Thesis (MSc)--University of Stellenbosch, 2007. / Bibliography. Also available via the Internet.

M.Ing. (Electrical and Electronic Engineering) / This work reports on a study in the fields of "High power pulsed power supplies and electromagnetic pulse compression". A background discussion on the application of pulsed power on lasers is given. Pulse compression and high power pulsed power supply configuration is described. The work concentrates on simultaneous energy transfer electromagnetic pulse compression which is a technique used to achieve a saving of core material in a pulse compressor. An analytical expression for the saving was derived and verified experimentally. This result is applied in different pulse compressors designed for the pulsed power supplies of copper vapour lasers. The test results are provided and discussed. A windfall from the practical work is a design solution for the voltage sharing compensation network needed when switching semiconductors in series.

M.Ing. (Electrical And Electronic Engineering) / The theme of this thesis is the time compression of energy in electrical systems with the aim of improving the efficiency and average power throughput capability of existing pulse compression systems. Recent success in the design of pulse lasers indicated successful operation at repetition rates of up to 2kHz, demanding more powerful pulse power supplies. These pulse lasers require unusually high peak power, not obtainable with conventional switching techniques. Typical pulse voltages of up to 40kV with peak currents of lOkA and rise times of 150ns are required. Traditionally thyratrons have been used for this application, but the limited lifetime of these devices is a major shortcoming, especially when operating at 2kHz. New switching techniques were needed. Excellent results have been obtained using saturable inductors as magnetic switches in pulse compressors, but notwithout limitations. The generation of heat due to core losses tends to limit the power throughput capability of these compressors, especially in the final stages. The minimisation of these losses is therefore of utmost importance. In order to improve the current systems, a study of the basic principles of compression was conducted. Loss-mechanisms, especially in the magnetic components, were modelled with the object of minimising losses. The limitations imposed on the system by flux conservation in magnetic and dielectric materials, led to the development of the new concept of bipolar compression. A full-scale bipolar compressor and pulse power supply was constructed and experimental results on this system are presented, showing an improvement in the efficiency. A new method for designing pulse compressors, incorporating core losses,evolved from this development

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A Rogowskl coil has been used to measure current in a pulsed discharge circuit. The frequency response of the coil has been determined experimentally, It has been established that the noise signal associated with the measurement of a pulsed discharge current is due principally to the electromagnetic radiation from the spark gap switches required in such a circuit. The radiation can be reduced by pressurizing the spark gap with argon, but at the same time the breakdown potential of the switch becomes erratic. A simple measuring system has been devised which completely eliminates the noise signal from the current waveform. The system consists of a balanced differential preamplifier which is fed by a completely balanced measuring circuit into which a 0.5µ sec delay line has been incorporated. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The primary purpose of this thesis is to determine experimentally if the attenuation of small -amplitude elastic waves in metals is governed principally by a linear mechanism (i.e. one for which the principle of superposition is valid). The secondary purpose is to interpret the attenuation measurements in terms of existing theories on acoustic dissipation in solids. Attenuation measurements of the Fourier components of a Rayleigh pulse were compared with those of sinusoidal Rayleigh waves of the same frequency. One copper, one aluminum and two α-brass circular cylindrical shells were used, and Rayleigh waves propagating along the truncated edges of these tubes were studied. Rayleigh pulses were detected at strain levels of approximately four and forty microstrain in order to test for any amplitude-dependent effects accompanying the attenuation. The sinusoidal Rayleigh waves were detected at strain amplitudes between three and ten microstrain. For three out of the four tubes the results indicated that the dominant attenuation mechanism is a linear process in the frequency range from 100 to 500 kc/s and in the strain region from four to forty microstrain. For the copper tube, however, the scatter in the results is such that no definite conclusion could be drawn. For all four tubes the internal friction, 1/Q, increases with frequency. For some of them there is evidence of a relaxation peak, probably as a result of the Zener effect, superimposed on the general trend. Dislocation damping, as proposed by Koehler (1952) and later generalized by Granato and Lucke (1956), is suggested as a dissipative mechanism which could account for this general trend. Most of the internal friction values are found to be between 20 X 10¯⁵ and 100 X 10¯⁵. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

When the message sink of a speech communication system is a human, the ultimate criterion of system performance is the subjective quality of the output speech. Unfortunately, no tractable mathematical function has been found to adequately relate speech quality to the physical system parameters. For this reason, empirical methods must be utilized to assess the interplay between the subjective quality and the objective parameters of a speech communication system. In this thesis the effects of transmission errors on the speech quality of pulse code modulation (PCM) and differential pulse code modulation (DPCM) voice communication systems are investigated. The subjective figure of merit adopted in this study is the listener's preference of the output speech with respect to a suitably chosen reference. Under some assumptions and restrictions, the system models of the two types of modulation systems of interest are formulated and simulated in an IBM System 360/Model 67 time-shared computer. With the aid of a special-purpose input/output interface, the simulated systems are used to process a recorded speech sample representative of English speech. The quality of the processed speech is then subjectively evaluated according to the isopreference method. The results of the subjective evaluation are presented in the form of isopreference contours. These contours indicate that in both PCM and DPCM systems, the speech quality is dominantly influenced by quantization error when the channel is relatively error-free; whereas if the channel is relatively noisy, finer quantization offers no improvement in quality. Furthermore, encoding the quantizer's output by either natural or folded binary coding yields virtually identical speech quality. In a comparison with contours of constant system signal-to-distortion power ratio, the isopreference contours reveal that under some conditions, the system signal-to-distortion power ratio may be a reasonably adequate measure of human preference of speech. In terms of the minimum channel capacity required to achieve a desired speech quality, DPCM is found to perform better than PCM for three important channel models: the binary symmetric channel, the additive white Gaussian channel, and the Rayleigh fading channel with additive white Gaussian noise. In the latter two cases, the performance improvement of DPCM over PCM increases with increasing desired speech quality for the range of speech quality considered in this study. Finally, the implications of the subjective tests' results in two suboptimal operations are discussed. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Within this educational endeavor instrumental development was explored through the investigation of microwave induce stable electromagnetic waves within a non-linear yttrium iron garnet ferromagnetic waveguide. The resulting magnetostatic surface waves were investigated as a possible method of rapid analytical evaluation of material composition. Initial analytical results indicate that the interaction seen between wave and material electric and magnetic fields will allow phase coherence recovery andanalysis leading to enhancement of analytical value. The ferromagnetic waveguide selected for this research was a high quality monocrystalline YIG (yttrium iron garnet) film. Magnetostatic spin waves (MSW) were produced within the YIG thin waveguide. Spin waves with desired character were used to analytically scan materials within the liquid and solid phase.

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