In this thesis, thermal effects on Cr4+:YAG fiber are studied through numerical modeling. Crystal fiber was used as the gain medium in amplified spontaneous emission(ASE) light source, lasers, or amplifiers. Because the absorbed pump power can not be completely turned to signal in energy transition, some of the absorbed pumping power will be converted into heat, which raises the fiber temperature. In continuous-wave regime, maximum temperature, the steady-state temperature profile, and thermal stresses in the host material under single end pump are obtained by using the commercial finite elements method software ANSYS. The pump power was propagated with exponential decay inside the fiber. Because more heat was generated at the light incident region, a maximum temperature of 397K was observed from the simulation result at the same region under single-end pump of 3W. Simultaneously, a maximum tensile stress of 39 MPa was reached at the border between YAG and Silica. Finally, temperature profiles and thermal stresses were calculated in the other conditions.

We have carried out a theoretical and an experimental study of thermal effects arising in bulk high-Tc superconductors. The theoretical study has allowed us to predict the self-heating behaviour. We have calculated the temperature evolution. We have shown the existence of a forbidden temperature window, and we have determined the analytical expression of a threshold field (Htr2) separating the « middle» and the «high» dissipation state . From a numerical modelling of a short cylinder, we have determined the time and spatial dependance of dissipated power and temperature within the sample. We have shown that the temperature rise is the highest along the corner location where the dissipated power is maximum. We have designed and constructed a susceptometer for characterizing large bulk superconductors (f →32 mm). The susceptometer allows a small temperature gradient (< 0.1K) to be achieved in the presence of large heating rates. It allows large AC and DC fields to be applied simultaneously, and was upgraded to measure simultaneously local temperatures and magnetic inductions. We have determined the heat transfer occuring in the susceptometer chamber. Magneto-thermal measurements with this system can be carried out with a high sensitivity and are found to be in very good agreement with the theoretical predictions. This work underlines the importance of the cooling conditions that can affect the distribution of the magneto-thermal properties within the superconductor.

The interface crack branching phenomena, including thermal effects, has been investigated by using complex variable method and Stroh's dislocation theory, extended to thermo-elasticity in matrix notation. As one of the most catastrophic failure modes in structures like laminated and sandwich composites in aerospace and marine construction, thin film in electronic packaging, rotators in high speed engine of aircraft and reactor in nuclear power station, the study of interface crack branching has become a topic not only having theoretical importance, but also having practical significance. A unified approach is presented to address the thermoelastic interface crack problems in dissimilar anisotropic bimaterial composites, and a compact closed form solution is formulated by analytical continuation principle of complex analysis. Employing the contour integral method, an explicit solution to the interaction between the dislocations and the interface crack is obtained. By modeling the branched portion as a continuous distribution of the dislocations, the thermoelastic interface crack branching problem is then converted to a set of semi-coupled singular integral equations and solved by Gauss-Jacobi integration schemes. The influence of material property mismatches between the two constituents and the thermal loading effects on the interface crack branching are demonstrated by extensive numerical simulation. Some useful criteria for predicting the interface crack branching growth and guidance for optimal composites design are suggested. Further, a contact model to eliminate the overlapping between the two surfaces of an interface crack is also proposed and some new parameters which could influence the interpenetrating phenomena are also discovered. The technique to extend the current method to three dimensional problems is also outlined. Furthermore, the C++ source code has been implemented to manipulate the complicated complex operations for numerically solving the singular integral equations in complex matrix form.

