Magnetic field-assisted freeze-casting was used to create porous B4C ceramic preforms. An optimum slurry consisted of a mixture of B4C powders with 6 wt.% Er2O3 powder in an H2O-PVA solution and was cooled at a rate of 1 °C/min from room temperature to -30 °C resulting in porous green state ceramic preform with vertical channels. The Er2O3 powder was added to improve the magnetic response of the slurry. The preform was then sublimated to remove H2O and then sintered. The sintered ceramic preform was then infiltrated in the most vertically aligned channel direction with molten Al (A356) metal through a vacuum-assisted pump to create the metal matrix composite (MMC). Finite element analysis simulations were used to analyze and predict the anisotropic effect of B4C channel alignment on mechanical properties. The mechanical properties of the composite were then experimentally found via compression testing, which was compared with rule-of-mixtures and finite element modeling simulations, to analyze the effect of anisotropy due to magnetic field-assisted freeze-casting. This study reinforces the viability of cost-effective magnetic field-assisted freeze-casting as a method to create highly directional ceramic preforms, which can be subsequently metal infiltrated to produce MMCs with highly anisotropic toughness.

The production and control of antihydrogen at very low temperatures provided a key tool to test the validity for the antimaterial of the fundamental principles of the interactions of nature such as the weak principle of equivalence (WEP), and CPT symmetry (Charge, Parity, and Time reversal). The work presented in this dissertation studies the collisions of electrons and positrons in strong magnetic fields that generate magnetobound positronium (positron-electron system temporarily bound due to the presence of a magnetic field) and its possible role in the generation of antihydrogen.

