JET EFDA Contributors

Ghent University, Gand, Flanders, Belgium

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Publications (387)535.42 Total impact

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    ABSTRACT: The Joint European Torus (JET, Culham, UK) is the largest tokamak in the world. JET has been upgraded over the years and recently it has also become a test facility of the components designed for ITER, the next step fusion machine under construction in Cadarache (France). At JET, the neutron emission profile of Deuterium (D) or Deuterium-Tritium (DT) plasmas is reconstructed using the neutron camera (KN3). In 2010 KN3 was equipped with a new digital data acquisition system (DAQ) based on Field Programmable Gated Array (FPGA). According to specifications, the DAQ is capable of high rate measurements up to 0.5 MCps. A new compact broadband spectrometer (KM12) based on BC501A organic liquid scintillating material was also installed in the same year and implements a similar DAQ as for KN3. This article illustrates the observations on the DAQ high count rate performance of both KN3 and KM12 in the latest JET D plasma experiments related to hybrid scenario and runaway electrons. For the latter, >1 MCps event rate was achieved with consequences on the behavior of the FPGA and on the reliability of the measurements.
    Physics Procedia 12/2015; 62. DOI:10.1016/j.phpro.2015.02.023
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    ABSTRACT: A consistent deterioration of global confinement in H-mode experiments has been observed in JET [1] following the replacement of all carbon plasma facing components (PFCs) with an all metal ('ITER-like') wall (ILW). This has been correlated to the observed degradation of the pedestal confinement, as lower electron temperature (Te) values are routinely measured at the top of the edge barrier region. A comparative investigation of core heat transport in JET-ILW and JET-CW (carbon wall) discharges has been performed, to assess whether core confinement has also been affected by the wall change.
    Plasma Physics and Controlled Fusion 06/2015; 57(6). DOI:10.1088/0741-3335/57/6/065002 · 2.39 Impact Factor
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    ABSTRACT: In a wide variety of natural and laboratory magnetized plasmas, filaments appear as a result of interchange instability. These convective structures substantially enhance transport in the direction perpendicular to the magnetic field. According to filament models, their propagation may follow different regimes depending on the parallel closure of charge conservation. This is of paramount importance in magnetic fusion plasmas, as high collisionality in the scrape-off layer may trigger a regime transition leading to strongly enhanced perpendicular particle fluxes. This work reports for the first time on an experimental verification of this process, linking enhanced transport with a regime transition as predicted by models. Based on these results, a novel scaling for global perpendicular particle transport in reactor relevant tokamaks such as ASDEX-Upgrade and JET is found, leading to important implications for next generation fusion devices. Full manuscript can be found in arXiv (http://arxiv.org/abs/1505.07741).
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    ABSTRACT: A global signature of the build-up to an intrinsic ELM is found in the phase of signals measured in full flux azimuthal loops in the divertor region of JET. Full flux loop signals provide a global measurement proportional to the voltage induced by changes in poloidal magnetic flux; they are electromagnetically induced by the dynamics of spatially integrated current density. We perform direct time-domain analysis of the high time-resolution full flux loop signals VLD2 and VLD3. We analyze plasmas where a steady H-mode is sustained over several seconds, during which all the observed ELMs are intrinsic; there is no deliberate intent to pace the ELMing process by external means. ELM occurrence times are determined from the Be II emission at the divertor. We previously found that the occurrence times of intrinsic ELMs correlate with specific phases of the VLD2 and VLD3 signals. Here, we investigate how the VLD2 and VLD3 phases vary with time in advance of the ELM occurrence time. We identify a build-up to the ELM in which the VLD2 and VLD3 signals progressively align to the phase at which ELMs preferentially occur, on a ~ 2 -5ms timescale. At the same time, the VLD2 and VLD3 signals become phase synchronized with each other, consistent with the emergence of coherent global dynamics in the integrated current density. In a plasma that remains close to a global magnetic equilibrium, this can reflect bulk displacement or motion of the plasma. This build-up signature to an intrinsic ELM can be extracted from a time interval of data that does not extend beyond the ELM occurrence time, so that these full flux loop signals could assist in ELM prediction or mitigation.
