In this thesis, a simple model of the stretched magnetic field lines in the nightside magnetotail was created. The nightside magnetosphere model contains four main regions: plasmasphere, plasma sheet, magnetic lobes, and low latitude boundary layers. The plasma sheet is split into three regions based on the shape of the closed field lines present: dipole plasma sheet, transition plasma sheet, and stretched plasma sheet (SPS). The SPS, the focus of this thesis, is split into two regions: disruption zones (DZs) and a central neutral sheet (NS). The shape of the stretched field lines contain four inflection points. The convex curvature regions form the DZs and the central concave curvature region forms the NS. The NS is split into two regions: outer neutral sheet (ONS) and inner neutral sheet (INS). Due to the reversal of the x-component of the magnetic field at the center line of the NS, the protons are magnetized in the ONS and "unmagnetized" in the INS. There are two main current systems in the SPS. The first is a double vortex current system consisting of eastward current in the DZs that closes westward in the NS. The second system is the NS field-aligned current (FAC) system. It is generated in the INS mainly by the earthward convective drift of the electrons while the "unmagnetized" protons have little convective drift and remain tailward of the electrons. This FAC system produces the pre-onset electron auroral arc during the growth phase of the substorm. A simple model of the stretched magnetic field lines was created in order to calculate the current systems present in the SPS. The simple model was based entirely upon the shape of the stretched field lines. It passed two physical tests, divergence of the magnetic field and limits at infinity, so it was used to calculate currents. The total current using Ampere's law and the curvature current was found. Both results agreed with the double vortex current system.

No description available.

The Swiss Free Electron Laser (SwissFEL) at the Paul Scherrer Institute is a national prestige project that will enable ground breaking new x-ray scattering experiments in areas such as biology, chemistry and physics. A plannedactivity is to generate possibility for x-ray diffraction under high pulsed magnetic fields to explore quantum mattermaterials. In fact, an entire beam line (CristallinaQ), dedicated to extreme sample environment (vacuum, electro-magnetic field, low temperature).This Master’s thesis project concerns the development of a magnet system for pulsed magnetic fields to be synchronised with the free electron laser pulses. The system is based on small-sized coils. This makes the systemtransportable and avoids the huge financial challenges and power requirements of the magnets at pulsed fields laboratories at Toulouse, Dresden or Tallahassee. Ultimately the magnet shall provide large pulsed fields of more than 30 T under conditions very similar to space, i.e. vacuum, low-temperature.The thesis presents the development of a complete coil manufacture and testing setup including a capacitor bank topower the magnet. With planned upgrades of the equipment, the coil manufacturing process is reaching reproduceable levels. I produce a first iteration of magnet coils. They follow a classical copper conductor design reinforcedwith an epoxy-Zylon matrix. During testing we produced 15 Tesla fields without degradation of the coils. At lastI analyse the observations from the tests and propose improvements and future steps for the further developmentof the magnet.

