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Institut de Recherche sur la Fusion par confinement Magnétique (CEA Cadarache)

Overview
Works: 13 works in 13 publications in 2 languages and 13 library holdings
Roles: Other
Publication Timeline
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Most widely held works by Institut de Recherche sur la Fusion par confinement Magnétique (CEA Cadarache)
Impact of geometry and shaping of the plasma facing components on hot spot generation in tokamak devices by Alex Grosjean( )

1 edition published in 2020 in English and held by 1 WorldCat member library worldwide

This PhD falls within ITER project support, aiming to study the thermal behavior of ITER-like PFC prototypes in two superconducting tokamaks: EAST (Hefei) and WEST (Cadarache). These prototypes correspond to castellated tungsten monoblocks placed along a cooling tube with small gaps (0.5 mm) between them, called plasma-facing units, to extract the heat from the components. The introduction of gaps between monoblocks (toroidal) and plasma-facing units (poloidal), to relieve the thermomechanical stresses in the divertor, implies that poloidal leading edges may be exposed to near-normal incidence angle. A local overheating is expected in a thin lateral band at the top of each monoblocks, which can be enhanced when the neighboring components are misaligned. In this work, we propose to study the impact of two geometries (sharp and chamfered LEs) of these components, as well as their misalignments on local hot spot generation, by means of embedded diagnostics (TC/FBG), and a submillimeter infrared system (~0.1 mm/pixel), whose emissivity varies with wavelength, and the temperature, but above all, the surface state of the component, which evolves under plasma exposure, during the experimental campaigns. The divertor Langmuir probes measure the plasma temperature, and thus estimate the ion Larmor radius that may play a role in the local heat flux distribution around poloidal and toroidal edges. The results presented in this thesis, confirming the modelling predictions by experimental measurements, support the final decision by ITER to include 0.5 mm toroidal beveling of monoblocks on the vertical divertor targets to protect poloidal leading edges from excessive heat flux
Pertes par couplages dans les câbles en conduit supraconducteurs des réacteurs à fusion thermonucléaire : modélisation théorique et investigations expérimentales by Maxime Chiletti( )

1 edition published in 2020 in English and held by 1 WorldCat member library worldwide

The objective of the present work is to generalize a purely analytical model used to assess the magnetization of a superconducting cable. These cables are assembled twisting several superconducting strands together in stages, making the electromagnetic description harder to handle due to this complex geometry. Starting from an already existing CEA model limited to the description of two consecutive stages, we strengthen its analytical basis in order to build a fully generalized model capable to represent complex cables with any number of stages. This new model, called N-stage COLISEUM, is confronted to experimental coupling losses measurements conducted on several cable samples in the JOSEFA facility, showing a fair agreement. The quality of the predictions from the N-stage COLISEUM is verified and also compared to an heuristic CEA model (MPAS), already established, opening the path for possible evolutions of both models and for stability applications
Study of impurity transport on ITER and of the associated choice of innovative soft x-ray detectors by Damien Colette( )

1 edition published in 2021 in English and held by 1 WorldCat member library worldwide

Due to the impact of fossil fuel and the limited amount of available fuel, it is crucial to develop clean and sustainable energy sources which can replace them. Controlled thermonuclear fusion is one of the main candidates. The ITER tokamak aims at demonstrating the feasibility of a fusion reactor generating more energy than it consumes (Q>10). The ITER plasma facing components will be the source of pollution by heavy impurities such as tungsten in the plasma. Such impurities lead to great radiative losses in the X-ray range. X-ray measurement is mandatory for impurity transport studies in order to be able to identify actuators preventing impurity accumulation in the plasma core. The ITER radiative environment limits the choice of X-ray detectors to gas detectors, of which the LVIC (Low Voltage Ionization Chamber) is the most promising candidate. This thesis aims at studying the capabilities of the LVIC for X-ray measurement on ITER. A synthetic diagnostic tool has been adapted from the Gas Electron Multiplier in order to simulate the detection with an LVIC. Tomographic inversion of the X-ray emissivity using LVIC is studied and additional lines-of-sight, compliant with the ITER integration constraints, are proposed. The possibility of energy discrimination is investigated through an innovative modification of the detector. An inversion method is specifically developed to deconvolute the X-ray spectrum. The electron temperature profile is successfully extracted from the X-ray spectrum. The capability of impurity transport study of the LVIC is demonstrated through the reconstruction of the tungsten convection and diffusion coefficients of an ITER plasma
Electron heat transport in tokamak H-mode pedestals by Myriam Hamed( )