Recycle/reuse networks are commonly used in industrial facilities to conserve natural resources, reduce environmental impact, and improve process economics. The design of these networks is a challenging task because of the numerous possibilities of assigning stream (process sources) to units that may potentially employ them (process sinks). Additionally, several fresh streams with different qualities and costs may be used to supplement the recycle of process streams. The selection of the type and flow of these fresh resources is an important step in the design of the recycle/reuse networks. This work introduces systematic approaches to address two new categories in the design of recycle/reuse networks: (a) The incorporation of thermal effects in the network. Two new aspects are introduced: heat of mixing of process sources and temperature constraints imposed on the feed to the process sinks iv (b) Dealing with variation in process sources. Two types of source variability are addressed: flowrate and composition For networks with thermal effects, an assignment optimization formulation is developed. Depending on the functional form of the heat of mixing, the formulation may be a linear or a nonlinear program. The solution of this program provides optimum flowrates of the fresh streams as well as the segregation, mixing, and allocation of the process sources to sinks. For networks with variable sources, a computer code is developed to solve the problem. It is based on discretizing the search space and using the concept of "floating pinch" to insure solution feasibility and optimal targets. Case studies are solved to illustrate the applicability of the new approaches.

The infrastructure of today is getting older and problems caused by deterioration over time is affecting the service life of these structures. In Sweden most of the existing bridges were constructed 60 to 70 years ago, rising the need to determine the state of health of the bridges as the maintenance costs will increase heavily. Part of the above-mentioned cut of the bridges owned by Trafikverket (The Swedish Transport Administration) that was constructed 60 – 70 years ago are pre-stressed concrete bridges. Pre-stressing of concrete structures is today a commonly used technology that utilizes the beneficial characteristic of concrete, the compressive strength, to a further extent than reinforced concrete.     This report will focus on the problems with pre-stressed concrete bridges and particularly on the thermal effects on the Abiskojokk railway bridge located in the northern part of Sweden. The pre-stressed box girder bridge spans in total 86 m in three lengths of 30 m, 35 m respectively 21 m starting from the east abutment and is part of the Iron-Ore Line starting in Kiruna and ending in Narvik, Norway. In an ocular(särskild) inspection of the bridge carried out the 18th of August in 2016 several crack patterns were mapped on the inside of the box girder along with some cracks about the top of the first column support starting from the east abutment. This thesis is focusing on the cracks that was mapped along the tendon positions on the inside of the box girder in the first span starting from the east abutment. The hypothesis is that the cracks are caused by temperature loads and normal forces obtained from the tendons at the thickening of the cross-sections. The research questions are; what the monitoring program shows and if it is possible to prove the hypothesis by using of a FE-model considering the gravity loads, temperature loads and the pre-stressing.     In order to determine the cause of the cracks on the inside of the box girder and investigate the behaviour of the bridge a monitoring program was installed, measuring the crack development over time along with the acceleration and temperatures of the bridge. Example data from the program were later used to analyse the behaviour of the bridge.     The results from the temperature data shows that the bridge has a slowness to temperature changes outside. This gives rise to temperature gradient acting over the bridge parts that may contribute to crack propagation. It also showed that the temperature correlates well with the strain of the cracks. The LVDT’s showed that the largest crack openings during train loading occurred in the second span of the bridge. The data also showed relatively large and unexpected negative peaks during the train loads. The strain gauges also show that the largest strain is occurring in the second span of the bridge. The crack envelopment during a train loads are more expected here and may prove that the negative peaks from the LVDT’s and accelerometers are caused by vibrations. The accelerometers showed that the largest transversal accelerations take place in the first and third span. This may be due to more restricted supports conditions at the column supports than the abutments. The accelerometer also showed correlating negative peaks with the LVDT’s that may be caused by vibrations in the bridge.     The conclusion from the monitoring program so far is that is not possible to prove the cause of the cracks so far, but it may be in the future.     The results from the non-linear FE-model showed that the thermal action of the Eurocode gradient was not enough to crack the concrete along with the pre-stressing load. However, the effects of the hypothesis were proven right.