In near-Earth space, highly spatio-temporally variant magnetic fields result from solar-terrestrial magnetic interaction. These near-Earth external fields currently represent the largest source of error in efforts to model the magnetic field produced in the Earth’s interior. Starting in 1999, the Decade of Geopotential Field Research (Friis-Christensen et al., 2009) has greatly increased the amount of available low-Earth orbit (LEO) satellite magnetic data. These data have driven many advances in field modelling, yet have highlighted that LEO measurements are particularly susceptible to contamination from external fields. This thesis presents a series of studies attempting to describe the external fields in more detail, in order that they can be more effectively separated from the internal fields in magnetic modelling efforts. A range of analysis methods, different for each study, are applied to satellite and ground-based observatory data. Mandea and Olsen’s (2006) method of estimating the secular variation (SV) of the internal field from satellite data via ‘Virtual Observatories’ (VOs) is applied to synthetic data from the upcoming Swarm constellation satellite mission of the European Space Agency. Beggan (2009) found VOs constructed from CHAMP satellite data to be contaminated with external field signals which appeared to have a significant local time (LT) dependence. I find that utilising the increased coverage of LT sectors offered by the Swarm constellation geometry does not significantly decrease the contamination. Following this surprising result I tested a wide range of methods aimed at reducing the VO contamination from each parameterised external field source region. In anticipation of future studies using real data, I used the results of the tests to provide a more complete description of the external field variations affecting analyses of geographically-fixed magnetic phenomena when using satellite data and spherical harmonic analysis (SHA). Ionospheric electric currents flowing at LEO altitudes are known to violate the assumption of measurements taken in a source-free space, required in SHA-based models of the magnetic field. In order to better describe the electromagnetic environment at LEO altitudes, I use data from the Ørsted and CHAMP satellites to calculate the current density from Amp`ere’s integral. Vector magnetic data from discrete overflights of the two satellites (at different altitudes) are rotated into the along-track frame to define the integral loop and its ‘surface area’, permitting estimation of the predominantly zonal current density flowing in the region between the two orbital paths. I designed selection criteria to extract geometrically-stable overflights spanning the range of LTs twice in the 6 years of mutually available satellite vector data. From these overflights I resolve current densities in the range 0:1 μA=m2, with the distribution of current largely matching the LT progression of the Appleton anomaly. I applied detailed tests to check for biases intrinsic to the method, and present results free of systematic errors. The results are compared with the predictions of the CTIP (Coupled Thermosphere-Ionosphere-Plasmasphere) model of ionospheric composition and temperature, showing a typically good spatiotemporal agreement. I find persistent current intensifications between geomagnetic latitudes of 30 and 50 in the post-midnight, pre-dawn sector, a region which has been previously considered to be relatively free of currents. External fields induce currents in the Earth’s conducting mantle, the magnetic fields of which add to the field measured at and above the Earth’s surface. The morphology of the long-period inducing field is poorly resolved on timescales of months to years, reducing the accuracy of mantle induction studies (a key part of the Swarm mission). I improve the description of its morphology via the method of Empirical Orthogonal Functions (EOFs), which I apply to over a decade of ground-based observatory data. EOFs provide a decomposition of the spatiotemporal structures contained in the magnetic field data, with partitions arising from the data themselves, overcoming the relatively simplistic assumptions made about the inducing field morphology in LT. The results of vector data EOF analyses are presented, but I rely primarily on scalar analyses which are more fitting for this study. I overcome the limitations of the irregular observatory distribution with a novel spatial weighting matrix, combining the output from multiple EOF analyses to greatly improve the data coverage in LT. I find that the seasonal variation of the inducing field is more important than the variation of the symmetric ring current on annual periods, and that dawn-dusk asymmetry should be accounted for to increase the accuracy of mantle conductivity estimates based on data covering the decadal timescales of the solar cycle.

La production des dipôles supraconducteurs de la machine LHC du CERN s'est terminée en automne 2006. Les aimants fonctionnent à la température cryogénique de 1.9 K et doivent produire un champ magnétique très uniforme permettant de conduire les protons dans la machine. Le champ magnétique a été contrôlé avec beaucoup de rigueur et différentes mesures ont été réalisées pendant les différentes phases de l'assemblage des dipôles. Le but a été la découverte de défauts de production et d'assemblage qui prouvent limiter les performances des aimants. Dans le travail de thèse les effets de la variation de la géométrie des composantes mécaniques comme les câbles supraconducteur, les cales et les colliers de soutien des bobines sur l'uniformité du champ magnétique ont été étudiés. Une méthode pratique pour identifier et corriger les problèmes d'usinage a été développée et utilisée dans la phase de production. La thèse consiste en trois parties principales :<br />-Analyse de la production des principales composantes mécaniques des dipôles et étude de l'influence de la géométrie et des procédures d'assemblage sur la qualité du champ magnétique.<br />-Utilisation pratique des mesures effectuées sur les dipôles dans l'atelier d'assemblage pour résoudre les problèmes de production et comprendre le comportement de la géométrie des bobines pendant les phases d'assemblage.<br />-Etude théorique des composantes aléatoires des harmoniques du champ magnétique pour qualifier la production des dipôles.

