This thesis explores the inverse problem of optical tomography applied to two-dimensional heterogeneous domains. The neutral particle transport equation was used as the forward model to simulate how neutral particles stream through and interact within these heterogeneous domains. A constrained optimization technique that uses Newton's method served as the basis of the inverse problem. The capabilities and limitations of the presented method were explored through various two-dimensional domains. The major factors that influenced the ability of the optimization method to reconstruct the cross sections of these domains included the locations of the sources used to illuminate the domains, the number of separate experiments used in the reconstruction, the locations where measurements were collected, the optical thickness of the domain, the amount of signal noise and signal bias applied to the measurements, and the initial guess for the cross section distribution. All of these factors were explored for problems with and without scattering. Increasing the number of sources, measurements and experiments used in the reconstruction generally produced more successful reconstructions with less error. Using more sources, experiments and measurements also allowed for optically thicker domains to be reconstructed. The maximum optical thickness that could be reconstructed with this method was ten mean free paths for pure absorber domains and two mean free paths for domains with scattering. Applying signal noise and signal bias to the measured fluxes produced more error in the reconstructed image. Generally, Newton's method was more successful at reconstructing domains from an initial guess for the cross sections that was greater in magnitude than their true values than from an initial guess that was lower in magnitude.

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Submitted by Emanuel Varela Cardoso (emanuel.varela@ufcg.edu.br) on 2018-10-16T16:53:51Z No. of bitstreams: 1 FRANKBELSON DOS SANTOS AZEVEDO – DISSERTAÇÃO (PPGFísica) 2015.pdf: 1064778 bytes, checksum: c929ffea2d7e7c51359bf2dfc237fbf9 (MD5) / Made available in DSpace on 2018-10-16T16:53:51Z (GMT). No. of bitstreams: 1 FRANKBELSON DOS SANTOS AZEVEDO – DISSERTAÇÃO (PPGFísica) 2015.pdf: 1064778 bytes, checksum: c929ffea2d7e7c51359bf2dfc237fbf9 (MD5) Previous issue date: 2015-09-24 / Capes / Este trabalho tem por objetivo estudar a dinâmica quântica planar de partículas neutras de spin-1/2 na presença de campos elétricos. A priori uma partícula com carga nula não devia interagir com campos eletromagnéticos, mas ao admitir partículas possuindo momento de dipolo magnético, vemos que a interação é possível. Tal estudo acontece através da equação de Dirac com acoplamento não mínimo, onde o termo de interação leva em conta o momento de dipolo magnético, spin da partícula e campo eletromagnético. A partir dessa equação, derivamos e resolvemos duas equações diferenciais de primeira ordem, mostrando que as soluções estão atreladas ao spin. As soluções para o efeito Aharonov-Casher é discutida em detalhes pela primeira vez neste trabalho. Também derivamos uma equação diferencial de segunda ordem, a partir da qual obtivemos o níveis de energia para uma partícula movimentando-se em um caminho circular de raio constante. Além disso, usando o método de extensão auto-adjunta, encontramos funções de onda de estados ligados e níveis de energia para o espaço completo, incluindo a região r = 0. Os níveis de energia obtidos são análogos aos níveis de Landau, e mostram uma dependência com o parâmetro projeção de spin. Por fim, tomamos o limite não relativístico para o espaço completo. / This work aims to study the planar quantum dynamics of neutral particles of spin- 1/2 in the presence of electric fields. A priori a particle with null charge should not interact with electromagnetic fields, but to admit particles having magnetic dipole moment, we see that the interaction is possible. Such study happen through the Dirac equation with non-minimal coupling, where the interaction term takes into account the magnetic dipole moment, spin of the particle and electromagnetic field. From this equation, we derive and solve two differential equations of first order, showing that the solutions is linked to spin. The solutions to the Aharonov-Casher effect is discussed in detail for the first time in this study. We also derive a differential equation of second order, from which we obtained the energy levels for a particle moving in a constant radius circular path. In addition, using the self-adjoint extension method, we find wave functions of bound states and energy levels to the full space, including the region r = 0. The Energy levels obtained are analogous to Landau levels, and show a dependence on the spin projection parameter. Finally, we take the non-relativistic limit for the full space.

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Examination of the paper insulation and copper stress braiding during stripdown of a number of Current Transformers (FMK type 400kV) has revealed the presence of dark deposits. Copper foils are often interspersed within layers of paper insulation and mineral oil found in transformer windings. The dark deposits were often found in association with these foils, affecting several layers of paper in addition to the layer in contact with the copper foil. This thesis describes the research undertaken to identify these deposits and establish a mechanism for the transportation through the paper layers. Preliminary investigation using scanning electron microscopy (SEM) in conjunction with energy dispersive X-ray analysis (EDX) has shown these dark deposits to be copper based. X-ray photoelectron spectroscopy was used to show that the transport of the copper deposit through the paper insulation was working under the influence of a diffusion controlled process, related to Fick's law. Laboratory studies in support of work designed to eliminate the problem have shown that corrosion of copper occurs in mineral oils containing a trace of oxygen. This corrosion is non protective in character and leads to migration of copper into adjacent layers of paper. It has been shown that the transport of copper through several layers of paper can be measured by XPS and that the concentration from one paper winding to the next declines in accord with Fick's law for non-steady state diffusion. Measurements of surface concentrations by XPS correlate well with measurements made with atomic absorption spectroscopy on solutions of extracts of the contaminated paper. The laboratory measurements have allowed determination of the diffusion coefficients and activation energy for the transport process and thus give a basis for interpretation of the diffusion profiles found in the transformer in terms of time and temperature of operation. The diffusion process is temperature dependant. The results have been used to produce long term prediction curves.

