With the transition towards renewable energy, and the deregulation of the electricity market, generation patterns and grid topology are changing. These changes increase the need for transfer capacity. One limiting factor, which sometimes leads to underutilization of the transmission grid, is interarea oscillations. These system-wide modes involve groups of generators oscillating relative to each other and are sometimes hard to control due to their scale and complexity. In this thesis we investigate how high-voltage direct current (HVDC) transmission can be used to attenuate interarea oscillations. The thesis has two main contributions. In the first contribution we show how the stability of two asynchronous grids can be improved by modulating the active power of a single interconnecting HVDC link. One concern with modulating HVDC active power is that the interaction between interarea modes of the two grids may have a negative impact on system stability. By studying the controllability Gramian, we show that it is always possible to improve the damping in both grids as long as the frequencies of their interarea modes are not too close. For simplified models, it is explicitly shown how the controllability, and therefore the achievable damping improvements, deteriorates as the frequency difference becomes small. The second contribution of the thesis is to show how coordinated control of two (or more) links can be used to avoid interaction between troublesome interarea modes. We investigate the performance of some multivariable control designs. In particular we look at input usage as well as robustness to measurement, communication, and actuator failures. Suitable controllers are thereby characterized. / Övergången till förnybar energi och avregleringen av elmarknaden leder till förändrade produktions-och överföringsmönster. Dessa förändringar medför behov av en ökad överföringskapacitet. En begränsande faktor, som kan leda till ett underutnyttjande av stamnätet, är interareapendlingar. Dessa systemövergripande pendlingar involverar grupper av generatorer som svänger i förhållande till varandra. Interareapendlingar är ibland svåra att styra på grund av deras skala och komplexitet. I denna avhandling undersöker vi hur förbindelser med högspänd likström, engleska high-voltage direct current (HVDC), kan användas för att dämpa interareapendlingar. Avhandlingen har två huvudbidrag. I det första bidraget visar vi hur stabiliteten hos två olika synkrona nät kan förbättras genom att modulera den aktiva effekten hos en enda HVDC-länk. Ett bekymmer med aktiv effektmodulering är att växelverkan mellan interareapendlingar hos de två näten kan ha en negativ inverkan på systemets stabilitet. Genom att studera styrbarhetsgramianen visar vi att det alltid är möjligt att förbättra dämpningen i båda näten så länge som frekvenserna hos deras interareapendlingar inte ligger för nära varandra. För förenklade modeller visas det uttryckligen hur styrbarheten och därmed de möjliga dämpningsförbättringarna, försämras då frekvensskillnaden blir liten. Avhandlings andra bidrag visar hur koordinerad styrning av två (eller fler) länkar kan användas för att undvika växelverkan mellan besvärliga interareapendlingar. Vi undersöker prestandan hos olika typer av flervariabla regulatorer. I synnerhet undersökers styrsignalsanvändning samt robusthet mot mät-, kommunikations- och aktuatorfel. Därigenom karakteriseras lämpliga regulatortyper. / <p>QC 20190308</p>

Quantum information technologies include secure quantum communications and ultra precise quantum sensing that are significantly more efficient than their classical counterparts. To enable such technologies, we need a scalable quantum platform in which qubits are con trollable. Color centers provide controllable optically-active spin qubits within the coherence time limit. Moreover, the nearby nuclear spins have long coherence times suitable for quantum memories. In this thesis, I present a theoretical understanding of and control protocols for various color centers. Using group theory, I explore the wave functions and laser pumping-induced dynamics of VSi color centers in silicon carbide. I also provide dynamical decoupling-based high-fidelity control of nuclear spins around the color center. I also present a control technique that combines holonomic control and dynamically corrected control to tolerate simultaneous errors from various sources. The work described here includes a theoretical understanding and control techniques of color center spin qubits and nuclear spin quantum memories, as well as a new platform-independent control formalism towards robust qubit control. / Doctor of Philosophy / Quantum information technologies promise to offer efficient computations of certain algorithms and secure communications beyond the reach of their classical counterparts. To achieve such technologies, we must find a suitable quantum platform to manipulate the quantum information units (qubits). Color centers host spin qubits that can enable such technologies. However, it is challenging due to our incomplete understanding of their physical properties and, more importantly, the controllability and scalability of such spin qubits. In this thesis, I present a theoretical understanding of and control protocols for various color centers. By using group theory that describes the symmetry of color centers, I give a phenomenological model of spin qubit dynamics under optical control of VSi color centers in silicon carbide. I also provide an improved technique for controlling nuclear spin qubits with higher precision. Moreover, I propose a new qubit control technique that combines two methods - holonomic control and dynamical corrected control - to provide further robust qubit control in the presence of multiple noise sources. The works in this thesis provide knowledge of color center spin qubits and concrete control methods towards quantum information technologies with color center spin qubits.

