D. Elbeze

Cea Leti, Grenoble, Rhône-Alpes, France

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Publications (29)40.24 Total impact

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    24.th IAEA Fusion Energy Conference; 10/2012
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    ABSTRACT: A 2D electron cyclotron emission (ECE) imaging system for Tore Supra is under design for studying the MHD physics of the magnetically confined plasma such as sawteeth, tearing modes, and turbulent fluctuations. Complex beam path due to the tight access in Tore Supra led to the design of reflective optics made of 6 or more large cylindrical∕flat mirrors. The total path length of the ECE beam is about 11 m, including almost 4 m inside the vacuum vessel. The imaging property of the optics has been estimated using the Gaussian beam simulation and ray transfer analysis. The possible setups for the optical alignment of the diagnostic and the operation scenarios with single- or dual-array measurement system are discussed.
    The Review of scientific instruments 10/2012; 83(10):10E318. · 1.52 Impact Factor
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    ABSTRACT: Tore Supra ECE diagnostic has been recently upgraded to study MHD modes driven by energetic particles up to 400 kHz. To improve the measurement sensitivity, the ECE signals of the 32 channels radiometer were amplified just below the saturation limit and sources of noise were investigated in order to keep it as low as possible. With such an improvement, fast particle driven modes with frequencies up to 200 kHz were detected. A 4-channel correlation ECE system using YIG filters with tuneable frequency was also installed. It allows fine radial scans of MHD modes and correlation length measurements. For the two kinds of YIG filter in use, the minimum frequency separation between two ECE channels that could be achieved was established measuring the correlation coefficient between the respective radiation noises. Finally, by modelling the ECE radiometer taking into account the antenna radiation pattern and the vertical position of the ECE beam relative to the plasma centre we improved the data analysis tools, thus giving a better determination of the phase radial structure of ECE oscillations. The poloidal structure of MHD modes can then be identified from ECE data and, for off axis ECE lines of sight, the direction of the plasma rotation can also be determined. This method allows identifying the occurrence of an inverse cascade of electron fishbone modes ranging from m/n=4/4 to 1/1 (m and n are the poloidal and toroidal mode numbers, respectively) which appears in lower hybrid current drive plasmas.
    09/2012;
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    ABSTRACT: This work presents a comparative study of the experimental observation of MHD instabilities identified as electron fishbone-like modes that appear in plasmas with lower hybrid current drive in FTU and Tore Supra tokamaks. Initially, the mode-induced electronic temperature oscillations measured by electron cyclotron emission were used to study the evolutions of the frequency and position of these modes. In FTU, where fishbones with and without bursting behaviour are observed, it was found that the evolutions of the mode position and frequency follow opposite trend in the two regimes. In Tore Supra, where abrupt changes between modes with different mode structures are often observed, it was found that the mode position evolves continuously and the wavenumbers follow an inverse cascade starting from an m/n = 4/4 mode and finishing in an 1/1 mode. In a second step, the energy of resonant electrons was estimated from the resonant condition of the precession drift frequency. It was found that in Tore Supra the resonant condition does not change during the frequency jumps. The relevance of the correction due to the pitch angle of the resonant electrons and the possible role of the energetic passing particles in the drive of these modes are discussed from the results obtained in both machines.
    Nuclear Fusion 01/2012; 52. · 2.73 Impact Factor
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    ABSTRACT: Experiments to study inward heat transport phenomena have been performed in the Tore Supra tokamak with off-axis electron cyclotron resonance heating (ECRH). Both power balance and perturbation transport analysis have been done for low-frequency (1 Hz) ECRH modulation experiments. Heat diffusivity and heat pinch have been separately determined by fitting the experimental data of the amplitude and phase of the Fourier transform of the modulated temperature with a linear transport model including convection term. Comparison with the critical gradient model has shown that the heat pinch previously obtained could include a pseudo-pinch due to the non-linearity of the diffusivity and an additional non-diffusive heat pinch. The pinch effect is reduced for higher densities.
    Nuclear Fusion - NUCL FUSION. 01/2012; 52(3).
