L’objectif de ce travail était d’étudier la variabilité de la circulation océanique méridienne aux échelles de temps pluri décennales dans l’océan Atlantique nord au cours des deux mille dernières années, ainsi que son lien avec la variation de l’extension des gyres subtropicales et subpolaires. J’ai donc étudié, à partir de carottes de sédiments marins à fort taux de sédimentation, l’évolution de la température des eaux de surface et de la stratification de la colonne d’eau en lien avec le fonctionnement des gyres subpolaire et subtropicale.Compte tenu des difficultés spécifiques à la période de temps considérée, la première partie de mon travail a consisté à contraindre le milieu et la période de calcification des principaux foraminifères utilisés, à partir de la composition isotopique de l’oxygène analysée dans la coquille des foraminifères planctoniques. J’ai également précisé la calibration en Magnésium et en Calcium en fonction de la température. La deuxième partie de mon travail a consisté à reconstruire les conditions hydrologiques dans des zones clés de l’océan Atlantique Nord sur les deux mille dernières années. J’ai ainsi construit un index de la gyre subpolaire à partir d’un gradient de température Est-Ouest, qui traduit l’intensité dynamique de la gyre subpolaire et de la gyre subtropicale. L’apport de l’analyse des foraminifères planctoniques profonds a permis de reconstituer les variations de la colonne d’eau supérieure. La stratification plus ou moins marquée de la colonne d’eau est reliée directement à l’intensité des vents d’Ouest. Les similitudes entre les vents et l’index de gyre m’ont amené à proposer un couplage entre l’océan et l’atmosphère aux échelles de temps pluri décennales.La dernière partie de ma thèse s’est focalisée sur les conséquences des variations de la dynamique des gyres océaniques sur le transport de chaleur vers les hautes latitudes ainsi que sur l’impact des variations des vents d’Ouest sur le climat européen. / The purpose of this thesis was to study the surface oceanic circulation in the North Atlantic Ocean during the last 2,000 years, and its link with the intensity of the subpolar and the subtropical gyres. To fulfill these objectives, I studied sediment cores with a high sedimentation rate to reconstruct the multidecadal variability of the temperature and the water column stratification, controlled by the dynamic of oceanic gyres. To improve the marine paleoclimatic signal recorded from planktonic foraminifera, I constrained their growing season and their calcification depth by analyzing the oxygen isotopic composition of their calcitic shells. I also established calibrations between Mg/Ca ratio and temperature for the main species used.I applied these calibrations to reconstruct the hydrological conditions in key areas of the North Atlantic Ocean. I constructed an index of the subpolar gyre that traduces the dynamic intensity of the subpolar gyre and the subtropical gyre. I also studied the variability of the upper water column based on the analysis of deep-dwelling foraminifera. I interpret past changes in the water column stratification as resulting from changes in the intensity of Westerly winds. The similarities between the wind forcing evolution and the index of the subpolar gyre dynamics led me to propose a coupling between the ocean and the atmosphere on the multidecadal time scale. The consequences of the gyres dynamic on heat transport and the impacts of the change in westerly wind strength on European climate are studied in the last part of the manuscript.

O objetivo deste trabalho consiste na análise teórica e experimental do desempenho térmico de dois arranjos de micro tubos de calor. Os arranjos diferem entre si pelo material base de fabricação, que são Acrilonitrila Butadieno Estireno (ABS) e latão, pelo número de canais e pelas dimensões. O fluido de trabalho utilizado foi R134a. A literatura indica micro tubos de calor como soluções proeminentes e de destaque para resfriamento de micro sistemas. Estes dispositivos podem ser adaptados a diferentes condições térmicas através da mudança de fluidos de trabalho, geometrias e materiais base do trocador. O estudo teórico foi baseado no modelo de circuitos térmicos proposto no presente trabalho, que visa calcular indicadores de desempenho térmico, sendo condutividade térmica efetiva e resistência térmica equivalente dos micro tubos de calor a partir das temperaturas obtidas experimentalmente. O estudo experimental foi baseado em obter distribuições de temperaturas dos dispositivos propostos sob diferentes condições de trabalho, variando-se a fração de enchimento de fluido de trabalho, inclinação e temperatura do condensador. Os resultados obtidos para o arranjo de micro tubos de calor em latão demonstrou uma razão de aumento de até 1482% na capacidade de transporte de calor, enquanto que o arranjo de micro tubos de calor em ABS apresentou uma razão de aumento de 247%. Os melhores desempenhos de ambos os casos foram observados para ângulos positivos com baixas frações de enchimento. / The purpose of this work is the theoretical and experimental study on the thermal performance of two micro heat pipes arrays. The differences between the arrays are the base substrate, which are Acrylonitrile Butadiene Styrene (ABS) and brass, the number of channels and their dimensions. The working fluid used was R134a. Previous works indicate micro heat pipes as prominent solutions for the cooling of micro systems. These devices can be adapted to different thermal conditions by changing the working fluids, geometries and base materials of the heat exchanger. The theoretical study was based on the thermal circuits model presented in this work, that aims on computing thermal performance indicators, such as the effective thermal conductivities and the equivalent thermal resistances of the micro heat pipes from the temperatures obtained experimentally. The experimental study was based on obtaining temperature distributions of the micro heat pipes under different working conditions, by varying the working fluid filling ratio, tilt angle and the temperature of the cooling water at the condenser. The results obtained for the brass micro heat pipe array showed a performance ratio increase up to 1482% in heat transfer capacity, while the ABS micro heat pipe array showed a performance ratio increase of 247%. The best performance for both cases were observed for positive tilt angles with low working fluid filling ratios.

