Advanced Simulations and Optimization of Intense Laser Interactions
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Advanced Simulations and Optimization of Intense Laser Interactions DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Joseph Richard Harrison Smith, B.A., M.S. Graduate Program in Physics The Ohio State University 2020 Dissertation Committee: Professor Chris Orban, Advisor Professor Enam A. Chowdhury Professor Douglass W. Schumacher Professor Richard J. Furnstahl c Copyright by Joseph Richard Harrison Smith 2020 Abstract 1This work uses computer simulations to investigate intense laser-plasma interactions. First, we use two-dimensional particle-in-cell (PIC) simulations and simple analytic models to in- vestigate the laser-plasma interaction known as ponderomotive steepening. When normally incident laser light reflects at the critical surface of a plasma, the resulting standing elec- tromagnetic wave modifies the electron density profile via the ponderomotive force, which creates peaks in the electron density separated by approximately half of the laser wave- length. What is less well studied is how this charge imbalance accelerates ions towards the electron density peaks, modifying the ion density profile of the plasma. Idealized PIC sim- ulations with an extended underdense plasma shelf are used to isolate the dynamics of ion density peak growth for a 42 fs pulse from an 800 nm laser with an intensity of 1018 W cm−2. These simulations exhibit sustained longitudinal electric fields of 200 GV m−1, which pro- duce counter-steaming populations of ions reaching a few keV in energy. We compare these simulations to theoretical models, and we explore how ion energy depends on factors such as the plasma density and the laser wavelength, pulse duration, and intensity. We also provide relations for the strength of longitudinal electric fields and an approximate timescale for the density peaks to develop. These conclusions may be useful for investigating the phe- nomenon of ponderomotive steepening as advances in laser technology allow shorter and more intense pulses to be produced at various wavelengths. We also discuss the parallels with other work studying the interference from two counter-propagating laser pulses. Next we investigate the development of ultra-intense laser-based sources of high energy ions, which is an important goal, with a variety of potential applications. One of the barriers to achieving this goal is the need to maximize the conversion efficiency from laser energy to ion energy. We apply a new approach to this problem, in which we use an evolutionary algorithm to optimize conversion efficiency by exploring variations of the target density profile with thousands of one-dimensional PIC simulations. We then compare this 1Some of this abstract is reprinted from Joseph R Smith, Chris Orban, Gregory K Ngirmang, John T Morrison, Kevin M George, Enam A Chowdhury, and WM Roquemore. Particle-in-cell simulations of density peak formation and ion heating from short pulse laser-driven ponderomotive steepening. Physics of Plasmas, 2019 [1] under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). ii \optimal" target identified by the one-dimensional PIC simulations to more conventional choices, such as with an exponential scale length pre-plasma, with fully three-dimensional PIC simulations. The optimal target outperforms the conventional targets in terms of maximum ion energy by 20% and shows a significant enhancement of conversion efficiency to high energy ions. This target geometry enhances laser coupling to the electrons, while still allowing the laser to strongly reflect from an effectively thin target. These results underscore the potential of this statistics-driven approach for optimizing laser-plasma simulations and experiments. Finally, we present computational fluid dynamic simulations that model the formation of thin liquid targets. These simulations allow us to explore new types of targets that may be beneficial for high repetition rate laser plasma interactions. iii In memory of my grandmother Theresa `Terry' Harrison and my grandfather Forrest E. `Smitty' Smith iv Acknowledgments I would like to thank my advisor Chris Orban for his guidance throughout the marathon that is a PhD. Chris has always been supportive and provided me with the academic freedom to explore a wide range of research projects. His advice is always welcome and I am very grateful that I have had the chance to work with him. Throughout my graduate studies at Ohio State, I have been fortunate to have had an abundance of positive role models and mentors to help me grow as a scientist. I'd like to thank Scott Feister for introducing me to computational plasma physics and it's always great to work with him on a project-even when the outcome is not what we hoped