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    ABSTRACT: Tore Supra routinely addresses the physics and technology of very long-duration plasma discharges, thus bringing precious information on critical issues of long pulse operation of ITER. A new ITER relevant lower hybrid current drive (LHCD) launcher has allowed coupling to the plasma a power level of 2.7 MW for 78 s, corresponding to a power density close to the design value foreseen for an ITER LHCD system. In accordance with the expectations, long distance (10 cm) power coupling has been obtained. Successive stationary states of the plasma current profile have been controlled in real-time featuring (i) control of sawteeth with varying plasma parameters, (ii) obtaining and sustaining a 'hot core' plasma regime, (iii) recovery from a voluntarily triggered deleterious magnetohydrodynamic regime. The scrape-off layer (SOL) parameters and power deposition have been documented during L-mode ramp-up phase, a crucial point for ITER before the X-point formation. Disruption mitigation studies have been conducted with massive gas injection, evidencing the difference between He and Ar and the possible role of the q = 2 surface in limiting the gas penetration. ICRF assisted wall conditioning in the presence of magnetic field has been investigated, culminating in the demonstration that this conditioning scheme allows one to recover normal operation after disruptions. The effect of the magnetic field ripple on the intrinsic plasma rotation has been studied, showing the competition between turbulent transport processes and ripple toroidal friction. During dedicated dimensionless experiments, the effect of varying the collisionality on turbulence wavenumber spectra has been documented, giving new insight into the turbulence mechanism. Turbulence measurements have also allowed quantitatively comparing experimental results with predictions by 5D gyrokinetic codes: numerical results simultaneously match the magnitude of effective heat diffusivity, rms values of density fluctuations and wavenumber spectra. A clear correlation between electron temperature gradient and impurity transport in the very core of the plasma has been observed, strongly suggesting the existence of a threshold above which transport is dominated by turbulent electron modes. Dynamics of edge turbulent fluctuations has been studied by correlating data from fast imaging cameras and Langmuir probes, yielding a coherent picture of transport processes involved in the SOL.
    Nuclear Fusion 08/2011; 51(9):094014. · 2.73 Impact Factor
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    ABSTRACT: In the Tore Supra tokamak, magnetohydrodynamic instabilities can be destabilized by energetic particles produced by radio-frequency heating systems. This paper reports on experimental observations of two types of energetic particle driven internal modes: the electron fishbone and the beta-induced Alfvén eigenmodes (BAEs). The evolutions of these modes in frequency and radial position during core relaxation cycles were determined by their induced electronic temperature and density fluctuations. Electron fishbones were observed in the so-called oscillation regime, while BAEs' observations were performed in the sawtooth regime. The results show that the frequency and radial position of these two instabilities vary substantially during the core relaxation cycles. It indicates that an accurate description of these evolutions must consider the equilibrium profile alterations during the core oscillations.
    Plasma Physics and Controlled Fusion 05/2011; 53(7):074012. · 2.37 Impact Factor
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    ABSTRACT: The main results of the Tore Supra experimental programme in the years 2007–2008 are reported. They document significant progress achieved in the domain of steady-state tokamak research, as well as in more general issues relevant for ITER and for fusion physics research. Three areas are covered: ITER relevant technology developments and tests in a real machine environment, tokamak operational issues for high power and long pulses, and fusion plasma physics. Results presented in this paper include test and validation of a new, load-resilient concept of ion cycotron resonance heating antenna and of an inspection robot operated under ultra-high vacuum and high temperature conditions; an extensive experimental campaign (5 h of plasma) aiming at deuterium inventory and carbon migration studies; real-time control of sawteeth by electron cyclotron current drive in the presence of fast ion tails; ECRH-assisted plasma start-up studies; dimensionless scalings of transport and turbulence; transport experiments using active perturbation methods; resistive and fast-particle driven MHD studies. The potential role of Tore Supra in the worldwide fusion programme before the start of ITER operation is also discussed.