O objetivo deste trabalho consiste na análise teórica e experimental do desempenho térmico de dois arranjos de micro tubos de calor. Os arranjos diferem entre si pelo material base de fabricação, que são Acrilonitrila Butadieno Estireno (ABS) e latão, pelo número de canais e pelas dimensões. O fluido de trabalho utilizado foi R134a. A literatura indica micro tubos de calor como soluções proeminentes e de destaque para resfriamento de micro sistemas. Estes dispositivos podem ser adaptados a diferentes condições térmicas através da mudança de fluidos de trabalho, geometrias e materiais base do trocador. O estudo teórico foi baseado no modelo de circuitos térmicos proposto no presente trabalho, que visa calcular indicadores de desempenho térmico, sendo condutividade térmica efetiva e resistência térmica equivalente dos micro tubos de calor a partir das temperaturas obtidas experimentalmente. O estudo experimental foi baseado em obter distribuições de temperaturas dos dispositivos propostos sob diferentes condições de trabalho, variando-se a fração de enchimento de fluido de trabalho, inclinação e temperatura do condensador. Os resultados obtidos para o arranjo de micro tubos de calor em latão demonstrou uma razão de aumento de até 1482% na capacidade de transporte de calor, enquanto que o arranjo de micro tubos de calor em ABS apresentou uma razão de aumento de 247%. Os melhores desempenhos de ambos os casos foram observados para ângulos positivos com baixas frações de enchimento. / The purpose of this work is the theoretical and experimental study on the thermal performance of two micro heat pipes arrays. The differences between the arrays are the base substrate, which are Acrylonitrile Butadiene Styrene (ABS) and brass, the number of channels and their dimensions. The working fluid used was R134a. Previous works indicate micro heat pipes as prominent solutions for the cooling of micro systems. These devices can be adapted to different thermal conditions by changing the working fluids, geometries and base materials of the heat exchanger. The theoretical study was based on the thermal circuits model presented in this work, that aims on computing thermal performance indicators, such as the effective thermal conductivities and the equivalent thermal resistances of the micro heat pipes from the temperatures obtained experimentally. The experimental study was based on obtaining temperature distributions of the micro heat pipes under different working conditions, by varying the working fluid filling ratio, tilt angle and the temperature of the cooling water at the condenser. The results obtained for the brass micro heat pipe array showed a performance ratio increase up to 1482% in heat transfer capacity, while the ABS micro heat pipe array showed a performance ratio increase of 247%. The best performance for both cases were observed for positive tilt angles with low working fluid filling ratios.