The DC-side floating capacitors in the floating power modules of power converters are subject to high voltage fluctuation, due to the presence of reactive harmonic components. Utilizing passive capacitors, as done in traditional methods, helps reduce the DC-bus voltage ripple but makes the system bulky. An active capacitor can be integrated with the floating H-bridge power modules to remove the effect of the ripple powers on the DC bus. The auxiliary circuit, which is much smaller in size compared to an equivalent passive capacitor, helps increase the power density of the system. This work focuses on the analysis of power components, and the extension of the active capacitor to the Perturbation Injection Unit (PIU), in which the DC side is highly distorted by multiple harmonic components. A control scheme is proposed to compensate for these multiple harmonics and balance the DC-link voltage in the active capacitor. Also, an equivalent DC-bus impedance model is introduced, which is more accurate than that in existing works. Simulation studies and evaluation of the design have verified the effectiveness of the active capacitor solution. / Single-phase power converters have been widely used in many applications such as electric vehicles, photovoltaic (PV) systems, and grid integration. Due to their popular application, there is a need to reduce the sizes and volumes while still maintaining good performances of the systems. One of the most effective methods, which is a subject in many research works, is to replace the bulky passive capacitor bank in a system by an active capacitor. The active capacitor is designed to absorb the ripple components in the DC side of the converters, which results in a constant DC-link voltage. In comparison to the passive capacitor solution, the active capacitor is much smaller in size but can give a better DC-bus ripple performance. Therefore, the active capacitor has become an attractive solution for the single-phase converters. The active capacitor for the traditional rectifier, where the DC side is directly connected to a load, has been intensively investigated in the past decade. However, there is limited research regarding the active capacitor for rectifiers with floating H-bridge power modules. This work extends the application of the active capacitor to the Perturbation Injection Unit (PIU), which is a grid-connected single-phase rectifier with floating H-bridge power modules. The selection of a suitable active capacitor for the PIU is based on the evaluation of various active capacitor banks. Limits in existing control schemes, which prevent the extension of the active capacitor to the PIU, are thoroughly studied. An effective voltage-mode control scheme is then proposed for the selected active capacitor, which makes it an attractive solution for the PIU. Moreover, limits of the DC-bus impedance analysis using traditional assumptions in existing works are investigated, and an improved DC-bus impedance model is proposed. Based on the operation conditions of the PIU and the proposed impedance model, the active capacitor's components can be properly designed, and improved configurations in terms of the equivalent impedance can be analyzed. Simulation results, as well as the design and evaluation of the active capacitor, demonstrate great improvements in terms of volume and weight over the traditional passive capacitor bank.