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    ABSTRACT: This paper reports the progress made at JET-ILW on integrating the requirements of the reference ITER baseline scenario with normalized confinement factor of 1, at a normalized pressure of 1.8 together with partially detached divertor whilst maintaining these conditions over many energy confinement times. The 2.5 MA high triangularity ELMy H-modes are studied with two different divertor configurations with D-gas injection and nitrogen seeding. The power load reduction with N seeding is reported. The relationship between an increase in energy confinement and pedestal pressure with triangularity is investigated. The operational space of both plasma configurations is studied together with the ELM energy losses and stability of the pedestal of unseeded and seeded plasmas. The achievement of stationary plasma conditions over many energy confinement times is also reported.
    Plasma Physics and Controlled Fusion 03/2015; 57(3). DOI:10.1088/0741-3335/57/3/035004 · 2.39 Impact Factor
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    ABSTRACT: The physical transport processes at the basis of JET typical inductive H-mode scenarios and advanced hybrid regimes, with improved thermal confinement, are analyzed by means of some of the newest and more sophisticated quasi-linear transport models: trapped gyro Landau fluid (TGLF) and QuaLiKiz. The temporal evolution of JET pulses is modelled by CRONOS where the turbulent transport is modelled by either QuaLiKiz or TGLF. Both are first principle models with a more comprehensive physics than the models previously developed and therefore allow the analysis of the physics at the basis of the investigated scenarios. For H-modes, ion temperature gradient (ITG) modes are found to be dominant and the transport models are able to properly reproduce temperature profiles in self-consistent simulations. However, for hybrid regimes, in addition to ITG trapped electron modes (TEM) are also found to be important and different physical mechanisms for turbulence reduction play a decisive role. Whereas E × B flow shear and plasma geometry have a limited impact on turbulence, the presence of a large population of fast ions, quite important in low density regimes, can stabilize core turbulence mainly when the electromagnetic effects are taken into account. The TGLF transport model properly captures these mechanisms and correctly reproduces temperatures.
    Plasma Physics and Controlled Fusion 03/2015; 57(3):035003. DOI:10.1088/0741-3335/57/3/035003 · 2.39 Impact Factor
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    ABSTRACT: Effectiveness of massive gas injection (MGI) for mitigation of disruptive wall damage has been investigated. Cross-reference analysis of the available JET experiments on MGI and their simulations with the TOKES code allow suggesting that in JET conditions one can convert into radiation the electron thermal energy and the plasma current energy, but the ion thermal energy does not convert into radiation because of very ineffective excitation of injected noble gas (NG) ions by D ions and long equipartition time between D ions and electrons. The model assumes rather high electron temperature during current quench (CQ), which contradicts with its time duration. Rough extrapolation of the result on ITER conditions shows that one can expect irradiation of total plasma energy if CQ duration in ITER is not shorter as in JET.
    Fusion Engineering and Design 02/2015; DOI:10.1016/j.fusengdes.2015.01.004 · 1.15 Impact Factor
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    ABSTRACT: In normal operation the JET neutral beam injectors have the operating gas supplied to the ion source and the neutraliser. For tritium operation the gas is supplied to both the ion source and neutraliser at a point close to the earth grid (“grid gas”) due to the difficulty in producing a gas line with a secondary containment and a ceramic break for high voltage standoff. In preparation for the next JET tritium campaign the JET EP2 PINIs have been characterised with grid gas flow. This paper reports measurements of arc efficiency, species and divergence in both normal and grid gas operation with hydrogen and deuterium. The data is used to predict the performance in tritium operation.
    Fusion Engineering and Design 02/2015; DOI:10.1016/j.fusengdes.2015.01.043 · 1.15 Impact Factor
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    ABSTRACT: Since the ITER-like wall in JET (JET-ILW) came into operation, dedicated ITER-like plasma current (I p) ramp-up (RU) and ramp-down (RD) experiments have been performed and matched to similar discharges with the carbon wall (JET-C). The experiments show that access to H-mode early in the I p RU phase and maintaining H-mode in the I p RD as long as possible are instrumental to achieve low internal plasma inductance (l i) and to minimize flux consumption. In JET-ILW, at a given current rise rate similar variations in l i (0.7–0.9) are obtained as in JET-C. In most discharges no strong W accumulation is observed. However, in some low density cases during the early phase of the I p &${\rm RU}(n_{\rm e}/n_{\rm e}^{\rm Gw} \sim 0.2)$ ; strong core radiation due to W influx led to hollow electron temperature (T e) profiles. In JET-ILW Z eff is significantly lower than in JET-C. W significantly disturbs the discharge evolution when the W concentration approaches 10−4; this threshold is confirmed by predictive transport modelling using the CRONOS code. I p RD experiments in JET-ILW confirm the result of JET-C that sustained H-mode and elongation reduction are both instrumental in controlling l i.
