Pulsed Breakdown Phenomena in Partial Vacuum
Type of Degreethesis
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In this thesis, the pulse generation and pulsed breakdown phenomena have been studied in detail. Pulsed power is widely used in food processing, biotechnology, military defense, and industrial applications. The first part of this work covers the simulation and analysis of common high voltage DC and pulse generator circuits. Then high power switching techniques which are essential to the generation of pulsed power are also explored. Due to the rapid development of high energy laser and accelerator, high power switches capable of handling powers up to terawatts are in great demand in industry. Pseudospark switch using the hollow cathode effect is an example for these applications. Pseudospark discharge is triggered by many means and is challenging. Carbon Nanotubes (CNTs) triggering is achieved in pseudospark switches operated in helium and tested in the laboratory. The delay time of the triggered breakdown with several CNTs samples under different trigger voltages is summarized. As the second part of this research, the pulsed breakdown phenomena in partial vacuum are investigated thoroughly. Solid insulators are widely employed in power systems operating in atmospheric pressure and partial vacuum. However, the hold-off capability of an insulator in vacuum is limited by the surface flashover problem due to the cathode ‘triple-junction’ effect. This is a common problem for space power and aerospace power systems. In this work, the surface flashover characteristics of nanodielectric materials composed of cast epoxy resin mixed with nano-dielectric particles in partial vacuum are studied. The samples except the control samples are made by adding nanoscale Al2O3 or TiO2 powder with predetermined ratios into the epoxy resin with known properties. The samples are cylindrical in shape (2.5 cm in diameter and 1 cm in thickness) and varying electrode configurations and spacing are used in this study. The surface flashover characteristics of the samples are investigated using both DC and 20 kHz pulsed-unipolar signals separately. The frequency and duty cycle of the pulsed signal are also varied in the experiments in order to obtain a clearer understanding of pulsed surface flashover in partial vacuum. Initially the cylindrical samples are sandwiched between two parallel plate electrodes, and the voltage, current, and light emission waveforms during the flashover events are recorded and analyzed. As a second test set, the electrodes laterally placed on the samples’ flat surface and surface breakdown experiments conducted and comparisons to the parallel plate case are made.