1 edition published in 2019 in English and held by 1 WorldCat member library worldwide

In H-mode plasmas, the modeling of the pedestal dynamics is an important issue to predict temperature and density profiles in the tokamak edge and therefore in the core. The EPED model, based on the stability of large scales MagnetoHydroDynamic (MHD) modes, is most commonly used to characterize the pedestal region. The EPED model has been successful until now. However, EPED model does not take into account small scales instabilities linked the the sharp pressure gradient and the pedestal characteristics prediction in terms of width and height is still open. Moreover, some recent analysis of JET plasmas suggest that another class of instabilities, called microtearing modes, may be responsible for electron heat transport in the pedestal, and thereby play some role in determining the pedestal characteristics. Microtearing modes belong to a class of instabilities where a modification of the magnetic field line topology is induced at the ion Larmor radius scale. This leads to the formation of magnetic islands, which can enhance the electron heat transport. The stability of MTMs has been theoretically studied in the past showing that a slab current sheet is stable in the absence of collisions. In contrast, recent gyrokinetic simulations in toroidal geometry found unstable MTMs, even at low collisionality. The purpose of our work is to improve the MTM stability understanding by comparing new analytical theory to linear gyrokinetic simulations. More precisely, physical mechanisms (magnetic drift, electric potential) are progressively included in the analytical description to recover the numerical simulations results and to "reconcile" numerical MTM investigations with theory
Model reduction for tokamak plasma turbulence : beyond fluid and quasi-linear descriptions by Camille Gillot( )

1 edition published in 2020 in English and held by 1 WorldCat member library worldwide

Optimal control of tokamak plasmas requires efficient and accurate prediction of heat and matter transport. Growing from kinetic resonant instabilities, turbulence saturates by involving many scales, from the small vortex up to the back-reaction on the density and temperature profiles. Self-organisation processes are of particular interest, encompassing spontaneous zonal flow genera- tion and transport by avalanche. First principle numerical simulation codes like GYSELA allow studying the gyro-kinetic evolution of the particle distribution function. The large model size and cost prompts the need for reduction. Removing velocity dimensions is the so-called collisionless closure problem for fluid equations. Earlier approaches are extended and generalised by calling to the dynamical systems and optimal control litterature. In particular, we apply the balanced truncation and rational interpolation to the one-dimensional linear VlasovPoisson problem. The interpolation method features a cheap and versatile formulation, opening the door to wider use for more complex phenomena. Quasi-linear theory is the reference model for turbulent effects. The GYSELA three-dimensional output is analysed to estimate the robustness of linear properties in turbulent filaments. Key quasi-linear quantities carry over to the non-linear regime. Effective velocities and shape of turbulent structures are computed, and match expected group velocities and linear eigenmode. Nevertheless, the turbulent potential spectrum must be specified externally to quasi- linear models. This results in radially travelling unstable linear solutions that share many properties of turbulent avalanches seen in numerical simulations
Study of tokamak plasma disruptions and runaway electrons in a metallic environment by Sundaresan Sridhar( )

1 edition published in 2020 in English and held by 1 WorldCat member library worldwide

Tokamaks are the devices currently closest to achieve nuclear fusion power and disruptions are unfavorable events in which the plasma energy is lost in a very short timescale causing damage to tokamak structures. RE beams are one of the consequence of disruptions and they carry the risk of in-vessel component damage. Thus, the prevention and control of the RE are of prime importance. The current strategy for runaway electrons is to avoid their generation by a massive material injection (MMI). If their generation cannot be avoided, a 2nd MMI will be used to mitigate the generated RE beam. After the 1st MMI to prevent RE generation, a background plasma of 1st MMI impurities is formed which make the second MMI inefficient to mitigate RE beams inefficient, as observed in the JET tokamak. In this thesis, the physics of the interaction between the RE beam and the mitigation MMI in the presence of a cold background plasma is studied
Apport de la simulation de mannequins virtuels biologiquement réalistes pour l'étude de la faisabilité de tâches d'assemblage et de maintenance d'une installation industrielle complexe by Céphise Louison( )

1 edition published in 2017 in French and held by 1 WorldCat member library worldwide