Denna studie undersöker hur unga kvinnor i Generation Z rationaliserar sina köp av fast fashion trots en växande medvetenhet om modeindustrins negativa aspekter gällande miljö och sociala konsekvenser. Genom att utforska gapet mellan konsumenternas intentioner kring hållbarhet och deras faktiska beteenden belyser studien faktorer som har en påverkan på konsumentens köpbeslut. Studien utgår från en kvalitativ metod där semistrukturerade intervjuer genomförts. Detta för att få en djupgående insikt i hur unga kvinnor inom Generation Z använder sig av rationaliseringar för att rättfärdiga sina köp av fast fashion. Analys av resultatet har utförts i form av en tematisk analys. Teorin Moralisk frikoppling har även tillämpats i ett försök att förklara det beteende som ligger till grund för gapet mellan konsumentens intention och handling. Studien visar att konsumenter i många fall använder sig av Moralisk frikoppling eftersom det ger en möjlighet att separera de negativa aspekterna av fast fashion, såsom miljöpåverkan, från de positiva aspekterna, exempelvis att plagget är billigt, snyggt eller trendigt. Resultatet visar att trots en ökad medvetenhet om fast fashions negativa aspekter, är det rationaliseringar som påverkar varför kvinnor inom Generation Z konsumerar, där pris och begär är de mest betydande drivkrafterna. Studien bidrar till en djupare förståelse av konsumentbeteende inom fast fashion samt en förståelse för de faktorer som påverkar gapet mellan konsumenters intentioner kring hållbarhet och dess faktiska handlingar. Ytterligare förväntas studien att bidra med en ökad självmedvetenhet hos konsumenter kring deras egen konsumtion av fast fashion och därav inspirera till mer medvetna val. Studien är skriven på svenska. / This study examines how young women in Generation Z rationalize their purchases of fast fashion despite a growing awareness of the fashion industry's negative aspects regarding environmental and social consequences. By exploring the gap between consumers' intentions regarding sustainability and their actual behaviors, the study highlights factors that have an impact on consumers' purchase decisions. The study is based on a qualitative method where semi-structured interviews were conducted. This is to gain an in-depth insight into how young women in Generation Z use rationalizations to justify their purchases of fast fashion. Analysis of the results has been done in the form of a thematic analysis. Additionally, the theory of Moral Decoupling has been applied in an attempt to explain the behavior that underlies the gap between the consumer's intention and action. The study shows that in many cases consumers use Moral Decoupling because it provides an opportunity to separate the negative aspects of fast fashion, such as environmental impact, from the positive aspects, for example that the garment is cheap, stylish or trendy. The result shows that despite an increased awareness of fast fashion's negative aspects, it is rationalizations that influence why women in Generation Z consume, where price and desire are the most significant driving forces. The study contributes to a deeper understanding of consumer behavior within fast fashion as well as an understanding of the factors that affect the gap between consumers' intentions regarding sustainability and its actual actions. Furthermore, the study is expected to contribute to an increased self-awareness among consumers regarding their own consumption of fast fashion and thereby inspire more conscious choices. The study is written in Swedish.

[ES] The automotive industry represents one of the most important sectors in the world. Given its socio-economic influence, research is aimed at reducing fuel consumption and emissions. Turbochargers provide several benefits including increased power for a given engine size, improved fuel economy and reduced emissions. The turbocharger is an important piece for the new generation of engines that must comply with the Euro 6 or in the U.S. Tier 3 vehicle emissions and fuel standard program. As more effort is made to increase efficiencies and reduce emissions, the complexity of the system increases. The high rotational speeds, pulsating flow conditions and high temperature differences between working fluids (exhaust gases, compressed air, lubricating oil, coolant fluids) make the turbo-charging a challenging task. Numerical simulation opens a range of possibilities to study the performance, efficiency and design of components in the turbocharger, but requires continued accuracy refinements. In this thesis, a great effort has been made to improve the overall understanding of the different physical phenomena that occur inside the turbocharger. Both, experimental and modelling efforts have been made to understand the thermal behaviour of the turbocharger under engine start/stop conditions. After state-of-the-art review of thermal studies and heat transfer