JET EFDA Contributors

Ghent University, Gand, Flanders, Belgium

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Publications (360)503.38 Total impact

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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). · 3.24 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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    ABSTRACT: The effective sputtering yield of Be &$(Y_{\rm Be}^{\rm tot})$ ; was determined in situ by emission spectroscopy of low ionizing Be as function of the deuteron impact energy (E in = 25–175 eV) and Be surface temperature (T surf = 200 °C–520 °C) in limiter discharges carried out in the JET tokamak. Be self sputtering dominates the erosion at high impact energies (E in > 150 eV) and causes &$Y_{\rm Be}^{\rm tot}$ ; far beyond 1. &$Y_{\rm Be}^{\rm tot}$ ; drops to low values, below 4.5%, at the accessible lowest impact energy (E in ≃ 25 eV) achievable in limiter configuration. At medium impact energies, E in = 75 eV, two contributors to the measured &$Y_{\rm Be}^{\rm tot}$ ; of 9% were identified: two third of the eroded Be originates from bare physical sputtering &$(Y_{\rm Be}^{\rm phys})$ ; and one third from chemical assisted physical sputtering &$(Y_{\rm Be}^{\rm chem})$ ;. 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 T surf dependence, obtained in a series of 34 identical discharges with ratch-up of T surf 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.The reduction of &$Y_{\rm Be}^{\rm chem}$ ; with T surf is also correlated to the reduction of the Be content in the core plasma providing information on the total source strength and Be screening. The chemical assisted physical sputtering, always present at the nominal limiter pre-heating temperature of T surf = 200 °C, is associated with an additional sputtering channel with respect to ordinary physical sputtering which is surface temperature independent. These JET experiments in limiter configuration are used to benchmark the ERO code and verify ITER first wall erosion prediction. The ERO code overestimates the observed Be sputtering in JET by a factor of about 2.5 which can be transferred to ITER predictions and prolong the expected lifetime of first wall elements.
    European Journal of Marketing 10/2014; 54(10). · 0.96 Impact Factor
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    ABSTRACT: The EDGE2D-EIRENE code is applied for simulation of divertor detachment during matched density ramp experiments in high triangularity, L-mode plasmas in both JET-Carbon (JET-C) and JET-ITER-Like Wall (JET-ILW). The code runs without drifts and includes either C or Be as impurity, but not W, assuming that the W targets have been coated with Be via main chamber migration. The simulations reproduce reasonably well the observed particle flux detachment as density is raised in both JET-C and JET-ILW experiments and can better match the experimental in-out divertor target power asymmetry if the heat flux entering the outer divertor is artificially set at around 2–3 times that entering the inner divertor. A careful comparison between different sets of atomic physics processes used in EIRENE shows that the detachment modelled by EDGE2D-EIRENE relies only on an increase of the particle sinks and not on a decrease of the ionization source. For the rollover and the beginning of the partially detached phase, the particle losses by perpendicular transport and the molecular activated recombination processes are mainly involved. For a deeper detachment with significant target ion flux reduction, volume recombination appears to be the main contributor. The elastic molecule-ion collisions are also important to provide good neutral confinement in the divertor and thus stabilize the simulations at low electron temperature (T e), when the sink terms are strong. Comparison between EDGE2D-EIRENE and SOLPS4.3 simulations of the density ramp in C shows similar detachment trends, but the importance of the elastic ion-molecule collisions is reduced in SOLPS4.3. Both codes suggest that any process capable of increasing the neutral confinement in the divertor should help to improve the modelling of the detachment. A further outcome of this work has been to demonstrate that key JET divertor diagnostic signals—Langmuir probe T e and bolometric tomographic reconstructions—are running beyond the limit of validity in high recycling and detached conditions and cannot be reliably used for code validation. The simulations do, however, reproduce the trend of the evolution of the line integrated bolometer chord measurements. The comparison between the code results and high-n Balmer line radiation intensity profiles confirms that a strong volume recombination is present during the experimental detachment and may play a role in this process, as suggested by the code.