The virtual reality field offers astounding opportunities throughout the design of complex assembly. One of the advantages is the possibility of immerse users in an ecological situation in the early stages of design. It's even more important during the design of complex industrial plant. It allows to validate accessibility, assembly, maintenance or even dismantling tasks. The ecological validity of the studies carried out in a virtual environment, requires the human-machine interfaces capable of transmitting and rendering realistic information. At present, virtual systems are not capable of providing a complete haptic interaction, in particular contact/collision between the user and virtual objects in the environment do not result in haptic feedback. This can become the cause of inappropriate behavior compared to reality. The aim of this thesis is to propose a way to reinforce spatial awareness in virtual environment. More specifically, we wish to substitute kinesthetic information with tactile information to increase spatial awareness and visuo-proprioceptive consistency. We developed a vibrotactile platform capable of providing spatial information on the environment. We also studied the role of the representation of the body in a virtual environment. Several experimental studies have been carried out in order to validate the developments we proposed
Impurity transport in tokamak plasmas : gyrokinetic study of neoclassical and turbulent transport by Peter Donnel( )

1 edition published in 2018 in English and held by 1 WorldCat member library worldwide

Impurity transport is an issue of utmost importance for tokamaks. Indeed high-Z materials are only partially ionized in the plasma core, so that they can lead to prohibitive radiative losses even at low concentrations, and impact dramatically plasma performance and stability. On-axis accumulation of tungsten has been widely observed in tokamaks.While the very core impurity peaking is generally attributed to neoclassical effects, turbulent transport could well dominate in the gradient region at ITER relevant collisionality. Up to recently, first principles simulations of corresponding fluxes were performed with different dedicated codes, implicitly assuming that both transport channels are separable and therefore additive. The validity of this assumption is questionned. Simulations obtained with the gyrokinetic code GYSELA have shown clear evidences of a neoclassical-turbulence synergy for impurity transport and allowed the identification of a mechanism that underly this synergy.An analytical work allows to compute the level and the structure of the axisymmetric part of the electric potential knowing the turbulence intensity. Two mechanisms are found for the generation of poloidal asymmetries of the electric potential: flow compressibility and the ballooning of the turbulence. A new prediction for the neoclassical impurity flux in presence of large poloidal asymmetries and pressure anisotropies has been derived. A fair agreement has been found between the new theoretical prediction for neoclassical impurity flux and the results of a GYSELA simulation displaying large poloidal asymmetries and pressure anisotropies induced by the presence of turbulence
Influence des conditions de surface sur le piégeage de l'hydrogène dans le tungstène by Mykola Ialovega( )

1 edition published in 2021 in English and held by 1 WorldCat member library worldwide

La rétention des isotopes de l'hydrogène (HI) et de l'hélium (He) dans les composants de la première paroi (PFC) est un enjeu majeur pour les futurs réacteurs tels ITER et DEMO, les conditions d'exposition au plasma de fusion pouvant entrainer la dégradation des matériaux. Les propriétés du tungstène (W), ont entrainés son choix pour le divertor d'ITER: lors des phases deutérium/tritium, les PFC W seront soumis à d'intenses flux de particules, HI, He, neutrons ou encore impuretés issues du plasma de bord. L'impact de l'He est particulièrement problématique, avec un endommagement significatif de la zone de proche surface dans le W: création de boucles de dislocations, bulles, ou W-fuzz. En outre, en présence d'oxygène résiduel, une oxydation de surface est possible du fait de la température élevée du divertor. La modification de la structure du W peut considérablement modifier les propriétés du matériau, et donc son espérance de vie face au plasma, ainsi que sa rétention en hydrogène, ce qui pose un problème de sureté dans le cas du tritium, qui est radioactif. Cette thèse a permis d'étudier les mécanismes fondamentaux du piégeage et la rétention de HI dans le W, grâce notamment à l'implantation par faisceau d'ions et la spectroscopie par thermo desorption (TDS), en fonction de différents états de surface: - Présence d'une couche d'oxide en surface d'un W polycristallin, formées dans des conditions pertinentes pour ITER; - Présence des modifications liées à une exposition à l'He proches de celles attendues dans ITER. Les mesures TDS ont été couplées avec des observations de microscopie afin de caractériser les modifications dans la surface et la structure du matériau
Utilisation du rayonnement X-mou pour l'étude du transport des impuretés dans les plasmas de tokamaks by Axel Jardin( )

1 edition published in 2017 in English and held by 1 WorldCat member library worldwide

