La faisabilite de la fusion magnetique est dependante de notre capacite a confiner l'energie des particules supra-thermiques liberees a haute energie par les reactions de fusion, dans les meilleures conditions de securite et d'efficacite. Dans ce but, il est necessaire de comprendre l'interaction entre les particules energetiques et le plasma thermo-nucleaire qui constitue l'environnement des reactions de fusion, afin de la controler. La these que nous presentons ici s'inscrit dans cet effort. Le coeur du travail mene est l'etude d'un type d'instabilite, le Beta Alfven Eigenmode (BAE), que peuvent exciter les particules energetiques, et dont on peut craindre qu'il degrade fortement non seulement le confinement des particules energetiques mais aussi le confinement du plasma dans sa globalite. Dans un premier temps, nous nous attacherons a decrire les caracteristiques de ce mode et nous deriverons sa relation de dispersion ainsi que sa structure. Dans une seconde partie, nous effectuerons l'etude de la stabilite lineaire de ce mode en presence de particules energetiques. Cette etude nous a permis de definir un critere analytique rendant compte de la capacite des particules energetiques a exciter le BAE. Ce critere sera discute et confronte aux resultats d'experiences menees durant la these. Cette etude lineaire presentant cependant quelques limites, il nous est apparu important de nous poser la question de la possibilite d'une modication de la stabilite du BAE liee a l'utilisation d'une description non-lineaire. Nous suggererons dans cette presentation un processus, verifie analytiquement et numeriquement, dont peut resulter l'existence d'etats meta-stables pour le BAE.

Une compréhension profonde du transport du plasma au bord d'un réacteur à fusion par confinement magnétique est obligatoire pour gérer l'extraction de puissance. Dans les dispositifs de fusion de nouvelle génération, des limites technologiques contraignent le flux de chaleur maximal au divertor. Pour une puissance d'échappement donnée le flux de chaleur maximal est déterminé par l'amplitude de l'empreinte du plasma au mur. Les profils de flux de chaleur au divertor peuvent être paramétrés par deux échelles de longueur du transport. Nous remettons en question l'interprétation actuelle de ces deux échelles de longueur en étudiant l'impact de la géométrie du divertor sur l'échappement. En particulier, un élargissement des profils de flux de chaleur avec la longueur de la jambe du divertor externe est diagnostiqué. Des efforts de modélisation ont montré que les simulations diffusives reproduisent les profils expérimentaux de flux de chaleur pour les plasmas à jambes courtes. Inversement, l'étalement du flux de chaleur pour une longe jambe du divertor est reproduit par un modèle turbulent, soulignant l'importance de la turbulence aussi dans le divertor. Ces résultats remettent en question l'interprétation de la largeur du flux de chaleur comme grandeur liée a la main SOL uniquement. Les configurations magnétiques avec une longe jambe du divertor mettent en évidence l'importance du transport asymétrique dans le divertor. Nous concluons que le transport dans la main SOL et celui dans le divertor ne sont pas à découpler et nous soulignons l'importance de la géométrie magnétique sur le transport turbulent avec l'avantage potentiel d'un inattendu étalement du dépôt de puissance. / A deep understanding of plasma transport at the edge of a magnetically confined fusion device is mandatory for a sustainable and controlled handling of the power exhaust. In the next-generation fusion device ITER, technological limits constrain the peak heat flux on the divertor. For a given exhaust power the peak heat flux is determined by the extent of the plasma footprint on the wall. Heat flux profiles at the divertor targets of X-point configurations can be parametrized by using two length scales for the transport of heat in SOL. In this work, we challenge the current interpretation of these two length scales by studying the impact of divertor geometry modifications on the heat exhaust. In particular, a significant broadening of the heat flux profiles at the outer divertor target is diagnosed while increasing the length of the outer divertor leg. Modelling efforts showed that diffusive simulations well reproduce the experimental heat flux profiles for short-legged plasmas. Conversely, the broadening of the heat flux for a long divertor leg is reproduced by a turbulent model, highlighting the importance of turbulent transport not only in the main SOL but also in the divertor. These results question the current interpretation of the heat flux width as a purely main SOL transport length scale. In fact, long divertor leg magnetic configurations highlighted the importance of asymmetric divertor transport. We therefore conclude that main SOL and divertor SOL transport cannot be arbitrarily disentangled and we underline the importance of the divertor magnetic geometry in enhancing asymmetric turbulent transport with the potential benefit of an unexpected power spreading.

