"RF excited CO2 lasers are widely used in industry. They provide relatively high power discharge levels while maintaining compactness, simplicity, and durability with respect to other competing laser technologies. To attain high power levels in the range of 5-10 KW, lasers with large electrode areas have to be designed. Unfortunately, due to the large electrode length requirements, transmission line effects make the discharge loading nonlinear, adversely affecting the efficiency of the CO2 laser. A standard approach to linearize the discharge loading is to introduce shunt inductors across the length of the electrodes in an effort to counter the capacitive nature of the discharge behavior. This thesis investigates and improves the theoretical models found in the literature in an effort to predict the discharge non-uniformity and allow for multiple shunt inductors installation. Specifically, we discuss the coupling of a CO2 laser discharge model with an electrical circuit solving scheme and how it can be characterized as one dimensional (1-D) and two dimensional (2-D) systems. The 1-D system is modeled using transmission line (TL) theory, where as the 2-D system is modeled using a finite difference time domain (FDTD) scheme. All our models were implemented in standard MATLAB code and the results are compared with those found in the literature with the goal to analyze and ascertain model limitations."

"This thesis presents an implementation of the Finite Difference Time Domain (FDTD) method on a massively parallel computer system, for the analysis of electromagnetic phenomenon. In addition, the implementation of an efficient mesh generator is also presented. For this research we selected the MasPar system, as it is a relatively low cost, reliable, high performance computer system. In this thesis we are primarily concerned with the selection of an efficient algorithm for each of the programs written for our selected application, and devising clever ways to make the best use of the MasPar system. This thesis has a large emphasis on examining the application performance."

A fundamental problem of microwave (MW) thermal processing of materials is the intrinsic non-uniformity of the resulting internal heating pattern. This project proposes a general technique to solve this problem by using comprehensive numerical modeling to determine the optimal process guaranteeing uniformity. The distinctive features of the approach are the use of an original concept of uniformity for MW-induced temperature fields and pulsed MW energy as a mechanism for achieving uniformity of heating. The mathematical model used to represent MW heating describes two component physical processes: electromagnetic wave propagation and heat diffusion. A numerical solution for the corresponding boundary value problem is obtained using an appropriate iterative framework in which each sub-problem is solved independently by applying the 3D FDTD method. Given a specific MW heating system and load configuration, the optimization problem is to find the experiment which minimizes the time required to raise the minimum temperature of the load to a prescribed goal temperature while maintaining the maximum temperature below a prescribed threshold. The characteristics of the system which most dramatically influence the internal heating pattern, when changed, are identified through extensive modeling, and are subsequently chosen as the design variables in the related optimization. Pulsing MW power is also incorporated into the optimization to allow heat diffusion to affect cold spots not addressed by the heating controlled by the design variables. The developed optimization algorithm proceeds at each time-step by choosing the values of design variables which produce the most uniform heating pattern. Uniformity is measured as the average squared temperature deviation corresponding to all distinct neighboring pairs of FDTD cells representing the load. The algorithm is implemented as a collection of MATLAB scripts producing a description of the optimal MW heating process along with the final 3D temperature field. We demonstrate that CAD of a practical applicator providing uniform heating is reduced to the determination of suitable design variables and their incorporation into the optimization process. Although uniformity cannot be attained using“static" MW heating, it is achievable by applying an appropriate pulsing regime. The functionality of the proposed optimization is illustrated by computational experiments which show that time-to-uniformity can be reduced, compared to the pulsing regime, by up to an order of magnitude.

