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This thesis describes a research investigation into novel designs of high voltage pulse transformers using magnetic insulation, which is the only practicable form of insulation for much of the equipment presently used in ultrahigh voltage pulsed-power work, including transmission lines and plasma opening switches. Although its use in transformers would bring important advantages in both size and weight reductions, a number of seemingly insurmountable problems have however so far prevented this. Two novel arrangements are presented in this thesis: one of these is a 500 kV transformer with self-magnetic insulation, and the other one is a 1 MV 'Tesla' transformer with external magnetic insulation. It is shown that both of these overcome the problems inherent in earlier designs and also offer considerable scope for further development in a number of important areas. It is believed that they represent the first working examples of magnetically-insulated transformers anywhere in the world. Modelling considerations of the transformers developed include both theoretical models and predicted characteristics. The filamentary technique used to describe mathematically the arrangements being investigated involves decomposition of the main conducting components into filamentary elements. The resulting equivalent electrical network includes all the mutual interactions that exist between the different filamentary elements, takes magnetic diffusion fully into account and enables the resistances and self and mutual inductances that are effective under fast transient conditions to be calculated. Theoretical results provided by the resulting mathematical models have been successfully validated by comparison with reliable experimental data. Much of the work detailed in the thesis has already been presented in high quality academic journals and at prestigious international conferences, and a solid theoretical and experimental basis has been laid down for future development and new progress into pulsed power system research.

This thesis is devoted to the detectability of an inhomogeneous region possibly embedded in a noisy environment. It presents models and algorithms using the theory of the longest significant run and percolation. We analyze the computational results based on simulation. We consider the length of the significant nodes in a chain with good continuation in a square lattice of independent nodes. Inspired by the percolation theory, we first analyze the problem in a tree based model. We give the critical probability and find the decay rate of the probability of having a significant run with length k starting at the origin. We find that the asymptotic rate of the length of the significant run can be powerfully applied in the area of image detection. Examples are detection of filamentary structures in a background of uniform random points and target tracking problems. We set the threshold for the rejection region in these problems so that the false positives diminish quickly as we have more samples. Inspired by the convex set detection, we also give a fast and near optimal algorithm to detect a possibly inhomogeneous chain with good continuation in an image of pixels with white noise. We analyze the length of the longest significant chain after thresholding each pixel and consider the statistics over all significant chains. Such a strategy significantly reduces the complexity of the algorithm. The false positives are eliminated as the number of pixels increases. This extends the existing detection method related to the detection of inhomogeneous line segment in the literature.

Evaporated thin film sandwich structures of Au-SiOX-Au have been studied. These normally show electroforming effects and subsequently electron emission, electroluminescence, negative resistance and thermal-voltage memory effects. Previous work in the field is critically reviewed. It was shown that the time dependence of the device current and emission current can be explained by making certain modifications to the filamentary conduction theory of Dearnaley. Detailed direct current-voltage measurements have revealed the existence of two different types of breakdown behaviour. At voltages less than 20 V single-hole breakdowns were observed, while in the voltage range 20-30 V large scale irreversible breakdown behaviour took place. The dependence of the voltage at which this occurs (Vß) on insulator thickness and temperature, together with measurements of the device temperature at breakdown and visual evidence of damage after breakdown, has led to the conclusion that this type of breakdown is a thermal effect. Such measurements also pointed to the existence of a high field region within the insulator, and potential distribution measurements confirmed this hypothesis. The high field region was also in evidence at low temperatures where the device current (Ic) showed a log Ic α Vb1/2 dependence on applied voltage (Vb). Measurements of electron attenuation lengths in SiOx gave values of 400-1000 Å irrespective of temperature. The temperature independence was consistent with the emitted electron energy distributions at 77 and 300 K. It was shown that electrons underwent Bragg diffraction through the top Au electrode. The angular distribution of emitted electrons became more isotropic with increasing voltage. Measurements on other systems showed that Al-SiOx/B2O3-Al devices could withstand very high voltages and give improved emission efficiency, while Au-CaBr2-Au and Au-Si3N4-Au devices showed very high initial currents and current-voltage characteristics which were irreversible.