simulation codes, this work presents an extensive experimental testing campaign that includes a thermal characterization of the turbocharger in stationary and transient conditions. Subsequently, several turbochargers were measured to assess the consequences that degraded oils can generate in the bearing system during endurance tests of oil-coking. To minimize the possibilities of coke formation, some theoretical studies were done. First, a 1D turbocharger model was used in GT-PowerTM for a detailed study of the temperature rise in the central housing during an engine hot-stop. The simulated cooling strategies aims to find an optimal in terms of minimizing extra energy consumption per K housing temperature reduction. After, a 2D radial model is proposed as improvement of an existing one-dimensional model developed at CMT - Universitat Politècnica de València. Aiming for a low computational cost, the radial model was developed to be compatible with fast one-dimensional engine simulations. Later, a detailed solution of heat fluxes was made by means of CFD using a 3D design of the turbocharger's central housing. The 3D model improved the results when temperature of the bearings/shaft is required. Additionally, thermal properties within the turbocharger can be obtained and therefore a reduction of the experimental tasks in the thermohydraulic test bench. Both 2D and 3D models were validated using experimental data, demonstrating predictive accuracy improvements on the results of previous models. / [CA] La industria automotriz representa uno de los sectores más importantes del mundo. Dada su influencia socioeconómica, la investigación está destinada a reducir el consumo de combustible y las emisiones. Los turbocompresores ofrecen varios beneficios, entre ellos, mayor potencia para un tamaño de motor determinado, mejor economía de combustible y reducción de emisiones. El turbocompresor es una pieza importante para la nueva generación de motores que deben cumplir con la normativa Euro 6 o en el programa estándar de emisiones y combustible de los EE. UU. Tier 3. A medida que se hacen más esfuerzos para aumentar la eficiencia y reducir las emisiones, la complejidad del sistema aumenta. Las altas velocidades de rotación, las condiciones de flujo pulsante y las altas diferencias de temperatura entre los fluidos de trabajo (gases de escape, aire comprimido, aceite lubricante, fluidos refrigerantes) hacen que la turbocarga sea una tarea desafiante. La simulación numérica abre un rango de posibilidades para estudiar el rendimiento, la eficiencia y el diseño de los componentes en el turbocompresor, pero requiere continuos refinamientos de precisión. En esta tesis, se ha hecho un gran esfuerzo para mejorar la comprensión global de los diferentes fenómenos físicos que ocurren al interior del turbocompresor. Se han hecho esfuerzos experimentales y de modelado para comprender el comportamiento térmico del turbocompresor en condiciones de arranque/parada del motor. Luego de una revisión de los estudios térmicos y de los códigos de simulación de transferencia de calor, éste trabajo presenta una extensa campaña de pruebas experimentales que incluye una caracterización térmica del turbocompresor en condiciones estacionarias y transitorias. Posteriormente, se midieron varios turbocompresores para evaluar las consecuencias que los aceites degradados pueden generar en el sistema de rodamientos durante pruebas de resistencia de coque de aceite. Para minimizar las posibilidades de formación de coque, se realizaron algunos estudios teóricos. En primer lugar, se usó un modelo de turbocompresor 1D en GT-PowerTM para un estudio detallado del aumento de temperatura de la carcasa central del turbocompresor durante un paro en caliente del motor. Las estrategias de enfriamiento simuladas apuntan a encontrar un óptimo en términos de minimizae el consumo de energía extra por reducción de la temperatura de la carcasa en Kelvin. Posteriormente, se propone un modelo radial 2D como mejora de un modelo unidimensional existente desarrollado en la CMT - Universitat Politècnica de València. Con el objetivo de conseguir un bajo costo computacional, el modelo radial 2D se desarrolló para ser compatible con simulaciones unidimensionales rápidas de motor. Después, se realizó una solución detallada de los flujos de calor mediante CFD utilizando un diseño 3D de la carcasa central del turbocompresor. El modelo 3D mejora los resultados cuando se requiere la temperatura de los cojinetes/eje. Además, con ésta campaña de CFD se pueden obtener propiedades térmicas dentro del turbocompresor y, por lo tanto, una reducción de las tareas experimentales en el banco de pruebas termohidráulico. Ambos modelos 2D y 3D fueron validados utilizando datos experimentales, demostrando mejoras de precisión de predicción sobre los resultados de modelos anteriores. / [EN] La indústria automotriu representa un dels sectors més importants