No description available.

No description available.

No description available.

The concept of Magneto-Hydrodynamic (MHD) propulsion can be used to implement a propeller-less propulsion system for marine vehicles. The basic principle behind MHD is to use the (Lorentz) force produced by the interaction of electric and magnetic fields to generate thrust on a conducting fluid in motion. Electrodes are lined up along the walls of the duct which act as the source of the electric field. Seawater acts as the conducting medium for the current when it passes through the duct. This medium is then subjected to a strong magnetic field within the duct, thereby producing an axial force, i.e., an axial thrust. Propulsion systems based on MHD require virtually no mechanical components, therefore a good application would be to design a propulsor which produces very little noise for small underwater vehicles. Results of a preliminary feasibility study on this application are presented in this thesis. An approximate, consistent MHD propulsion theoretical model to assess the performance of a MHD propulsor for small underwater vehicles is introduced and analyzed. The model is generalized from the hydrodynamic point of view to consider inlet and outlet diffusers. The general model is applied systematically varying the main design parameters with respect to a given autonomous underwater vehicle (AUV) size. The results show that larger magnetic fields, longer propulsor lengths and smaller inlet flow speeds are preferred to get the highest propulsion efficiency and thrust. To check the consistency of the theoretical model, experiments are conducted. The results of these experiments show an approximate relation between the theoretical equations and the actual phenomenon. / Master of Science / In recent years, there has been an increase in the usage of small autonomous (unmanned) underwater vehicles (AUV) for various purposes such as exploration, mining and military applications. Most of these AUVs use the conventional system of a motor and propeller to drive the vehicle. This thesis proposes a different method of propulsion, one without any mechanical moving parts such as a rotor or a motor, for certain applications of these AUVs. The proposed system uses the concept of Magneto-Hydrodynamics (MHD) to propel the vehicle using an interaction between the applied magnetic and electric fields inside the propulsion channel. These applied fields produce a force (Lorentz) on the fluid that is present in the channel, thereby creating thrust to propel the vehicle. In the present case, the fluid is the electrically conducting seawater. Since, propulsion systems based on MHD require no mechanical components, they produce very little noise and are ideal for applications that require stealth. A feasibility study on this application is introduced, analyzed and presented in this thesis. Parameters such as applied fields, propeller configurations, and propeller shape and size are varied with respect to a given AUV size, to understand how each variable effects the system. The results show that larger magnetic fields, longer propulsor lengths and smaller inlet flow speeds are preferred to get the highest propulsion efficiency and thrust. To check the consistency of the theoretical model, experiments are conducted. The results of these experiments show an approximate relation between the theoretical equations and the actual phenomenon.

Neste trabalho, utilizando técnicas magneto-ópticas, estudamos tempos de relaxação spin-rede (T1) do estado fundamental de centros \'F\' e, cristais de halogenetos alcalinos (KCl-KBr). Descrevemos um sistema semi-automático para medidas ópticas de T1, capaz de medir tempos de relação curtos (~1mS), baseado na medida do Dicroísmo Circular magnético (DCM) que apresentam aqueles sais quando portadores de centros paramagnéticos. Obtivemos a dependência de T1 com o campo magnético H (até 65 Kgauss), bem como os espectros de DCM para diferentes concentrações nas matrizes mistas. Uma teoria desenvolvida por Panepucci e Mollenauer (1) para matrizes puras, foi adaptadas para explicar a relaxação spin-rede nos cristais mistos. Os resultados obtidos para o processo direto (T~2.0 K), confrontados com auqela teoria, mostram que o mecanismo de relaxação dominante até 25 KGauss continua sendo a modulação por fônons da interação hiperfins entre o elétron \'F\' e os núcleos vizinhos. / Using magneto-optic techniques we have studied the ground state spin- lattice relaxation times (T1) of \'F\' centers in mixed Alkali Halide cristals (KCl-KBr). We describe a computer assisted system to optically measure short relaxation times (~1mS). The technique is based on the measurement of the Magnetic Circular Dicroism (MCD) presented by F centers. We obtained the T1 magnetic field dependency at 2 K up to 65 kGauss), as well as the MCD spectra for different relative concentration at the mixed matrices. The theory developed by Panepucci and Mollenauer for F centers spin-lattice relaxation in pure matrices was modified to explain the behavior of T1 in mixed cristais. The Direct Process results (T~2.0 K) compared against that theory shows that the main relaxation mecanism, Up to 25 kGauss, continues to be phonon modulation of the hyperfine interaction between \'F\' electrons and surrounding nuclei.

