The ocean currents flowing through the earth's main magnetic field are known to induce secondary magnetic fields. Hence, variations in the ocean circulation induce variations in the net magnetic field. This research is aimed at exploring the potential for using geomagnetic data to study variability in ocean circulation and climate. / First, general relativity theory is used to formally establish the proper set of electromagnetic equations to be used for observers in a rotating (accelerating) frame of reference observing a medium (the ocean, in this case) with relative velocity. Extra terms due to rotation are derived and described and a generalized Schiff's charge density is shown to be potentially significant for the application to ocean circulation. / We extend the theory of electromagnetic fields generated by ocean currents. Many analytical solutions are found for idealized ocean features including sheared flow, jets, and a Stommel gyre. Results indicate that the ocean-induced magnetic fields will typically have magnitudes of 10's-100's of nT within the ocean. Outside of the ocean, the magnitudes are smaller (typically 1-10 nT) but decay on scales set by the horizontal scale of the ocean feature. / We investigate the time-scales associated with the adjustment of electromagnetic fields generated by low-frequency ocean currents. We find that the time scales can be quite long, prohibiting a quasistatic assumption in the treatment of the electromagnetic fields generated by the important tidal, inertial, and diurnal-frequency ocean currents. / Three-dimensional explicit time-dependent and steady-state finite-difference numerical models are constructed to study the electromagnetic fields generated by more realistic ocean current and conductivity features. / The ocean currents generate electromagnetic forces on the fluid at the surface of the earth's core. If these forces lead to significant core motion, the effect of the oceans on the generation and variability of the earth's magnetic field may be nontrivial. We estimate the form and magnitude of these forces and make comparisons with observations. Despite many uncertainties, we find evidence to suggest the ocean forcing mechanism may be significant and conclude that this process should be further investigated in the context of a larger study. / This work indicates that it is likely that the geomagnetic record has captured oceanic signals. From a preliminary data analyses we find that aside from the oceanic tidal signals, the magnetic record shows other signals of possible oceanic origin including an apparent correlation between magnetic records from the equatorial Pacific and the Southern Oscillation Index. We discuss the prerequisites that are necessary to extract information about ocean circulation variability from the geomagnetic record.

Ferromagnetic/ferromagnetic Ni/Co multilayers with component layer thicknesses ranging from 40 A down to 5 A were prepared by DC-magnetron sputtering. Due to the fact that Ni and Co alloys share a common d band, it is expected that the total resistance of the multilayers, including the elemental resistance of the layers and the resistance of the interdiffused alloyed region at the interfaces, will be lower than for other 3d transition metal combinations. Consequently, the magnetoresistance ratio $ Delta rho/ rho$ is expected to be enhanced. / Structural characterization by grazing-angle X-ray reflectivity reveals high-quality layered structures with a well-defined composition modulation along the film growth direction. Wide-angle X-ray diffraction scans display the polycrystalline nature of the Ni/Co multilayers which grow in an FCC phase with a preferred (111) orientation and a fraction of (200) structural domains. / Measurements of the magnetotransport properties of these multilayers indicate that the magnetoresistance (MR) effect, $ Delta rho sim0.35 mu Omega cdot$cm, is roughly constant over the entire compositional range. The MR ratio $ Delta rho/ rho,$ which is as high as 3.0% in a Si/(Ni40A/Co5A) $ times$ 6 multilayer, is therefore more strongly dependent on the zero-field resistivity $ rho.$ By fitting a semi-classical model to the resistivity compositional variation, we determined the interface contribution to the resistivity. The MR measurements as well as the magnetic anisotropy of the films, studied by vibrating sample magnetometry (VSM) and magneto-optical Kerr effect (MOKE) magnetometry, are consistent with the origin of the observed MR effect being anisotropic magnetoresistance (AMR). The highest magnetic sensitivity measured at zero-field and constant in the range from ${ sim}{-}10$ Oe to +10 Oe was 0.1%/Oe. This value compares well with other alloys being developed as magnetic sensors. (Abstract shortened by UMI.)