    Nuclear Fusion 01/2015; 55(1). DOI:10.1088/0029-5515/55/1/013009 · 3.24 Impact Factor
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    Physics of Plasmas 01/2015; 22(6):-. DOI:10.1063/1.4922846 · 2.25 Impact Factor
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    ABSTRACT: The evolution of the parameters of the plasma in the termination phase of high confinement H-modes at JET with carbon fibre composite plasma facing components (JET-C) has been analysed with a view to predict the dynamics of the plasma energy decrease for sudden terminations of the ITER Q DT = 10 scenario caused by malfunction of additional heating systems. JET-C experiments show that the rate of decay of the plasma energy in the high performance H-mode termination phase is predominantly determined by the duration of the type III ELMy H-mode phase after the end of the type I ELMy H-mode regime. Longer type III ELMy H-mode phase durations lead to slower plasma energy decay rates. The duration of the type III ELMy H-mode phase is itself determined by the margin of the edge power flow (dominated by the rate of collapse of the plasma energy) over the H-mode threshold power in the termination phase, with larger margins leading to longer type III ELMy H-mode phase durations. For most of the JET-C discharges analysed the timescale for the plasma energy decrease in the termination of high energy confinement H-modes is comparable to the energy confinement time of the plasma in the high confinement phase rather than half of this value, which is to be expected for instantaneous H–L transitions. Modelling of the termination phase of ITER Q DT = 10 H-modes (with transport assumptions in this phase validated against JET-C experiments) shows that similar to JET-C results the timescale for the decrease of the plasma energy is comparable and can even be longer than the energy confinement time of the burning phase, provided that ELM control can be maintained. This is due to the long sustainment of the type III ELMy H-mode by the substantial edge power flow compared to the H-mode threshold power during this phase. The large edge power flow in the termination phase of ITER high Q DT plasmas is provided by the decrease of the plasma energy and the slow collapse of the alpha heating. Operational strategies in ITER to control the energy decay rate as well as the consequences of the lack of ELM control in the high Q DT termination phase are presented.
    Nuclear Fusion 12/2014; 54(12). DOI:10.1088/0029-5515/54/12/123014 · 3.24 Impact Factor
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    ABSTRACT: The transition reported in L-mode plasmas leading to the formation of a density “shoulder” in the Scrape-off Layer (SOL) of tokamaks is investigated in the three ITER stepladder machines: COMPASS, ASDEX Upgrade (AUG) and JET. In AUG and JET, the broadening of the profiles can be observed at similar relative densities (ne/nGW ∼ 0.4–0.5) and coincides with a clear increase in the size of SOL filaments and the onset of the outer divertor detachment. In COMPASS, no transition is observed in the same density range: the divertor does not detach and no clear effect is observed neither in the profiles nor in the turbulence. This is explained as the result of an increase of intermittent transport triggered by the increased resistivity along filaments associated to divertor detachment. The transition conditions in AUG and JET are compared to foreseen ITER SOL parameters, and the implications are briefly discussed.
