Frédéric Brochard

Frédéric Brochard
University of Lorraine | UdL · Institut Jean Lamour

PhD

About

69
Publications
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782
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Introduction
My current research is focused on: - unipolar arcing mechanisms and dynamics; - statistical investigation of tomographically reconstructed fast video data of tokamak edge/SOL turbulence. I am also leading the SPEKTRE project (Sheath, Plasma Edge Kinetic Turbulence Radiofrequency Experiment).

Publications

Publications (69)
Article
COMPASS addressed several physical processes that may explain the behaviour of important phenomena. This paper presents results related to the main fields of COMPASS research obtained in the recent two years, including studies of turbulence, L-H transition, plasma material interaction, runaway electron, and disruption physics: Tomographic reconstru...
Article
COMPASS addressed several physical processes that may explain the behaviour of important phenomena. This paper presents results related to the main fields of COMPASS research obtained in the recent two years, including studies of turbulence, L–H transition, plasma material interaction, runaway electron, and disruption physics: • Tomographic reconst...
Article
Full-text available
The sheath properties are studied by using 1d3V particle-in-cell simulations in a plasma bounded by two conductive electrodes between which is applied a constant voltage, Vw. A magnetic field tilted by θ with respect to the wall is considered in the simulations. Elastic collisions with neutrals are modeled by an operator that randomly reorients ion...
Article
Rayleigh–Bénard convection (RBC) is studied in an elasto-viscoplastic (EVP) gel, the widely used and studied Carbopol gel. The transition from conductive to convective regime is determined by the Schmidt–Milverton principle which leads to a critical value of the inverse of the Yield number 1∕Yc. However, an oscillatory motion in the gel is observed...
Article
The ASDEX Upgrade (AUG) programme, jointly run with the EUROfusion MST1 task force, continues to significantly enhance the physics base of ITER and DEMO. Here, the full tungsten wall is a key asset for extrapolating to future devices. The high overall heating power, flexible heating mix and comprehensive diagnostic set allows studies ranging from m...
Article
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Following the lead of [Nguyen van Yen Nucl. Fus. 52 (2012) 013005], this article tackles the problem of tomographic inversion with one camera assuming a constant emissivity of light along the magnetic field lines. In this way, the 3D problem reduces to 2D by helical symmetry, allowing for the reconstruction of any poloidal plane in the field of vie...
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Fast video data recorded from 2008 to 2012 in full-tungsten ASDEX Upgrade have been analyzed with the TRACE algorithm developed to automatically detect and track dust particles. The purpose of the work presented in this paper is to complete earlier study of the influence of various discharge conditions on dust rates with an investigation of dust mo...
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The first comparison of dust radius and surface temperature estimates, obtained from spectroscopic measurements of thermal radiation, with simulations of dust heating and vaporization by the MIGRAINe dust dynamics code is reported. The measurements were performed during controlled tungsten dust injection experiments in the cold and dense plasmas of...
Article
The ASDEX Upgrade (AUG) programme is directed towards physics input to critical elements of the ITER design and the preparation of ITER operation, as well as addressing physics issues for a future DEMO design. Since 2015, AUG is equipped with a new pair of 3-strap ICRF antennas, which were designed for a reduction of tungsten release during ICRF op...
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Full-text available
Integrating the plasma core performance with an edge and scrape-off layer (SOL) that leads to tolerable heat and particle loads on the wall is a major challenge. The new European medium size tokamak task force (EU-MST) coordinates research on ASDEX Upgrade (AUG), MAST and TCV. This multi-machine approach within EU-MST, covering a wide parameter ran...
Article
Full-text available
The ASDEX Upgrade (AUG) programme is directed towards physics input to critical elements of the ITER design and the preparation of ITER operation, as well as addressing physics issues for a future DEMO design. Since 2015, AUG is equipped with a new pair of 3-strap ICRF antennas, which were designed for a reduction of tungsten release during ICRF op...