del món. Donada la seua influència socioeconòmica, la investigació està destinada a reduir el consum de combustible i les emissions. Els turbocompressors oferixen diversos beneficis, entre ells, major potència per a una grandària de motor determinat, millor economia de combustible i reducció d'emissions. El turbocompressor és una peça important per a la nova generació de motors que han de complir amb la normativa Euro 6 o en el programa estàndard d'emissions i combustible dels EE. UU. Tier 3. A mesura que es fan més esforços per a augmentar l'eficiència i reduir les emissions, la complexitat del sistema augmenta. Les altes velocitats de rotació, les condicions de flux polsen-te i les altes diferències de temperatura entre els fluids de treball (gasos de fuga, aire comprimit, oli lubricant, fluids refrigerants) fan que la turbocarga siga una tasca desafiador. La simulació numèrica obri un rang de possibilitats per a estudiar el rendiment, l'eficiència i el disseny dels components en el turbocompressor, però requerix continus refinaments de precisión. En aquesta tesi, s'ha fet un gran esforç per a millorar la comprensió global dels diferents fenòmens físics que ocorren a l'interior del turbocompressor. S'han fet esforços experimentals i de modelatge per a comprendre el comportament tèrmic del turbocompressor en condicions d'arranque/parada del motor. Després d'una revisió dels estudis tèrmics i dels codis de simulació de transferència de calor, este treball presenta una extensa campanya de proves experimentals que inclou una caracterització tèrmica del turbocompressor en condicions estacionàries i transitòries. Posteriorment, es van mesurar uns quants turbocompressors per a avaluar les conseqüències que els olis degradats poden generar en el sistema de rodaments durant proves de resistència de coc d'aceite. Per a minimitzar les possibilitats de formació de coc, es van realitzar alguns estudis teòrics. En primer lloc, es va usar un model de turbocompressor 1D en GT- Power \textsuperscript{TM} per a un estudi detallat de l'augment de temperatura de la carcassa central del turbocompressor durant una desocupació en calent del motor. Les estratègies de refredament simulades apunten a trobar un òptim en termes de minimizae el consum d'energia extra per reducció de la temperatura de la carcassa en Kelvin. Posteriorment, es proposa un model radial 2D com a millora d'un model unidimensional existent desenrotllat en la CMT - Universitat Politècnica de València. Amb l'objectiu d'aconseguir un baix cost computacional, el model radial 2D es va desenrotllar per a ser compatible amb simulacions unidimensionals ràpides de motor. Después, es va realitzar una solució detallada dels fluxos de calor per mitjà de CFD utilitzant un disseny 3D de la carcassa central del turbocompressor. El model 3D millora els resultats quan es requerix la temperatura dels cojinetes/eje. A més, amb esta campanya de CFD es poden obtindre propietats tèrmiques dins del turbocompressor i, per tant, una reducció de les tasques experimentals en el banc de proves termohidráulico. Ambdós models 2D i 3D van ser validats utilitzant dades experimentals, demostrant millores de precisió de predicció sobre els resultats de models anteriores. / Rodriguez Usaquén, YT. (2019). Experimental study of oil coking problem and contribution to the modelling of heat transfer in turbochargers [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/117314

The illicit practice of money laundering (ML) poses global challenges, causing harmful repercussions for society. Legislators worldwide have placed financial institutions at the forefront of initiatives aimed at preventing ML. Even with the adoption of Anti-Money Laundering (AML) measures, financial institutions continue to face challenges in combating illicit financial activity. Prior investigations primarily concentrated on evaluating the regulatory framework itself, leaving gaps in understanding other aspects causing ineffective AML efforts. Going further in-depth, this study aims to explore how institutional pressure leads to ineffective ML prevention within Finnish financial institutions. To address these challenges, the institutional theory and several concepts supported the exploration of the institutional environment, with due consideration given to social constructs. Furthermore, a qualitative approach is employed due to the exploratory nature of this study. It consists of the analysis of both classified and public official documents from the Finnish Financial Supervisory Authority (FIN-FSA), interviews held with representatives actively involved in financial institutions' AML efforts, and other official AML guidelines. The findings of this study acknowledge the suboptimal regulatory pressure and reveal instances of data deficiencies. However, the social context emerged as the most influential factor, highlighting the gap between AML efforts and ML activities.