for. I'd like to thank Gregory Ngirmang for challenging the simulation paradigm in our group, and the field, and for inspiring me to explore paths less taken with my research. Enam Chowdhury is a formidable force in science, and I'd like to thank him for sharing his knowledge and intuition, and for being a strong advocate for those he works with. The past and present scientists working with the Extreme Light Group at AFRL rep- resent a special supportive community that I am honored to have collaborated with. John Morrison is an excellent scientist and I would like to thank him for pushing me to find my own way in research (and of course to graduate in a timely manner). Kevin George exemplifies the qualities of an excellent collaborator, bringing a fresh perspective to every project and pushing us to make excellent work. Mel Roquemore's unrivaled passion for sci- ence and warm attitude towards others encouraged me to be a better scientist. I would also like to thank Kyle Frische for keeping the lab running and warmly welcoming the invasion of theorists from OSU. Joseph Snyder is a talented scientist and provided me with welcome advice for succeeding in graduate school and afterwards. One cannot escape the positive influence of Doug Schumacher and the HEDP group at OSU. Doug encourages people to succeed and his high expectations inspire excellent work. The entire HEDP group is extraordinarily welcoming and helped to broaden my knowledge. Alexander Klepinger, Anthony Zingale, Derek Nasir, Preston Pozderac, and Nick Czapla are always up for a lively discussion on physics or a variety of other topics. It's a pleasure to work with them and the others in the HEDP group. Of course, my work is only possible due to those like Becky Daskalova, Anthony, Derek, Nick, German, and others in the lab who v keep those of us playing with computers in check. I would also like to thank Ginny Cochran for all of her effort organizing journal clubs while she was at OSU. The previous members of the group are still making great impressions at their current jobs, and are always welcoming to current students at conferences. I would like to thank my undergraduate research mentors John Ramsay, Jennifer Bowen, and Susan Lehman for introducing me to research and helping me become a competent researcher. I would also like to thank John Grove, Jim Hill, Karlene Maskaly, and the other scientists and students who made my summer in New Mexico enjoyable (along with chances to meet with Alexander Klepinger while he was in Albuquerque, and with other HEDP alumni in the state). I would like to thank Dick Furnstahl for his computational physics and Bayesian methods courses, which were excellent preparations for actual research. I'd also like to thank Andrew Heckler, Tom Gramila, and Ralf Bundschuh for helping me grow as an educator. I would like to thank Chris Porter, Bart Snapp, Jim Fowler, Jon Brown, and others for the surprising quality of work we were able to accomplish with the BuckeyeVR project, despite limited resources. The arduous endeavor of a PhD wouldn't have been possible without the continual support of my friends and family. My friends have made the last five years a much more enjoyable experience. I'm not sure how I would have made it through the first years of graduate school without the revolving set of officemates Ethel, Jose, Andr´es,Daniella, Franz, Emilio, Bruce, and Philip. I'd also like to thank Mike, Estefany, Humberto, Paulo, and Catherine, and the others for welcoming me into the extended Bridge family. I cherish experiences with my friends like semi-spontaneous trips to Canada, to see the total eclipse, and moving a friend across the country. I'd like to thank Jose, Ethel, Bruce, and others for joining me in the culinary exploration of Columbus. I'd like to thank Alexander, Matt, Andr´es,and Emily for expanding my repertoire of board games and being wonderful friends! I'd also like to thank my officemates Zach and Derek for interesting conversations. Andr´es, thank you for being a great housemate, especially in these times. Alexander, thank you for bringing people together and always having something interesting to talk about. Ethel and Bruce thanks for extending your friendship and being awesome. Jose, thanks for making work interesting and being a great friend. I could keep going, but I shouldn't, so I'd like to thank all of the others at OSU and elsewhere who have had a strong impact on my life. Lastly I would like to thank my family for their support and encouragement. My mother and father have also been so supportive of me and my goals, I can't thank you enough for everything they do! I'd also like to thank my brother Jacob, the political scientist, for blazing the path before me, and for my incredibly intelligent sister Heidi who will do great things. vi Vita May 2015 .