Le dioxyde de carbone, principal gaz à effet de serre lié aux activités humaines, est considéré comme l’un des gaz les plus problématiques pour notre environnement ces dernières années, principalement à cause du réchauffement climatique qu’il engendre. C’est pour cette raison que l’augmentation de sa teneur dans l’atmosphère nous concerne tous quant aux conséquences futures pour notre planète. Afin de limiter l’émission de CO2, sa conversion en composés à valeur ajoutée présente un grand intérêt et est possible notamment via des procédés plasmas. Plus particulièrement, les décharges à barrière diélectrique (DBD) sont utilisées depuis quelques années pour générer des plasmas froids opérant à pression atmosphérique, principalement pour des applications en traitement de surface, mais également pour le traitement d’effluents gazeux.Lors de cette thèse, nous nous sommes focalisés sur le processus de dissociation du CO2 en CO et O2 via un réacteur DBD à flux continu et avons analysé sa conversion et son efficacité énergétique via différentes études. Celles-ci ont été réalisées grâce à plusieurs méthodes de diagnostic, comme par exemple la spectrométrie de masse utilisée pour déterminer la conversion et l’efficacité du processus, la spectroscopie d’émission optique, l’oscilloscope pour une caractérisation électrique, etc. afin d’avoir une meilleure compréhension du comportement des décharges CO2.Dans un premier temps, nous avons réalisé une étude détaillée d’un plasma CO2 pur où nous avons fait varier différents paramètres, tels que le temps de résidence, la fréquence, la puissance, la pulsation de la haute tension et l’épaisseur et la nature du diélectrique. Le CO2 donne lieu généralement à une décharge filamentaire, consistant en de nombreuses microdécharges réparties au niveau de la zone du plasma. Celles-ci constituent la principale source de réactivité dans une DBD. Un aperçu détaillé de l’aspect physique de ces microdécharges a été réalisé grâce à la caractérisation électrique, permettant de mieux comprendre les propriétés électriques de la décharge et des microdécharges. En effet, nous avons pu déterminer l’importance de la tension présente au niveau du plasma, de l’intensité du courant plasma, du nombre de microdécharges et de leur temps de vie sur l’efficacité du processus de dissociation de CO2.Ensuite, nous avons conclu ce travail avec des études combinant le CO2 en phase plasma avec de l’eau ou du méthane afin de produire des molécules à valeur ajoutée telles que les syngas (CO et H2), mais aussi des hydrocarbures (C2H6, C2H4, C2H2 et CH2O) dans le cas de l’ajout du méthane. A travers ces études, nous avons obtenu une meilleure connaissance de la chimie et de la physique qui ont lieu dans ce type de plasma. / Carbon dioxide appears as one of the most problematic gases for the environment, mostly because it is responsible for global warming. This is why its increasing concentration into the atmosphere, mainly due to anthropogenic activities, is a real concern for planet Earth. In order to prevent the release of large amounts of CO2, its conversion into value-added products is of great interest. In this context, plasma-based treatments using dielectric barrier discharges (DBDs) are nowadays more and more used for the conversion of this gas. In this thesis, we investigated the CO2 splitting process into CO and O2 via a flowing cylindrical DBD and we studied its conversion and energy efficiency by means of several diagnostic methods, such as mass spectrometry to determine the conversion and energy efficiency of the process, optical emission spectroscopy for gas temperature measurements, and an oscilloscope for electrical characterization, in order to obtain a better understanding of the CO2 discharge itself.First, we focused on an extensive experimental study of a pure CO2 plasma where different parameters were varied, such as the gas residence time, the operating frequency, the applied power, the pulsation of the AC signal, the thickness and the nature of the dielectric. CO2 discharges typically exhibit a filamentary behavior, consisting of many microdischarges, which act as the main source of reactivity in a DBD. A detailed insight in the physical aspects was achieved by means of an in-depth electrical characterization, allowing more insight in the electrical properties of the discharge and more specifically in the microdischarges, which are