La consommation mondiale d'énergie a fortement augmenté durant le siècle dernier et va continuer de croître au cours des prochaines décennies. Le développement d'énergies durables et alternatives aux énergies fossiles constitue un enjeu crucial pour les générations futures. Dans ce contexte, la fusion thermonucléaire contrôlée serait un candidat de premier choix pour assurer la transition énergétique. Le tokamak, basé sur la fusion par confinement magnétique, est actuellement la solution la plus en vue pour contrôler la réaction de fusion et utiliser cette énergie à des fins civiles.Dans les plasmas de tokamak, les impuretés lourdes comme le tungstène présent dans les éléments de la paroi face au plasma, peuvent migrer vers le plasma de cœur et fortement dégrader les performances fusion par rayonnement. L'objectif de cette thèse est d'utiliser ce rayonnement dans la gamme des X-mous afin d'en déduire des informations sur le transport du tungstène dans le plasma de cœur. Le but est de contrôler à terme cette concentration en impuretés et d'identifier les actuateurs pouvant agir sur cette distribution
Impact of 3D non axisymmetrical magnetic perturbations on the transport and turbulence in the edge plasma of tokamaks by Benjamin Luce( )

1 edition published in 2020 in English and held by 1 WorldCat member library worldwide

The power exhaust and the increase of the confinement time are two major challenges for tokamaks. These are linked to the perpendicular, mostly turbulent transport toward the wall. The H-mode reduces the perpendicular transport but is associated with deleterious relaxation events, the ELMs, which lead to unreasonnable transient heat fluxes. In order to control the ELMs. 3D magnetic perturbations (MP) have been added. If some impacts of MPs have been studied, few data exist on their impact on the turbulent transport.In this thesis, we model the edge plasma, from the outer core to the Scrape-Off Layer (SOL), with a 3D electrostatic fluid turbulent code, TOKAM3X and a mean-field one, SOLEDGE3X_HDG. With the TOKAM3X, we observe the impact of 3D MPs on edge turbulence. We start with an electrostatic isothermal model with single mode MP in a limiter circular geometry. This study reproduces some experimental observations. The MPs only moderately impact turbulence properties. A complexification of the model is done, with non isothermal simulations. We show that the decoupling of particles and energy is important. Experimental trends are recovered. The impact on turbulence is still moderated. An analysis of the consequences of such findings for mean-field simulations is then proposed through a direct comparison. It shows a significant difference with turbulentsimulations. Finally, first results in the direction of more realistic MPs spectra are presented, using TOKAM3X mean-field cases as a start. An extension of this work is done on SOLEDGE3X_HDG in more complex geometries and even simulating a realistic ripple in WESTgeometry
A new modelling of the cross-field transport in diverted edge plasma : application to 2D transport simulations with SolEdge2D-EIRENE by Serafina Baschetti( )

1 edition published in 2019 in English and held by 1 WorldCat member library worldwide