Komplexe Lebenszyklen sind eine häufige, aber anspruchsvolle Lebensweise von Parasiten. Parasiten mit komplexen Lebenszyklen infizieren nacheinander mehrere Wirte und passen sich an verschiedene ökologische Nischen an. Werden in diesem Szenario dieselben Gene in allen Wirten einheitlich exprimiert? Die Hypothese der adaptiven Entkopplung besagt, dass verschiedene Stadien in einem komplexen Lebenszyklus genetisch und evolutionär unabhängig sind, was bedeutet, dass die Selektion auf Parasitenmerkmale in einem Wirt keine Auswirkungen auf Merkmale in anderen Wirten hat. Wir haben diese Hypothese anhand von zwei Parasitenarten getestet: dem Bandwurm Schistocephalus solidus und dem Fadenwurm Camallanus lacustris. Die Transkriptomsequenzierung wurde an Proben während ihrer komplexen Lebenszyklen durchgeführt. Die Genexpressionsanalyse wurde durchgeführt, um Gene zu identifizieren, die zwischen den Wirten, zwischen den Funktionsstadien und zwischen den Bedingungen unterschiedlich exprimiert werden. Darüber hinaus wurde mit einer Analyse der Anreicherung von Gensätzen untersucht, ob ähnliche biologische Funktionen von ähnlichen Genen in verschiedenen Stadien kodiert werden. Unsere Ergebnisse zeigen signifikante Unterschiede in der Genexpression zwischen den verschiedenen Stadien, wobei keine positive Korrelation zwischen Stadien mit derselben Aufgabe oder demselben Wirt beobachtet wurde. Gene, die in einem Stadium hochreguliert (oder herunterreguliert) werden, werden in anderen Stadien nicht in ähnlicher Weise exprimiert. Dies spricht für die Unabhängigkeit der einzelnen Stadien bei der Genexpression, die es den Parasiten ermöglicht, ihren Phänotyp als Reaktion auf unterschiedlichen Selektionsdruck zu modulieren. Diese Ergebnisse bestätigen die Hypothese der adaptiven Entkopplung bei parasitären Würmern und bieten Einblicke in den evolutionären Erfolg dieser Lebensweise. / Complex life cycles are a common but demanding lifestyle among parasites. Parasites with complex life cycles sequentially infect multiple hosts, adapting to various ecological niches using information encoded within a single genome. In this scenario, are the same genes expressed consistently across all hosts, as might occur when parasite stages perform similar functions? The adaptive decoupling hypothesis posits that different stages in a complex life cycle are genetically and evolutionarily independent, meaning selection on parasite traits in one host does not affect traits in other hosts. We tested this hypothesis using two species of parasites: the tapeworm Schistocephalus solidus and the nematode Camallanus lacustris, both of which have three-host life cycles. Transcriptome sequencing was performed on samples throughout their complex life cycles, generating transcriptomes from 10 stages of S. solidus and 7 stages of C. lacustris. Gene expression analysis was conducted to identify genes differentially expressed between hosts, between functional stages, and between conditions. Additionally, gene set enrichment analysis assessed whether similar biological functions are encoded by similar genes across stages. Our findings demonstrate significant differences in gene expression across stages, with no positive correlation observed between stages sharing the same task or host. The highest correlation occurred between stages sampled close in time. In conclusion, the lack of positive correlation between different life cycle stages indicates that genes up-regulated (or down-regulated) in one stage are not similarly expressed in other stages. This evidence supports the independence of each stage in gene expression, enabling parasites to modulate their phenotype in response to different selective pressures. These findings corroborate the adaptive decoupling hypothesis in parasitic worms with complex life cycles, offering insights into the evolutionary success of this lifestyle.

The recent economic and financial turmoil has led central banks around the world to heavily utilize unconventional monetary policy measures. Unconventional in this sense means a deflection from traditional central bank policy measures, i.e. interest rate innovations. Although these measures were widely discussed, the uniformed, coherent and comprehensive framework of such measures is still missing. The aim of this thesis is to establish the framework for possible classification of such policies together with transmission channels to the real economy. The empirical part examines the impacts of unconventional policies on real data using vector autoregression and vector error correction models. This analysis is based on monthly data period between 1999 and 2013, which is strongly affected by implementation of the unconventional policies in its second half. The last section examines the possible future of these policies as a normal instrument of central banks and describes their main challenges and shortcomings. JEL classification: C32, E40, E44, E50, E52, E58, E60 Keywords: Unconventional monetary policy, Interest rate, Decoupling principle, Balance sheet policy stratification, Quantitative easing, Channels of transmission, Vector Autoregression, Vector error correction model Author's e-mail:...