One of the most demanding engineering issues in Inertial Fusion Energy (IFE) reactors is the design of a reaction chamber that can withstand the intense photons, neutrons and charged particles due to the fusion event. Rapid pulsed deposition of energy within thin surface layers of the fusion reactor components such as the first wall may cause severe surface erosion due to ablation. One particularly innovative concept for the protection of IFE reactor cavity first walls from the direct energy deposition associated with soft X-rays and target debris is the thin liquid film protection scheme. In this concept, a thin film of molten liquid lead is fed through a silicon carbide first wall to protect it from the incident irradiations. Numerous studies have been reported in the literature on the thermal response of the liquid film to the intermittent photon and ion irradiations, as well as on the fluid dynamics and stability of liquid films on vertical and upward-facing inclined surfaces. However, no investigation has heretofore been reported on the stability of thin liquid films on downward-facing solid surfaces with liquid injection through (i.e. normal to the surface of) the bounding wall. This flow models the injection of molten liquid lead over the upper end cap of the reactor chamber. The hydrodynamics of this flow can be interpreted as a variation of the Rayleigh-Taylor instability due to the effect of the bounding wall which is continuously fed with the heavier fluid. In order to gain additional insight into the thin liquid film protection scheme, experiments have been conducted to investigate the critical issues associated with this concept. To this end, an experimental test facility has been designed and constructed to simulate the hydrodynamics of thin liquid films injected normal to the surface of and through downward-facing flat walls. In this doctoral thesis, the effect of different design parameters (film thickness, liquid injection velocity, liquid properties and inclination angle) on liquid film stability has been examined. The results address the morphology of the film free surface, the frequency of droplet formation and detachment, the size and penetration depth of the detached droplets, and the interface wave number. These experimental data have been used to validate a novel mechanistic numerical code based on a level contour reconstruction front tracking method over a wide range of parameters. The results of this investigation will allow designers of IFE power plants to identify appropriate windows for successful operation of the thin liquid film protection concept for different coolants.

The A15-based Nb3Sn and V3Ga superconducting compounds are an integral part of synchrotrons and magnetic fusion reactor technology, especially where a magnetic field higher than 10 T is required, which lies beyond the limit of conventional Nb-Ti superconductors (~8 T). These brittle intermetallic compounds are difficult to manufacture in the form of wires, required for the application purpose, using the traditional wire-drawing process. Hence, bronze technique is adopted to fabricate such filamentary wires. This is based on the solid-state diffusion where A3B compound (A=Nb or V, B=Sn or Ga) forms during the interaction of Cu(B) and A. The operation of pure superconducting wires gets restricted to the field of 12 T, however, the ever-increasing demands for an improved efficiency have promoted the development of these A15 wires with the addition of alloying elements such as Ti and Zr. Many important physical and mechanical properties of such wires depend on the growth behaviour of these compounds. Therefore, understanding the growth of such compounds necessitates an in-depth analysis on diffusion behaviour of various elements in both bronze-based solid solutions as well as A15-intermetallics. Estimation of diffusion parameters makes use of the most commonly used diffusion couple technique. There are mainly three methods available for the estimation of the interdiffusion coefficients, proposed by Matano-Boltzmann (MB), Den Broeder (dB), same as Sauer-Freise (SF) and Wagner. Among these three, MB treatment is known to be the least accurate method, especially when there is a deviation of molar volume in a system from the ideality. At the same time molar volume might affect the estimation process differently for dB and Wagner’s approach. MB method is still being used neglecting the actual molar volume variation. On the other hand, the implementation of dB or Wagner’s approach for the estimation remains to be random. For the first time, we have critically examined the role of molar volume on estimated diffusion parameters and indicated the more accurate approach. Similar analysis for the estimation of the intrinsic diffusion coefficient is conducted considering Heumann and van Loo’s methods. Furthermore, the discussion is extended to the estimations of various diffusion parameters considering the measured composition profile in the V-Ga system. A detailed diffusion study has been conducted on Cu(Ga) and Cu(Sn) solid solutions to examine the role of the vacancy wind effect on interdiffusion. The interdiffusion, intrinsic and impurity diffusion coefficients are determined to facilitate the discussion. It is found that Ga and Sn are the faster diffusing species in the respective systems. The trend of the interdiffusion coefficients is explained with the help of the driving force. Following that, the tracer diffusion coefficients of the species are calculated with and without consideration of the vacancy wind effect. We found that the role of the vacancy wind is negligible on the minor element in a dilute solid solution, which is the faster diffusing species in this system and controls the interdiffusion process. However, consideration of this effect is important to understand the diffusion rate of the major element, which is the slower diffusing species in this system. Major drawback of studying diffusion in multi-component systems is the lack of suitable techniques to estimate the diffusion parameters. In this study, a generalized treatment to determine the intrinsic diffusion coefficients in multi-component systems is developed utilizing the