<p>Demands for energy have been increasing in the whole world. According to higher consumption, the price of energy rises yearly too. This evokes usage of insulating products in a wider range. By adding insulation, we lower the amount of energy needed to heat our homes, resulting in fewer associated greenhouse gas emissions and a lower monthly heating bill. Savings depend on insulation thicknesses and on conditions, in which the insulant is kept. Mineral insulation based on stone wool is also a member of building insulants that defends buildings and constructions against temperature changes of the ambient. However, even when we use modern technologies and building techniques to reduce high energy losses, we can never provide unimpeachable protection of stone wool from damage. During a construction process on a building site or at fast climate changes, it often happens that stone wool is exposed to rain precipitaions or other climate effets. This brings water to the insulating structure. Besides the loss of insulating qualities, the stone wool is left permanently wet. Even the fibres of stone wool are inorganic, they still can be attacked by degradation processes due to organic agents fixing fibres together. Analysis of damaged flat-roof constructions using stone wool and verification of material properties is a starting point of this licentiate thesis.</p><p>The attached paper section can be divided into two parts:</p><p>In-situ practice that notes troubles with insulating materials based on stone wool with inbuilt moisture on a building site</p><p>Laboratory measurement that observe material properties of stone wool under varying conditions</p>

Veränderungen des Klimas, zunehmende Grundwassernutzungen sowie die Verdichtung der städtischen Strukturen wirken sich auf Temperaturen, Mengenbilanzen und den Wasserspiegel des Grundwassers aus. Schon heute lassen sich anthropogene Einflüsse wie tief liegende Gebäudestrukturen und Einleitungen von Wasser-Wasser-Wärmepumpen auf das Temperaturniveau des Grundwassers nachweisen. Zielstellung der vorliegenden Arbeit war die Untersuchung der Auswirkungen dieser natürlichen und anthropogenen Effekte in Dresden und die Aufstellung von Ansätzen eines Grundwasser-Temperaturmanagements. Auf der Grundlage aktueller Daten zu Grundwassernutzung und zur Grundwasserneubildung wurden Ist-Zustands-Berechnungen sowie Projektionen künftiger Systemzustände mit dem Grundwassermodell Dresden realisiert. Aufgrund des von TESCH (2013) in Szenarienberechnungen projizierten Rückgangs der Grundwasserneubildung um ca. zwei Drittel bis zum Ende des 21. Jahrhunderts ist ein deutlich geringeres Grundwasserdargebot zu erwarten. Defizite in der Grundwasserbilanz werden jedoch durch einen höheren Anteil an Uferfiltrat zum Teil ausgeglichen. Wesentlich ist, dass in den Szenarienberechnungen die maximale Entnahmemenge einen größeren Einfluss auf die Grundwasserbilanzen ausübt als der projizierte Rückgang der Grundwasserneubildung. Die Gewinnung von ausreichend Grundwasser für die Deckung des Trink- und Brauchwasserbedarfs von Bevölkerung, Gewerbe und Industrie scheint auch künftig sicher. Wärmeeinträge in das Grundwasser wurden anhand von Daten aus Stichtagsmessungen der Jahre 2009 und 2011 (FUGRO HGN; 2009 und SCHOLZ UND LEVIS, 2011) identifiziert. Hierbei konnte eine Reihe von Temperaturanomalien im Stadtgebiet den Quellen eindeutig zugeordnet werden. Anhand der Untersuchung von drei Teilgebieten zeigte sich, dass das Temperaturniveau des Grundwassers im Stadtzentrum (Teilgebiet Altstadt) gegenüber den anderen untersuchten Teilgebieten (Elbbogen Übigau und Johannstadt/Striesen) erhöht ist, was zuerst auf die Vielzahl von Bauwerken zurück geführt wird, die bis in das Grundwasser reichen. Des Weiteren zeigte sich ein deutlicher Zusammenhang von Messstellendichte und Ergebnisqualität. Während in der Altstadt die Identifikation von Wärmequellen gut möglich war, sind die Ergebnisse zu anderen Teilgebieten aufgrund der deutlich geringeren Messstellendichte weniger belastbar. Temperaturen im Boden und in der Luftsäule einer Grundwassermessstelle in der Dresdner Altstadt wurden über einen Zeitraum von ca. 2 Jahren ausgewertet. Die Untersuchungen belegen die Durchprägung des Jahresgangs der Lufttemperatur bis zum Grundwasser mit einer zeitlichen Verzögerung des Eintreffens der Extremwerte von ca. drei Monaten. Mit den Untersuchungen konnte nachgewiesen werden, dass das gleichzeitig angewendete Verfahren der Messung von Temperaturen in der Luftsäule einer Grundwassermessstelle zur Identifizierung der vertikalen Temperaturverteilung im Boden praktisch anwendbar ist. Der Wärmetransport im Boden wurde mit dem Programm HYDRUS 1-D für den Ist-Zustand auf Basis der Bodentemperaturmesswerte und für die Zukunftsszenarien auf der Basis von WETTREG 2010-Daten abgebildet. Die Berechnungen ergaben im Vergleich zum Ist-Zustand erhöhte Bodentemperaturen. Besonders interessant ist, dass die Änderungssignale der Bodentemperaturen für alle berechneten Tiefen bei den Minima deutlicher ausfallen als bei den Maxima. Des Weiteren zeigt sich ein signifikanter Unterschied zwischen den Berechnungsergebnissen der beiden untersuchten Zeitscheiben (2021 bis 2050 und 2071 bis 2100). Die höheren Bodentemperaturen im Winter bieten gegebenenfalls Ansatzpunkte zur Nutzung dieses Wärmeangebots, die erhöhten Temperaturen im Sommer können gegebenenfalls zu einer Erhöhung der Temperaturen des Wassers in Abschnitten des Trinkwassernetzes mit zeitweise größeren Aufenthaltsdauern führen. Die gefundenen Ergebnisse implizieren zudem künftig höhere Grundwassertemperaturen. Die Auswirkungen von Wärmeeinträgen auf das Grundwasser wurden mit Hilfe von MODFLOW/SEAWAT-Konzeptmodellen untersucht. Für den Ist-Zustand berücksichtigen diese