    Nuclear Fusion 09/2014; 54(9). · 3.24 Impact Factor
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    ABSTRACT: The behaviour of tungsten in the core of hybrid scenario plasmas in JET with the ITER-like wall is analysed and modelled with a combination of neoclassical and gyrokinetic codes. In these discharges, good confinement conditions can be maintained only for the first 2–3 s of the high power phase. Later W accumulation is regularly observed, often accompanied by the onset of magneto-hydrodynamical activity, in particular neoclassical tearing modes (NTMs), both of which have detrimental effects on the global energy confinement. The dynamics of the accumulation process is examined, taking into consideration the concurrent evolution of the background plasma profiles, and the possible onset of NTMs. Two time slices of a representative discharge, before and during the accumulation process, are analysed with two independent methods, in order to reconstruct the W density distribution over the poloidal cross-section. The same time slices are modelled, computing both neoclassical and turbulent transport components and consistently including the impact of centrifugal effects, which can be significant in these plasmas, and strongly enhance W neoclassical transport. The modelling closely reproduces the observations and identifies inward neoclassical convection due to the density peaking of the bulk plasma in the central region as the main cause of the accumulation. The change in W neoclassical convection is directly produced by the transient behaviour of the main plasma density profile, which is hollow in the central region in the initial part of the high power phase of the discharge, but which develops a significant density peaking very close to the magnetic axis in the later phase. The analysis of a large set of discharges provides clear indications that this effect is generic in this scenario. The unfavourable impact of the onset of NTMs on the W behaviour, observed in several discharges, is suggested to be a consequence of a detrimental combination of the effects of neoclassical transport and of the appearance of an island.
    Nuclear Fusion 08/2014; 54(8). · 3.24 Impact Factor
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    ABSTRACT: The termination of the current and the loss of runaway electrons following runaway current plateau formation during disruptions have been investigated in the JET, DIII-D and FTU tokamaks. Substantial conversion of magnetic energy into runaway kinetic energy, up to ∼10 times the initial plateau runaway kinetic energy, has been inferred for the slowest current terminations. Both modelling and experiment suggest that, in present devices, the efficiency of conversion into runaway kinetic energy is determined to a great extent by the characteristic runaway loss time, τ diff, and the resistive time of the residual ohmic plasma after the disruption, τ res, increasing with the ratio τ diff/τ res. It is predicted that, in large future devices such as ITER, the generation of runaways by the avalanche mechanism will play an important role, particularly for slow runaway discharge terminations, increasing substantially the amount of energy deposited by the runaways onto the plasma-facing components by the conversion of magnetic energy of the runaway plasma into runaway kinetic energy. Estimates of the power fluxes on the beryllium plasma-facing components during runaway termination in ITER indicate that for runaway currents of up to 2 MA no melting of the components is expected. For larger runaway currents, minimization of the effects of runaway impact on the first wall requires a reduction in the kinetic energy of the runaway beam before termination and, in addition, high plasma density n e and low ohmic plasma resistance (long τ res) to prevent large conversion of magnetic into runaway kinetic energy during slow current terminations.
    Nuclear Fusion 08/2014; 54(8). · 3.24 Impact Factor
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    ABSTRACT: When the ITER-like wall was installed in JET, one of the 218 Be inner wall guard limiter tiles had been enriched with 10Be as a bulk isotopic marker. During the shutdown in 2012–2013, a set of tiles were sampled nondestructively to collect material for accelerator mass spectroscopy measurements of 10Be concentration. The letter shows how the marker experiment was set up, presents first results and compares them to preliminary predictions of marker redistribution, made with the ASCOT numerical code. Finally an outline is shown of what experimental data are likely to become available later and the possibilities for comparison with modelling using the WallDYN, ERO and ASCOT codes are discussed.
    Nuclear Fusion 08/2014; 54(8). · 3.24 Impact Factor
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    ABSTRACT: Recent developments in theory-based modelling of core heavy impurity transport are presented, and shown to be necessary for quantitative description of present experiments in JET and ASDEX Upgrade. The treatment of heavy impurities is complicated by their large mass and charge, which result in a strong response to plasma rotation or any small background electrostatic field in the plasma, such as that generated by anisotropic external heating. These forces lead to strong poloidal asymmetries of impurity density, which have recently been added to numerical tools describing both neoclassical and turbulent transport. Modelling predictions of the steady-state two-dimensional tungsten impurity distribution are compared with experimental densities interpreted from soft X-ray diagnostics. The modelling identifies neoclassical transport enhanced by poloidal asymmetries as the dominant mechanism responsible for tungsten accumulation in the central core of the plasma. Depending on the bulk plasma profiles, neoclassical temperature screening can prevent accumulation, and can be enhanced by externally heated species, demonstrated here in ICRH plasmas.