Two neutral beam injectors (NBIs) were installed on the COMPASS tokamak as a main upgrade after its move from the UK to Prague. The thesis deals with general experimental specifications of additionally heated plasmas by the NBI under the wide range of conditions on the COMPASS tokamak. The geometrical parameters of the beams are determined from the spectro- scopic measurements. Consequently, the NBI power passing through the narrow duct connecting NBI and tokamak chamber is investigated. The passing fraction of the NBI power is confirmed by the power balance analysis, which provides information about the delivered power into the plasma based on the plasma parameters. Once the delivered power is well known a description of beams interaction with plasma via global parameters such as the plasma temperatures, the plasma density, the energy confinement time and neutron yield is given. It is shown, that the NBIs heat mostly ions, which can reach ∼ 1 keV in the plasma core. Moreover, the impact of the fast ions presence on the commonly observed MHD activities, the sawtooth instability and the edge localized modes (ELMs), is discussed and compared with results from other tokamaks. The sawtooth period is prolonged by the influence of the NBI from 2 ms up to 4.5 ms. The knowledge of the NBI passing power into...

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O esquema iterativo de fonte de espalhamento (SI) é tradicionalmente aplicado para a convergência da solução numérica de malha fina para problemas de transporte de nêutrons monoenergéticos na formulação de ordenadas discretas com fonte fixa. O esquema SI é muito simples de se implementar sob o ponto de vista computacional; porém, o esquema SI pode apresentar taxa de convergência muito lenta, principalmente para meios difusivos (baixa absorção) com vários livres caminhos médios de extensão. Nesta dissertação descrevemos uma técnica de aceleração baseada na melhoria da estimativa inicial para a distribuição da fonte de espalhamento no interior do domínio de solução. Em outras palavras, usamos como estimativa inicial para o fluxo escalar médio na grade de discretização de malha fina, presentes nos termos da fonte de espalhamento das equações discretizadas SN usadas nas varreduras de transporte, a solução numérica da equação da difusão de nêutrons em grade espacial de malha grossa com condições de contorno especiais, que aproximam as condições de contorno prescritas que são clássicas em cálculos SN, incluindo condições de contorno do tipo vácuo. Para aplicarmos esta solução gerada pela equação da difusão em grade de discretização de malha grossa nas equações discretizadas SN de transporte na grade de discretização de malha fina, primeiro implementamos uma reconstrução espacial dentro de cada nodo de discretização, e então determinamos o fluxo escalar médio em grade de discretização de malha fina para usá-lo nos termos da fonte de espalhamento. Consideramos um número de experimentos numéricos para ilustrar a eficiência oferecida pela presente técnica (DSA) de aceleração sintética de difusão. / The scattering source iterative (SI) scheme is traditionally applied to converge finemesh numerical solutions to fixed-source discrete ordinates neutron transport problems. The SI scheme is very simple to implement under a computational viewpoint. However, the SI scheme may show very slow convergence rate, mainly for diffusive media (low absorption) with several mean free paths in extent. In this work we describe an acceleration technique based on an improved initial guess for the scattering source distribution within the slab. In other words, we use as initial guess for the fine-mesh average scalar flux in the scattering source terms of the SN discretized equations used in the transport sweeps, the coarse-mesh solution of the neutron diffusion equation with special boundary conditions to account for the classical SN prescribed boundary conditions, including vacuum boundary conditions. To apply this coarse-mesh diffusion solution into the fine-mesh SN transport sweep discretized equations, we first perform within-node spatial reconstruction, and then we determine the fine-mesh average scalar flux for use in the scattering source terms. We consider a number of numerical experiments to illustrate the efficiency of the offered diffusion synthetic acceleration (DSA) technique.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Um método numérico nodal livre de erros de truncamento espacial é desenvolvido para problemas adjuntos de transporte de partículas neutras monoenergéticas em geometria unidimensional com fonte fixa na formulação de ordenadas discretas (SN). As incógnitas no método são os fluxos angulares adjuntos médios nos nodos e os fluxos angulares adjuntos nas fronteiras dos nodos, e os valores numéricos gerados para essas quantidades são os obtidos a partir da solução analítica das equações SN adjuntas. O método é fundamentado no uso da convencional equação adjunta SN discretizada de balanço espacial, que é válida para cada nodo de discretização espacial e para cada direção discreta da quadratura angular, e de uma equação auxiliar adjunta não convencional, que contém uma função de Green para os fluxos angulares adjuntos médios nos nodos em termos dos fluxos angulares adjuntos emergentes das fronteiras dos nodos e da fonte adjunta interior. Resultados numéricos são fornecidos para ilustrarem a precisão do método proposto. / A numerical nodal method that is free from all spatial truncation errors is developed for one-speed slab-geometry discrete ordinates (SN) fixed-source adjoint neutral particle transport problems. The unknown in the method are the node-edge and the node-average adjoint angular fluxes, and the numerical values obtained for these quantities are those of the analytic solution of the adjoint SN equations. The method is based on the use of the standard spatially discretized SN balance adjoint equation, which holds in each spatial node and for each discrete ordinates direction, and a nonstandard adjoint auxiliary equation that contains a Greens function for the node-average adjoint angular fluxes in terms of the exiting adjoint angular fluxes from the node edges and the adjoint interior source. Numerical results are given to illustrate the methods accuracy.