EVALUATION OF A HIGH-PRESSURE, COAXIAL SPARK GAP FOR PULSED RING-DOWN APPLICATIONS by COLT JAMES A MASTER THESIS IN ELECTRICAL ENGINEERING Submitted to the Graduate Faculty of Texas Tech University in Partial Ful llment of the Requirements for the Degree of MASTER OF SCIENCE Approved Dr. James Dickens Committee Chairman Dr. John Mankowski Fred Hartmeister Dean of the Graduate School December, 2007 Copyright c 2007, Colt James Texas Tech University, Colt James, December 2007 ACKNOWLEDGMENTS I would like to thank my adviser, Dr. Dickens, for guidance and help throughout this project. I would also like to thank the other member of my committee, Dr. Mankowski, for his support and knowledge. Special thanks go out to the Dr. John Krile and Shad Holt, who provided a great deal of advice and knowledge. I would also like to thank the sta of the Center for Pulsed Power and Power Electronics. This project could not have been accomplished without the help of Danny Garcia, Dino Castro, Elmer Thornton, and Shannon Gray. I would also like to thank all of my colleagues in the lab, speci cally my oce mates Greg Edmiston, Thomas Holt, Andrew Young, and Mohamed Elsayed, for all their advice and help. Finally, I would like to thank my friends and family for their invaluable support. ii Texas Tech University, Colt James, December 2007 CONTENTS ACKNOWLEDGMENTS ........................... ii ABSTRACT .................................. iv LIST OF TABLES ............................... v LIST OF FIGURES .............................. vi CHAPTER I INTRODUCTION ........................... 1 II BACKGROUND THEORY ..................... 4 2.1 Closing Switches ......................... 4 2.1.1 Breakdown Mechanisms ................... 5 2.1.1.1 Townsend Discharge ................. 5 2.1.1.2 Streamer Breakdown ................. 6 2.1.2 Erosion Mechanisms ..................... 10 2.2 Previous Work .......................... 13 III EXPERIMENTAL SETUP ..................... 15 3.1 Spark Gap ............................. 15 3.1.1 Electrostatic Modeling .................... 17 3.2 Test Bed .............................. 21 3.2.1 Trigger System ........................ 22 3.3 Diagnostics ............................ 23 IV EXPERIMENTAL RESULTS .................... 25 4.1 Preliminary Results ........................ 26 4.2 Final Results ........................... 30 V CONCLUSIONS ............................ 39 BIBLIOGRAPHY ............................... 41 iii Texas Tech University, Colt James, December 2007 ABSTRACT The design and jitter performance of a high pressure, coaxial spark gap for use in pulse ring-down applications is presented. Additional comparisons with trigatron style triggering are also presented. The spark gap is triggered by eld distortion of a center plane electrode. The switch was tested up to 75 pulses per second (pps) with a maximum switching voltage of 50 kV in nitrogen. Analysis will focus on jitter measurements taken over the full lifetime of the switch. This paper presents the results of this analysis along with comparisons from the literature. Speci cally, switch jitter and lifetime will be evaluated as a function of switch geometry as a whole and as a function of trigger electrode geometry. iv Texas Tech University, Colt James, December 2007 LIST OF TABLES 2.1 Summary of jitter experiments. ..................... 13 4.1 Summary of electrode erosion data for preliminary tests. Negative values indicate a loss of mass, while positive numbers indicate an increase in mass. ............................. 27 4.2 Summary of switch jitter for the preliminary tests. .......... 28 4.3 Summary of electrode erosion data for nal tests. Negative values indicate a loss of mass, while positive numbers indicate an increase in mass. .................................. 30 4.4 Summary of electrode erosion data for preliminary tests. Negative values indicate a loss of mass, while positive numbers indicate an increase in mass. ............................. 32 4.5 G{10 mechanical and electrical properties. ............... 37 v Texas Tech University, Colt James, December 2007 LIST OF FIGURES 1.1 Diagram of a phased array antenna system. The variable delay is made up of all the electrical connections necessary for array operation. 1 1.2 Example of pulse ring-down. Sample waveform used for phased array testing. .................................. 2 2.1 Diagram of a three-electrode triggered spark gap.[1] ......... 4 2.2 Diagram of a trigatron spark gap.[1] .................. 5 2.3 Diagram depiction of the streamer breakdown process, taken from Nasser[2]. (a) Formation of primary electron avalanche. (b) Photons are emitted from excited gas. (c) Secondary avalanches are formed from photo-electrons emitted near the cathode. (d) Positive space charge begins to build up at the anode. (e) Streamer propagating due to successive avalanches. (f) Some branches die out while avalanches continue to feed others. (g) Completed streamer channel. ...... 