Metamaterials display unusual electromagnetic properties, such as, a negative effective permeability and negative effective permittivity. This has sparked much interest due to possibility of negative refraction which was later confirmed by experiments. The ability of magnetically coupled resonant circuits to display an effective permeability lead to the discovery of magneto-inductive waves. These waves are only supported on arrays of magnetically coupled resonant circuits. Research into magneto-inductive waves has been largely concentrated on their use in filters and their potential use in magnetic resonance imaging. However, some work has proposed the use of magneto-inductive waveguides as a data transfer medium. This report builds on previous work which found that an optimum existed for terminal-waveguide coupling, and aims to investigate the application of magneto-inductive waves in data transfer systems. A brief overview of the topic is given along with a description of the underlying characteristics. Factors that affect the capacity of magneto-inductive wave data transfer systems, such as inter element coupling, were identified. Two novel structures, both with the intent of increasing the bandwidth via different methods, are studied. One, by making a pseudo one-dimensional channel from a two-dimensional structure, and the other by using a dual-layer design to increase the coupling between adjacent elements. Both systems are modelled, using simple circuit theory and the impedance matrix method, and a comparison between simulated behaviour and experimental observation was made. There is discussion about the differences between experiment and simulation as well as their limitations. Magneto-inductive wave data transfer systems are eventually expected to support multiple terminals and as previous research only considered two-terminal systems, an investigation into the response of a one- and two-dimensional system with a blocking terminal was undertaken. The system was modelled, again using simple circuit theory and the impedance matrix method, and simulation and experiment were compared. As a whole, the simulations showed good agreement with experiments, after some initial adjustments. Both one- and two-dimensional systems showed that their performance was not severely effected by a blocking terminal. This suggests that magneto-inductive waveguides could support more terminals.

In this dissertation, the extraordinary magnetoresistance (EMR) effect in semiconductor/metal hybrid structures is studied to improve the performance in sensing applications. Using two-dimensional finite element simulations, the geometric dependence of the output sensitivity, which is a more relevant parameter for EMR sensors than the magnetoresistance (MR), is studied. The results show that the optimal geometry in this case is different from the geometry reported before, where the MR ratio was optimized. A device consisting of a semiconductor bar with length/width ratio of 5~10 and having only 2 contacts is found to exhibit the highest sensitivity. A newly developed three-dimensional finite element model is employed to investigate parameters that have been neglected with the two dimensional simulations utilized so far, i.e., thickness of metal shunt and arbitrary semiconductor/metal interface. The simulations show the influence of those parameters on the sensitivity is up to 10 %. The model also enables exploring the EMR effect in planar magnetic fields. In case of a bar device, the sensitivity to planar fields is about 15 % to 20 % of the one to perpendicular fields. 5 A “top-contacted” structure is proposed to reduce the complexity of fabrication, where neither patterning of the semiconductor nor precise alignment is required. A comparison of the new structure with a conventionally fabricated device shows that a similar magnetic field resolution of 24 nT/√Hz is obtained. A new 3-contact device is developed improving the poor low-field sensitivity observed in conventional EMR devices, resulting from its parabolic magnetoresistance response. The 3-contact device provides a considerable boost of the low field response by combining the Hall effect with the EMR effect, resulting in an increase of the output sensitivity by 5 times at 0.01 T compared to a 2-contact device. The results of this dissertation provide new insights into the optimization of EMR devices for sensor applications. Two novel concepts are presented, which are promising for realizing EMR devices with high spatial resolution and for opening new applications for EMR sensors in the low-field regime.

