Wong Wan Yan = 光學中消光截面佯謬之解釋 / 黃蘊殷. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2003. / Includes bibliographical references (leaves 58-60). / Text in English; abstracts in English and Chinese. / Wong Wan Yan = Guang xue zhong xiao guang jie mian yang miu zhi jie shi / Huang Yunyin. / Chapter 1 --- Introduction --- p.1 / Chapter 2 --- Scattering of a Plane Wave by an Infinitely Long Cylinder --- p.4 / Chapter 2.1 --- Dielectric cylinder (with no absorption) --- p.8 / Chapter 2.2 --- Perfectly conducting cylinder --- p.10 / Chapter 3 --- Scattering of a Gaussian Beam by a Dielectric Cylinder (Non- Resonance) --- p.13 / Chapter 3.1 --- Configuration and equations used --- p.13 / Chapter 3.2 --- Numerical result and discussion --- p.17 / Chapter 3.2.1 --- a <w --- p.19 / Chapter 3.2.2 --- a>w --- p.21 / Chapter 4 --- Scattering of a Gaussian Beam by a Dielectric Cylinder (Res- onance) --- p.24 / Chapter 4.1 --- Algorithm --- p.26 / Chapter 4.2 --- Numerical result and discussion --- p.28 / Chapter 5 --- Scattering of a Plane Wave by a Conducting Strip --- p.33 / Chapter 5.1 --- Configaration and equations used --- p.34 / Chapter 5.2 --- Numerical calculation --- p.39 / Chapter 5.3 --- Numerical results and discussion --- p.42 / Chapter 6 --- Actual Scattering Cross Section --- p.49 / Chapter 6.1 --- Configaration and Equations used --- p.49 / Chapter 6.2 --- Numerical result and disscusions --- p.51 / Chapter 7 --- Conclusions --- p.56 / Bibliography --- p.58 / Chapter A --- Bessel functions --- p.61 / Chapter B --- A plane wave incident on a single-layered medium --- p.63

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An energy analyzer for electron beams, based on a retarding-potential method, has been studied both experimentally and theoretically. In this method a potential energy barrier at right-angles to the beam acts as a high-pass energy filter allowing electrons having energies greater than the barrier height to pass through while turning back electrons of lower energy. The potential barrier in the present study was created by applying a negative potential to an electrode consisting of a 600/inch hexagonal copper mesh or an electrode having a pair of single apertures in series (compound aperture). Two different compound apertures, of diameters 0.4 mm and 1.0 mm, were used. The retarding electrode was mounted between two grounded outer electrodes having openings centered on the axis for entrance and exit of electrons. The barrier height was adjusted by means of a small variable bias voltage between the electron gun cathode and the retarding electrode. Auxiliary electron lenses external to the retarding module were used to satisfy the conditions that the beam be normal to the retarding electrode and, for an imaging filter, that an image plane be focused at the retarding electrode. A beam having a narrow distribution of energies was used to calibrate the energy analyzer as a function of bias voltage for the three different configurations of retarding electrode. The calibration curves were then compared with the transmission curve for a beam having a broadened energy distribution. The feasibility of obtaining a filtered image was explored by observing the image of a fine mesh focused into an aperture of the retarding electrode. The experiments were carried out for a beam voltage of 15 kV. At this beam voltage the energy resolution is poor, being 3. 6 volts for the smaller compound aperture, 3. 7 volts for the mesh, and 5.2 volts for the larger compound aperture. Typically, electrostatic analyzers operate on a beam which has been decelerated to low energies before entering the analyzer, in which case the energy resolution is proportionately better. For example, at 150 V the above resolution figures would be 0.036, 0.037, and 0.052, respectively. In view of the potential for high energy-resolution and the attractive features of in-line, rotationally-symmetric systems, it is felt that the retarding-potential analyzer is a promising candidate for further development.

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Surface engineering provides solutions to wear and corrosion degradation of engineering components. Laser cladding is a surfacing process used to produce wear and corrosion resistant surfaces by covering a particular part of the substrate with another material that has superior properties, producing a fusion bond between the two materials with minimal dilution of the clad layer by the substrate. The advantages of laser cladding compared to conventional techniques include low and controllable heat input into the workpiece, a high cooling rate, great processing flexibility, low distortion due to the low heat input to the workpiece and minimal post-treatment. The main processing parameters of laser cladding include laser power, laser spot size, processing speed, and powder feed rate. Within an optimized operational window, all these variables have some effect on the temperature of the clad interaction zone. The laser cladding technique is very complicated because it involves metallurgical and physical phenomena, such as laser beam-materials interaction, heat transfer between the clad and the substrate, and the interdiffusion of the clad and the substrate materials. Laser cladding is currently an open-loop process, relying on the skills of the operator and requiring dedication to specialty to make it successful. Unless the required expertise is provided, attempts to make the process successful will be futile. The objective in conducting the project was to investigate and develop prototype sensors to monitor and control Nd:YAG laser cladding process. Through a LabVIEW software based monitoring program, real-time process monitoring of optical emissions in the form of light and heat radiation was carried out, and correlated with the properties of the produced clad layers. During various experiments, single- and multiple-track laser cladding trials were performed. The responses of such sensors to the selected conditions were examined and an in depth analysis of detected heat and optical radiation signals was carried out. The results of these experiments showed the ability of such sensors to recognize changes in process parameters, and detected defects on layer surfaces along with the presence of oxides. A multi-function closed-loop laser power and CNC motion table feed rate control interface based on a LabVIEW platform has been designed and built, which is capable of accepting and interpreting sensors� data and adjusting accordingly the laser power and CNC motion table feed rate to produce sound clad layers. The developed dual control strategy utilized in this study forms a relatively inexpensive and less-complicated system that allows end-users to achieve lower failure rates during laser cladding (within its own limitations) and, therefore, through successful concurrent control of melt pool temperature and motion table feed rate provide better productivity and quality in the experimentally produced clad layers.

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A low velocity, low temperature beam of atoms is produced from a two-dimensional magneto-optic trap known as an atomic funnel. The funnel provides simultaneous spatial and velocity compression of atoms and will provide a source for a three-grating atomic interferometer. Rubidium atoms from an oven are slowed by chirped cooling and loaded into the trap. Atoms are ejected from the trap using frequency offsets in optical molasses. The resultant beam has a controllable velocity in the range of 3 to 10 m/s with temperatures of order 500 ��K. / Graduation date: 1996

Thesis (Ph. D.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.