Our Sun is a variable star. The magnetic fields in the Sun play an important role for the existence of a wide variety of phenomena on the Sun. Among those, sunspots are the slowly evolving features of the Sun but solar ares and coronal mass ejections are highly dynamic phenomena. Hence, the solar magnetic fields could affect the Earth directly or indirectly through the Sun's open magnetic flux, solar wind, solar are, coronal mass ejections and total solar irradiance variations. These large scale magnetic fields originate due to Magnetohydrodynamic dynamo process inside the solar convection zone converting the kinetic energy of the plasma motions into the magnetic energy. Currently the most promising model to understand the large scale magnetic fields of the Sun is the Flux Transport Dynamo (FTD) model. FTD models are mostly axisymmetric models, though the non-axisymmetric 3D FTD models are started to develop recently. In these models, we assume the total magnetic fields of the Sun consist of poloidal and toroidal components and solve the magnetic induction equation kinematicaly in the sense that velocity fields are invoked motivated from the observations. Differential rotation stretches the poloidal field to generate the toroidal field. When toroidal eld near the bottom of the convection zone become magnetically buoyant, it rises through the solar convection zone and pierce the surface to create bipolar sunspots. While rising through the solar convection zone, the Coriolis force keeps on acting on the flux tube, which introduces a tilt angle between bipolar sunspots. Since the sunspots are the dense region of magnetic fields, they diffuse away after emergence. The leading polarity sunspots (close to equator) from both the hemisphere cancel each other across the equator and trailing polarity sunspots migrate towards the pole to generate effective poloidal fields. This mechanism for generation of poloidal field from the decay of sunspots is known as Babcock-Leighton process. After the poloidal field is generated, the meridional flow carries this field to the pole and further to the bottom of the convection zone where differential rotation again acts on it to generate toroidal field. Hence the solar dynamo goes on by oscillation between the poloidal field and toroidal field, where they can sustain each other through a cyclic feedback process. Just like other physical models, FTD models have various assumptions and approximations to incorporate these different processes. Some of the assumptions are observationally verified and some of them are not. Considering the availability of observed data, many approximations have been made in these models on the theoretical basis. In this thesis, we present various studies leading to better understanding of the different processes and parameters of FTD models, which include magnetic buoyancy, meridional circulation and Babcock-Leighton process. In the introductory Chapter 1, we first present the observational features of the solar magnetic fields, theoretical background of the FTD models and motivation for investigating different processes. Most of the results of our work are presented in Chapters 2 - 7. In the Chapters 2 - 5, we explain various important issues regarding the treatment of magnetic buoyancy, irregularities of the solar cycle during descending phase, effect of different spatial structure of meridional flow on the dynamo and how dynamo generated fields would a ect the meridional ow using 2D axisymmetric Flux Transport Dynamo model. In the Chapters 6 & 7, the build up of polar fields from the decay of sunspots and a proper treatment of Babcock-Leighton process by invoking realistic convective flows, are presented using 3D Flux Transport Dynamo model. Finally the conclusions and future works are given in the Chapter 8. In 2D axisymmetric Flux Transport Dynamo models, the rise of the toroidal magnetic field through the convection zone due to magnetic buoyancy and then the generation of the poloidal magnetic field from these bipolar sunspots, has been treated mainly in two ways|a non-local method and a local method. In Chapter 2, we have analyzed the advantages and disadvantages of both the methods. We find that none of them are satisfactory to depict the correct picture of magnetic buoyancy because it is an inherently 3D process. Unless we go to the 3D framework of Flux Transport Dynamo models, we have to treat the magnetic buoyancy in such simplistic way. We find that the non-local treatment of magnetic buoyancy is very robust for a large span of parameter space but it does not take into account the depletion of flux from the bottom of the convection zone which has a significant importance in irregularity study of the solar cycle. The local treatment of magnetic buoyancy includes the flux depletion from the bottom of the convection zone and treats the magnetic buoyancy much realistically than the non-local treatment. But this local treatment of magnetic buoyancy is not so robust. We also pointed out that the long-standing issue about appearance of sunspots in the low-latitudes needs to be studied carefully. In Chapter 3, we have studied various irregularities of the solar cycle during its decaying phase. We have reported that the decay rate of the cycle is strongly correlated with amplitude of the same cycle as well as the amplitude of the next cycle from different sunspot proxies like sunspot number, sunspot area and 10.7 cm radio flux data. We explain these correlation from flux transport dynamo models. We nd that the correlations can only be reproduced if we introduce stochastic fluctuations in the meridional circulations. We also reproduced most of the correlation found in ascending and descending phase of the solar cycle from century long sunspot area data (Mandal et al., 2017) from Kodaikanal observatory, India which are in great agreement with the correlations found earlier from Greenwich sunspots data. In most of the FTD models, a single cell meridional circulation is assumed within the solar convection zone, with the equatorward return flow at its bottom. But with recent development in helioseismology, plenty of results have come out about various spatial structure of meridional circulation (Zhao et al., 2013; Schad et al., 2013; Rajaguru & Antia, 2015; Jackiewicz et al., 2015). Some