Konzeptmodelle bereits Wärmeeinträge durch Gebäude und thermische Grundwassernutzungen (MIX, 2013). In den Szenarienberechnungen wurden projizierte erhöhte mittlere Lufttemperaturen aufgeprägt und weitere, zum Teil fiktive Nutzungen und Wärmeeinträge durch Gebäude implementiert. Die mit dem Anstieg der Lufttemperatur erwartete Erhöhung der mittleren Grundwassertemperatur und somit die Wirkung der natürlichen Anteile der Wärmeeinträge wird für die weniger anthropogen beeinflussten Grundwasserleiterabschnitte am deutlichsten. Die Modellergebnisse zeigen, dass unter den angenommenen Voraussetzungen mittlere Grundwassertemperaturen über 20°C nicht erreicht werden und modellgestützte Managementmaßnahmen für größere Grundwasserleiterabschnitte hinsichtlich der Bewertung energetischer Nutzungen des Grundwassers zielführend sind. Aufgrund des heutigen Standes der Forschungen zur Auswirkung von Wärmeeinträgen auf die Grundwasserqualität kann noch kein Handlungszwang abgeleitet werden, gleichsam fehlt ohne verbindliche Temperaturrichtwerte ein rechtlicher Rahmen. In der Klärung dieser Fragen, der verstärkten Wärmerückgewinnung aus dem Grundwasser und dem modellgestützten Grundwasserwärmemanagement sind zukünftige Aufgabenfelder der Grundwasserbewirtschaftung erkennbar. / Climate change, the rise of energetic groundwater use and the compact city structures cause an impact to the groundwater temperatures, groundwater quantity balance and the groundwater table. Today impacts of anthropogenic influences like deep basements of big buildings and the infiltration of heated or cooled water from groundwater using heat pumps were already detected. The target of this dissertation has been the investigation of these natural and anthropogenic effects in Dresden and planning steps for a groundwater temperature management. Basing on existing data of groundwater use and recharge in Dresden, a modelling of the recent and future system status scenarios with the three-dimensional model has been done. According to the latest results of the regional climate model WETTREG 2010 and a work by Tesch about the groundwater recharge until the end of the 21st Century, a significant reduction in resources are expected. Partly the balance deficit will be regulated by bank filtration. It is an important fact that the maximum discharge rate, which is larger than the permitted real use, has a bigger influence in the balance than the lower groundwater recharge. The water catchment to supply inhabitants and industrial units seems to be secure in the future. Heat impacts to the groundwater were detected by measurements in 2011 and 2012. With the results of these measurements anomalies of the temperature field and the emission points of heat inputs were distinctly located. Based on the investigation of three subareas, a higher level of groundwater temperatures in the city center (subarea Altstadt) compared to the other subareas (Übigau and Jogannstadt/Striesen) was detected. The reason of this fact is the multitude of big buildings which are reaching the aquifer. The investigation has also showed the relationship between the quantity of the measuring points and the quality of the results. In the subarea Altstadt an identification of heat inputs could be very well found. The results in the other subareas with a lower amount of sampling points have not the same level of validity. Information from time series over two years about soil and air column temperatures of a close-by groundwater measurement point were analyzed. The research documents the heat transport from the air to the groundwater with a retardation of the extreme values along about three months. With this analysis, the method of measurement air column temperatures in groundwater measurement points aiming to identify the vertical soil temperature distribution could be attested. The measured heat transport in the unsaturated soil was reproduced with the HYDRUS 1-D program. After this, future scenarios on the basic of WETTREG 2010 results were computed. The findings are higher soil temperature levels in the future with higher alteration signals in the minimum than in the maximum values. The modeling results have also showed a significant difference in the investigated time series (2021 - 2050 and 2071 - 2100). The higher temperatures in winter could be a chance to use this heat. In the summer it could partly affect parts of the water supply. Furthermore the findings implicate higher ground water temperatures in the future. To investigate heat impacts to the ground water concept, models of MIX (2013) were used for the heat transport in the aquifer which combines the heat impact of buildings and heat pumps with the natural air temperature rise. The WETTREG2010 result (air temperatures), heat inputs and possible new energetic groundwater use systems were implemented in the conceptual models. Results of the modeling has showed that the expected rise of the ground water temperature will be more significant for the less anthropogenic influenced parts of the urban aquifer than the parts with high initial level of heat pollution. In the model results, the temperatures do not reach mean values of 20°C (LAWA guideline). An important finding is also that these models could be used for a more efficient groundwater heat management and for the evaluation of energetic groundwater projects of its use. Because of the recent stand of research on the impacts of higher ground water temperatures to the ground water quality, a need for action can’t be indicated at the moment. At present there are no guideline values neither standard of law for the energetic use of groundwater. This facts and the question of heat recycling from the urban aquifer are fields for the groundwater management in the future.