<p> Mapping of radio frequency (RF) energy deposition has been challenging for 50+ years, especially, when scanning patients in the magnetic resonance imaging (MRI) environment. As result, electromagnetic simulation software is often used for estimating the specific absorption rate (SAR), the rate of RF energy deposition in tissue. The thesis work presents challenges associated with aligning information provided by electromagnetic simulation and MRI experiments. As result of the limitations of simulations, experimental methods for the quantification of SAR were established. A system for quantification of the total RF energy deposition was developed for parallel transmit MRI (a system that uses multiple antennas to excite and image the body). The system is capable of monitoring and predicting channel-by-channel RF energy deposition, whole body SAR and capable of tracking potential hardware failures that occur in the transmit chain and may cause the deposition of excessive energy into patients. Similarly, we demonstrated that local RF power deposition can be mapped and predicted for parallel transmit systems based on a series of MRI temperature mapping acquisitions. Resulting from the work, we developed tools for optimal reconstruction temperature maps from MRI acquisitions. The tools developed for temperature mapping paved the way for utilizing MRI as a diagnostic tool for evaluation of RF/microwave emitting device safety. Quantification of the RF energy was demonstrated for both MRI compatible and non-MRI-compatible devices (such as cell phones), while having the advantage of being noninvasive, of providing millimeter resolution and high accuracy.</p>

<p> We present a fourth-order accurate, multilevel Maxwell solver, discretized in space with a finite volume approach and advanced in time with the classical fourth-order Runge Kutta method (RK4). Electric fields are decomposed into divergence-free and curl-free parts; we solve for the divergence-free parts of Faraday's Law and the Amp&egrave;re-Maxwell Law while imposing Gauss' Laws as initial conditions. We employ a damping scheme inspired by the Advanced Weather Research and Forecasting Model to eliminate non-physical waves reflected off of coarse-fine grid boundaries, and Kreiss-Oliger artificial dissipation to remove standing wave instabilities. Surprisingly, artificial dissipation appears to damp the spuriously reflected waves at least as effectively as the atmospheric community's damping scheme.</p>

<p> Historically, the National Aeronautics and Space Administration (NASA) has used rocket-powered vehicles as launch vehicles for access to space. A familiar example is the Space Shuttle launch system. These vehicles carry both fuel and oxidizer onboard. If an external oxidizer (such as the Earth's atmosphere) is utilized, the need to carry an onboard oxidizer is eliminated, and future launch vehicles could carry a larger payload into orbit at a fraction of the total fuel expenditure. For this reason, NASA is currently researching the use of air-breathing engines to power the first stage of two-stage-to-orbit hypersonic launch systems. Removing the need to carry an onboard oxidizer leads also to reductions in total vehicle weight at liftoff. This in turn reduces the total mass of propellant required, and thus decreases the cost of carrying a specific payload into orbit or beyond. However, achieving hypersonic flight with air-breathing jet engines has several technical challenges. These challenges, such as the mode transition from supersonic to hypersonic engine operation, are under study in NASA's Fundamental Aeronautics Program. </p><p> One propulsion concept that is being explored is a magnetohydrodynamic (MHD) energy- bypass generator coupled with an off-the-shelf turbojet/turbofan. It is anticipated that this engine will be capable of operation from takeoff to Mach 7 in a single flowpath without mode transition. The MHD energy bypass consists of an MHD generator placed directly upstream of the engine, and converts a portion of the enthalpy of the inlet flow through the engine into electrical current. This reduction in flow enthalpy corresponds to a reduced Mach number at the turbojet inlet so that the engine stays within its design constraints. Furthermore, the generated electrical current may then be used to power aircraft systems or an MHD accelerator positioned downstream of the turbojet. The MHD accelerator operates in reverse of the MHD generator, re-accelerating the exhaust flow from the engine by converting electrical current back into flow enthalpy to increase thrust. Though there has been considerable research into the use of MHD generators to produce electricity for industrial power plants, interest in the technology for flight-weight aerospace applications has developed only recently. </p><p> In this research, electromagnetic fields coupled with weakly ionzed gases to slow hypersonic airflow were investigated within the confines of an MHD energy-bypass system with the goal of showing that it is possible for an air-breathing engine to transition from takeoff to Mach 7 without carrying a rocket propulsion system along with it. The MHD energy-bypass system was modeled for use on a supersonic turbojet engine. The model included all components envisioned for an MHD energy-bypass system; two preionizers, an MHD generator, and an MHD accelerator. A thermodynamic cycle analysis of the hypothesized MHD energy-bypass system on an existing supersonic turbojet engine was completed. In addition, a detailed thermodynamic, plasmadynamic, and electromagnetic analysis was combined to offer a single, comprehensive model to describe more fully the proper plasma flows and magnetic fields required for successful operation of the MHD energy bypass system. </p><p> The unique contribution of this research involved modeling the current density, temperature, velocity, pressure, electric field, Hall parameter, and electrical power throughout an annular MHD generator and an annular MHD accelerator taking into account an external magnetic field within a moving flow field, collisions of electrons with neutral particles in an ionized flow field, and collisions of ions with neutral particles in an ionized flow field (ion slip). In previous research, the ion slip term has not been considered. </p><p> The MHD energy-bypass system model showed that it is possible to expand the operating range of a supersonic jet engine from a maximum of Mach 3.5 to a maximum of Mach 7. The inclusion of ion slip within the analysis further showed that it is possible to 'drive' this system with maximum magnetic fields of 3 T and with maximum conductivity levels of 11 mhos/m. These operating parameters better the previous findings of 5 T and 10 mhos/m, and reveal that taking into account collisions between ions and neutral particles within a weakly ionized flow provides a more realistic model with added benefits of lower magnetic fields and conductivity levels especially at the higher Mach numbers. (Abstract shortened by UMI.)</p>