L’imagerie par résonance magnétique (IRM) est un outil d’investigation majeur donnant accès de manière non invasive à des nombreuses informations quantitatives et fonctionnelles. La qualité des images obtenues (rapport-signal-sur-bruit, RSB) est cependant limitée dans certaines applications nécessitant des résolutions spatiales et/ou temporelles poussées. Afin d’améliorer la sensibilité de détection des équipements d’IRM, diverses orientations peuvent être suivies telles qu’augmenter l’intensité du champ magnétique des imageurs, améliorer les performances des systèmes de détection radiofréquence (RF), ou encore développer des séquences d’acquisition et des techniques de reconstruction d’images plus efficaces. La thématique globale dans laquelle s’inscrit cette thèse concerne le développement des systèmes de détection RF à haute sensibilité pour l’IRM à haut champ chez l’homme. En particulier, des antennes auto-résonantes basées sur le principe des lignes de transmission sont utilisées parce qu’elles peuvent être réalisée sur substrat souple. Cette adaptabilité géométrique du résonateur permet d’ajuster précisément sa forme aux spécificités morphologiques de la zone anatomique observée, et ainsi d’augmenter le RSB. La première visée technologique de ce projet concerne le développement, de la conception jusqu’à la mise en œuvre dans un appareil 7 T corps entier, d’un système de détection RF flexible à haute sensibilité, utilisant des antennes miniatures associées en réseau. L’utilisation d’un réseau d’antennes miniatures permet d’obtenir des images sur un champ de vue élargi tout en conservant la haute sensibilité inhérente à chaque antenne miniature. De plus, la technologie de l’imagerie parallèle devient accessible, ce qui permet d’accélérer l’acquisition des images. De surcroît, un nouveau schéma de résonateur de ligne transmission avec un degré de liberté supplémentaire est introduit, ce qui permet de réaliser de grands résonateurs multi-tours pour l’IRM à haut champ. Cette thèse décrit le développement, la mise en œuvre et l’évaluation des nouveaux systèmes de détection RF au moyen de simulations analytiques et numériques, et des études expérimentales. / Magnetic resonance imaging (MRI), among other imaging techniques, has become a major backbone of modern medical diagnostics. MRI enables the non-invasive combined, identification of anatomical structures, functional and chemical properties, especially in soft tissues. Nonetheless, applications requiring very high spatial and/or temporal resolution are often limited by the available signal-to-noise ratio (SNR) in MR experiments. Since first clinical applications, image quality in MRI has been constantly improved by applying one or several of the following strategies: increasing the static magnetic field strength, improvement of the radiofrequency (RF) detection system, development of specialized acquisition sequences and optimization of image reconstruction techniques. This work is concerned with the development of highly sensitive RF detection systems for biomedical ultra-high field MRI. In particular, auto-resonant RF coils based on transmission line technology are investigated. These resonators may be fabricated on flexible substrate which enables form-fitting of the RF detector to the target anatomy, leading to a significant SNR gain. The main objective of this work is the development of a flexible RF coil array for high-resolution MRI on a human whole-body 7 T MR scanner. With coil arrays, the intrinsically high SNR of small surface coils may be exploited for an extended field of view. Further, parallel imaging techniques are accessible with RF array technology, allowing acceleration of the image acquisition. Secondly, in this PhD project a novel design for transmission line resonators is developed, that brings an additional degree of freedom in geometric design and enables the fabrication of large multi-turn resonators for high field MR applications. This thesis describes the development, successful implementation and evaluation of novel, mechanically flexible RF devices by analytical and 3D electromagnetic simulations, in bench measurements and in MRI experiments.

Existem métodos de integração de equações algébrico diferenciais não lineares (DAEs) considerados clássicos pela literatura. Porém, neste trabalho, através uma abordagem geométrica, apresenta-se um método de integração de DAEs. Tal método é inspirado na teoria de desacoplamento de sistemas não lineares explícitos, quando se considera que as saídas são restrições algébricas. Neste caso, a DAE pode ser identificada como dinâmica zero. O resultado principal desta abordagem é que, dada uma DAE, sob certas condições, é possível a construção de um sistema explícito, de tal maneira, que as soluções desse sistema explícito convergem para as soluções da DAE. / Classical methods for numerical integration of diferential algebraic equations (DAEs) can be formal in the literature. In this work, using a diferential geometric approach, a numerical method of integration of DAEs is established. This method is inspired in the decoupling theory of nonlinear explicit systems, when one considers that the outputs are algebraic constraints. The main result is the construction of an explicit system, whose solutions converge to the solutions of the DAE.