    Plasma Physics and Controlled Fusion 07/2014; 57(1). · 2.39 Impact Factor
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    EPS 2014; 06/2014
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    ABSTRACT: The former all-carbon wall on JET has been replaced with beryllium in the main torus and tungsten in the divertor to mimic the surface materials envisaged for ITER. Comparisons are presented between Type I H-mode characteristics in each design by examining respective scans over deuterium fuelling and impurity seeding, required to ameliorate exhaust loads both in JET at full capability and in ITER.
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    ABSTRACT: For a two week period during the Joint European Torus (JET) 2012 experimental campaign, the same high confinement plasma was repeated 151 times. The dataset was analysed to produce a probability density function (pdf) for the waiting times between edge-localised plasma instabilities ("ELMS"). The result was entirely unexpected. Instead of a smooth single peaked pdf, a succession of 4-5 sharp maxima and minima uniformly separated by 7-8 millisecond intervals was found. Here we explore the causes of this newly observed phenomenon, and conclude that it is either due to a self-organised plasma phenomenon or an interaction between the plasma and a real-time control system. If the maxima are a result of "resonant" frequencies at which ELMs can be triggered more easily, then future ELM control techniques can, and probably will, use them. Either way, these results demand a deeper understanding of the ELMing process.
    Plasma Physics and Controlled Fusion 06/2014; 56(7). · 2.39 Impact Factor
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    ABSTRACT: The mirror-linked divertor spectroscopy diagnostic on JET has been upgraded with a new visible and near-infrared grating and filtered spectroscopy system. New capabilities include extended near-infrared coverage up to 1875 nm, capturing the hydrogen Paschen series, as well as a 2 kHz frame rate filtered imaging camera system for fast measurements of impurity (Be II) and deuterium Dα, Dβ, Dγ line emission in the outer divertor. The expanded system provides unique capabilities for studying spatially resolved divertor plasma dynamics at near-ELM resolved timescales as well as a test bed for feasibility assessment of near-infrared spectroscopy.
    HTPD 2014, Atlanta, GA; 06/2014
  • Physics of Plasmas 06/2014; 21(6). · 2.25 Impact Factor
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    ABSTRACT: In this paper numerical simulations with COREDIV code of JET discharges with ITER-like wall are presented. We concentrate on the JET H-mode and hybrid scenarios with nitrogen seeding and all simulations have been performed with the same transport model and only the discharge input parameters like auxiliary heating Paux, line average plasma density nline, confinement factor H98, nitrogen input flux ΓpuffN were changed in the calcu lations. The separatrix density nsepe is an input parameter in our model and has been kept equal to 0.4÷0.5nlinein the simulations, with the recycling coefficient adjusted accordingly. It has been shown that COREDIV is able to reproduce basic parameters of nitrogen seeded discharges for both H-mode and hybrid scenarios. We have achieved reasonable agreement with global plasma parameters like radiations levels, Zeff and tungsten concentrations and the plasma profiles, including density, temperature and radiation are in very good agreement with experimental data. The agreement of the code results with the experimental data might be even better, if the simulations are further tuned taking into account uncertainties to the sputtering model, separatrix density or SOL transport. Simulations show that the observed Zeff level is defined mostly by the low Z impurity content, Be and N2 in the considered shots. It has been found that the tungsten radiation plays always very important role and can not be mitigated even by strong influx of nitrogen. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Beiträge aus der Plasmaphysik 06/2014; 54(4‐6).
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    ABSTRACT: After the code parameters have been fixed by the numerical modeling of a well diagnosed JET pulse, the electron density and the input power have been changed, resulting in 4 density scans (〈ne〉 in the range 3.8 – 8.2 x 1019m-3) at Pin = 17, 22, 27, 32 MW. At any given power level, W flux decreases with increasing 〈ne〉 as a consequence of the decrease in Te at the target plates. Also the W concentration in the core (cW) decreases, but this not necessarily leads to reduced core radiation. Indeed, while at high Pin the core radiation decreases with density, at low Pin it increases. At high 〈ne〉 the increase in the input power leads to enhanced PradPrad, leaving, however, nearly unchanged the power radiated fraction frad Indeed, the increase in frad withPin is observed only at low 〈ne〉, up to a level of about frad = 0.4. These numerical results, linked to the non-linear self-consistent physics of W production and transport, suggest the best conditions are achieved when the level of the electron density is adapted to the level of the available Pin. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Beiträge aus der Plasmaphysik 06/2014; 54(4‐6).