    Journal of Nuclear Materials 10/2014; 463:123. DOI:10.1016/j.jnucmat.2014.10.019 · 2.02 Impact Factor
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    T Craciunescu, A Murari, M Gelfusa, I Tiseanu, V Zoita, Jet Efda Contributors
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    ABSTRACT: In magnetic confinement nuclear fusion devices such as JET, the last few years have witnessed a significant increase in the use of digital imagery, not only for the surveying and control of experiments, but also for the physical interpretation of results. More than 25 cameras are routinely used for imaging on JET in the infrared (IR) and visible spectral regions. These cameras can produce up to tens of Gbytes per shot and their information content can be very different, depending on the experimental conditions. However, the relevant information about the underlying physical processes is generally of much reduced dimensionality compared to the recorded data. The extraction of this information, which allows full exploitation of these diagnostics, is a challenging task. The image analysis consists, in most cases, of inverse problems which are typically ill-posed mathematically. The typology of objects to be analysed is very wide, and usually the images are affected by noise, low levels of contrast, low grey-level in-depth resolution, reshaping of moving objects, etc. Moreover, the plasma events have time constants of ms or tens of ms, which imposes tough conditions for real-time applications. On JET, in the last few years new tools and methods have been developed for physical information retrieval. The methodology of optical flow has allowed, under certain assumptions, the derivation of information about the dynamics of video objects associated with different physical phenomena, such as instabilities, pellets and filaments. The approach has been extended in order to approximate the optical flow within the MPEG compressed domain, allowing the manipulation of the large JET video databases and, in specific cases, even real-time data processing. The fast visible camera may provide new information that is potentially useful for disruption prediction. A set of methods, based on the extraction of structural information from the visual scene, have been developed for the automatic detection of MARFE (multifaceted asymmetric radiation from the edge) occurrences, which precede disruptions in density limit discharges. An original spot detection method has been developed for large surveys of videos in JET, and for the assessment of the long term trends in their evolution. The analysis of JET IR videos, recorded during JET operation with the ITER-like wall, allows the retrieval of data and hence correlation of the evolution of spots properties with macroscopic events, in particular series of intentional disruptions.
    Plasma Physics and Controlled Fusion 10/2014; 56(11):114006. DOI:10.1088/0741-3335/56/11/114006 · 2.39 Impact Factor
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    B Cannas, A Fanni, A Murari, A Pau, G Sias, Jet Efda Contributors
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    ABSTRACT: Identifying a low-dimensional embedding of a high-dimensional data set allows exploration of the data structure. In this paper we tested some existing manifold learning techniques for discovering such embedding within the multidimensional operational space of a nuclear fusion tokamak. Among the manifold learning methods, the following approaches have been investigated: linear methods, such as principal component analysis and grand tour, and nonlinear methods, such as self-organizing map and its probabilistic variant, generative topographic mapping. In particular, the last two methods allow us to obtain a low-dimensional (typically two-dimensional) map of the high-dimensional operational space of the tokamak.
    Plasma Physics and Controlled Fusion 10/2014; 56(11):114005. DOI:10.1088/0741-3335/56/11/114005 · 2.39 Impact Factor
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    A. J. Webster, J. Morris, T. N. Todd, S. Brezinsek, JET EFDA Contributors
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    ABSTRACT: A unique experiment in the Joint European Torus (JET) consecutively produced 120 almost identical plasma pulses, providing two orders of magnitude more data than is usually available. This allows the statistical detection of previously unobservable phenomena such as a sequence of resonant-like waiting times between edge-localised instabilities (ELMs). Here we investigate the causes of this phenomenon. By synchronising data to the 1000s of ELM times and averaging the results, random errors are reduced by a factor of 50, allowing unprecedentedly detailed behaviour to be described. A clear link can then be observed between plasma confinement, ELM occurrence, vertical plasma oscillations, and an otherwise unobservable oscillation in a control coil current that is not usually associated with ELM occurrence. The results suggest a strong and unanticipated edge-plasma dependence on control system behaviour.
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    25th IAEA Fusion Energy Conference, St. Peterburg 2014; 10/2014
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    ABSTRACT: Wall conditioning will be required in ITER to control fuel - and impurity recycling and to improve plasma performance and reproducibility. In the nuclear phase, wall conditioning will also contribute to the control of the tritium (T) inventory within the fuelling cycle. This paper reviews experimental and modelling research activities on Glow Discharge Conditioning (GDC) and Ion Cyclotron Wall Conditioning (ICWC), wall conditioning techniques which ITER will use. A novel 2D multi-fluid model has been developed and benchmarked against experimental data, with the aim to assess uniformity and wall coverage with the ITER glow discharge system currently re-designed. Results show that H2-GDC in ITER will be fairly homogeneous in terms of electron density and temperature and toroidal distribution of the ion fluxes to the wall, determining the rate of cleaning. The efficiency of isotopic exchange with GDC or ICWC for Tritium removal has been in particular assessed in JET-ILW. Extrapolated to ITER, fuel removal efficiencies are found to be comparable to the estimated T-retention per 400 s long ITER D:T shots.
