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Applications, converter topologies, and control schemes are examined for superconductive magnetic energy storage (SMES) systems. Diurnal load leveling for electric utilities and compensation for fluctuations in photovoltaic (PV) power generation are the primary applications discussed. It is demonstrated that a SMES system implemented with standard AC/DC converters offers energy storage capacity large enough, and dynamic response fast enough, to compensate for PV fluctuations due to changes in weather conditions. The method of control is developed so that the charging and discharging of the SMES system are changed in response to PV fluctuations, and the combined SMESIPV power output is smooth and controllable. An innovative control scheme is introduced for SMES that can simultaneously regulate real power and voltage independently without hardware modifications to the standard ACIDC bridge arrangement normally used for coordinated control of real and reactive power. The combination of SMES and PV systems could benefit from DCIDC converters that take advantage of the DC nature of both. It is established that DClDC converters can respond with sufficient speed to handle variations in PV power. A converter topology is devised where two DC/DC converters in cascade effectively maintain a PV array at its maximum power point and simultaneously control a SMES system to compensate for PV fluctuations. An alternative cascade configuration of an AC/DC converter with a DCIDC converter is proposed that could significantly reduce the reactive power requirements and improve the operational characteristics of a large scale SMES system connected to the utility grid. / Master of Science

Tam Ka Ming. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2002. / Includes bibliographical references (leaves 142-145). / Text in English; abstracts in English and Chinese. / Tam Ka Ming. / Acknowledgement --- p.ii / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Models for Many Body Systems --- p.1 / Chapter 1.2 --- High Temperature Superconductor --- p.5 / Chapter 1.3 --- Reasons of Study Double Layers --- p.9 / Chapter 1.4 --- "Incommensurate Magnetic Excitation, Order Parameters" --- p.10 / Chapter 1.5 --- Thesis Organization --- p.10 / Chapter 2 --- Monte Carlo Methods --- p.12 / Chapter 2.1 --- General Aspects of Monte Carlo Simulation --- p.12 / Chapter 2.2 --- "Markov Chain, Metropolis Algorithm, Heat-Bath Algorithm" --- p.18 / Chapter 2.3 --- Quantum Monte Carlo --- p.21 / Chapter 2.4 --- Variational Quantum Monte Carlo --- p.22 / Chapter 2.5 --- Green Function Monte Carlo --- p.27 / Chapter 2.6 --- Auxiliary Field Monte Carlo --- p.35 / Chapter 2.7 --- Summary of Quantum Monte Carlo --- p.38 / Chapter 3 --- Constrained Path Monte Carlo --- p.40 / Chapter 3.1 --- Various Formulas Used in Constrained Path Monte Carlo --- p.40 / Chapter 3.2 --- Constrained Path Monte Carlo --- p.47 / Chapter 3.3 --- Constrained Path Approximation --- p.51 / Chapter 3.4 --- Measurements --- p.52 / Chapter 3.5 --- Method of Avoiding Numerical Instability --- p.55 / Chapter 4 --- Parallelisation of CPMC Program --- p.57 / Chapter 4.1 --- Architecture of Parallel Machine --- p.57 / Chapter 4.2 --- Communication Methods in Parallel Program --- p.65 / Chapter 4.3 --- Comparison of Parallel and Serial Program --- p.74 / Chapter 4.4 --- Measure of Parallel Program --- p.85 / Chapter 4.5 --- Analysis of Serial CPMC Program --- p.89 / Chapter 4.6 --- Analysis of the Parallel CPMC Program --- p.93 / Chapter 5 --- Results from CPMC --- p.101 / Chapter 5.1 --- Hubbard Model with Next Nearest Neighbour Hopping --- p.101 / Chapter 5.2 --- Spin and Charge Correlations --- p.104 / Chapter 5.3 --- Vertex Contributions --- p.111 / Chapter 6 --- Mean Field Calculations of bilayer --- p.119 / Chapter 6.1 --- Objective --- p.119 / Chapter 6.2 --- Theoretical Model --- p.121 / Chapter 6.3 --- Method of Solving the Model --- p.122 / Chapter 6.4 --- Finding Susceptibility by RPA --- p.131 / Chapter 6.5 --- Results --- p.136 / Chapter 6.6 --- Summary --- p.140 / Chapter 7 --- Summary --- p.141 / Bibliography --- p.142

