can be both clockwise and counter-clockwise. The machine is equipped with two 40-keV NBIs with heating power up to 350 kW each which allow to reach H-mode, however COMPASS is able to reach even an ohmic-heating-driven H-mode as well [5]. For overview of the recent ITER-related results ranging from detachment studies to disruption analysis, see poster by Pánek et al. at the Conference [6]. The RE research on COMPASS gains from the machine flexibility and cooperation with groups working on relevant diagnostics and models. The RE generation in quiescent scenarios and losses related to various MHD phenomena and external field errors were studied as well as massive gas injection (MGI) triggered disruptions in the ramp-up scenario [1,7] – see the description in the next section.
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