Le fonctionnement à l'équilibre du réacteur à fusion de prochaine génération, ITER, nécessitera le développement d'outils numériques fiables permettant d'estimer les paramètres d'ingénierie clés à un coût de calcul raisonnable. Les codes de transport répondent à cette exigence car ils reposent sur des équations fluides bidimensionnelles qui sont moyennées sur les fluctuations temporelles, de la même manière que les modèles « Reynolds Averaged Navier-Stokes » couramment utilisés dans la communauté des fluides neutres. De plus, les codes de transport peuvent rassembler la plupart des ingrédients physiques régissant le comportement du plasma de bord, ainsi que une topologie magnétique réaliste et la géométrie du mur. Cependant, leur prévisibilité est limitée par une description inadéquate des flux turbulents perpendiculaires aux lignes de champ magnétique, qui influent fortement e confinement du plasma sur de longues périodes. En effet les flux perpendiculaires, supposés diffusifs, sont grossièrement déterminés par des coefficients de diffusion homogènes ou "ad-hoc", ou par des procédures à boucle de rétroaction appliquées "a-posteriori" à des données expérimentales. Motivés par ces questions, nous présentons dans ce travail un nouveau modèle pour estimer de manière cohérente la distribution des flux perpendiculaires dans les codes de transport, lorsque les plasmas en régime permanent sont concernés. La stratégie consiste à introduire des outils numériques efficaces largement utilisés dans la communauté de la turbulence neutre en physique des plasmas. Deux concepts clés sont inspirants dans la communauté des fluides neutres. Le premier est "l'hypothèse de Boussinesq". Elle consiste à linéariser le tenseur de contraintes de Reynolds dansl'équation de Navier-Stokes moyennée dans le temps via une relation de diffusion dans laquelle le terme de proportionnalité est appelé « eddy viscosity ». Le deuxième concept est le modèle "k-epsilon", dans lequel les équations de transport pour l'énergie turbulente cinétique moyenne et le taux d'échange d'énergie entre les structures turbulentes sont conçues de manière semi-empirique. A l'équilibre, k et epsilon permettent une estimation auto-cohérente de l'« eddy viscosity », intégrant ainsi l'impact de la turbulence sur les flux moyennés à l'état d'équilibre. Ces concepts ne peuvent pas être appliqués directement pour enrichir la modélisation des flux perpendiculaires dans les plasmas en raison de différentes propriétés de turbulence. Par conséquent, nous suggérons une adaptation du modèle k-epsilon pour les flux neutres à des plasmas à confinement magnétique, où deux équations de transport pour l'énergie cinétique turbulente et son taux de dissipation sont dérivées algébriquement, y compris la physique de l'instabilité d'interchange linéaire, responsable de la distribution "ballonnée" du transport perpendiculaire dans le bord du plasma. Différentes approches sont décrites pour fermer les paramètres libres : premièrement, une procédure de boucle de rétroaction pour optimiser les résultats numériques comparés avec un test expérimental. Ensuite, on assume une loi d'échelle de référence pour la largeur du profil de flux de chaleur dans la SOL, déterminée empiriquement à partir des mesures expérimentales du flux de chaleur sur le divertor externe dans diverses machines. Le nouveau modèle est intégré au package de transport SolEdge2D-EIRENE, développé en collaboration entre le CEA et le laboratoire M2P2 de l'Université d'Aix-Marseille. Les résultats numériques à l'état d'équilibre sont discutés et on démontre qu'ils se comparent favorablement aux données expérimentales soit à l'outer midplane que au divertor externe. De plus, on montre que les distributions de diffusivité présentent des asymétries poloïdales cohérentes avec la distribution "ballonnée" du transport perpendiculaire observée dans les mêmes conditions dans les codes de premier principe et les expériences
Modelling of plasma-antenna coupling and non-linear radio frequency wave-plasma-wall interactions in the magnetized plasma device under ion cyclotron range of frequencies by LingFeng Lu( )

1 edition published in 2016 in English and held by 1 WorldCat member library worldwide

Ion Cyclotron Resonant Heating (ICRH) by waves in 30-80MHz range is currently used in magnetic fusion plasmas. Excited by phased arrays of current straps at the plasma periphery, these waves exist under two polarizations. The Fast Wave tunnels through the tenuous plasma edge and propagates to its center where it is absorbed. The parasitically emitted Slow Wave only exists close to the launchers. How much power can be coupled to the center with 1A current on the straps? How do the emitted radiofrequency (RF) near and far fields interact parasitically with the edge plasma via RF sheath rectification at plasma-wall interfaces? To address these two issues simultaneously, in realistic geometry over the size of ICRH antennas, this thesis upgraded and tested the Self-consistent Sheaths and Waves for ICH (SSWICH) code. SSWICH couples self-consistently RF wave propagation and Direct Current (DC) plasma biasing via non-linear RF and DC sheath boundary conditions (SBCs) at plasma/wall interfaces. Its upgrade is full wave and was implemented in two dimensions (toroidal/radial). New SBCs coupling the two polarizations were derived and implemented along shaped walls tilted with respect to the confinement magnetic field. Using this new tool in the absence of SBCs, we studied the impact of a density decaying continuously inside the antenna box and across the Lower Hybrid (LH) resonance. Up to the memory limits of our workstation, the RF fields below the LH resonance changed with the grid size. However the coupled power spectrum hardly evolved and was only weakly affected by the density inside the box. In presence of SBCs, SSWICH-FW simulations have identified the role of the fast wave on RF sheath excitation and reproduced some key experimental observations. SSWICH-FW was finally adapted to conduct the first electromagnetic and RF-sheath 2D simulations of the cylindrical magnetized plasma device ALINE
 
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Audience level: 0.99 (from 0.96 for Modelling ... to 0.99 for Model redu ...)

Alternative Names
CEA/DRF/IRFM

CEA/DSM/IRFM

CEA. IRFM

Commissariat à l'énergie atomique (France). Institut de Recherche sur la Fusion par confinement Magnétique

Institut de recherche sur la fusion magnétique (Cadarache)

IRFM

Languages
English (12)

French (1)