spread out throughout the active zone of the plasma. It was found throughout this work that the plasma voltage, which reflects the electric field and thus determines how the charged particles are accelerated, the plasma current, which reflects the electron density, but also the number of microdischarges and their average lifetime, play an important role in the efficiency of the CO2 dissociation process. It was revealed that the microdischarge number is important as it represents the repartition of the locations of reactivity. Indeed, as the microfilaments are more spread out in the same discharge volume, the probability for the CO2 molecules to pass through the reactor and interact with at least one microdischarge filament becomes more important at a larger number of microfilaments.The second part of the thesis was dedicated to discharges combining CO2 and H2O or CH4, both being hydrogen source molecules. The combined CO2/H2O or CO2/CH4 conversion allows forming value-added products like syngas (CO and H2), but also hydrocarbons (C2H6, C2H4, C2H2 and CH2O), at least in the presence of methane. Throughout this study, we tried to obtain a better knowledge of the chemistry and physic behind these conversion processes. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

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Les supraconducteurs à hautes températures sont considérablement utilisés dans des applications telles que des machines électriques. La conception de telles machines requiert une évaluation précise des pertes AC générées par leurs bobinages de câbles supraconducteurs multifilamentaires torsadés. Ainsi, des outils numériques robustes et précis sont indispensables afin de modéliser rapidement en 3D le comportement de tels câbles. Dans ce manuscrit, deux approches numériques dont une approche de type galerkin discontinue furent développées afin de simplifier considérablement la modélisation 3D de ce type de câbles. Elles furent appliquées à la formulation en H couplée aux effets thermiques. La modélisation de cas simples a permis de valider ces approches en les comparant à celle des éléments finis implémentée sur Comsol. Puis, une transformation géométrique fut proposée afin de modéliser de façon équivalente en 3D, quelque soit la configuration en champ magnétique, des câbles à filaments supraconducteurs droits à la place de filaments torsadés. De cette transformation, un modèle d'ordre réduit fut développé et validé afin de simplifier considérablement la modélisation 3D de ce type de câbles. À partir de ce modèle réduit, l'influence du champ elliptique sur les pertes par aimantation d'un échantillon de 100 câbles de 54 filaments de MgB ₂ torsadés fut modélisé en moins d'une journée. Par ailleurs, des premières études empiriques montrant la faisabilité d'une homogénéisation furent brièvement présentées et validées pour des câbles ayant une seule couche de filament. / High temperature superconductors are increasingly being used for several applications such as electrical machines. Thus the design of suchdevices ultimately requires an accurate evaluation of AC losses generated by superconducting coils sometimes made of twisted multifilamentary wires.The development of robust numerical tools geared towards the 3 D modeling of such wires is therefore needed. In this manuscript, the main objective isto develop numerical approaches allowing considerable simplifications of the modelling of twisted multi-filamentary superconducting wires in 3 D. First, two numerical approaches such as the discontinuous galerkin method applied on the thermally coupled H -formulation were developed. They were compared and validated on simple cases using the finite element method implemented in Com-sol. Then, a well-defined mapping was proposed to simply model in 3 D, for any magnetic field configuration, straight multifilamentary wires instead oftwisted ones. An application of this mapping allowed the development of a model-order reduction approach which simplify considerably the mod-elling of twisted wires in 3 D. Thus, the elliptical magnetic fields impact on magnetization losses was accurately modeled in less than a day with the reduced approach applied on 100 MgB ₂ wires with 54 twisted filaments. As a research perspective on the reduced model advanced applications, an homogenization, empirically defined, was briefly presented for wires with single layer only.