A grande maioria da população mundial está crescentemente exposta à radiação eletromagnética proveniente de fontes que muitas vezes estão localizadas nas proximidades do corpo. A radiação eletromagnética é considerada um agente possivelmente cancerígeno para as pessoas, classificação 2B indicada pela Organização Mundial da Saúde-OMS (WHO/IARC, 2011). Devido às preocupações em relação aos riscos associados a esta exposição existem normas que recomendam os valores máximos de exposição permitidos (ICNIRP, 1998; FCC, 2001). A correta avaliação das doses de radiação é, portanto, relevante. Este trabalho tem a finalidade de avaliar o impacto dos resultados do cálculo da dose da Taxa de Absorção Específica (SAR) em usuários expostos a radiação por tablets operando na faixa de radiocomunicações Wi-fi. Os três modelos existentes de cabeça humana utilizados serão um manequim homogêneo SAM phantom e dois modelos de cabeça realistas heterogêneos: um adulto masculino e uma criança masculina. Será também utilizado nas simulações um modelo masculino de criança que foi desenvolvido através de imagens de tomografia computadorizada (TC) pelo processo de segmentação feito no software AMIRA. Será utilizado um modelo genérico de tablet. Os parâmetros dosimétricos usados para simulação da SAR serão computados pelo software SEMCAD X que é baseado no Método das Diferenças Finitas no Domínio do Tempo (FDTD). Será criado também um código do Método FDTD através do software MATLAB que servirá para a escolha dos parâmetros do SEMCAD X. A distância entre o tablet e os modelos de cabeças varia de 50 mm a 300 mm. Os resultados da SAR serão comparados com os limites de exposição recomendados pelas normas internacionais. Também serão simuladas diferentes posições da antena no tablet. Da análise dos resultados foi constatado que os valores de SAR são muito baixos e todos os resultados ficaram dentro dos limites do psSAR recomendados pela FCC de 1,6 W/kg em cada 1 g de tecido e de 2 W/kg em cada 10 g de tecido estabelecidos pela ICNIRP. Comparando os valores de SAR do modelo SAM com o modelo DUKE, o modelo SAM se mostra conservador, porém quando a comparação é feita com as crianças o SAM deixa de ser conservador. / The vast majority of the world population is increasingly exposed to electromagnetic radiation from sources which are often located near to the body. Electromagnetic radiation is considered a possible carcinogen for people, classification 2B indicated by the World Health Organization-WHO (WHO/IARC, 2011). Due to concerns regarding the risks associated with this exposition there are regulations suggesting maximum allowed exposure values (ICNIRP, 1998; FCC, 2001). The correct evaluation of radiation doses is therefore relevant. This work aims to assess the impact of the results of the calculation of Specific Absorption Rate dose (SAR) in users exposed to radiation from tablets operating in the Wi-fi band. The three existing models of human head used are a homogeneous dummy SAM phantom and two heterogeneous realistic head models: a male adult and a male child. It will also be used in the simulations a male child model which was developed from computed tomography (CT) imaging using the AMIRA software for the segmentation process. A generic model of tablet is used. Dosimetric parameters used for simulation of the SAR are computed using the SEMCAD X software which is based on the Finite Difference Method in Time Domain (FDTD). A FDTD code was developed using the MATLAB software in order to help to choose the input SEMCAD X parameters. The distances between the tablet and the head of the models varies from 50 mm to 300 mm. SAR results are compared with the exposure limits recommended by international standards. Different antenna positions on the tablet are simulated too. Examining the results it was found that the SAR values are very low and all results are within the psSAR limits recommended by FCC (1,6 W/kg averaged over 1 g of tissue) and by ICNIRP (2 W/kg in 10 g of tissue). Comparing the SAR in the SAM model with the SAR in the DUKE model, the SAM model shows to be conservative. However, when compared with the children, the SAM is not conservative.