We compute conductance fluctuations in a variety of disordered mesoscopic systems through direct numerical evaluation of the Kubo-Greenwood formula for the conductivity. / Our model Hamiltonian is quite different from the Anderson tight-binding Hamiltonian which has been most commonly used in studies of electronic structure and properties of disordered systems. It is reminiscent of the Kronig-Penney model in that $ delta$-like atomic potentials are specified by a single parameter. / Our model structures range from substitutional binary alloys to topologically disordered "glasses", and include systems where the disorder is caused by random small displacements of atoms from their crystalline lattice positions. / We test the universal nature of conductance fluctuations for the model with substitutional disorder. We study systems which are always larger than the elastic mean free path, but not always smaller than the localization length of the electron wavefunctions. In those systems where universality is expected, we confirm the ergodic theorem of Lee and Stone and observe the universal amplitudes of the fluctuations both in two-dimensional systems with and without a magnetic field, and in quasi-one-dimensional geometry. / We have also performed the first studies of conductance fluctuations in strongly disordered systems and observed a universal relationship between the amplitude of the fluctuations and the value of the conductance itself: this relationship does not depend upon the nature of the disorder.

A theoretical and experimental study of the spatial spread of electron dose distributions for pencil and broad electron beams in various media is presented. We discuss in detail two methods for measurement of electron beam kinetic energies, describe our technique for the measurement of the optical density versus dose relationship for radiographic films, and present the experimental set-up for the measurement of the spatial spread of electron dose distributions for pencil and broad electron beams. / The Fermi-Eyges theory for the small angle multiple Coulomb scattering describes the spatial electron distribution in scattering media. The spatial spread of a pencil electron beam in a phantom as predicted by the Fermi-Eyges theory is an increasing function of depth in phantom irrespective of the depth. Our experiments, on the other hand, show that the spatial spread indeed increases with depth until depths close to 2/3 of the practical electron range, but at larger depths the spatial spread saturates, then decreases and vanishes at depths greater than the range of electrons in the material. To describe the observed saturation and the decrease of the spatial spread at depths beyond 2/3 of the practical electron range we introduce an electron absorption term containing four empirical parameters into the original Fermi differential equation and show that its solution describes the experimental results obtained for pencil electron beams of various energies in polystyrene, cork and aluminum phantoms. We also compare with experimental results the spatial electron dose distributions predicted by the Fermi-Eyges theory and the modified Fermi-Eyges theory for collimated broad electron beams.

Thesis (Ph.D.)--The University of Wisconsin - Madison, 2007. / (UMI)AAI3261460. Source: Dissertation Abstracts International, Volume: 68-04, Section: B, page: 2532. Adviser: Thomas A. Lipo.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006. / Source: Dissertation Abstracts International, Volume: 67-11, Section: B, page: 6633. Adviser: Weng Cho Chew. Includes bibliographical references (leaves 143-146) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007. / Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 1068. Adviser: Paul T. Debevec. Includes bibliographical references (leaves 94-100) Available on microfilm from Pro Quest Information and Learning.