Ce travail de thèse a pour but le développement d'une source de pompe pour l'OPCPA nanoseconde qui sera inclus au sein du laser 10 PW Apollon.Dans un premier temps nous présentons les spécificités des systèmes multi-pétawatts et l'intérêt de l'amplification OPCPA pour ce type de systèmes. Le choix du matériau amplificateur Yb:CaF2 pour le développement de la source de pompe OPCPA est alors justifié après exposition de ses caractéristiques. Nous concluons enfin le premier chapitre par le design préliminaire de la source de pompe, design adapté aux spécificités du cristal Yb:CaF2.Dans le deuxième chapitre nous commençons par étudier les effets de lentilles thermiques et de dépolarisation du milieu amplificateur sous forte puissance de pompe et en fonction de la polarisation ou de l'orientation cristalline. Cela permet de valider le choix de l'orientation [111] du cristal amplificateur pour les systèmes de forte puissance. Dans un second temps nous nous attardons sur les céramiques Yb:CaF2 car leur isotropie mécanique présente a priori un intérêt pour les systèmes de forte puissance. Nous étudions les céramiques fabriquées par voie humide et mettons en évidence un fort échauffement limitant leur utilisation. Plusieurs hypothèses quant à cet effet sont alors discutées.Dans le dernier chapitre, nous complétons le design de la pompe par des simulations d'absorption, de gain et d'extraction d'énergie. Nous exposons enfin les résultats expérimentaux obtenus en amplification pour les architectures régénérative et multipassage géométrique, utilisant des cristaux Yb:CaF2 en configuration miroir actif. / This PHD work is aiming at the development of a pump source for the nanosecond OPCPA of the 10 PW laser Apollon.First, we present the characteristics of the multipetawatt systems and the interest of OPCPA architecture for this type of systems. After detailing its characteristics, we then justify the choice of Yb:CaF2 as active media for the development of the nanosecond OPCPA pump source. We finally end the first chapter by the preliminary design of the pump source which is adapted to the specificities of the Yb:CaF2 crystal.The second chapter begins by the study of thermal lenses and thermal induced depolarization under high-power pumping in function of polarization or crystal orientation. The results allow us to validate the choice of [111] orientation for active media in the frame of high power laser systems. We then focus on ceramics because their mechanical isotropy appears to be interesting for high power systems. We study the ceramics made by wet route synthesis and highlight a strong heating, restricting their utilization. Several hypothesis are then investigated to explain this effect.In the last chapter we finished the design of the pump source with simulations of absorption, gain and energy extraction. We finally detailed the experimental results for two different architectures (regenerative and multipass), using Yb:CaF2 crystals in active mirror configuration.

Le laser Mégajoule, dédié à l’interaction laser-matière pour la physique des hautes énergies est composé de modules pré-amplificateurs (MPA) devant respecter une qualité de faisceau exigeante. Les MPA actuels utilisent des matériaux verre phosphate dopé au Néodyme pour un fonctionnement à 1053 nm. Leur cadence de tir est cependant limitée à 1 tir / 5 min à cause de leur faible conductivité thermique. Il serait intéressant d’augmenter cette cadence pour optimiser le temps alignement des optiques ou réaliser des diagnostics sur l’installation laser. Pour augmenter la cadence de tir des MPA, nous proposons de changer le matériau actuel par du Nd :Lu :CaF2 de conductivité thermique dix fois plus élevée en vue d’atteindre une cadence de répétition de 10 Hz. Dans ce travail de thèse, nous présentons une étude des effets thermomécaniques induits par un pompage diode inhomogène et transverse, de fluence 13 J/cm2. Nous présentons les mesures de biréfringence résolues spatialement avec un montage en polariseur-analyseur croisés. La distribution des contraintes thermomécaniques associées est reconstruite par un modèle de simulation avec le logiciel COMSOL®. Enfin, une étude de l’influence de l’orientation cristalline du Nd :Lu :CaF2 est développée afin d’expliquer la disparité de biréfringence induite mesurée entre les échantillon de Nd :Lu :CaF2. / Laser facility such as the Megajoule Laser dedicated to laser-matter interaction including inertial fusion need pre-amplifier modules (PAM) which must respect a high beam quality. The current PAM use Phosphate glass doped with Neodymium material to work at 1053 nm with a repetition rate of 1 shot / 5 min limited by a low thermal diffusion. However, it would be interesting to increase the shot rate for alignment or diagnostic purposes. Therefore, we propose to change this amplification material by crystal Nd :Lu :CaF2 with a thermal diffusion ten times higher in view of achieving a repetition rate of 10 Hz. In this PhD work, we report a characterization of the thermal induced effects under a diode inhomogeneous and transverse pump with an energy density of 13 J/cm2. We begin by studying the spatially resolved induced birefringence with a cross polarizer-analyzer setup. We reconstruct the stress pattern of our samples by simulating the global setup with COMSOL® software which includes the thermal and mechanic Multiphysics interaction. This model allows us first to compare with experimental results and then to entirely simulate the mechanical behavior of this new material. A detailed study of the influence of the crystal orientation on the induced birefringence is presented to explain the disparity observed on the different samples Nd :Lu :CaF2 measurements