Article
Full-text available
Fast SA5 camera operating in the visible range has been installed on the COMPASS tokamak and used to study oscillations of visible light emission at plasma periphery. The work proved capability of the camera to detect several types of oscillatory modes in the COMPASS plasma in the frequency range 500 Hz – 50 kHz, such as sawteeth (~400 Hz), tearing...
Article
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Fast video data recorded during seven consecutive operation campaigns (2008–2012) in full-tungsten ASDEX Upgrade have been analyzed with an algorithm developed to automatically detect and track dust particles. A total of 2425 discharges have been analyzed, corresponding to 12 204 s of plasma operation. The analysis aimed at precisely identifying an...
Article
PEO was conducted on Al by applying a pulsed bipolar current. The role of the cathodic polarization on the appearance of micro-discharges (MDs) and on the subsequent formation of the PEO oxide layers is investigated. Various ratios of the charge quantity RCQ = Qp/Qn (defined as the anodic Qp to cathodic Qn charge quantity ratio over one current pul...
Article
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Mechanical dust-wall collisions are unavoidable in fusion devices and their accurate modeling is essential for the understanding of dust transport. The MIGRAINe dust dynamics code features analytical models addressing all facets of dust-surface impacts, some aspects of which have not been experimentally validated thus far. Dedicated dust injection...
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The first combined experimental and theoretical studies of dust remobilization by plasma forces are reported. The main theoretical aspects of remobilization in fusion devices under steady state conditions are analyzed. In particular, the dominant role of adhesive forces is highlighted and generic remobilization conditions - direct lift-up, sliding,...
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This paper summarizes the status of the COMPASS tokamak, its comprehensive diagnostic equipment and plasma scenarios as a baseline for the future studies. The former COMPASS-D tokamak was in operation at UKAEA Culham, UK in 1992-2002. Later, the device was transferred to the Institute of Plasma Physics of the Academy of Sciences of the Czech Republ...
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Stereoscopic fast camera observations of pre-characterized carbon and tungsten dust injection in TEXTOR are reported, along with the modelling of tungsten particle trajectories with MIGRAINe. Particle tracking analysis of the video data showed significant differences in dust dynamics: while carbon flakes were prone to agglomeration and explosive de...
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Dust trajectories have been recorded with an unprecedented, under fusion-relevant plasma conditions, spatial resolution of 9 μm/pixel in Pilot-PSI. The optical setup allowed the use of fast cameras as a basic microscope. It is demonstrated that such a resolution is essential for the correct interpretation of experiments on several aspects of dust-s...
Article
The first combined experimental and theoretical studies of dust remobilization by plasma forces are reported. The main theoretical aspects of remobilization are analyzed. In particular, the dominant role of adhesive forces is highlighted and generic remobilization conditions - detachment, sliding, rolling - are formulated. A novel experimental tech...
Article
Dust-surface collisions impose size selectivity on the ability of dust grains to migrate in scrape-off layer and divertor plasmas and to adhere to plasma-facing components. Here, we report first experimental evidence of dust impact phenomena in plasma environments concerning low-speed collisions of tungsten dust with tungsten surfaces: re-bouncing,...
Article
Langmuir probe measurements in front of high power ion cyclotron resonant frequency antennas are not possible or simply too noisy to be analyzed properly. A linear experiment is a radio frequency (RF) magnetized plasma discharge reactor designed to probe the rectified potential in front of such antennas but at low power level (1 kW) to next improve...
Article
We report the results of an experimental investigation of the impact of Supersonic Molecular Beam Injection in the Tore Supra tokamak. Several diagnostics were synchronised with the injection to extract a global picture of the physics at play from the time scale of turbulence (∼10 μs) to the full-recovery time (∼1 s). As previously reported, a stro...
Article
Two fast visible range camera systems with vertically and tangentially oriented crossed viewing fields in the divertor region of ASDEX Upgrade were used to observe tungsten (W) droplets ejected from a melting W-pin into the divertor plasma. To obtain the spatial (3D) trajectories of the tungsten droplets, the trajectory of a given droplet in the (2...