    Nuclear Fusion 09/2009; 49(10):104010. · 2.73 Impact Factor
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    ABSTRACT: Modes oscillating at the acoustic frequency and identified as beta Alfvén eigenmodes (BAEs) have been observed in Tore-Supra under ion cyclotron resonant heating. In this paper, the linear excitation threshold of these modes, thought to be driven by suprathermal ions, is calculated and compared with Tore-Supra observations. Similar studies of the linear excitation threshold of energetic particles driven modes were carried out previously for toroidal Alfvén eigenmodes or fishbones. In the case of BAEs, the main point is to understand whether the energetic particle drive is able to exceed ion Landau damping, which is expected to be important in the acoustic frequency range. For this, the BAE dispersion relation is computed and simplified in order to derive a tractable excitation criterion suitable for comparison with experiments. The observation of BAEs in Tore-Supra is found to be in agreement with the calculated criterion and confirms the possibility to trigger these modes in the presence of ion Landau damping. Moreover, the conducted analysis clearly puts forward the role of the global tunable parameters which play a role in the BAE excitation (the magnetic field, the density etc), as well as the role of some plasma profiles. In particular, the outcome of a modification of the shear or of the heating localization is found to be non-negligible and it is discussed in the paper.
    Plasma Physics and Controlled Fusion 08/2009; 51(9):095002. · 2.37 Impact Factor
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    ABSTRACT: Energetic particles driven modes are one of the concerns for burning plasmas. On Tore-Supra, fast ions and electrons are generated by the RF heating systems and fast particles driven modes are detected with ECE and reflectometry diagnostics. In ICRH heated plasmas, modes are observed in the acoustic frequency range 30–70 kHz. The observed frequency agrees with the frequency predicted for both geodesic acoustic modes and beta Alfvén eigenmodes (BAE), but their structure and their excitation by fast ions advocate for an identification as BAE. Experimental analysis displays the existence of an excitation threshold depending on various parameters such as the ICRH power, the minority fraction and the density, in agreement with the theoretical prediction resulting from a balance between the fast ion drive and Landau damping by thermal ions. In lower hybrid current drive plasmas, electron fishbones have been detected below 20 kHz. Evolution of the fishbone modes with LH power was studied. Precession fishbones are observed at moderate power to be replaced by a mode at diamagnetic frequency at higher power. Precession fishbone frequency exhibits periodical jumps (0.1 s). These jumps are linked to spontaneous transitions between modes at different wavenumbers and a redistribution of the fast electrons in resonance with these modes. The number of excited modes and their wave number change with the LH power.
    Nuclear Fusion 07/2009; 49(8):085033. · 2.73 Impact Factor
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    ABSTRACT: MHD instabilities driven by fast electrons identified as fishbonelike modes have been detected on Tore Supra during lower hybrid current drive discharges. Direct experimental evidence is reported of a novel feature: the regular redistribution of suprathermal electrons toward external tokamak regions which are correlated to periodic mode frequency jumps. Sharp drops of the electron temperature time trace are factually linked to the cyclical deterioration of the fast electron confinement.
    Physical Review Letters 05/2009; 102(15):155005. · 7.73 Impact Factor
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    ABSTRACT: Temperature fluctuation diagnostics are being used to detect and study the fast particle instabilities, which could destabilize Magnetohydrodynamic (MHD) modes in tokamak plasmas. As the thermal noise is higher than the temperature fluctuation amplitude, correlation of two adjacent Electron Cyclotron Emission (ECE) channels is needed to recover the electron temperature fluctuations using long time integration. On Tore-Supra, we are developing a multi-channel ECE correlation diagnostic to observe MHD modes at 4 plasma positions with a radial resolution of about 2 centimeters and the poloidal resolution of about 4 cm (1/e folding width). A 1-channel prototype using 2 bandpass YIG filters has experimentally identified MHD instabilities related to fast particles dynamics. This first channel is currently being upgraded to reduce crosstalk and a second channel will be operational in 2008. The probing frequencies can be changed during the discharges, with 1s dead time between two modifications. Programmable attenuators are used to optimize the signal dynamic according to the frequency sensitivity. At nominal condition (B = 3.8 T), the diagnostic accessibility covers the outer plasma part up to a normalized radius r/a = 0.2 on the inner side. Two more channels will be implemented in 2009 and will allow us to measure the radial profile of MHD modes, or to study their inner/outer side asymmetry.