concept of pseudo-binary approach. This is explained with the help of experimentally developed diffusion profile in the Cu(Sn, Ga) solid solution. Based on an interdiffusion study using an incremental diffusion couple in the V-Ga binary system, we have shown that V diffuses via lattice, whereas Ga does so via grain boundaries for the growth of the V3Ga phase. We could estimate the contributions from two different mechanisms, which are, usually, difficult to delineate in an interdiffusion study. Available tracer diffusion studies and the atomic arrangement in the crystal structure have been considered for a discussion on the diffusion mechanisms. Diffusion–controlled growth rate of V3Ga at the Cu(Ga)/V changes dramatically because of a small change in Ga content in Cu(Ga). One atomic percent increase in Ga leads to more than double the product phase layer thickness and a significant decrease in activation energy. Kirkendall marker experiment indicates that V3Ga grows because of diffusion of Ga. Role of different factors influencing the diffusion rate of Ga and high growth rate of V3Ga are discussed. The growth of Nb3Sn by bronze technique on two different single crystals and deformed Nb is studied. The grain boundary diffusion-controlled growth rate is found to be different for each of these three specimens. The difference is explained on the basis of the grain size of Nb3Sn. Elemental additions such as Ti and Zr to either bronze or metal are found to improve the superconducting properties. We have examined their effects on the growth rates of A15-phase formed in Cu(B,x)/A and Cu(B)/(A,x), where x is Ti or Zr. In either cases Ti and Zr-additions result in an improved growth rate of the product phase and reduces activation energy with increase in alloying addition; however few precipitates are formed in the interdiffusion zone for Cu(B,x)/A. Wavelength dispersive spectrometry (WDS)-mapping reveals these to be x-rich. Scanning transmission electron microscopy (STEM)-analysis suggests having composition gradient inside a single precipitate. TEM-diffraction demonstrates these to be Ti(A) solid solution crystallizing as BCC-structure for Cu(B,Ti)/A. These are located on grain boundaries of A15-phase. Electron back-scattered diffraction (EBSD)-analysis demonstrates grain morphology of product phase and found the average grain size to exhibit a decreasing trend with increasing x content. Columnar grains, on Ti and Zr addition tend to form as equiaxed ones. Based on the morphology and grain size pattern, the role of grain boundary diffusion is speculated to have a dominant effect with increase in elemental additions. The texture evolution of the product phase is also investigated and found the product phase to grow as a strongly textured one with the elemental additions. A peculiar pattern is observed for the texture of the product phase and its adjacent A or A(x) grains.

Dans ce travail, nous étudions la stabilité thermique et les effets des irradiations par un plasma d'hélium ou de deutérium de films minces de WO3 d’intérêt pour la fusion magnétique (projet ITER). L’objectif est de comprendre comment une oxydation du divertor modifie les interactions plasma paroi. Pour cela, nous avons synthétisé des films de WO3 par oxydation thermique de substrats de W à 400°C et caractérisé les effets du type de substrat, de la pression d’oxygène et du temps d’oxydation sur la structure et sur l’épaisseur des oxydes formés. La structure (monoclinique nanocristalline), la morphologie et les défauts des échantillons ont été analysés avant et après traitement, à différentes échelles, en utilisant la microscopie électronique, la microscopie Raman, la diffraction de rayons X, et la microscopie à force atomique.Le chauffage sous vide (400 - 800°C) a conduit à la formation de WO2. Le bombardement aux ions D+ (11 eV) a mené à une diffusion profonde du deutérium à travers le film d’oxyde, engendrant un effet électrochimique, observé ici pour la première fois sous irradiation plasma. Cet effet, réversible, est associé à la formation de bronzes de tungstène (DxWO3) et à une transition de phase vers une structure hexagonale. Des bombardements aux ions He+ (20 eV) ont été réalisés afin de dissocier les effets physiques et chimiques. A température ambiante, le bombardement a causé peu de changements morphologiques et structuraux. Par contre, le autre bombardement à 400°C a causé une érosion du film d’oxyde accompagnée d’un changement de couleur, une amorphisation en surface et la formation de bulles à l’interface W / WO3. / As part of laboratory studies devoted to magnetic fusion we have investigated the thermal stability and the effects of helium and deuterium plasma irradiation on tungsten oxide thin films. The objective is to predict the consequences of the oxidation of the W plasma facing component (divertor) for plasma wall interactions.To this aim, we have synthesized WO3 films by thermal oxidation of W substrates at 400°C and we have characterized the effects of the W substrate, the oxygen pressure and the oxidation duration on the structure and the thickness of the oxide films. The sample crystalline structure (monoclinic nanocrystalline), defects and morphologies were characterized before and after treatment using scanning and transmission electron microscopies, Raman microscopy, X-Ray diffraction and atomic force microscopy. Heating under vacuum up to 800°C leads to changes in the film structure and composition which results in the formation of WO2. D+ bombardment (11 eV) leads to D+ diffusion throughout the oxide film and to an electrochromic effect, here observed for the first time under plasma irradiation. This effect - which turned out to be reversible - is related to the formation of W bronzes (DxWO3) and to a phase transition of the oxide toward a hexagonal structure. Helium bombardments (20 eV) have then been performed to unravel physical and chemical processes at play. He+ bombardment at room temperature causes slight structural and morphological changes. On the contrary, He+ bombardment at 400°C leads to a significant erosion of the oxide film, accompanied by a colour change, the surface amorphisation and the formation of bubbles at the W / WO3 interface.