helioseismology group (Zhao et al., 2013) reported that the meridional circulation has a double cell structure in solar convection zone and some groups (Schad et al., 2013; Jackiewicz et al., 2015) have reported a multi-cellular structure of meridional circulation in the convection zone. By probing the supergranular motion Hathaway (2012) estimated that the meridional ow has an equatorward return ow at the upper convection zone 70 Mm below the surface. In view of the above observed results, we have discussed in Chapter 4 what would happen to Flux Transport Dynamo model if we consider other structure of meridional circulation instead of single cell meridional circulation encompassing whole convection zone. We nd that the our dynamo model works perfectly ne as long as there is an equatorward propagation at the bottom of the convection zone. Our model also works with shallow meridional circulation as found by Hathaway (2012), if we consider the latitudinal pumping in our model. The temporal variation of meridional circulation on the surface is also observed from various measurement techniques. Chou & Dai (2001) rst observed a variation of meridional circulation with the solar cycle from their helioseismic measurements. Hathaway & Rightmire (2010) also found a variation up to 5 m s 1 for the solar cycle 23 by measuring the magnetic elements on the surface of the Sun. Recently Komm et al. (2015) have analyzed MDI and HMI Dopplergram data and reported a solar cyclic variation with detail latitudinal dependence. To explain this variation of the meridional circulation with the solar cycle, we construct a theoretical model by coupling the equation of the meridional circulation (the component of the vorticity equation within the solar convection zone) with the equations of the flux transport dynamo model in Chapter 5. We consider the back reaction due to the Lorentz force of the dynamo-generated magnetic fields and study the perturbations produced in the meridional circulation due to it. This enables us to model the variations of the meridional circulation without developing a full theory of the meridional circulation itself. We obtain results which reproduce the observational data of solar cycle variations of the meridional circulation reasonably well. We get the best results on assuming the turbulent viscosity acting on the velocity field to be comparable to the magnetic diffusivity (i.e. on assuming the magnetic Prandtl number to be close to unity). We have to assume an appropriate bottom boundary condition to ensure that the Lorentz force cannot drive a flow in the sub-adiabatic layers below the bottom of the tachocline. Our results are sensitive to this bottom boundary condition. We also suggest a hypothesis how the observed inward flow towards the active regions may be produced. In Chapter 6 and Chapter 7, we have studied some of the aspects of solar magnetic eld generation process using 3D dynamo model that were not possible to study earlier using axisymmetric 2D Flux Transport dynamo models. We have used the 3D dynamo model developed by Mark Miesch (Miesch & Dikpati, 2014; Miesch & Teweldebirhan, 2016) and study how polar fields build up from the decay of sunspots more realistically in Chapter 6. We first reproduce the observed butter y diagram and periodic solution considering higher diffusivity value than earlier reported results and use it as a reference model to study the build up polar fields by putting a single sunspot pair in one hemisphere and two sunspot pairs in both the hemispheres. The build up of the polar fields from the decay of sunspots are studied earlier using Surface Flux Transport model (Wang et al., 1989; Baumann et al., 2004; Cameron et al., 2010) which solve only radial component of the induction equation on the surface of the Sun ( | plane). But these 2D SFT models have some inherent limitation for not considering the 3D vectorial nature of the magnetic fields and subsurface processes. We have shown that not considering the vectorial nature and subsurface process has an important effect on the development of the polar fields. We have also studied the effect of a few large sunspot pairs violating Hale's law on the strength of the polar field in this Chapter. We nd that such ant-Hale sunspot pairs do produce some effect on the polar fields, if they appear at higher latitudes during the mid-phase of the solar cycle|but the effect is not dramatic. In Chapter 7, we have incorporated observed surface convective ows directly in our 3D dynamo model. As we know that the observed convective flows on the photosphere (e.g., supergranulation, granulation) play a key role in the Babcock-Leighton (BL) process to generate large scale polar fields from sunspots fields. In most surface flux transport (SFT) and BL dynamo models, the dispersal and migration of surface fields is modeled as an effective turbulent diffusion. Recent SFT models have incorporated explicit, realistic convective flows in order to improve the fidelity of convective transport but, to our knowledge, this has not yet been implemented in previous BL models. Since most Flux-Transport (FT)/BL models are axisymmetric, they do not have the capacity to include such flows. We present the first kinematic 3D FT/BL model to explicitly incorporate realistic convective flows based on solar observations. Though we describe a means to generalize these flows to 3D, we find that the kinematic small-scale dynamo action they produce disrupts the operation of the cyclic dynamo. Cyclic solution is found by limiting the convective flow to surface flux transport. The results obtained are generally in good agreement with the observed surface flux evolution and with non-convective models that have a turbulent diffusivity on the order of 3 1012 cm 2 s 1 (300 km2 s 1). However, we nd that the use of a turbulent diffusivity underestimates the dynamo efficiency, producing weaker mean fields than in the convective models. Also, the convective models exhibit mixed polarity bands in the polar regions that have no counterpart in solar observations. Also, the explicitly computed turbulent electromotive force (emf) bears little resemblance to a diffusive flux. We also find that the poleward migration speed of poloidal flux is determined mainly by the meridional flow and the vertical diffusion.