The Earth’s magnetic field is generated by fluid motion of liquid iron in the outer core. Flows at the top of the outer core are believed to be responsible for the secular variation (SV) observed at the surface of the Earth. Modelling of this flow is open to considerable ambiguity, though methods adopting different physical assumptions do lead to similar flow velocity regimes. Some aspects of the ambiguities are investigated in this thesis. The last decade has seen a significant improvement in the capability to observe the global field at high spatial resolution. Several satellite missions have been launched, providing a rich new set of scalar and vector magnetic measurements from which to model the global field in detail. These data complement the existing record of groundbased observatories, which have continuous temporal coverage at a single point. I exploit these new data to model the secular variation (SV) globally and attempt to improve the core flow models that have been constructed to date. Using the approach developed by Mandea and Olsen (2006) I create a set of evenly distributed ‘Virtual Observatories’ (VO), at 400km above the Earth’s surface, encompassing satellite measurements from the CHAMP satellite over seven years (2001-2007), inverting the SV calculated at each VO to infer flow along the core-mantle boundary. Direct comparison of the SV generated by the flow model to the SV at individual VO can be made. Thus, the residual differences can be investigated in detail. Comparisons of residuals from flow models generated from a number of VO datasets provide evidence that they are consistent with internal and external field effects in the satellite data. I also show that the binning and processing of the VO data can induce artefacts, including sectorial banding, into the residuals. By employing the core flows from the inversion of SV data it may be possible to forecast the change of the present magnetic field (as measured) forwards in time for a short time period (e.g. less than five years) within an acceptable error budget. Using simple advection of steady or non-steady flows to forecast magnetic field change gives reasonably good fit to field models such as GRIMM, POMME or xCHAOS (< 50nT root mean square difference after five years). The forecast of the magnetic field change can be improved by optimally assimilating measurements of the field into the forecast from flow models at discrete points in time (e.g. annually). To achieve this, an Ensemble Kalman Filter (EnKF) can be used to the capture non-linearity of the model and delineate the error bounds by means of a Monte Carlo representation of the field evolution over time. In the EnKF model, an ensemble of probable state vectors (Gauss coefficients) evolve over time, driven by SV derived from core flows. The SV is randomly perturbed at each step before addition to the state vectors. The mean of the ensemble is chosen as the most likely state (i.e. field model) and the error associated with the estimate can be gauged from the standard deviation from the mean. I show an implementation of the EnKF for steady and non-steady flows generated from ‘Virtual Observatory’ field models, compared to the field models GRIMM and xCHAOS over the period 2002–2008. Using the EnKF, the maximum difference never exceeds 25nT over the period. This promising approach allows measurements to be included into model predictions to improve the forecast.

The influence of the horizontal magnetic field has not been sufficiently studied in contrast to study activity on the influence of the vertical magnetic field by researchers. The reason was that the influence of horizontal magnetic field to change the wear mass loss of ferromagnetic materials is smaller compared to the vertical magnetic field. However, the influence of horizontal magnetic field on rolling contact changes the subsurface crack initiation point toward surface is postulated by a researcher. Therefore, it is significance finding out how the horizontal magnetic field influences the tribological characteristics. This thesis presents a study on the influence of the horizontal magnetic field on wear in sliding contacts contributes for ascertainment the effect and mechanism of horizontal magnetic field on tribological characteristics of sliding contacts, through the experimental approach.The static magnetic field with densities of 0 and 1.1 Tesla and different orientations was applied to different contact conditions, different surface modifications and two sliding frequencies, using a ball-on-plate contact configuration. In conclusion, the presence of magnetic field enhances the chemical adsorption between iron or oxide iron and oxygen, and causes the transition of adhesive wear to oxidative wear. Besides, the presence of magnetic field combined with low sliding frequency forms the bulging on the wear surface and weakens the prevailing wear mechanism due to the low frictional temperature. On the other hand, the presence of magnetic field combined with high sliding frequency induces the transition to the oxidative wear mechanism and reduces the wear. Also, distinctly different appearances of wear surface are created by different magnetic field orientations. In the lubricated sliding contact, the magnetic field causes the reduction of wear and induction of oxide. It is postulated that the presence of magnetic field enhances the oxygen adsorption on the wear track by iron wear particles and hence varies the tribological behaviour. The influence of magnetic field on carbon steel coating consists in changes of oxide iron layer and steel layer, alterations of mechanical properties of the coating, and decrease in the mass loss and the surface roughness on the dry sliding contact. All these could be suggested the influence of adhesive strength of the interface between the base material and coating.