Neste trabalho, utilizando técnicas magneto-ópticas, estudamos tempos de relaxação spin-rede (T1) do estado fundamental de centros \'F\' e, cristais de halogenetos alcalinos (KCl-KBr). Descrevemos um sistema semi-automático para medidas ópticas de T1, capaz de medir tempos de relação curtos (~1mS), baseado na medida do Dicroísmo Circular magnético (DCM) que apresentam aqueles sais quando portadores de centros paramagnéticos. Obtivemos a dependência de T1 com o campo magnético H (até 65 Kgauss), bem como os espectros de DCM para diferentes concentrações nas matrizes mistas. Uma teoria desenvolvida por Panepucci e Mollenauer (1) para matrizes puras, foi adaptadas para explicar a relaxação spin-rede nos cristais mistos. Os resultados obtidos para o processo direto (T~2.0 K), confrontados com auqela teoria, mostram que o mecanismo de relaxação dominante até 25 KGauss continua sendo a modulação por fônons da interação hiperfins entre o elétron \'F\' e os núcleos vizinhos. / Using magneto-optic techniques we have studied the ground state spin- lattice relaxation times (T1) of \'F\' centers in mixed Alkali Halide cristals (KCl-KBr). We describe a computer assisted system to optically measure short relaxation times (~1mS). The technique is based on the measurement of the Magnetic Circular Dicroism (MCD) presented by F centers. We obtained the T1 magnetic field dependency at 2 K up to 65 kGauss), as well as the MCD spectra for different relative concentration at the mixed matrices. The theory developed by Panepucci and Mollenauer for F centers spin-lattice relaxation in pure matrices was modified to explain the behavior of T1 in mixed cristais. The Direct Process results (T~2.0 K) compared against that theory shows that the main relaxation mecanism, Up to 25 kGauss, continues to be phonon modulation of the hyperfine interaction between \'F\' electrons and surrounding nuclei.

Magnetorheological dampers, or as they are more commonly called, MR dampers, are being developed for a wide variety of applications where controllable damping is desired. These applications include dampers for automobiles, heavy trucks, bicycles, prosthetic limbs, gun recoil systems, and possibly others. This thesis first introduces MR technology through a discussion of MR fluid and then by giving a broad overview of MR devices that are being developed. After giving the reader an understanding of MR technology and devices, MR damper basics are presented. This section includes a discussion of MR damper types, mathematical fundamentals, and an approach to magnetic circuit design. With the necessary background information covered, MR dampers for automotive use are then discussed. Specifically, designs for MR dampers that were built for a Mercedes ML-430 and for a Ford Expedition are presented along with their respective test results. These test results are presented and compared with the original equipment hydraulic dampers. After discussing automotive MR dampers, designs for gun recoil applications are presented. Specifically, two different MR damper designs are discussed along with live-fire test results for the first damper. Finally, two hybrid dampers that were based on a modified adjustable hydraulic damper are presented. These hybrid dampers, if pursued further, may develop into controllable replacements for large hydraulic dampers such as those installed on large vehicles and field Howitzers. In conclusion, recommendations are made for materials as well as for seal selection and other design aspects. / Master of Science

The main idea behind the work presented in this thesis is to investigate if it is possible to find a mechanism that leads to surface magnetic field concentrations and could operate under solar conditions without postulating the presence of magnetic flux tubes rising from the bottom of the convection zone, a commonly used yet physically problematic approach. In this context we study the ‘negative effective magnetic pressure effect’: it was pointed out in earlier work (Kleeorin et al., 1989) that the presence of a weak magnetic field can lead to a reduction of the mean turbulent pressure on large length scales. This reduction is now indeed clearly observed in simulations. As magnetic fluctuations experience an unstable feedback through this effect, it leads, in a stratified medium, to the formation of magnetic structures, first observed numerically in the fifth paper of this thesis. While our setup is relatively simple, one wonders if this instability, as a mechanism able to concentrate magnetic fields in the near surface layers, may play a role in the formation of sunspots, starting from a weak dynamo-generated field throughout the convection zone rather than from strong flux tubes stored at the bottom. A generalization of the studied case is ongoing. / <p>At the time of the the doctoral defence the following paper was unpublished and had a status as follows: Paper nr 7: Submitted</p>