The field of low-dimensional quantum magnets has received lot of attention owing to the possibility of studying phenomena associated with the quantum nature of matter. Many materials that realize low-dimensional spin arrangements in their structure have been synthesized in the past twenty years due to the emergence and development of crystal growth techniques. These materials have been studied using various experiments in order to explore the wide range of interesting properties predicted theoretically for low-dimensional systems. In this pursuit, novel properties have been observed and many open questions have been raised. One such property that is typically observed in many low-dimensional quantum magnets is heat transport via magnetic excitations. Large magnitudes of magnetic heat conductivity has been found experimentally in materials belonging to this class in addition to the conventionally known phononic heat conduction, and interesting theoretical predictions like the divergence of heat conductivity in certain spin models exist, that have stimulated research in this field. This experimental work mainly deals with the crystal growth and heat transport properties of low-dimensional quantum magnets that include one-dimensional (1D) spin chain systems Sr$_2$CuO$_3$ and SrCuO$_2$, two-dimensional (2D) Heisenberg antiferromagnet La$_2$CuO$_4$, and a five-leg spin ladder La$_8$Cu$_7$O$_{19}$, with a view to understand propagating low-energy magnetic excitations and their interaction amongst themselves, other quasiparticles and impurities present in the systems. These interactions result in scattering processes that govern the magnitude and temperature dependence of heat conductivity. In spite of considerable theoretical and experimental work in the field of heat transport, a complete understanding of the scattering mechanisms is lacking. The work tries to add to the experimental knowledge about magnetic heat transport in such systems and presents cases which motivate the need for theoretical understanding of aspects of heat transport. The focus of this work was twofold. One part focusses on the single crystal growth using the travelling-solvent floating zone (TFSZ) method of materials which realize low-dimensional spin systems in their structure. The TFSZ method is indispensable for growing large single crystals of extraordinary purity, which can be used for investigations using neutrons and other techniques like heat conductivity measurements that probe anisotropic properties. The other part deals with the experimental results on heat transport and other thermodynamic properties of these materials. In order to study the behaviour of the magnetic heat conductivity at high temperatures, and the effect of small amount of magnetic and non-magnetic impurities on the heat transport of 2D Heisenberg antiferromagnet La$_2$CuO$_4$, single crystals of pure La$_2$CuO$_4$, and Ni- and Zn-doped versions, La$_2$Cu$_x$Ni$_{1-x}$O$_4$ and La$_2$Cu$_x$Zn$_{1-x}$O$_4$ for $x$ = 0.001 and 0.003, were grown using the TFSZ method. Heat transport in the pure compound was experimentally investigated for the first time up to very high temperatures of 813 K using two methods, namely the steady state method for low temperatures and the dynamic flash method for measuring high temperature conductivity. Analysis of the magnon mean-free path using empirical models based on semi-classical theories, and qualitative comparison to theoretical calculations seems to suggest that scattering between magnons might play an important role in addition to scattering of magnons with phonons and defects, and that the spin-spin correlation length could be crucial in limiting the mean free path of magnons at high temperatures. These experimental results and indications of probable scattering mechanisms based on non-rigorous analyses and comparisons, strongly motivate the need for theoretical studies. Heat conductivity measurements on the Ni- and Zn- doped versions of La$_2$CuO$_4$ are still incomplete and inconclusive, and hence have not been reported in this work. Heat transport experiments on Ni- and Ca-doped Sr$_2$CuO$_3$ were performed, with a motivation to investigate the role of disorder induced by impurities lying within the spin chains (Ni) and those lying outside the spin chains (Ca), on the heat transport in this system. In both the cases, the magnetic heat transport is observed to be strongly suppressed upon doping. Empirical analysis of the data seems to suggest that in the temperature regime of 100-300 K, the temperature dependence of the mean-free path of magnetic excitations for the Ni- and Ca-doped samples can be described by scattering with defects (Ni and Ca impurities) and phonons alone. However, surprisingly, a strong increase of phononic conductivity is observed perpendicular and parallel to the spin chains of the Ni-doped compounds compared to the pure compounds, whose explanation seems to lie in the existence of an additional dissipative scattering mechanism present in the pure compounds that is lifted upon doping, possibly due to the presence of a spin gap in the doped compounds. The effect of Ni on the Sr$_2$CuO$_3$ and SrCuO$_2$ was also investigated by studying the low energy regime of the spin excitation spectrum using other microscopic probes like nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS). Large single crystals of SrCu$_x$Ni$_{1-x}$O$_2$, with $x$ = 0.01 were grown and used in these experiments that observed the presence of a spin gap in the Ni-doped sample. Further theoretical investigations are however required to understand the possible role of the spin gap in influencing the spin-phonon scattering mechanism, and its relevance to the observed enhancement in phononic conduction. Although we observe that in the case of both 1D and 2D systems, a semi-classical kinetic model for heat transport along with empirical models of scattering processes describe the temperature dependence of the measured heat conductivity surprisingly well in the temperature regime up to 300 K and 800 K respectively, interpretations based on these analyses must be treated as only preliminary, and as a step towards understanding microscopically the scattering mechanisms involved in low-dimensional systems such as the ones discussed in this work. In the direction of exploratory research towards synthesis of novel low-dimensional materials, two cuprate compounds were synthesized in the form of single crystals using the floating zone method for the first time, namely, a five leg $S=tfrac{1}{2}$ antiferromagnetic spin ladder compound La$_8$Cu$_7$O$_{19}$ and an insulating delafossite LaCuO$_{2}$. A bulk 3D antiferromagnetic ordering is observed in La$_8$Cu$_7$O$_{19}$. Heat conductivity of La$_8$Cu$_7$O$_{19}$ is observed to be purely phononic and no contribution from magnetic excitations seem to exist, although the measurements indicates that there is a large anisotropy in heat transport. However, detailed diffraction experiments using x-rays and neutrons indicate that both the crystal and magnetic structures are complicated, and that the details of the structure prevent La$_8$Cu$_7$O$_{19}$ from being a perfect realization of a five-leg spin ladder.

La fusion nucléaire est une solution technologique prometteuse pour une nouvelle source d'énergie. Cependant, utiliser la par fusion nucléaire confinement magnétique comme source d'énergie constitue un challenge scientifique et technologique car cela requière à la fois un bon confinement du plasma de cœur et un contrôle des flux de chaleurs arrivant à la paroi. Ce travail est motivé par la problématique de la gestion des flux de chaleur dans les réacteurs de fusion. Cela est nécessaire pour éviter d'endommager les coûteux composants faisant face au plasma. La compréhension des mécanismes physiques régissant le transport de la chaleur dans le plasma de bord est une tâche critique pour le design des futures machines. Dans ce contexte, il est nécessaire de faire des prédictions fiables de l'étalement de la chaleur dans le but de dimensionner correctement ces futures machines. Cela appelle à un fondement théorique décrivant la manière dont l'énergie s'échappe du plasma. Des études théoriques et expérimentales ont tenté aboutir à cette fin, cependant les mécanismes en jeux ne sont toujours pas clairs. Pour atteindre ce but, la modélisation numérique est un complément nécessaire aux expériences. Ce travail de thèse est dédié à l'étude numérique des différents aspects du transport de la chaleur dans le plasma de bord un utilisant les approches fluides. Une attention particulière est porté à deux mécanismes suspectés de joué un grand rôle dans le transport de la chaleur : le transport intermittent due à la turbulence et le transport convectif à large échelle par les vitesses dérives. Le problème a été traité avec une approche graduelle en utilisant différent outils numériques. / Fusion devices are a promising solution for a new source of energy. However, using fusion reaction to produce power within a magnetic confinement is a scientific and technological challenge as it requires a high confinement in the core plasma at the same time as a good control of plasma exhaust on the material walls. This work is motivated by the key problematic of power handling in fusion power plants necessary to avoid damaging the expensive plasma facing components (PFC). The understanding of the physics underlying the heat transport, and more specifically is a critical task for the engineering design of future Tokamak devices. In this context, it is mandatory to make reliable predictions of the power spreading in order to correctly size the future Tokamaks. This calls for a theoretical ground describing the way energy escapes the core plasma through the separatrix and deposits on the PFCs. Some theoretical and experimental studies attempt to achieve such a task, however no definitive conclusion have been drawn yet. To achieve this goal, numerical modelling is a necessary complement to experimental results. This PhD work has been dedicated to the study of the different aspects of the heat transport in the edge plasma using a numerical fluid approach. Special focus was devoted to two types of mechanisms suspected to play an important role in the heat transport: intermittent turbulence; the large-scale convective transport.

