GAS PHASE SWITCHING FOR PULSED POWER APPLICATIONS A Dissertation by WILLIAM NICHOLS POLLARD Submitted to the Office of Graduate and Professional Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Chair of Committee, David Staack Committee Members, Gerald Morrison Sy-Bor Wen Edward White Head of Department, Andreas Polycarpou May 2016 Major Subject: Mechanical Engineering Copyright 2016 William Nichols Pollard ABSTRACT This dissertation serves to increase the understanding of applied pulsed plasma. Pulsed plasmas are experimentally studied in three contexts 1) fundamentally as switches, 2) applied in a dense plasma focus (DPF), and 3) applied as flow actuators. In these contexts the systems were studied with voltage, repetition frequency, energy, and pulse duration, ranges from 10 – 100 kV, 1 to 20 kHz, 1 mJ to 1 J, and 5 to 100 ns respectively depending on the requirements of ultimate application. These high voltages at high frequency push the current limits of spark switches. Attaining desired conditions required an understanding of the plasma switching characteristics, electrical coupling with the load, and the ultimate application. In the first experimental study plasma pulsing limitation were determined. In these experiments a DC pulsed plasma is pushed to its limits (within constraints) to determine maximum pulsing frequency while maintaining 10 kV available to drive novel applications. In these experiments 137 mJ of energy were pulsed at a frequency of 20 kHz with full-width half-max of 8 ns. Similar pulsing at 42 kHz was observed while maintaining roughly 5 kV. Operating performance was governed by electrode material, discharge gas/flow, chamber pressure, and circuit elements, both intentional and parasitic. In the second experiment, the dense plasma focus (DPF) compresses stored energy through dynamic plasma processes to initiate fusion reactions. This requires controlled short duration pulsing. The DPF in this work is, at the time of this writing, the smallest of its kind to show evidence of pinching based on ns resolution image analysis. ii The reduction in size was made possible by introducing Knudsen scaling to DPF design criteria. The motivation for investigating microscale DPFs is that neutron production efficiency may scale as a-3, where a is the DPF anode radius. Smaller sized systems are also more amenable to portability. Because these devices can fuse, they are capable of generating 15 MeV neutrons and other high energy particles, and they otherwise make for convenient observation of complex plasma dynamics. The plasma actuator for flow control was used for the delay of separation. Short duration plasma pulses were required to produce high velocity synthetic jets. Geometrically and electrically varied sets of individual actuators were computationally and experimentally investigated, compared and characterized. Also a 93 kV, 700 A peak current, 100 ns pulse duration linear array of 23 pulsed plasma actuators in series was designed and tested on a 58 cm span airfoil at Re = 400,000. At a pulsing frequency of 40 Hz and 11° stall was prevented and a drag reduction of 12% was measured. This study supplements current literature on individual actuators and is unique in reporting on full span actuated airfoils. All of the aforementioned studies show both the capabilities and limitations of plasma-pulsed pulsed-plasmas. But, also the necessary understanding of coupling between the pulsing mechanism and its applications. A better understanding of breakdown processes, specifically charged particle generation and diffusion, both in the switch and the load will advance capabilities of such pulsed plasma applications. iii ACKNOWLEDGEMENTS I would first like to express my thanks to Prof. David Staack and Dr. Andrew Duggleby. Their insight, guidance and support have been invaluable resources throughout my education. I always enjoyed the occasional close of business “Just one thing before I go…” conversations that developed into 30+ minute technical discussions on all manner of topics. I named such occurrences ‘Staack attaacks,’ but I can assure you this is a tongue in cheek moniker. These casual chats encouraged critical thinking and were fantastic for improving my engineering intuition. It is satisfying to look back and see how far I have progressed as an engineer/scientist. I am grateful to have been provided such conditions for growth. I would like to thank Prof. Gerald Morrison, Prof. Sy-bor Wen, and Prof. Ed White for their participation as my committee members. Your comments and cross-field expertise have been helpful in preparing this research. I would like to thank my fellow PEDL students, both past and present, for their comments and assistance. Discussions with you were always enlightening and often entertaining. Special thanks goes to Nick Gawloski, John LaSalle and Matthew Burnette for their help with experimentation and data analysis on the HV switch project. Last but certainly not least, I would like to thank both my father and my wife for their support and encouragement. Any future success will have been made possible because of them. iv TABLE OF CONTENTS Page ABSTRACT .......................................................................................................................ii ACKNOWLEDGEMENTS .............................................................................................. iv TABLE OF CONTENTS ................................................................................................... v LIST OF FIGURES ........................................................................................................ viii LIST OF TABLES .......................................................................................................... xix 1. INTRODUCTION .......................................................................................................... 1 1.1 Background .............................................................................................................. 1 1.2 Dissertation objective and motivation ...................................................................... 5 1.3 Dissertation overview ............................................................................................... 8 2. REQUISITE PLASMA KNOWLEDGE ..................................................................... 11 2.1 Description of plasmas ........................................................................................... 11 2.2 Breakdown using DC external E-fields .................................................................. 17 2.3 Continuous DC discharges and the load line ......................................................... 20 2.4 Pulsed DC discharges ............................................................................................. 21 2.5 Streamer breakdown mechanisms .......................................................................... 23 2.6 The thermal instability ........................................................................................... 25 3. HIGH VOLTAGE KHZ PLASMA SWITCH ............................................................. 27 3.1 Introduction to pulsed plasma and applications ..................................................... 27 3.2 Dissertation objectives ........................................................................................... 35 3.3 Experimental setup ................................................................................................. 36 3.3.1 Gen1 DC switch .............................................................................................. 37 3.3.2 Gen2 DC switch .............................................................................................. 43 3.3.3 Gen3 DC switch .............................................................................................. 45 3.3.4 AC switching ................................................................................................... 47 3.4 Results .................................................................................................................... 50 3.4.1 Gen1 DC results .............................................................................................. 51 3.4.2 Gen 2 DC results ............................................................................................. 56 3.4.3 Gen3 DC results .............................................................................................. 59 3.4.4 AC results ........................................................................................................ 68 3.4.5 Conclusions ..................................................................................................... 72 v 3.5 Electrode degradation ............................................................................................. 76 3.6 DBD load characterization ..................................................................................... 78 3.6.1 Load characterization theory – Lissajous plots ............................................... 78 3.6.2 Sinusoidal driven DBD experimental setup .................................................... 80 3.6.3 AC DBD characterization ............................................................................... 82 3.6.4 DC ns-pulsed experimental setup .................................................................... 94 3.6.5 Monopolar DBD characterization ..................................................................