This thesis describes a research of suppression of superconducting gap in a superconducting island of a Ferromagnetic-Superconducting-Ferromagnetic Single-Electron-Transistor due to the fringing magnetic fields produced by the ferromagnetic leads. The devices are working below the critical temperature of the superconducting gap. A model is proposed to explain how the fringing magnetic field produced by the leads is strong enough to suppress the superconducting gap. The peak of the fringing magnetic field produced by one lead reaches 5000 oe. It is observed an inverse tunneling magneto resistance during the suppression of the superconducting gap, obtaining a maximum absolute value 500 times greater than the TMR in the normal state where the efficiency of the spin injection is low. It is concluded that the suppression of the superconducting gap is due to fringing magnetic field and not to the spin accumulation because the low efficiency of the spin injection. It is suggested a new geometry to reduce the effect of the fringing magnetic field so it can be obtained a suppression of the superconductivity due to the spin accumulation. It is described the qualitatively behavior of the IV characteristic when the suppression of the superconductivity is due to spin accumulation.

PhD (Physics) / Department of Physics / Understanding how and why superconductivity (SC) occurs in a given material has been very challenging for physicists for more than a hundred years, notwithstanding the major milestones, such as the London theory, the Landau-Ginzburg theory, and the BCS theory. The extreme challenge to predict the occurrence of SC is symbolized by the long string of unanticipated but breathtaking advances, i.e., the unexpected discoveries of cuprates and Fe-pnictides being the dramatic modern examples. Because of their incompatibility, the nucleation of SC near a ferromagnet is di cult and has never been realized except for the case that another superconductor provides proximity-boosted Cooper pairs. This perceived necessity to start with another superconductor is engrained in the exten- sive study of the proximity e ect in superconductor/ferromagnet (S/F) powder sample, where all the structures involve a superconductor with either stable or metastable struc- ture. Compounding the di culty, it is also generally recognized that SC with substantial Tc is favourable in low dimensionality because of strong quantum uctuation. In this thesis, we report a serendipitous nding of SC that emerges under the most implausible circumstances in low eld microwave absorption measurement. This new revelation may lead to unconventional avenues to explore novel SC for applications in superconducting spintronics. By means of a varienty of techniques, including EPR, SEM, FTIR, PPMS/VSM and XRD, nanonickel incorporated YBCO in di erent weighting factors have been studied. With its complex chemical structure and magnetic properties, Ni-YBCO is far from well understood and the magentic behavior of the system under di erent conditions is investi- gated. From the dilute mixture of nanonickel particles, it is found that groups of normal Josephson junctions (JJs) and JJs due to YBCO-nickel-YBCO interparticle weaklinks form as nickel is ferromagnetic. We experimentally show, for the rst time multiple phase reversals in the non-resonant microwave absorption (NRMA) spectra from Ni-YBCO pos- sibly, due to the formation of JJs. We also show that these multiple phase reversals then vii depend on microwave power and temperature. We argue that microwave power induced coherence among some groups of JJs and breaking of some of the weaker JJs can then lead to the disappearance of multiple phase reversals at higher microwave power levels. Further, we also report a role of pair breaking e ects that shall give a linear eld de- pendence of the derivative microwave absorption signal, which is essentially the NRMA signal. This pair-breaking e ect dominates at temperatures closer to Tc as expected thermodynamically. The presence of two peaks in the system, results in high permeability ferromagnet which acts as a magnetic short circuit for magnetic ux density and creates low reluctance path. A transition from normal to anomalous does not occur in this work, because of the possibility of junction in the sample. As predictable at the region around the origin where the weaklinks are supposed to be very strong for a very low doping or low nanonickel addition ( 0.5 % wt), not much e ect was observed. However, when the nanonickel addition is increased to 2 % and 3% we see a signi cant change in the magnetization and the associated hysteresis, indicating ux pinning. / NRF