In dieser Arbeit beschreiben wir wie nicht-störungstheoretischer Wärmetransport im Rahmen von ab initio-Simulationen und linearer Antworttheorie formuliert werden kann. Die daraus resultierende ab initio-Green-Kubo-Methode ermöglicht die Simulation von Wärmetransport in Festkörpern beliebiger Anharmonizität und ist besonders geeignet um "stark anharmonische" Systeme zu beschreiben in denen störungstheoretische Ansätze unzuverlässig werden. Um die systematische Unterscheidung von harmonischen und anharmonischen Materialien zu ermöglichen führen wir ein "Anharmonizitätsmaß" ein, welches die anharmonischen Beiträge zu den interatomaren Kräften unter thermodynamischen Bedingungen quantifiziert. Mit diesem Anharmonizitätsmaß untersuchen wir typische dynamische Effekte die in stark anharmonischen Materialien auftreten, sowie die Grenzen störungstheoretischer Methoden zur Berechnung von Wärmetransporteigenschaften. Wir zeigen, dass eine negative Korrelation des Anharmonizitätsmaßes mit der Wärmeleitfähigkeit einfacher Kristalle besteht, was die intuitive Auffassung bestärkt, wonach harmonische Materialien bessere Wärmeleiter sind und umgekehrt. Auf diesen Erkenntnissen aufbauend identifizieren wird anharmonische Materialien als Kandidaten für Wärmetransport-Simulationen auf der Suche nach neuen thermischen Isolatoren. Auf diesem Wege identifizieren wir mehrere neue thermische Isolatoren welche potentielle technologische Relevanz als thermische Barrieren oder Thermoelektrika aufweisen könnten, und schlagen diese zur experimentellen Untersuchung vor. / In this work, we describe how a non-perturbative heat transport formalism for solids emerges in the framework of ab initio simulations coupled with linear response theory. The resulting ab initio Green Kubo method allows for studying heat transport in solids of arbitrary anharmonic strength, and is particularly suited to describe “strongly anharmonic” systems where per- turbative approaches become unreliable. In order to discern harmonic from anharmonic materials in a systematic way, we introduce an “anharmonicity measure” which quantifies the anharmonic contribution to the interatomic forces under thermodynamic conditions. Using this anharmonicity measure, we investigate typical dynamical effects occurring in strongly anharmonic compounds and investigate the limits of perturbative approaches for the study of thermal transport. We show that this measure negatively correlates with bulk thermal conductivities in simple solids, supporting the intuitive notion that more harmonic materials are better heat conductors and vice versa. Based on these findings, we identify anharmonic compounds as candidates for thermal transport simulations in the search for novel thermal insulators. In this way, we identify several new thermal insulators of potential technological relevance as thermal barriers or thermoelectric materials which we suggest for experimental study.

Dans le cadre de la Fusion par Confinement Inertiel, la maîtrise de la propagation des faisceaux laser intenses, qui se propagent dans le plasma sous-dense d'une cavité d'ignition, reste un enjeu majeur. En effet, durant leur propagation, les faisceaux vont modifier les paramètres du plasma, ces paramètres contrôlant eux-mêmes la propagation. Cette rétroaction entre le plasma et le laser est potentiellement instable et produit de l'autofocalisation, de la filamentation, etc. Ces mécanismes peuvent alors dégrader fortement la propagation et in fine l'uniformité du dépôt d'énergie recherché.Dans cette étude, expérimentale et numérique, nous nous intéressons à la propagation d'un filament laser (tache focale d'une dizaine de micromètres, impulsion de 1,5 ns et d'intensité variant de 1014 W.cm-2 à 1016 W.cm-2) dans un milieu très sous dense (quelques pour-cents de la densité électronique critique). Deux expériences ont été réalisées sur l'installation de puissance LULI2000 et ont permis à la fois d'observer la transmission laser et de caractériser les gradients de température. Ces gradients sont produits par le transport électronique et influencent la réponse du plasma au laser. Afin de modéliser le rôle des gradients de température dans les instabilités d'auto-focalisation et de filamentation, nous avons associé un code d'hydrodynamique-radiative (FCI2) à un code détaillé de propagation (Héra). Pour compléter les observables précédentes, nous avons aussi mis en œuvre un diagnostic de déflectométrie protonique. Ce diagnostic permet de mesurer les champs électriques présents dans le plasma, ces champs étant issus du chauffage et de l'interaction laser. Un nouvel outil, associant le code de propagation laser et un code Monte-Carlo de transport de protons, a donc été mis en place pour modéliser les déflexions des protons. Cet outil ouvre en plus de nouvelles perspectives pour discuter de l'influence des effets 3D dans l'exploitation de ce diagnostic. Les résultats obtenus confirment l'intérêt à sonder les champs électriques au cœur même du plasma pendant la propagation laser. / The understanding and the control of high-power laser propagation into under-dense plasma is important to achieve inertial confinement fusion. During this process, the interaction of the laser with the plasma filling the hohlraum can lead to significant losses of laser energy which prevent ignition. Self-focusing or filamentation of the laser light is one of these phenomena which are desired to be mitigated since they also affect the uniformity of the laser illumination on the hohlraum wall.In order to improve our understanding of the laser-plasma interaction phenomena at play, we describe an experimental and numerical study involving an intense laser pulse between 1014 W.cm-2 and 1016 W.cm-2 , and which interacts with millimetric and under-dense plasma (having density of few % of the critical density). This work presents two experiments fielding a series of diagnostics aimed at well characterizing the laser propagation (Hisac camera) together with heat deposition in plasmas using Thomson scattering. Experimental results will be presented and discussed in the light of detailed simulations performed with the 3D laser propagation code Hera. In order to take into account the temperature gradients within the plasma during the laser propagation, Hera (laser propagation code) and FCI2 (radiation-hydrodynamic code) have been coupled. Besides, proton radiography has been used in order to access to electric fields. The measurements led to the implementation of a new and promising numerical tool using the Hera and Diane codes (Diane is a Monte Carlo particle tracing code). 3D proton radiography modelling opens new possibilities for users of this temporally and spatially resolved diagnostic.