Šiame darbe apžvelgiama šiluminių reiškinių lazerio aktyvioje terpėje teorija, pristatomi temperatūros bei įtempių ir deformacijų pasiskirstymo aktyviojioje lazerio terpėje kompiuterinio modeliavimo metodika. Temperatūros ir įtempių skaičiavimai atlikti trims aktyviosios terpės konfigūracijoms – Nd:YVO4 plokštelės, dviejų Nd:YAG plokštelių, kaupinamų zigzagu ir plokščiojo kompozitinio keraminio YAG elemento. Ištirta kaupinimo galios, legiruoto sluoksnio storio, aušintuvo ir aktyvios terpės šiluminių parametrų įtaka temperatūros pasiskirstymui, įtempiams ir deformacijoms aktyviojoje terpėje. / Theory of thermal effects in laser active medium is observed in this work. The metodology of temperature, thermal strain and stresses distribution in laser active medium numerical simulation is presented. Modeling of temperature, strain and stress is done by three active medium configurations: Nd:YVO4 plate, thin Nd:YAG elements and composite YAG ceramic slab. The influence to temperature, stresses and strain distribution of pump power and doped layer thickness in laser active medium has been investigated.

abstract: In this work, an advanced simulation study of reliability in millimeter-wave (mm-wave) GaN Devices for power amplifier (PA) applications is performed by means of a particle-based full band Cellular Monte Carlo device simulator (CMC). The goal of the study is to obtain a systematic characterization of the performance of GaN devices operating in DC, small signal AC and large-signal radio-frequency (RF) conditions emphasizing on the microscopic properties that correlate to degradation of device performance such as generation of hot carriers, presence of material defects and self-heating effects. First, a review of concepts concerning GaN technology, devices, reliability mechanisms and PA design is presented in chapter 2. Then, in chapter 3 a study of non-idealities of AlGaN/GaN heterojunction diodes is performed, demonstrating that mole fraction variations and the presence of unintentional Schottky contacts are the main limiting factor for high current drive of the devices under study. Chapter 4 consists in a study of hot electron generation in GaN HEMTs, in terms of the accurate simulation of the electron energy distribution function (EDF) obtained under DC and RF operation, taking into account frequency and temperature variations. The calculated EDFs suggest that Class AB PAs operating at low frequency (10 GHz) are more robust to hot carrier effects than when operating under DC or high frequency RF (up to 40 GHz). Also, operation under Class A yields higher EDFs than Class AB indicating lower reliability. This study is followed in chapter 5 by the proposal of a novel π-Shaped gate contact for GaN HEMTs which effectively reduces the hot electron generation while preserving device performance. Finally, in chapter 6 the electro-thermal characterization of GaN-on-Si HEMTs is performed by means of an expanded CMC framework, where charge and heat transport are self-consistently coupled. After the electro-thermal model is validated to experimental data, the assessment of self-heating under lateral scaling is considered. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2018

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