Ces travaux abordent la problématique des mesures de sonde de Langmuir dans un environnement RF. Les mesures expérimentales ont montré que des courants DC négatifs (électroniques) étaient collectés sur la structure d'une antenne ICRF sous tension, pendant que des courants DC positifs (ioniques) sont recueillis par une sonde de Langmuir à l'autre bout du tube de flux magnétique ouvert connecté à l'antenne, la sonde étant au potentiel de la machine. Un modèle de tube de flux asymétrique, de type de sonde double, est présenté. Celui-ci modélise un plasma, confiné le long des lignes de champ magnétique, ayant à chaque extrémité une électrode dont l'une est polarisée à un potentiel RF et l'autre à la masse. L'électrode polarisée modélise le potentiel RF résultant de l'intégration, le long d'une ligne champ magnétique, du champ électrique rayonné par les straps d'une antenne ICRF, tandis que l'autre électrode modélise la sonde au potentiel de la machine. Ce modèle permet d'expliquer l'apparition de courants DC en émettant simplement l'hypothèse qu'il faut à la fois une asymétrie de la source RF par rapport à une masse fixe, une conductivité RF transverse non nulle autorisant des courants RF transverses ainsi qu'une caractéristique courant-tension non linéaire due aux gaines pour favoriser des courants négatifs du côté RF et des courants positifs côté sonde. Ce modèle permet également de modéliser les caractéristiques Courant DC - Tension DC d'une sonde en présence de RF et ainsi d'évaluer les propriétés du plasma. Dans ce cas l'électrode modélisant la sonde n'est plus à la masse, mais à un potentiel donné. Des résultats analytiques sont trouvés dans certaines limites / This work investigates the problematic of probe measurements in RF environment. DC currents flowing along magnetic field lines connected to powered ICRF antennas have been observed experimentally. Negative (i.e. net electron) current is collected on the powered ICRF antenna structure, while positive (i.e. net ion) current is collected by magnetically connected Langmuir probes. An asymmetric model based upon a double probe configuration was developed. The ICRF near field effect is mimicked by a ?driven? RF electrode at one extremity of an "active" open magnetic flux tube, where a purely sinusoidal potential is imposed. The other connection point is maintained at ground potential to model a collecting probe. This "active" flux tube can exchange transverse RF currents with surrounding "passive" tubes, whose extremities are grounded. With simple assumptions, an analytical solution is obtained. We can thus explain how DC currents are produced from RF sheaths. This model also makes it possible to model the characteristics DC Current' DC Voltage of a probe in the presence of RF and thus to evaluate some plasma properties. In this case the electrode at ground potential (probe) is polarized at a given potential. Analytical results are found within certain limits

Divertors are in-vessel, plasma-facing, components in magnetic-confinement fusion reactors. Their main function is to remove the fusion reaction ash (α-particles), unburned fuel, and eroded particles from the reactor, which adversely affect the quality of the plasma. A significant fraction (~15 %) of the total fusion thermal power is removed by the divertor coolant and must, therefore, be recovered at elevated temperature in order to enhance the overall thermal efficiency. Helium is the leading coolant because of its high thermal conductivity, material compatibility, and suitability as a working fluid for power conversion systems using a closed high temperature Brayton cycle. Peak surface heat fluxes on the order of 10 MW/m^2 are anticipated with surface temperatures in the region of 1,200°C to 1,500°C. Recently, several helium-cooled divertor designs have been proposed, including a modular T-tube design and a modular finger configuration with jet impingement cooling from perforated end caps. Design calculations performed using the FLUENT® CFD software package have shown that these designs can accommodate a peak heat load of 10 MW/m^2. Extremely high heat transfer coefficients (~50,000 W/(m^2 K)) were predicted by these calculations. Since these values of heat transfer coefficient are considered to be outside of the experience base for gas-cooled systems, an experimental investigation has been undertaken to validate the results of the numerical simulations. Attention has been focused on the thermal performance of the T-tube and the finger divertor designs. Experimental and numerical investigations have been performed to support both divertor geometries. Excellent agreement has been obtained between the experimental data and model predictions, thereby confirming the predicted performance of the leading helium-cooled divertor designs for near- and long-term magnetic fusion reactor designs. The results of this investigation provide confidence in the ability of state-of-the-art CFD codes to model gas-cooled high heat flux plasma-facing components such as divertors.