Article
This work presents the physics design for a simple quasi-axially symmetric stellarator. A plasma configuration described by a modest number of Fourier coefficients was found to establish this symmetry with good accuracy. The low rotational transform results in a relatively simple coil set exhibiting low curvatures and comfortable clearance between...
Article
Two fast visible range camera systems with vertically and tangentially oriented crossed viewing fields in the divertor region of ASDEX Upgrade were used to observe tungsten (W) droplets ejected from a melting W-pin into the divertor plasma. To obtain the spatial (3D) trajectories of the tungsten droplets, the trajectory of a given droplet in the (2...
Article
Full-text available
Images acquired by cameras installed in tokamaks are difficult to interpret because the three-dimensional structure of the plasma is flattened in a non-trivial way. Nevertheless, taking advantage of the slow variation of the fluctuations along magnetic field lines, the optical transformation may be approximated by a generalized Abel transform, for...
Article
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This paper presents full cross-section measurements of drift waves in the linear magnetized plasma of the Mirabelle device. Drift wave modes are studied in regimes of weakly developed turbulence. The drift wave modes develop azimuthal space-time structures of plasma density, plasma potential, and visible light fluctuations. A fast camera diagnostic...
Article
2D dust particle trajectories are extracted from fast framing camera videos of ASDEX Upgrade (AUG) by a new time- and resource-efficient code and classified into stationary hot spots, single-frame events and real dust particle fly-bys. Using hybrid global and local intensity thresholding and linear trajectory extrapolation individual particles coul...
Article
The present work focuses on the influence of the distance between the substrate and the counter electrodes on the plasma electrolytic oxidation process. Depending on the waveform parameters of the applied current, two working regimes may be achieved referred to as “soft PEO regime” and “conventional (arcing) PEO regime.” At constant process duratio...
Article
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Spectral analysis and bicoherence analysis are applied to fast camera data in order to study the nonlinear mode coupling in the low-β cylindrical magnetized plasma device Mirabelle [ T. Pierre et al., Rev. Sci. Instrum. 58, 6 (1987) ]. The camera provides a measurement of electron plasma density fluctuations in a complete plasma column cross-sectio...
Article
An algorithm has been developped, which makes it possible to automatically extract trajectories of a large number of particles from fast imaging data, allowing a statistical analysis of particles trajectories under various plasma conditions, a better understanding of their influence on plasma properties, and a better characterization of the plasma...
Article
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This paper deals with the characterization of a RF reactor with a helicon configuration by studying a pure argon plasma. Tunable diode-laser induced fluorescence (TD-LIF) is performed in the center of the diffusion chamber to determine relative Ar+ metastable density as a function of RF power and static magnetic fields. A diode laser tuned at λ0 =...
Article
A generally available and easy-to-use viewer for the simultaneous visualisation of the ASDEX Upgrade vacuum vessel computer aided design models, diagnostics and magnetic geometry, solutions of 3D plasma simulation codes and 2D camera images was developed. Here we report on the working principle of this software and give several examples of its tech...
Article
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A method enabling automatic detection and tracking of large amounts of individual dust particles in plasmas is presented. Individual trajectories can be found with a good spatiotemporal resolution, even without applying any external light source to facilitate detection. Main advantages of this method is a high portability and the possibility of mak...
Article
Full-text available
Fast visible imaging is used on a cylindrical magnetized argon plasma produced by thermionic discharge in the Mirabelle device. To link the information collected with the camera to a physical quantity, fast camera movies of plasma structures are compared to Langmuir probe measurements. High correlation is found between light fluctuations and plasma...
Article
Full-text available
Plasma wall interactions studies are of primary importance for increasing the life time of the first wall in fusion devices. In ITER, the divertor target plates will receive on a small surface a significant part of the power during operation, and carbon materials will be used. Although carbon has several advantages than the materials used at other...
Article
Methods for the characterization of radial convective transport in magnetized plasma devices are depicted. Perspectives for the investigation of transport using the linear low-beta plasma device Mirabelle are presented. The utility of time resolved data analysis, using the wavelet transform of the signals, is demonstrated. These techniques are ill...