    25th IAEA Fusion Energy Conference, St. Peterburg 2014; 10/2014
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    ABSTRACT: The effective sputtering yield of Be was determined in situ by emission spectroscopy of low ionizing Be as function of the deuteron impact energy (Ein = 25–175 eV) and Be surface temperature (Tsurf = 200 °C–520 °C) in limiter discharges carried out in the JET tokamak. Be self sputtering dominates the erosion at high impact energies (Ein > 150 eV) and causes far beyond 1. drops to low values, below 4.5%, at the accessible lowest impact energy (Ein 25 eV) achievable in limiter configuration. At medium impact energies, Ein = 75 eV, two contributors to the measured of 9% were identified: two third of the eroded Be originates from bare physical sputtering and one third from chemical assisted physical sputtering . The later mechanism has been clearly identified by the appearance of BeD A–X emission and quantified in cause of a temperature dependence at which the BeD practically vanishes at highest observed Be limiter temperatures. The recorded Tsurf dependence, obtained in a series of 34 identical discharges with ratch-up of Tsurf by plasma impact and inertial cooling after the discharge, revealed that the reduction of BeD is correlated with an increase of D2 emission. The release mechanism of deuterium in the Be interaction layer is exchanged under otherwise constant recycling flux conditions at the limiter.
    European Journal of Marketing 10/2014; 54(10). DOI:10.1088/0029-5515/54/10/103001 · 0.96 Impact Factor
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    25th IAEA Fusion Energy Conference, St. Peterburg 2014; 10/2014
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Publication Stats

2k Citations
535.42 Total Impact Points

Institutions

  • 2007–2014
    • Ghent University
      • Department of Applied Physics
      Gand, Flanders, Belgium
    • Princeton University
      • Princeton Plasma Physics Laboratory
      Princeton, NJ, United States
    • Lomonosov Moscow State University
      • Faculty of Computational Mathematics and Cybernetics
      Moskva, Moscow, Russia
    • Max Planck Institute for Plasma Physics
      Arching, Bavaria, Germany
  • 2011
    • Fusion for Energy
      Barcino, Catalonia, Spain
    • École Polytechnique Fédérale de Lausanne
      • Center for Research In Plasma Physics
      Lausanne, Vaud, Switzerland
  • 2010
    • Lawrence Livermore National Laboratory
      • Physics Division
      Livermore, California, United States
    • MIT Portugal
      Porto Salvo, Lisbon, Portugal
    • University of Rome Tor Vergata
      • Department of Industrial Engineering
      Roma, Latium, Italy
  • 2006–2009
    • Massachusetts Institute of Technology
      • Plasma Science and Fusion Center (PSFC)
      Cambridge, MA, United States
    • Neesa Infrastructure Limited
      Amadavad, Gujarat, India
    • The University of Warwick
      • Department of Physics
      Warwick, ENG, United Kingdom
    • KTH Royal Institute of Technology
      Tukholma, Stockholm, Sweden
  • 2005
    • The Police Academy of the Czech Republic in Prague
      Praha, Praha, Czech Republic
    • INO - Istituto Nazionale di Ottica
      Florens, Tuscany, Italy
    • Imperial College London
      Londinium, England, United Kingdom
  • 2004–2005
    • Forschungszentrum Jülich
      • Zentralabteilung für Chemische Analysen (ZCH)
      Düren, North Rhine-Westphalia, Germany
    • Queen's University Belfast
      Béal Feirste, Northern Ireland, United Kingdom
    • University of Strathclyde
      • Department of Physics
      Glasgow, Scotland, United Kingdom
    • ENEA
      Roma, Latium, Italy
  • 2003–2004
    • Culham Centre for Fusion Energy
      Abingdon-on-Thames, England, United Kingdom
    • University of Innsbruck
      Innsbruck, Tyrol, Austria
    • Chalmers University of Technology
      • Department of Applied Mechanics
      Goeteborg, Västra Götaland, Sweden