    04/2009;
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    ABSTRACT: On the Tore Supra tokamak, the ten-channel far infrared interferometer consists of a double color (119 and 195 microm) system with two detectors for each channel to measure the plasma density. The phase measurement is obtained by combining a 100 kHz shifted reference beam with the probing beam that has crossed the plasma. The achieved precision--a few percent of a fringe--is very good compared with the expected variations due to plasma, which are on the order of several fringes. However, the counting of the fringe variations can be affected when the signal is perturbed by electromagnetic interferences or when it deviates in the presence of strong plasma refraction changes occurring during ICRH breakdowns, pellet injections, or disruptions. This induces a strong decrease in the reliability of the measurement, which is an important concern when the diagnostic is used for density control. We describe in this paper the renewing of the electronics that has been achieved to reduce and correct the number of the so-called fringe jumps. A new zero crossing method for phase measurement is used, together with a field programmable gate array semiconductor integration, to measure the phase and activate the algorithm of corrections every 10 micros. Comparisons between a numerical oscilloscope analysis and the corrected acquired data in the case of laboratory amplitude modulation tests and in the case of real plasma perturbations are also discussed.
    The Review of scientific instruments 11/2008; 79(10):10E710. · 1.52 Impact Factor
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    ABSTRACT: On Tore Supra, five far infrared (FIR) retro-reflected channels have been installed to double the existing traversing ones, in order to increase the spatial resolution of the electron and current density measured by the interfero-polarimeter diagnostic.The new channel geometries are similar to those of the intended ITER polarimeter and have been designed for long pulse plasma operation. Inside the tokamak, the beams are reflected on edge-cooled retro-reflectors. During the 2003–2004 TS campaign, a prototype and reference plan mirrors had been exposed to plasma. After their removals, quantitative SIMS deposit analysis estimate the thickness of the carbon up to 700nm and FIR measurements confirmed that the surface changes were too small to modify the reflectivity of the mirrors.The transport of the FIR beams by mirrors to the detectors is then discussed. Ray tracings through plasma and mirrors have been done to rightly design and position the windows, the detectors and the separation grids and minimize the misalignment effects.First results of both interferometric and polarimetric reflected channel signals show a good consistency with the traversing ones and allow resolving more accurately the profiles.
    Fusion Engineering and Design - FUSION ENG DES. 01/2007; 82(5):1238-1244.
  • C. Gil, D. Elbeze
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    ABSTRACT: On the Tore Supra (TS) tokamak, a coupled FIR interfero-polarimeter diagnostic has been routinely used for electron density measurements [1-2]. For the more difficult current density evaluation, the used polarimetric technique is classical, with two detectors for each channel. It is a direct method to fully characterize the polarisation of the signal. As it needs amplitude measurements, it is very sensitive to the differential variations of the alignment during plasma, which are induced by moves of mirrors or refraction. To avoid this last inconvenience and thus improve the precision of the measurements, a prototype of a compact double detector has been designed and tested in the diagnostic environment without plasma, then during a TS campaign. The results showed that it was a very easy device to use, but that spurious depolarisations can strongly damage the rotation measurements. As a consequence, the mechanical design of the mounting needs to be reviewed.
    01/2007;
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    ABSTRACT: Integrated dimensionless confinement studies have been performed recently on JET, C-Mod, JT-60U, DIII-D, and Tore Supra. Analysis of ELMy H-mode identity experiments on JET and C-Mod shows that, despite results indicating confinement falls as density approaches the Greenwald limit, Greenwald fraction is not a relevant parameter for confinement scaling, but that collisionality is. Studies on JT-60U show a fall in ELMy H- mode confinement with increasing β , contrasting with those on DIII-D and JET which showed a negligible effect. Analysis of a multi-machine database indicates that the differing results may be due to a change in β dependency with plasma shape. Tore Supra experiments show a negligible β effect on L-Mode confinement, suggesting that any β dependence observed in ELMy H-modes is related to the edge pedestal. Statistical studies of multi- machine core and edge confinement databases lead to an improved explanation of these results. The impact of these results on the understanding of plasma transport and its extrapolation to ITER will be discussed.
    01/2007;
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    ABSTRACT: During fully noninductively driven discharges in the Tore Supra tokamak, large spontaneous oscillations of the core electron temperature (DeltaTe/Te>50%) have been observed for the first time. They occurred during the standard O regime, which is itself characterized by periodic oscillations of much smaller amplitude. The "giant" oscillations appear to involve distinct mechanisms with respect to the O regime and provide a spectacular example of the complex nonlinear interactions between energy confinement, noninductive current sources, and MHD that may occur in a tokamak plasma during steady-state operation.