Submitted by camilla martins (camillasmmartins@gmail.com) on 2017-02-10T17:21:33Z No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Dissertacao_ModelagemAtravesMetodo.pdf: 7457965 bytes, checksum: fbaa083e9a9d17f829e6a8c47780f985 (MD5) / Approved for entry into archive by Edisangela Bastos (edisangela@ufpa.br) on 2017-02-14T14:30:00Z (GMT) No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Dissertacao_ModelagemAtravesMetodo.pdf: 7457965 bytes, checksum: fbaa083e9a9d17f829e6a8c47780f985 (MD5) / Made available in DSpace on 2017-02-14T14:30:00Z (GMT). No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Dissertacao_ModelagemAtravesMetodo.pdf: 7457965 bytes, checksum: fbaa083e9a9d17f829e6a8c47780f985 (MD5) Previous issue date: 2015-08-31 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / FAPESPA - Fundação Amazônia de Amparo a Estudos e Pesquisas / Neste trabalho, o solo amazônico da região de Belém é modelado como um material dispersivo, no qual a condutividade elétrica varia com a frequência. Para isto, utiliza-se um aproximador de Padé no domínio da frequência, o qual é aplicado à equação de Ampére. Equações especiais para utilizar o campo elétrico no domínio do tempo são obtidas através médias temporais e da transformada inversa de Fourier. Os resultados obtidos através das simulações numéricas realizadas com o método FDTD são comparados com transitórios obtidos experimentalmente. Pulsos padronizados por normas para descargas atmosféricas são injetados em sistemas de aterramento clássicos. Excelente concordância foi observada, validando o método numérico desenvolvido. / The Amazonian soil of Belém is modeled as a dispersive material, of which the electrical conductivity is dependent on the frequency. For this aim, we use the Padé's approxima-tion in frequency domain, which is applied to Ampère's law. Special update equations for electric field in time domain are obtained by applying time averaging and the in-verse Fourier transform. The results obtained by numerical simulations performed with the FDTD method are compared to transient responses measured in field. Lightning-patterned pulses are injected in classical grounding systems. Excellent agreement is ob-served, which validates the developed numerical method.

This thesis aims to determine the distribution of the relative permittivity for random mixtures of material using electromagnetic simulations. The algorithm used in the simulations is the FDTD method which solves Maxwell's equations numerically in the time-domain. The material is modeled as randomly shaped particles with radius 12 ± 10 micrometre in x- and y-direction and radius 3 ± 1 micrometre in zdirection. The scattering parameters from the transmitted and reflected electric field when a plane wave interacts with the material are measured. The relative permittivity is determined from the scattering parameters using the iterative Baker-Jarvis method. The simulations shows that both the distribution and the value of the relative permittivity is low when the particles have non conducting layers to force interruptions to prevent percolation, a conducting path between the particles. The most important result is of the kind where the simulations do not have any boundaries to prevent percolation. These simulations reflects how the relative permittivity distributes in real measurements. It is established that the value of the relative permittivity has a large distribution and also that percolation occurs because of the periodic structures.

Les antireflets permettent d'accroître l'efficacité des cellules photovoltaïques, d'augmenter la sensibilité des détecteurs optroniques, et même d'améliorer l'extraction lumineuse des diodes électroluminescentes. Traditionnellement, des empilements de matériaux en couches minces sont utilisés pour les fabriquer. Nous avons étudié une technique alternative qui s'appuie sur la microstructuration de l'interface air-substrat. Il s'agit, plus précisément, de modéliser et de fabriquer des surfaces microstructurées bi-périodiques sur silicium et sur germanium présentant un effet antireflet très efficace dans l'infrarouge en bandes II et III respectivement. Ces structures nécessitent une description rigoureuse des phénomènes de propagation de la lumière. L'influence des paramètres opto-géométriques est examinée sous le point de vue des cristaux photoniques en utilisant les diagrammes de bandes. Pour réaliser ces structures, des techniques à bas coût, basées sur une gravure humide anisotrope du semi-conducteur cristallin à travers un masque obtenu par photolithographie, ont été utilisées. Nous obtenons expérimentalement sur silicium un facteur de réflexion inférieur à 4% sur l'ensemble du spectre IR II. Un très bon accord calcul/mesure permet de valider les résultats numériques obtenus précédemment.