Student Number : 9807515F - MSc Dissertation - School of Computational and Applied Mathematics - Faculty of Science / Statistical and algebraic techniques of subspace tracking were tested for filtering the earth’s response from airborne magnetic field gradients in order to discriminate the relatively small response (dipole) of objects on the earth’s surface, such as UXO. Filtering the data was not very effective with these methods but a subspace was found in the data for the magnitude of the magnetic moment of the dipole. This subspace is easily obtained using the singular value decomposition and can be used for an approximate location, without depth estimation, as well as the relative size of the dipole.

Thesis advisor: Cyril P. Opeil / Thermoelectric (TE) materials are of great interest to contemporary scientists because of their ability to directly convert temperature differences into electricity, and are regarded as a promising mode of alternative energy. The TE conversion efficiency is determined by the Carnot efficiency, η_C and is relevant to a commonly used figure of merit ZT of a material. Improving the value of ZT is presently a core mission within the TE field. In order to advance our understanding of thermoelectric materials and improve their efficiency, this dissertation investigates the low-temperature behavior of the p-type thermoelectric Cu2Se through chemical doping and nanostructuring. It demonstrates a method to separate the electronic and lattice thermal conductivities in single crystal Bi2Te3, Cu, Al, Zn, and probes the electrical transport of quasi 2D bismuth textured thin films. Cu2Se is a good high temperature TE material due to its phonon-liquid electron-crystal (PLEC) properties. It shows a discontinuity in transport coefficients and ZT around a structural transition. The present work on Cu2Se at low temperatures shows that it is a promising p-type TE material in the low temperature regime and investigates the Peierls transition and charge-density wave (CDW) response to doping [1]. After entering the CDW ground state, an oscillation (wave-like fluctuation) was observed in the dc I-V curve near 50 K; this exhibits a periodic negative differential resistivity in an applied electric field due to the current. An investigation into the doping effect of Zn, Ni, and Te on the CDW ground state shows that Zn and Ni-doped Cu2Se produces an increased semiconducting energy gap and electron-phonon coupling constant, while the Te doping suppresses the Peierls transition. A similar fluctuating wave-like dc I-V curve was observed in Cu1.98Zn0.02Se near 40 K. This oscillatory behavior in the dc I-V curve was found to be insensitive to magnetic field but temperature dependent [2]. Understanding reducing thermal conductivity in TE materials is an important facet of increasing TE efficiency and potential applications. In this dissertation, a magnetothermal (MTR) resistance method is used to measure the lattice thermal conductivity, κ_ph of single crystal Bi2Te3 from 5 to 60 K. A large transverse magnetic field is applied to suppress the electronic thermal conduction while measuring thermal conductivity and electrical resistivity. The lattice thermal conductivity is then calculated by extrapolating the thermal conductivity versus electrical conductivity curve to a zero electrical conductivity value. The results show that the measured phonon thermal conductivity follows the e^(Δ_min⁄T) temperature dependence and the Lorenz ratio corresponds to the modified Sommerfeld value in the intermediate temperature range. These low-temperature experimental data and analysis on Bi2Te3 are important compliments to previous measurements and theoretical calculations at higher temperatures, 100 – 300 K. The MTR method on Bi2Te3 provides data necessary for first-principles calculations [4]. A parallel study on single crystal Cu, Al and Zn shows the applicability of the MTR method for separating κ_e and κ_ph in metals and indicates a significant deviation of the Lorenz ratio between 5 K and 60 K [3]. Elemental bismuth is a component of many TE compounds and in this dissertation magnetoresistance measurements are used investigate the effect of texturing in polycrystalline bismuth thin films. Electrical current in bismuth films with texturing such