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Electric Oxygen-Iodine Laser Discharge Scaling and Laser Performance

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Electric Oxygen-Iodine Laser Discharge Scaling and Laser Performance ELECTRIC OXYGEN-IODINE LASER DISCHARGE SCALING AND LASER PERFORMANCE BY BRIAN S. WOODARD DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Aerospace Engineering in the Graduate College of the University of Illinois at Urbana-Champaign, 2012 Urbana, Illinois Doctoral Committee: Professor Wayne C. Solomon, Chair and Director of Research Professor Rodney L. Burton Professor Gregory S. Elliott Professor James J. Coleman Professor Joseph T. Verdeyen Abstract In 2004, a research partnership between the University of Illinois and CU Aerospace demonstrated the first electric discharge pumped oxygen-iodine laser referred to as ElectricOIL. This exciting improvement over the standard oxygen-iodine laser utilizes a gas 1 discharge to produce the necessary electronically-excited molecular oxygen, O2(a ), that serves as the energy reservoir in the laser system. Pumped by a near-resonant energy 2 2 transfer, the atomic iodine lases on the I( P1/2) → I( P3/2) transition at 1315 nm. Molecular oxygen diluted with helium and a small fraction of nitric oxide flows through a radio- 1 frequency discharge where O2(a ) and many other excited species are created. Careful investigations to understand the benefits and problems associated with these other states in the laser system allowed this team to succeed where other research groups had failed, and after the initial demonstration, the ElectricOIL research focus shifted to increasing the efficiencies along with the output laser energy. Among other factors, the laser power scales 1 with the flow rate of oxygen in the desired excited state. Therefore, high yields of O2(a ) are desired along with high input oxygen flow rates. In the early ElectricOIL experiments, the pressure in the discharge was approximately 10 Torr, but increased flow rates forced the pressure to between 50 and 60 Torr requiring a number of new discharge designs in order to 1 produce similar yields of O2(a ) efficiently. Experiments were conducted with only the electric discharge portion of the laser system using emission diagnostics to study the effects of changing the discharge geometry, flow residence time, and diluent. The power carried by 1 O2(a ) is the maximum power that could be extracted from the laser, and the results from 1 these studies showed approximately 2500 W stored in the O2(a ) state. Transferring this energy into the atomic iodine has been another challenge in ElectricOIL as experiments have shown that the iodine is pumped into the excited state slower than is predicted by the known kinetics, resulting in reduced output power. An elementary model is presented that may partially explain this problem. Larger laser resonator volumes are employed to improve power extraction by providing more flow time for iodine pumping. The results presented in this work in conjunction with the efforts of others led to ElectricOIL scaling from 200 mW in the initial demonstration to nearly 500 W. ii Acknowledgments The work throughout the past many years that culminated in this document required support in many ways. Financially, funding was provided through various contracts: AFRL Phase I SBIR #FA9451-06-M-0138, HEL-JTO BAA #FA9451-06-D-0016, AFOSR MRI #F49620-02-1-0357, MDA Phase II SBIR #DASG60-03-C-0098, and DARPA #HR0011-09- C-0025 (as subcontract from Physical Sciences, Inc.). Additional support came from CU Aerospace internal research and development funds and the Directed Energy Professional Society Scholarship Program. I thank my advisor, Professor Wayne Solomon for his guidance, motivation, and patience throughout this process. Many years have passed since he provided me with a summer job when I was an undergraduate student. At the time, I had no idea how many more years I would spend in that laboratory, but I am grateful for all the opportunities that working with this research group has provided to me. I have been fortunate enough to have two other mentors working along with my advisor to guide me throughout my graduate career, so I thank Dr. David Carroll and Professor Joseph Verdeyen for all their assistance. Dr. Joseph Zimmerman and Gabriel Benavides embarked on the journey toward a Ph.D. on the same day as me, and the past several years of courses and research would have been even more challenging without them going through the same process. Darren King has worked with our research group throughout my educational tenure and taught me many practical aspects of working in a research laboratory. Many other members of our research group, both past and present, contributed to this work. Andrew Palla, Michael Day, Tyler Field, Kirk Kittell, Neil Hejmanowski, Curtis Woodruff, Daniel Weber, Scott Tuttle, Casey Hoercher, Brad Wheaton, Wayne Neumaier, Alayna Roberts, and Brian Shea all deserve thanks. I also generally thank all of my family and friends for keeping me sane during this process by providing the necessary distractions from work. The encouragement and confidence provided by my parents and my sister throughout all of my endeavors has been greatly appreciated. Finally, I thank my wife, Julia, for her patience, encouragement, patience, support, and patience while I completed graduate school. She assisted in every aspect of finally, actually completing this document. iii Table of Contents Page 1. Introduction and Background .............................................................................................1 1.1. Motivation, Background, and Uses for High Energy Lasers ....................................2 1.2. Discharge Driven Oxygen-Iodine Lasers ..................................................................6 1.3. The ElectricOIL Device ............................................................................................8 1.4. Thesis Overview ......................................................................................................11 2. Experimental Setup and Diagnostics .................................................................................15 2.1. General laboratory hardware ...................................................................................15 2.2. Emission diagnostics ...............................................................................................16 2.3. Absorption diagnostics ............................................................................................22 2.4. Discharge Types and Geometries ............................................................................24 2.5. Laser Cavities ..........................................................................................................26 3. Discharge Experiments ......................................................................................................31 3.1. O2(a) Production Theory .........................................................................................31 3.2. NO Discussion .........................................................................................................40 3.3. Unsuccessful attempts to create O2(a) more efficiently ..........................................42 3.4. Ozone Measurements ..............................................................................................46 3.5. Helium Diluent ........................................................................................................49 3.6. Materials ..................................................................................................................54 3.7. Discharge Electrode Gap .........................................................................................55 3.8. Discharge Length and Power Density .....................................................................59 3.9. High Flow Rate Discharge Configurations .............................................................63 4. Nitrogen diluted ElectricOIL .............................................................................................72 4.1. Discharge Investigations .........................................................................................72 4.2. Gain and Laser Experiments ...................................................................................76 5. Gain Recovery ...................................................................................................................80 5.1. Iodine Kinetics in ElectricOIL ................................................................................80 5.2. Gain Recovery Experiments ....................................................................................83 5.3. Simple Time-Dependent Gain Recovery Model .....................................................86 6. ElectricOIL Scaling Results ...............................................................................................96 6.1. Cav6 Gain and Laser Performance ..........................................................................96 6.2. Cav7 Laser Performance .......................................................................................102 6.3. ElectricOIL Scaling Summary ..............................................................................104 iv 7. Concluding Remarks ........................................................................................................108 7.1. Summary of Completed Work ..............................................................................108 7.2. Recommendations for Future ElectricOIL Studies ...............................................110 v 1. Introduction and Background First demonstrated
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