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Pulsed Breakdown Phenomena in Partial Vacuum

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Pulsed Breakdown Phenomena in Partial Vacuum PULSED BREAKDOWN PHENOMENA IN PARTIAL VACUUM by Zhenhong Li A thesis submitted to the Graduate Faculty of Auburn University in partial fulfillment of the requirements for the Degree of Master of Science Auburn, Alabama December 8, 2012 Copyright 2012 by Zhenhong Li Approved by Hulya Kirkici, Chair, Professor of Electrical and Computer Engineering Thaddeus Roppel, Associate Professor of Electrical and Computer Engineering Michael Baginski, Associate Professor of Electrical and Computer Engineering Abstract 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 ii 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. iii Acknowledgments The author would like to thank his advisor Dr. Hulya Kirkici for her patience and guidance on this research work during the past two years. Without her support this work would be impossible to achieve. The author would like to thank Dr. Baginski and Dr. Roppel for serving as the committee members. The author also would like to express appreciation to Ms. Linda Barresi and Mr. Calvin Cutshaw for their material assistance during this work. Thanks are also due to his group members, especially Rujun Bai, Haitao Zhao and Huirong Li who help him run the experiments over and over again. Finally, the author wishes to thank his mother and sisters, for their unfailing support and understanding. ξ Work partially sponsored by AFOSR grant # AF-FA9550-08-10051 iv Style manual or journal used Graduate School's Thesis and Dissertation Guide Computer software used Microsoft Office 2007 v Table of Contents Abstract ......................................................................................................................................... ii Acknowledgments ....................................................................................................................... iv List of Tables ............................................................................................................................... ix List of Figures ............................................................................................................................... x Chapter I INTRODUCTION & BACKGROUND ....................................................................... 1 1.1 Introduction ................................................................................................................ 1 1.2 Electrical Breakdown of Gases ................................................................................... 4 1.2.1 Townsend Mechanism ....................................................................................... 4 1.2.2 Paschen‘s Law ................................................................................................... 6 1.2.3 High Frequency Breakdown .............................................................................. 9 1.2.4 Pulsed Breakdown ........................................................................................... 11 1.2.5 Glow Discharge ............................................................................................... 13 1.2.6 Hollow Cathode Discharge .............................................................................. 14 1.3 Pulsed Power ............................................................................................................. 15 1.3.1 Pulsed Power Generation ................................................................................. 15 1.3.2 Power Switches ................................................................................................ 17 1.4 Surface Flashover...................................................................................................... 20 1.4.1 Space Environment & Spacecraft .................................................................... 20 1.4.2 Surface Flashover in Vacuum & Partial Vacuum ............................................ 22 vi Chapter II HIGH VOLTAGE GENERATOR SIMULATION & APPLICATION ................... 24 2.1 High Voltage DC Power Supply .............................................................................. 24 2.2 High Voltage Pulse Generator ................................................................................. 27 2.3 Application: CNTs Initiated Hollow Cathode Discharge ......................................... 31 2.3.1 Experiments Setup ........................................................................................... 33 2.3.2 Experiment Results & Discussions .................................................................. 33 Chapter III SURFACE FLASHOVER IN PARTIAL VACUUM ............................................. 36 3.1 Sample Preparation .................................................................................................. 36 3.2 Experimental Setup ................................................................................................... 38 3.3 Experimental Procedure ............................................................................................ 40 3.4 Breakdown Voltage versus Pressure ......................................................................... 41 3.4.1 DC Breakdown Voltage versus Pressure ......................................................... 43 3.4.2 Pulsed Breakdown Voltage versus Pressure .................................................... 46 3.5 Electrode Configurations Effect on Breakdown Voltage ......................................... 49 3.6 Breakdown Voltage versus Pulse Parameters ........................................................... 51 3.6.1 Breakdown Voltage versus Frequency ............................................................ 52 3.6.2 Breakdown Voltage versus Duty Cycle ........................................................... 54 3.7 Surface Effect on Breakdown Voltage ..................................................................... 54 3.8 Conclusions ............................................................................................................... 57 Chapter IV CONCLUSIONS ...................................................................................................... 59 References ................................................................................................................................... 61 Appendix A SPICE Listing for Pulse Generator Circuit .......................................................... A-1 Appendix B Surface Breakdown Voltages vs. Pressure (Sandwiched Electrode) .................... B-1 vii Appendix C Surface Breakdown Voltages vs. Pressure (Lateral Electrode) ............................ C-1 Appendix D Surface Breakdown Voltages vs. Pressure (Machined Samples) ......................... D-1 Appendix E Surface Breakdown Voltages vs. Frequency ........................................................ E-1 Appendix F Surface Breakdown Voltages vs. Duty Cycle ........................................................ F-1 viii List of Tables Table 2.1 CNTs sample list ......................................................................................................... 34 Table 3.1 Nanodielectric sample
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