that all grains are oriented with the trigonal axis normal to the film plane is found to flow in an isotropic manner. By contrast, bismuth films with no texture such that not all grains have the same crystallographic orientation exhibit anisotropic current flow, giving rise to preferential current flow pathways in each grain depending on its orientation. Textured and non-textured bismuth thin films are examined by measuring their angle-dependent magnetoresistance at different temperatures (3 – 300 K) and applied magnetic fields (0 – 90 kOe). Experimental evidence shows that the anisotropic conduction is due to the large mass anisotropy of bismuth and is confirmed by a parallel study on an antimony thin film [5]. [1] Mengliang Yao, Weishu Liu, Xiang Chen, Zhensong Ren, Stephen Wilson, Zhifeng Ren, and Cyril Opeil, J. Alloys Compd. 699, 718 (2017). [2] Mengliang Yao, Weishu Liu, Xiang Chen, Zhensong Ren, Stephen Wilson, Zhifeng Ren, and Cyril P. Opeil, J. Materiomics 3, 150 (2017). [3] Experimental determination of phonon thermal conductivity and Lorenz ratio of single crystal metals: Al, Cu and Zn, Mengliang Yao, Mona Zebarjadi, and Cyril P. Opeil, under review. [4] Experimental determination of phonon thermal conductivity and Lorenz ratio of single crystal bismuth telluride, Mengliang Yao, Stephen Wilson, Mona Zebarjadi, and Cyril Opeil, under review. [5] Albert D. Liao, Mengliang Yao, Ferhat Katmis, Mingda Li, Shuang Tang, Jagadeesh S. Moodera, Cyril Opeil, Mildred S. Dresselhaus, Appl. Phys. Lett. 105, 063114 (2014). / Thesis (PhD) — Boston College, 2017. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Este trabalho consistiu na implementação de um método de homogeneização de campo, denominado shimming passivo. Esse método é caracterizado pela inserção de pequenas peças ferromagnéticas no interior do magneto; a interação destas peças com o campo magnético principal produz campos locais capazes de corrigir inomogeneidades indesejadas, quando várias peças são estrategicamente combinadas. Embora esse método já tenha sido bem discutido, implementado e publicado por D.I.Hoult na década de 80, ele ainda não havia sido desenvolvido para um magneto resistivo com núcleo de ferro e peças polares, como é o caso do Artro-ToRM. Nosso objetivo era, através do Artro-ToRM, encontrar uma metodologia de modelagem e correção de campo que fosse útil para magnetos com a mesma geometria. Foram utilizados métodos computacionais de ajuste numérico que, a partir de mapas originais de campo, foram capazes de encontrar configurações de peças magnéticas que pudessem reduzir as inomogeneidades presentes. Um dos maiores desafios do trabalho, foi encontrarmos elementos passivos com comportamento previsível quando submetidos ao campo magnético principal, já que os programas de otimização presupunham que trabalhávamos com dipolos magnéticos, para efeitos de simplificação. Finalmente, considerando uma região cilíndrica com raio de 5 cm e comprimento de 10 cm, mostramos uma melhora de 390 ppm para 250 ppm na homogeneidade, após a correção passiva. / This work presents the implementation of a method for field correction, called passive shimming. The method is characterized by the insertion of small iron pieces in magnet bore; the interaction of these pieces with the main magnetic field produces local fields capable of correcting undesired inhomogenity, when some parts are strategically combined. Although this method has been already proposed by D.I.Hoult in the eighties, it still has not been developed for resistive magnets with polar pieces, such as the Artro-ToRM. Our objective were to find a method for field modeling and correction that could be useful for magnets with similar geometry. Computational methods of numerical adjustment were used from the original field maps, it was possible to find the optimal the configurations of magnetic parts for reducing the field inhomogenity. One of the biggest challenges of the work, was to find passive elements with previsible behavior when submitted to magnetic field main, since the optimization programs preassumed that we worked with magnetic dipoles, for the purpose of simplification. Finally, considering a cilindrical region, we show an improvement of 390 ppm to 250 ppm in the homogenity, after the passive correction.