    Physical Review Letters 02/2006; 96(4):045004. · 7.73 Impact Factor
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    ABSTRACT: Polarimetry measurements of Faraday rotation on Tore Supra are systematically affected by spurious oscillations that may seriously hinder the reconstruction of plasma current profiles. The same behavior has also been regularly observed on other experiments, such as JET and as RFX. Causes of this phenomenon are interferences between the probing beam and stray radiations having the same frequency. It perturbs the intensity of the signals from the detectors installed on the two orthogonal polarization channels. Experimental tests confirm the dependence of the oscillation amplitudes on the Faraday angle as predicted by the analytical and numerical description of the phenomenon. Special care must be taken to eliminate stray beams: When the stray beam polarization is parallel to the incident one, a relative ratio of the electric field amplitudes <1% has been found to be acceptable (error <0.06°) but a much lower ratio must be achieved when polarization is perpendicular. Such oscillations can be also present on polarimeters using beams with modulating polarization. For existing and future polarimeters, special care should be taken to make sure that all the possible sources of stray beams are sufficiently minimized.
    Review of Scientific Instruments 09/2004; 75(10):3405-3407. · 1.60 Impact Factor
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    ABSTRACT: Scientific and technical activities on JET focus on the issues likely to affect the ITER design and operation. Our understanding of the ITER reference mode of operation, the ELMy H-mode, has progressed significantly. The extrapolation of ELM size to ITER has been re-evaluated. Neoclassical tearing modes have been shown to be meta-stable in JET, and their beta limits can be raised by destabilization (modification) of sawteeth by ion cyclotron radio frequency heating (ICRH). Alpha simulation experiments with ICRH accelerated injected 4 (He) beam ions provide a new tool for fast particle and magnetohydrodynamic studies, with up to 80-90% of plasma heating by fast 4 He ions. With or without impurity seeding, a quasi-steady-state high confinement (H-98 = 1), high density(n(e)/n(GW) = 0.9-1) and high beta (betaN = 2) ELMy H-mode has been achieved by operating near the ITER triangularity ( similar to 0.40-0.5) and safety factor (q(95) similar to 3), at Z(eff) similar to 1.5-2. In advanced tokamak (AT) scenarios, internal transport barriers (ITBs) are now characterized in real time with a new criterion, rhoT(*). Tailoring of the current profile with T lower hybrid current drive provides reliable access to a variety of q profiles, lowering access power for barrier formation. Rational q surfaces appear to be associated with ITB formation. Alfven cascades were observed in reversed shear plasmas, providing identification of q profile evolution. Plasmas with 'current holes' were observed and modelled. Transient high confinement AT regimes with H-89 = 3.3, beta(N) = 2.4 and ITER-relevant q < 5 were achieved with reversed magnetic shear. Quasi-stationary ITBs are developed with full non-inductive current drive, including similar to 50% bootstrap current. A record duration of ITBs was achieved, up to 11 s, approaching the resistive time. For the first time, pressure and current profiles of AT regimes are controlled by a real-time feedback system, in separate experiments. Erosion and co-deposition studies with a quartz micro-balance show reduced co-deposition. Measured divertor thermal loads during disruptions in JET could modify ITER assumptions.
    Nuclear Fusion 01/2003; · 2.73 Impact Factor
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    ABSTRACT: The recent development of real-time measurements and control tools in JET has enhanced the reliability and reproducibility of the relevant ITER scenarios. Diagnostics such as charge exchange, interfero-polarimetry, electron cyclotron emission have been upgraded for real-time measurements. In addition, real-time processes like magnetic equilibrium and q profile reconstruction have been developed and applied successfully in real-time q profile control experiments using model based control techniques. Plasma operation and control against magnetohydrodynamic instabilities are also benefiting from these new systems. The experience gained at JET in the field of real-time measurement and control experiments operation constitutes a very useful basis for the future operation of ITER scenarios.
    Plasma Physics and Controlled Fusion, v.45, A367-A383 (2003). 01/2003;