Presented dissertation discusses the influence of electromagnetic fields on the weight of the particles, which are considered in this work as substances with the hub. Theoretical knowledge was practically tested on plant organisms (early somatic embryos, fungal pathogens), theoretically the influence of electromagnetic field on cell structure was modeled. It was subsequently confirmed by a practical experiment assumption about the influence of electromagnetic fields on the weight of the particles. These experiments were preceded by a theoretical study of the problem and the formulation of a solution using Maxwell's equations, from which other descriptive equations and formulas were derived. The results of the experiments were presented at several professional conferences and published in professional journals and proceedings.

This study focused on determining the groundwater flow paths in the crystalline subsurface rocks of the Bulawayo metropolitan, Zimbabwe, through analysing the discontinuities of the electrical properties of the ground, as well as in the magnetic field of the underlying rocks. Further, borehole water quality was analysed by measuring and mapping several chemical parameters, specifically TDS, salinity and the electrical conductivity. The electrical and magnetic anomalies were measured at two field sites within the Bulawayo province, the Harry Allen Golf Course and the Barbour Fields dumpsite, while 120 boreholes were sampled for water quality in a large part the province. Two magnetometers were used to measure the magnetic field and the time and location of the measurements, which resulted, after processing in SURFER, into the magnetic field map of these areas. This was used, in conjunction with the geologic map of the Bulawayo province, in order to determine useful locations for the electrical resistivity surveys. These included electrical resistivity tomography and vertical electrical sounding and were performed with an earth resistivity/induced polarization (IP) meter in order to measure the electrical resistivity of the ground. Inverse modelling was used in the RES2DINV software program to produce the topographic image of resistivity. The results for the borehole sampling showed that the maps for the three chemical parameters were very similar, with the western and northern parts of the mapped area displaying higher concentration values. The results of the electrical resistivity surveying showed probable areas of groundwater flow with its relationship to electric conductivity.

In the present PhD work, three sophisticated models based on the "Engineering" modeling approach have been utilized to conveniently describe and thoroughly analyze details of Lightning events at the CN Tower. Both the CN Tower and the Lightning Channel are represented by a number of connected in series Transmission Line sections in order to account for the variations in the shape of the tower and for plasma processes that take place within the Lightning Channel. A sum of two Heidler functions is used to describe the "uncontaminated" Return Stroke current, which is injected at the attachment point between the CN Tower and the Lightning Channel. Reflections and refractions at all points of mismatched impedances are considered until their contribution becomes less than 1% of the originally injected current wave. In the proposed models, the problem with the current discontinuity at the Lightning Channel front, commonly taken care of by introducing a "turn-on" term when computing radiation fields, is uniquely treated by introducing reflected and transmitted components. For the first time, variable speed of propagation of the Return Stroke current front has been considered and its influence upon the predicted current distributions along the whole Lightning Channel path and upon the radiated distant fields analyzed. Furthermore, as another novelty, computation of the electromagnetic field is accomplished in Cartesian Coordinates. This fact permits to relax the requirement on the verticality of the Lightning Channel, normally imposed in Cylindrical Coordinates. Therefore, it becomes possible to study without difficulty the influence of a slanted Lightning Channel upon the surrounding electromagnetic field. Since the proposed sophisticated Five-Section Model has the capability to represent very closely the structure of the CN Tower and to emulate faithfully the shape of, as well as physical processes within the Lightning Channel, it is believed to have the potential of truthfully reproducing observed fields. The developed modeling approach can be easily adapted to study the anticipated radiated fields at tall structures even before construction.

Arc Flash events occur due to faults in electrical equipment combined with a significant release of energy across an electrical arc. Due to the large energy release, plasma is generated, pressures increase, and the plasma expands. Under these conditions the plasma becomes excited enough to liquefy metal causing physical damage to equipment and any humans in the vicinity. This thesis investigates the state of art for detection of arc flash events and investigates a method of improving detection reliability, and speed by monitoring the high frequency voltage / current patterns utilizing methods similar to arc flash circuit interrupters (AFCI). A second alternative detection approach is determined through analysis of the physics of plasma development. The current state of art is based upon light detection. However this thesis experimentally investigates what happens before the arc event emits visible light. The results show that current flows to ground during an arc event slightly prior to the production of light. Further it shows through analysis of the physics of plasma that a high speed plasma detector has the potential to identify an arc event before the presence of visible light. Through the design and construction of experimental test setups, and physics analysis, this thesis provides new paths for detecting arc events that present opportunities to improve detection time.