La filamentation laser femtoseconde dans l'air est un phénomène qui implique une riche famille d'effets optiques non linéaires. Effet laser de filaments a émergé comme un phénomène nouveau en 2011. Il a été activement étudiée au cours des dernières cinq années, non seulement en raison de ses applications potentielles dans les techniques de télédétection mais aussi la physique pour découvrir. Cette thèse est consacrée principalment à l'étude de deux types d'effets laser à partir du plasma filamentaire généré par 800 nm impulsions laser femtosecondes dans l'air ou l'azote pur. Le premier est l'émission spontanée amplifiée à 337 nm longueur d'ondes de molécules d'azote neutre qui est bidirectionnel, activée uniquement par des impulsions polarisées circulairement. Le mécanisme d'inversion de population est attribuée à électrons-molécules collisions inélastiques entre les électrons énergiques et les molécules d'azote neutres sur l'état du sol. La caractérisation complète de 337 nm impulsion laser vers l'avant et vers l'arrière est réalisée. En particulier, les mesures de profil temporelles sont comparées à des simulations numériques basées sur l'équation de Maxwell-Bloch à une dimension, qui se révèle être en bon accord. Un autre type d'effet laser est lié à ions d'azotes excités, émettant à 391 nm et 428 nm longueurs d'onde. Ce type d'effet laser est observée avec laser pompe polarisée linéairement. Il est caractérisé systématiquement dans des domaines spatiaux, temporels et spectrales. Les résultats du profil temporel prouve que l'émission laser ionique est fondamentalement superradiance. Un nouveau mécanisme, à savoir le processus d'excitation recollision d'électrons, est proposé pour la réalisation de la distribution de la population dans le niveau supérieur de la transition. Il est soutenu par deux mesures expérimentales consistant en la dépendance de 391 nm émission laser à l'ellipticité et la dépendance à longueur d'onde de laser pompe. Des simulations numériques donnent un bon accord avec l'observation expérimentale. / Femtosecond laser filamentation in air is a phenomenon that involves a rich family of nonlinear optic effects. Lasing effect from filaments has emerged as a new phenomenon in 2011. It has been actively studied in recent 5 years not only because of its potential applications in remote sensing techniques but also the fruitful physics involved. This thesis is devoted to the study of two types of lasing effect from filament plasma generated by 800 nm femtosecond laser pulses in air or pure nitrogen. The first is the bidirectional amplified spontaneous emission at 337 nm wavelength of neutral nitrogen molecules, only enabled by circularly-polarized pulses. The population inversion mechanism is attributed to inelastic electron-molecule collisions between energetic electrons and neutral nitrogen molecules on the ground state. Full characterization of both forward and backward 337 nm lasing pulse is conducted. Particularly the temporal profile measurements is compared to numerical simulations based on one-dimensional Maxwell-Bloch equation, which turns out to be in good agreement. Another type of lasing effect is related to excited nitrogen ion, emitting at 391 nm and 428 nm wavelengths. This type of lasing effect can only be observed with linearly-polarized pump laser. It is systematically characterized in spatial, temporal and spectral domains. The temporal profile results proves that ionic lasing emission is fundamentally superradiance. A new mechanism, namely the electron recollision excitation process, is proposed for the achievement of population distribution in the upper level of transition. It is supported by two experimental measurements consisting of pump ellipticity dependence and pump wavelength dependence of 391 nm lasing intensity. Numerical simulations give good agreement with the experimental observation.