La modélisation de la diffusion par des matériaux hétérogènes rugueux est un sujet complexe tant au niveau théorique que numérique. Dans notre travail, les dimensions des différents éléments constitutifs du milieu sont dans le domaine résonant et leurs propriétés sont caractéristiques du domaine optique ou infrarouge. Nous chercherons à calculer numériquement la Fonction de Distribution de la Réflectance Bidirectionnelle pour une géométrie 2D. Nous développons une approche de Monte Carlo basée sur la combinaison d'une méthode des différences finies dans le domaine temporel (FDTD) pour le calcul électromagnétique et de méthodes de génération de milieux hétérogènes rugueux (Metropolis). <br /><br />Nous appliquons cette approche pour étudier la diffusion par des surfaces rugueuses de fonctions d'auto-corrélation gaussienne ou exponentielle. Les résultats des calculs sont comparés avec ceux donnés par la Méthode des Moments. Ensuite, nous proposons une méthode originale de détermination d'un indice effectif relié à la propagation de l'énergie cohérente. Nous l'utilisons pour déterminer les domaines de validité de modèles approchés : Maxwell-Garnett, Bruggeman, Foldy-Twersky et Keller. Nous utilisons ces résultats pour étudier la diffusion en champ lointain par un ensemble bidisperse de particules, et notamment la possibilité de remplacer l'influence des petites particules par le milieu homogénéisé décrit précédemment. Enfin, nous étudions le problème du couplage surface/volume. Nous vérifions une hypothèse de découplage basée sur l'utilisation de l'indice effectif défini précédemment pour la diffusion surfacique notamment dans le cas où la rugosité est directement corrélée à la position des particules.

L'objectif de cette thèse est de concevoir et élaborer des outils innovants, baptisés nano-antennes (NAs), pour sonder le champ électromagnétique optique au voisinage des structures de la nanooptique. Une NA est ici une nanostructure métallique placée à l'extrémité d'une sonde locale fonctionnant en mode collection. Une telle structure joue le rôle de relais entre le champ électromagnétique optique à la surface de l'échantillon et la sonde locale, attribuant à la sonde une sensibilité spécifique vis-à-vis du champ électromagnétique vectoriel. Les nano-antennes seront conçues de manière à fournir une information optique encore inaccessible par le biais de la microscopie en champ proche conventionnelle, pour répondre aux besoins actuels et futurs de la nano-optique. Dans la première partie, nous proposons une nouvelle configuration de sonde locale la nano-ouverture papillon complément d'une nano-antenne papillon et nous exposons sa fabrication aux dimensions spécifiques, à l'extrémité d'une fibre optique en polymère. Dans la deuxième partie, nous présentons une nouvelle configuration de microscopie en champ proche permettant l'accès aux lignes de champs électriques et magnétiques optiques 3D au dessus d'un échantillon, avec une résolution sub-longueur d'onde. Le microscope proposé permet de cartographier simultanément et indépendamment les distributions des amplitudes complexes des deux composantes du champ électrique transverse. La dernière partie est consacrée à l'application d'une méthode numérique dite FDTD afin d'étudier le comportement spectral et l'origine de l'exaltation au niveau de la partie centrale d'une nano-ouverture papillon.

De récentes études ont montré que les ondes électromagnétiques, proche d'une structure diffusante telle qu'une pointe de microscope à force atomique (AFM), peuvent être diffusées et détectées en champ lointain. Ainsi, la détection d'ondes de surface par microscopie optique en champ proche (SNOM) est une technique prometteuse dans le cadre des mesures thermiques aux petites échelles. Une telle technologie prend alors le nom de microscope TRSTM (Thermal Radiation Scanning Tunnelling Microscopy). Le travail de thèse présenté dans ce manuscrit se scinde en deux étapes. La première a trait à nos travaux expérimentaux basés sur le montage d'un dispositif TRSTM. Nous en décrivons les différentes composantes, ainsi que les difficultés rencontrées liées à son fonctionnement. En outre, divers outils numériques, destinés à détecter et extraire tout signal périodique utile, sont développés. La seconde étape se concentre sur nos travaux numériques. Nous y proposons un modèle de diffusion d'ondes électromagnétiques basé sur la FDTD (Finite-Difference Time-Domain) et la transformation champ proche/champ lointain. Ce modèle a été validé par l'étude de dipôles, puis de sphères dispersives à proximité d'un substrat diélectrique. Alors, un certain nombre de simulations de diffusion d'ondes évanescentes par une pointe, de diverses formes et de divers matériaux, proche d'une interface, est présenté.