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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

A Retarding-potential Analyzer for Measuring Energy Distributions in Electron Beams

Zhou, Li 20 July 1993 (has links)
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.

Hinge rotation capability of prestressed concrete beams.

Chai, Neville Malcolm. January 1970 (has links)
No description available.

An investigation of the behaviour of a three-dimensional reinforced concrete connection.

Khan, Abdul Qaseem January 1969 (has links)
No description available.

An Investigation of combined stresses in reinforced concrete beams.

Mirza, Muhammad Saeed. January 1967 (has links)
No description available.

Sensing and control of Nd:YAG laser cladding process

Salehi, Dariush, ds_salehi@yahoo.com January 2005 (has links)
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.

Torsion in concrete framed structures

Shepherd, Peter Noel. January 1975 (has links) (PDF)
No description available.

A rubidium atomic funnel

Swanson, Thomas B. 11 July 1995 (has links)
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

Retrofitting of reinforced concrete coupling beams by bolted side steel plates for strength and deformability

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

Propagation of laser radiation through atmospheric turbulence /

Dunphy, James R. January 1974 (has links)
Thesis (Ph. D.)--Oregon Graduate Center, 1974.

Noise effects, emittance control, and luminosity issues in laser wakefield accelerators /

Cheshkov, Sergey Valeriev, January 2001 (has links)
Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references (leaves 97-103). Available also in a digital version from Dissertation Abstracts.

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