8 2.4 Potential distribution in a three-electrode spark gap.[1] Top - Main electrode at high, positive voltage with the trigger electrode oating. Middle - Incoming trigger pulse causes an increase in potential dif- ference between main electrode and trigger electrode. Bottom - Es- tablishment of arc between main and trigger electrodes brings them to the same potential and increases the potential di erence between trigger electrode and ground electrode. ................. 10 3.1 Spark gap cross section ......................... 16 3.2 Machined Bruce pro le electrodes. ................... 17 vi Texas Tech University, Colt James, December 2007 3.3 Comparison of electrode pro les. Left - Standard Bruce pro le, Right - Modi ed Bruce pro le ......................... 18 3.4 Electrostatic eld simulation of Bruce pro le electrodes. ....... 19 3.5 Electrostatic eld simulation. ...................... 19 3.6 Potential distribution throughout the gap for each stage of the break- down process. Top - Main electrode at high, positive voltage with the trigger electrode oating. Middle - Incoming trigger pulse causes an increase in potential di erence between main electrode and trigger electrode. Bottom - Establishment of arc between main and trig- ger electrodes brings them to the same potential and increases the potential di erence between trigger electrode and ground electrode. 20 3.7 Test bed equivalent circuit ........................ 21 3.8 Test Bed Load .............................. 22 3.9 PFL Trigger System ........................... 23 4.1 Top - Annular midplane, Left - Point midplane, Right - Radius mid- plane .................................... 26 4.2 Overlay of ve voltage waveforms. The magnitude of the trigger voltage was scaled down to allow the waveform to t in the graph. 27 4.3 Image of the anode and cathode taken after 10 million shots. The baes can be seen surrounding the electrodes. ............ 28 4.4 Standard deviation jitter recorded over 10 million shots. ....... 29 4.5 Statistical delay times over the recorded lifetime of the electrodes. 29 4.6 Comparison photo of the second tested anode after 10 million shots. 31 4.7 Neutral biased jitter vs. cathode biased jitter of the annular midplane over tested lifetime of the switch .................... 33 vii Texas Tech University, Colt James, December 2007 4.8 Comparison of midplane geometries. All measurements taken with the midplane in the cathode biased position. ............. 34 4.9 Comparison of midplane geometries. All measurements taken with the midplane in the neutral biased position. .............. 34 4.10 Close-up image of the e ects of breakdown inside the G{10 housing. 38 viii Texas Tech University, Colt James, December 2007 CHAPTER 1 INTRODUCTION The goal of this project is to evaluate a long-lifetime, low-jitter spark gap for pulsed ring-down phased array applications. Phased array antenna systems pro- vide a means of removing the mechanical problems involved with large high gain antennas by connecting many smaller antenna elements together. The trade o with array antennas is that, while mechanically simpler, electrically they are much more complex and the connections between them become very important. This is why precise switching components are necessary. An array usually consists of a source, a number of antenna elements, and the electrical connections between them, Fig.1.1. If the source is connected to all the elements at the same time a beam will be produced with a gain proportional to the number of elements and phase normal to the antenna elements. The radiated electric eld pattern can be changed by varying the time delay in which the antenna elements are switched. By switching the elements in di erent orders (Fig.1.1), the directed phase angle can be changed. This also means that an inability or variation in ring the array elements can cause an unwanted phase angle shift and loss of power on target. Figure 1.1: Diagram of a phased array antenna system. The variable delay is made up of all the electrical connections necessary for array operation. 1 Texas Tech University, Colt James, December 2007 Array systems can either be continuous or pulsed. In this project a modi ed pulsed system will be built to transmit a ringing pulse. The pulse forming network that would be used to produce a single square pulse to propagate on a matched antenna load in a typical pulsed array system will be removed. This will result in a reduction of size and weight. Instead, a high-voltage switch will be placed within each antenna element. The antenna elements will be charged to