Cette thèse de doctorat porte sur l'étude théorique de la conductivité thermique du réseau dans le bismuth semi-métallique et sur les stratégies pour réduire la conductivité thermique en vue d'applications pour réduire l'échauffement dans les circuits électroniques, et pour la thermoélectricité. J'ai utilisé des méthodes avancées de résolution de l'équation de transport de Boltzmann pour les phonons, et de calcul ab initio des éléments de matrice de l'interaction phonon-phonon. J'ai calculé la dépendance en température de la conductivité thermique du réseau dans le matériau en volume en excellent accord avec les rares expériences disponibles. J'ai obtenu une description très précise, à l'échelle microscopique, du transport de la chaleur et j'ai quantifié la contribution des porteurs de charge à la conductivité thermique totale. J'ai démontré que la nano-structuration et la photo-excitation sont des moyens très efficaces dans le bismuth de contrôler la diffusion des phonons qui portent la chaleur, respectivement par interaction avec les bords de l'échantillon, et par interaction phonon-phonon. En contrôlant l'équilibre entre ces deux derniers effets, j'ai prédit de façon exhaustive l'effet de réduction pour différentes températures et tailles de nanostructures, pour des mono et poly-cristaux, semi-conducteurs ou semi-métalliques. Enfin, j'ai étudié l'élargissement anharmonique des phonons acoustiques et optiques, et j'ai déterminé pour chacun les interactions majeures qui contribuent à l'élargissement. L'atténuation du son a été prédite dans le bismuth pour de futures expériences. L'approximation des grandes longueurs d'ondes [long-wave approximation (LWA)] a été validée pour le bismuth et ses limites ont été déterminées. / This work is devoted to the theoretical investigation of the heat conduction in bulk bismuth and the possible strategies for its reduction. Thermal properties of Bi are extremely interesting because of its low thermal conductivity that makes this material suitable for the thermal management applications. Moreover, bismuth is an excellent model substance for the study of thermoelectricity and bismuth-based compounds such as Bi2 Te3 and Bi2 Se3 which are typical thermoelectric materials used in industrial applications.In collaboration with L. Paulatto (IMPMC), G. Fugallo (Ecole Polytechnique), F. Mauri(IMPMC) and M. Lazzeri (IMPMC) I have applied the recently developed advanced methods of the solution of the Boltzmann transport equation (BTE) and of the phonon-phonon matrix elements calculation to describe thermal transport in bismuth. I have obtained the temperature dependence of the lattice thermal conductivity which is in excellent agreement with experiment. Moreover I am able to predict the lattice thermal conductivity (LTC) at temperatures at which it has not been measured. I have found that most of heat is carried by the acoustic phonons. However, the optical phonons were shown to play an important role by modulating the magnitude of the acoustic-optical phonon interaction (AOPI) and thus the value of the lattice thermal conductivity. Furthermore, I have shown that the available experimental data for the lattice thermal conductivity for polycrystalline thin-films are remarkably explained by my calculations, which enables me to predict the effect of the LTC size reduction for various temperatures and nanostructure shapes and sizes.The methods I use contain no empirical fitting parameters and give a direct insight into the microscopic mechanisms determining the transport and anharmonic properties of the materials. This allows me to analyze the anharmonic broadening that is inversely proportional to the phonon lifetime, for the various phonon modes along the high symmetry directions in the Brillouin zone and show what are the major scattering channels for coalescence/decays of phonons that govern the thermal transport in Bi.