Les dispositifs DBD se répandent dans un grand nombre d’applications industrielles. Utilisés depuis plus de 150 ans pour la production d’ozone afin de décontaminer l’eau à grande échelle, ils ont depuis la fin du XXème siècle investi les domaines du traitement de surface polymère, du dépôt de couche mince sur substrat et de l’émission lumineuse pour la décontamination ainsi que la médecine. Ces dispositifs sont mis en oeuvre avec un générateur électrique dont les caractéristiques impactent fortement la qualité de la décharge. Ce travail s’inscrit en partie dans le cadre du développement d’une application de traitement de surface à pression atmosphérique. Il aborde la problématique de l’augmentation de la vitesse de dépôt de couche mince au travers des paramètres de l’alimentation électrique. Plus précisément, ce travail s’intéresse aux apports d’une alimentation en courant rectangulaire et aborde également les problématiques liées à la conception et à la fabrication de ce convertisseur. En particulier, une grande attention est portée sur l’étude du transformateur élévateur, car au travers de ses éléments parasites capacitifs, ce dernier peut limiter le transfert de puissance entre la source électrique et le dispositif DBD. Un deuxième aspect de cette étude consiste à entrevoir l’intérêt que revêtent deux convertisseurs statiques dédiés à l’alimentation de dispositifs DBD. Le premier consiste en une alimentation résonante en régime de conduction discontinue dont la particularité est de posséder trois degrés de liberté (fréquence, tension d’entrée et largeur d’impulsion), ce qui lui confère un intérêt exploratoire. Le second convertisseur consiste en une alimentation résonante haute tension et haute fréquence permettant l’éviction du transformateur élévateur, et mettant en oeuvre des interrupteurs au nitrure de gallium (GaN) afin d’atteindre une fréquence de fonctionnement supérieure au mega-Hertz avec un faible niveau de pertes. / DBD devices are widely used in industrial applications. 150 years ago, they were only employed in ozoners for water decontamination. In recent decades, the progress of knowledge and technology allowed to use them in many other applications like surface treatment, medical applications and light emission. Actually, these devices are supplied with an electrical source which parameters can strongly impact the discharge behaviour. An important part of this work comes within the framework of the development of an atmospheric pressure surface treatment involving DBDs. The issue of the influence of the generator's electrical parameters on the treatment speed is discussed. In particular, this work focuses on the merits of a rectangular shaped current source concerning the behavior of an atmospheric pressure discharge in nitrogen ; the problems related to the design and the fabrication of such a converter are highlighted. The design of the high voltage transformer is then described in detail since its lumped elements play an important role as they can strongly limit the power transfer between the electrical source and the DBD device. A second aspect of this work is to establish the interests of two particular power converters. The first one is a resonant converter operating in a discontinuous conduction mode ; its merits is to exhibit three degrees of freedom (input voltage, frequency, current pulse width) instead of two, which is a tremendous asset for exploring purposes. The second one is a high-frequency resonant converter where a resonant inductance and the DBD device structural capacitances are used instead of a high voltage transformer to perform the voltage amplification, which circumvents the issue related to the transformer parasitic elements. This converter is based on GaN HEMT switches in order to reach a low semiconductor losses level and a fairly high operating frequency (above the mega-Hertz).

<p>A small plasma generation system was first developed using a ring-pin electrode configuration with the goal of producing a plasma disk at the burner outlet. Two distinct plasma regimes were identified: diffused and filamentary. Diffuse discharges were generated at low frequencies while filamentary discharges were generated at moderate to high frequencies. The induced flow fields generated by both diffuse and filamentary plasma discharges were investigated using high-speed schlieren visualization and particle image velocimetry. The rise in gas temperature was measured using optical emission spectroscopy. Lastly, the electrical properties for both types of plasma discharges was measured. The measurements provided a set of pulse parameters for the investigation of the plasma-flame interaction on the atmospheric pressure burner. </p> An atmospheric pressure plasma-assisted burner with a ring-pin electrode geometry was designed and fabricated to investigate the effect of nanosecond repetitively pulsed discharges on methane-air flames. The burner can produce both Bunsen-type and swirl-stabilized flames (helical vane swirlers, swirl number of 0.62) with a modular design to allow for a removable block swirler component. Flame chemiluminescence and direct imaging of flame structure and dynamics was done to understand the burner’s operating limits. The burner can operate 6 – 13 kW flames, with flames stabilizing at approximately 2 inches above the burner exit. The effect of air flow rate on plasma formation was investigated and it was found that the high velocity of the incoming gas changes the plasma regime and electrical properties. Finally, the plasma discharge was applied on lifted, swirled flames and used for plasma-assisted ignition. For lifted swirled flames, we found that a minimum of 100 pulses is required to generate a filamentary discharge in the air stream. Higher number of pulses at high frequencies appeared to extinguish the primary flame. A minimum of 6000 was used for ignition. The plasma-assisted burner will allow for future studies to investigate the plasma flame coupling for various conditions using a wide variety of diagnostics. <br>