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    ABSTRACT: Resonant magnetic perturbation experiments at JET with the ITER-like wall have shown the formation of radially propagating pre-ELM structures in the heat flux profile on the outer divertor. These appear a few milliseconds before the major divertor heat load, caused by type-I edge-localized modes (ELMs). The formation of the pre-ELM structures is accompanied by an increase in the Dα emission. For some pronounced examples, the propagation appears to end at the positions where an increased heat load is seen during the ELM crash a few milliseconds later. These observations are presented and discussed along with a comparison of a thermoelectric edge currents model.
    Nuclear Fusion 04/2014; 54(7):072004. · 3.24 Impact Factor
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    ABSTRACT: The ITER-like wall recently installed in JET comprises solid beryllium limiters and a combination of bulk tungsten and tungsten-coated carbon fibre composite divertor tiles without active cooling. During a beryllium power handling qualification experiment performed in limiter configuration with 5 MW neutral beam injection input power, accidental beryllium melt events, melt layer motion and splashing were observed locally on a few beryllium limiters in the plasma contact areas. The Lorentz force is responsible for the observed melt layer movement. To move liquid beryllium against the gravity force, the current flowing from the plasma perpendicularly to the limiter surface must be higher than 6 kA m−2. The thermo-emission current at the melting point of beryllium is much lower. The upward motion of the liquid beryllium against gravity can be due to a combination of the Lorentz force from the secondary electron emission and plasma pressure force.
    Physica Scripta 04/2014; 2014(T159):014041. · 1.03 Impact Factor
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    ABSTRACT: In this paper, the nature of the primary instability present in the pedestal forming region prior to the transition into H mode is analysed using a gyrokinetic code on JET-ILW profiles. The linear analysis shows that the primary instability is of resistive nature, and can therefore be stabilized by increased temperature, hence power. The unstable modes are identified as being resistive ballooning modes. Their growth rates decrease for temperatures increasing towards the experimentally measured temperature at the L-H transition. The growth rates are larger for lower effective charge Zeff. This dependence is shown to be in qualitative agreement with recent and past experimental observations of reduced Zeff associated with lower L-H power thresholds.
    Nuclear Fusion 01/2014; 54(2). · 3.24 Impact Factor
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    ABSTRACT: An overview of the recent results of Lower Hybrid (LH) experiments at JET with the ITER-like wall (ILW) is presented. Topics relevant to LH wave coupling are addressed as well as issues related to ILW and LH system protections. LH wave coupling was studied in conditions determined by ILW recycling and operational constraints. It was concluded that LH wave coupling was not significantly affected and the pre-ILW performance could be recovered after optimising the launcher position and local gas puffing. SOL density measurements were performed using a Li-beam diagnostic. Dependencies on the D2 injection rate from the dedicated gas valve, the LH power and the LH launcher position were analysed. SOL density modifications due to LH were modelled by the EDGE2D code assuming SOL heating by collisional dissipation of the LH wave and/or possible ExB drifts in the SOL. The simulations matched reasonably well the measured SOL profiles. Observations of arcs and hotspots with visible and IR cameras viewing the LH launcher are presented.
    01/2014; 1580(1).

Publication Stats

2k Citations
503.38 Total Impact Points


  • 2005–2014
    • Ghent University
      • Department of Applied Physics
      Gand, Flanders, Belgium
  • 2011
    • Fusion for Energy
      Barcino, Catalonia, Spain
    • École Polytechnique Fédérale de Lausanne
      • Center for Research In Plasma Physics
      Lausanne, Vaud, Switzerland
  • 2010
    • MIT Portugal
      Porto Salvo, Lisbon, Portugal
    • University of Rome Tor Vergata
      • Department of Industrial Engineering
      Roma, Latium, Italy
  • 2009
    • Massachusetts Institute of Technology
      • Plasma Science and Fusion Center (PSFC)
      Cambridge, MA, United States
  • 2007
    • Princeton University
      • Princeton Plasma Physics Laboratory
      Princeton, NJ, United States
  • 2006
    • The University of Warwick
      • Department of Physics
      Warwick, ENG, United Kingdom
  • 2002–2005
    • Forschungszentrum Jülich
      • • Zentralabteilung für Chemische Analysen (ZCH)
      • • Plasmaphysik (IEK-4)
      Jülich, North Rhine-Westphalia, Germany
  • 2003
    • Chalmers University of Technology
      Goeteborg, Västra Götaland, Sweden
    • Culham Centre for Fusion Energy
      Abingdon-on-Thames, England, United Kingdom