After nearly 15 years of research effort, High Temperature Superconductors (HTS) are finding a wide range of practical applications. A clear understanding of the factors controlling the current carrying capacity of these materials is a prerequisite to their successful technological development. The critical current density (Jc) in HTS is directly dependent on the structure and pinning of the Flux Line Lattice (FLL) in these materials. This thesis presents an investigation of the Jc anisotropy in HTS. The use of thin films grown on off c-axis (vicinal) substrates allowed the effect of current directions outside the cuprate planes to be studied. With this experimental geometry Berghuis, et al. (Phys. Rev. Lett. 79,12, pg. 2332) observed a striking flux channelling effect in vicinal YBa2Cu3O7-δ (YBCO) films. By confirming, and extending, this observation, it is demonstrated that this is an intrinsic effect. The results obtained, appear to fit well with the predictions of a field angle dependent cross-over from a three dimensional rectilinear FLL to a kinked lattice of strings and pancakes. The pinning force density for movement of strings inside the cuprate planes is considerably less than that on vortex pancake elements. When the FLL is entirely string-like this reduced pinning leads to the observed channelling minima. It is observed that anti-phase boundaries enhance the Jc in vicinal YBCO films by strongly pinning vortex strings. The effect on the FLL structure cross-over of increasing anisotropy has been elucidated using de-oxygenated vicinal YBCO films. Intriguingly, the counter intuitive prediction that the range of applied field angle for which the kinked lattice is fully developed reduces with increasing anisotropy, appears to be confirmed. Although vortex channelling cannot be observed in c-axis YBCO films, the pinning force density for vortex string channelling has been extracted by observing string dragging. By studying the effect of rotating the applied field at a constant angle to the cuprate planes, it is possible to observe the cross-over into the string pancake regime in c-axis films. In the 3D region, the observed behaviour is well explained by the anisotropic Ginzburg-Landau model. Measurements were also made on thin films of the much more anisotropic Bi 2Sr2CaCu2O8+x material, grown on vicinal substrates. The absence of any flux channelling effect and clear adherence to the expected Kes-Law behaviour in the observed Jc characteristics does not provide evidence for the existence of the predicted 'crossing lattice' in Bi 2Sr2CaCu2O8+x.

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Anomalous electrical behaviour may appear in the d.c. and a.c. characteristics of metals or semiconductors in which the current is determined by both voltage and temperature. Theoretical investigations have been carried out by assuming different models of heat flow and the conditions for the appearance of thermal breakdown and Negative Resistance (NR) have been obtained for both metals and semi-conductors. For purely longitudinal heat flow, NR is predicted for metals while for semi-conductors, the d.c. characteristic is of the "breakdown" type without NR. On the other hand, the radial heat flow model predicts NR for semi-conductors and the conductivity modulation due to the radial temperature distribution causes a concentration of current-density along the axis, giving rise to the "thermal pinch" effect. For metals, this model does not predict NR and the resistivity modulation confines the current-density within a small depth from the surface giving rise to the "thermal skin effect". NR is also predicted for the model considering longitudinal heat flow with surface heat loss, the d.c. λ-K thermal theory, in semi-conductors whereas for metals, the theory does not predict NR. For the applicability of the λ-K thermal theory, the specimen must be thin enough to ensure an isothermal cross-section. The a.c. impedance of the specimen with a small a.c. voltage superimposed on the d.c. bias has been obtained for the λ-K thermal theory. Theoretical analysis shows that a non-zero surface loss parameter λ is essential for attaining NR in semiconductors. Experiments were performed with metals and semiconductors in an attempt to check the d.c. and a.c. λ-K thermal theories. Comparison of the experiments with the theory shows that for semi-conductors, the λ-K thermal theory is valid for current-density J ≤20 amps.cm-² while for metals, it is valid for J ≤5 x 10⁴ amps.cm-². The measured a.c. characteristics at both low and high frequencies are interpreted on the basis of the a.c. λ-K thermal theory but over the intermediate frequency region, the theory offers no explanation for the "circular arc" locus of impedances observed experimentally for both metals and semi-conductors. The present investigation enables one to determine the character of heat flow from measurements of the electrical characteristics of the specimen and also to distinguish the thermal effects which may be present in other experiments (e.g. on the "magnetic pinch"). / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A mathematical model is developed which extends the theory of rip currents developed by Bowen (1969b) for a straight beach to curved beaches where radii of curvature are large relative to the width of the surf zone. Nine forcing terms are found to cause rip current systems. The terms are functions of the longshore variation in wave height and angle of incidence of the incoming waves at the breakers. The model is applied to the case of a circular beach with conical nearshore bottom topography. A large rip current component is found to exist which is inversely proportional to the radius of curvature of the beach. Another significant rip current component is found to be proportional to the variation in the angle of incidence of the waves at the breakers. This component would cause rip currents on a straight beach where some irregular offshore topography caused some variation in the incident angle of the incoming waves. Another component rip current was found which was essentially the same as the one predicted by Bowen (1969b). / Science, Faculty of / Physics and Astronomy, Department of / Graduate

This thesis investigates two separate but related problems. In Part I a study is made of the propagation of continental shelf waves and barotropic Rossby waves in a steady, laterally sheared current of the form V + Є W, where W is a centred random function and Є << 1. If the correlation length of W is small compared with the characteristic horizontal length scale of the system; for example, the shelf width or a channel width, the waves are unstable. Their growth rate is largely determined by the magnitude of the correlation length, while the phase speed is given by the sum of weighted averages of the mean current V and the lateral gradient of potential vorticity. Application of the theory to the Brooks and Mooers (1977a) model of the Florida Straits yields wave parameters that are in accord with those measured by Duing (1975). In Part II, an attempt is made to understand the dynamics governing observed low-frequency currents in the Strait of Georgia (GS). A simple two-layer model indicates that the mean currents in GS are probably baroclinically stable. A barotropic stability model implies that a shear instability might be of some importance. However, the analysis of current meter data shows that the velocity components of the fluctuations are either nearly in phase or close to 180° out of phase; this means that the motions are not due to the type of waves considered here. Analysis of the relationship between the winds and currents in both the frequency and time domains implies that the wind may play an indirect role in forcing GS motions. It is conjectured that the wind and tide interact with the Fraser River outflow to modulate the estuarine circulation in the system and force low-frequency currents. Direct nonlinear interaction between tidal constituents produces a coherent fortnightly variation in the currents, but cannot account for the observations. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

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This is Part B of the Thesis. / <p> The design and performance of a flat emitting ring on-axis electron gun suitable for use in a small reflected beam accelerator was investigated. The design constraints include a low emittance (approximately 5π cm mrad), and a small beam size with a focus approximately 13 cm from the emitting surface.</p> <p> A suitable geometry was determined theoretically and was tested with a dispenser cathode. A beam with a focus at 12.7 ± 1 cm and an emittance of approximately 7π cm mrad was obtained. However, the dispenser cathode response time to heater current changes is too large for the required gun current control.</p> <p> Experiments were done to study the mechanical and thermal properties of flat emitting foil rings, since a directly heated foil has a fast response time. Two foils were tested: 1.27 x 10^-3 cm thick tungsten and 4.57 x 10^-4 cm thick tantalum. The present simple design requires impractically thin foils (≤ 0.25 microns thick) to reach emission temperatures at feasible heater currents.</p> / Thesis / Master of Engineering (MEngr)

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