Article
Spatiotemporal mode coupling is studied experimentally in a cylindrical plasma device. For that purpose, a bicoherence analysis is applied to spatially resolved measurements of drift wave fluctuations in order to study nonlinear coupling in the wave number spectrum. The use of the k bicoherence is shown to be much more accurate and straightforward...
Article
Full-text available
An open-loop spatiotemporal synchronization method is applied to flute modes in a cylindrical magnetized plasma. It is demonstrated that synchronization can be achieved only if the exciter signal rotates in the same direction as the propagating mode. Moreover, the efficiency of the synchronization is shown to depend on the radial properties of the...
Article
The spatiotemporal transition scenario of flute instabilities from a regular to a turbulent state is experimentally investigated in the plasma column of a thermionic discharge. The same transition scenario, i.e., the Ruelle-Takens route to turbulence, is found for both the Kelvin-Helmholtz and the Rayleigh-Taylor instabilities. It is demonstrated t...
Article
The spatiotemporal transition scenario of flute instabilities from a regular to a turbulent state is experimentally investigated in the low-β plasma column of a thermionic discharge. The same transition scenario, i.e., the Ruelle-Takens route to turbulence, is found for both the Kelvin-Helmholtz and the Rayleigh-Taylor instabilities. It is demonstr...
Article
We report on experiments performed on the low‐β plasma device MIRABELLE. Using a limiter, we recently observed that when increasing the magnetic field strength transitions between various gradient driven instabilities occur. New thorough measurements allow to identify unambiguously three instability regimes. At low magnetic field the strong E r×B v...
Article
Intermittent coherent structures are a common feature of edge magnetized plasmas. Such structures, large compared to typical turbulent scales, can significantly contribute to the high cross-field particles and energy transport in fusion experiments. Since a net energy transfer must be accompanied by a finite phase coherence between the involved str...
Article
Recent experiments performed on the low β plasma device Mirabelle [ T. Pierre, G. Leclert, and F. Braun, Rev. Sci. Instrum. 58, 6 (1987) ] using a limiter have shown that transitions between various gradient driven instabilities occurred on increasing the magnetic field strength. New thorough measurements allow to identify unambiguously three insta...
Article
Full-text available
In this contribution we report on experiments performed on the low-β plasma device MIRABELLE. Using a limiter, we recently got experimental evidence that when increasing the magnetic field transitions between various gradient driven instabilities are observed. The characterization is based on: (i) comparison between the poloidal propagation velocit...
Article
A detailed analysis of the phase velocity of unstable low-frequency waves is performed in a laboratory magnetized plasma column. The measurement of the radial profiles of the density, electron temperature and plasma potential allows to determine the radial profile of the electric drift velocity and electron diamagnetic drift for increasing values o...
Article
The spatiotemporal structure of nonlinear low frequency waves in a magnetized plasma is studied. The m=2 azimuthal mode exhibits clearly a rotating spiral structure when the diameter of the plasma column is smaller than the diameter of the containing tube. The measurement of the plasma potential profile leads to the conclusion that the magnitude an...
Conference Paper
Full-text available
Nous avons etudie les regimes d’intermittence que l’on observe sur le bord d’une colonne droite de plasma soumise a un champ magnetique. Dans un premier temps, les regimes reguliers sont etudies et une analyse spatio-temporelle met en evidence une structure spirale qui se developpe autour de la colonne. L’analyse lineaire du systeme s’appuie sur le...
Article
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Numerous plasma instabilities with frequencies ranging from the kHz to the GHz are ob-served in Hall thrusters [1]. Besides the large amplitude low frequency (10-30kHz) oscillations (the so-called "breathing mode") high-frequency (> 1 MHz) bursty fluctuations are also fre-quently observed. These high frequency bursts are correlated to decay periods...
Article
Full-text available
The broadband spectrum of fluctuations observed in fusion plasmas can result from nonlinear high order wave-wave interactions, which can transfer the energy contained in a few modes to a multitude of modes. A nonlinear energy transfer involves at least three modes and requires a frequency and phase match between the modes. A well-known tool to meas...