<p>Neuronal current MRI (ncMRI) is a field of study to directly map electrical activity in the brain using MRI, which has many benefits over functional MRI. One potential ncMRI method, Lorentz effect imaging (LEI), has shown promise but needs a better theoretical understanding to improve its use.</p><p>We develop three computational models to simulate the LEI experiments of an electrolyte filled phantom subject to a current dipole based on: ion flow, particle drift, and electromagnetohydrodynamics (EMHD). With comparative experimental results, we use the EMHD model to better understand the Lorentz effect over a range of current strengths. We also quantify the LEI experimental images and assess ways to measure the underlying current strength, which would greatly benefit comparative brain mapping.</p><p>EMHD is a good predictor of LEI signal loss. We can measure the underlying current strength and polarity in the phantom using LEI images. We can also use trends from the EMHD model results to predict the required current density for signal detection in future LEI experiments. We can also infer the electric field strength, flow velocity, displacement, and pressure from the predicted current magnitude in an LEI experiment.</p><p>The EMHD model provides information that greatly improves the utility and understanding of LEI. Future study with our EMHD model should be performed using shorter dipole lengths, higher density and lower strength of current sources, and varying current source frequencies to understand LEI in the setting of mapping brain activity.</p> / Dissertation

Magnetic fields are fundamental to the structure and dynamics of the Sun’s corona. Observations show them to be locally complex, with highly sheared and twisted fields visible in solar filaments/prominences. The free magnetic energy contained in such fields is the primary source of energy for coronal mass ejections, which are important—but still poorly understood drivers of space weather in the near-Earth environment. In this thesis, a new model is developed for the evolution of the large-scale magnetic field in the global solar corona. The model is based on observations of the radial magnetic field on the solar photosphere (visible surface). New active regions emerge, and their transport and dispersal by surface motions are simulated accurately with a surface flux transport model. The 3D coronal magnetic field is evolved in response to these photospheric motions using a magneto-frictional technique. The resulting sequence of nonlinear force-free equilibria traces the build-up of magnetic helicity and free energy over many months. The global model is applied to study two phenomena: filaments and coronal mass ejections. The magnetic field directions in a large sample of observed filaments are compared with a 6-month simulation. Depending on the twist of newly-emerging active regions, the correct chirality is simulated for up to 96% of filaments tested. On the basis of these simulations, an explanation for the observed hemispheric pattern of filament chirality is put forward, including why exceptions occur for filaments in certain locations. Twisted magnetic flux ropes develop in the simulations, often losing equilibrium and lifting off, removing helicity. The physical basis for such losses of equilibrium is demonstrated through 2D analytical models. In the 3D global simulations, the twist of emerging regions is a key parameter controlling the number of lift-offs, which may explain around a third of observed coronal mass ejections.

In this thesis, we investigate the collective electron dynamics in single and coupled superlattice systems under the influence of a DC electric field. Firstly, we illustrate that Bloch oscillations suppress electron transport and the resulting charge domains form self-sustained current oscillations. Upon the application of a tilted magnetic field, stochastic web structures are shown to form in the phase space of the electron trajectory. This occurs only when the Bloch and cyclotron frequencies are commensurate allowing the electrons to demonstrate chaotic unbounded trajectories, leading to an increase in transport. The charge domain dynamics also present additional peaks during such resonances. The rapid changes in the dynamical states found is an example of non-KAM chaos. We show then the amplitude and frequency of current oscillations in a single superlattice can be controlled. Secondly, two models are designed to mutually couple two semiconductor superlattices by a common resistive load. We examine the effects of coupling strength and frequency detuning on the collective current dynamics. The devices are considered to be arranged together on a single substrate as well as on individual substrates. Large AC power is witnessed during anti-phase and in-phase synchronization between current oscillations. Finally, two superlattices are coupled through a resonance circuit incorporating single mode resonances from external influences in the circuit. In this system, chaotic current dynamics are induced with regions of chaos separating different regions of synchronization. High frequency oscillations with minimal phase difference cause the largest power generation. In all three coupling models high frequency components are found in the Fourier power spectra. The power generated in the coupled systems is found greater and at times more than double the power generated in the autonomous superlattice. Thus this thesis provides innovative methods of enhancing and controlling powerful high-frequency signals. This effectively gives manipulation over the intensity of the electromagnetic radiation produced by the superlattice.