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Copyright by Joseph Michael Shaw 2019 The Thesis Committee for Joseph Michael Shaw certifies that this is the approved version of the following thesis: Experimental Studies on High-Energy Radiation Sources from Laser Wakefield Accelerators APPROVED BY SUPERVISING COMMITTEE: Michael C. Downer, Supervisor Aaron C. Bernstein Experimental Studies on High-Energy Radiation Sources from Laser Wakefield Accelerators by Joseph Michael Shaw Thesis Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of Master of Arts The University of Texas at Austin May 2019 To my parents and the rest of my family for their unwavering support and love. Acknowledgments My sincerest gratitude goes to my adviser, Professor Downer, for introducing me to the incredibly interesting world of laser-plasma physics and allowing me to play with his expensive laser. I would like to thank Xioaming Wang and Hai-En Tsai whose initial tutelage in laser maintenance and laser plasma diagnostics would prove invalu- able throughout my entire time at UT. I would like to thank Rafal Zgadzaj and Aaron Bernstein for the countless hours of time donated towards setting up experiments, proofreading publications and presentations, and providing general advice of all kinds. Thank you to Vincent Chang, Andrea Hannasch, Max LaBerge, Kathleen Weichman, Jake Welch, Xiantao Cheng, Ganesh Ti- wari, and Luc Lisi for all their help setting up and conducting experiments. And thanks for being great company during late-night data runs. I would like to thank Farbod Shafiei, Neil Fazel, and Loucas Loumakos for all the lively conversations and being great office mates. Thanks to Watson Henderson for all of his advice on a myriad of subjects. Finally, I would like to thank the entire Texas Petawatt staff for their hard work over several month- long experimental runs. The data and analysis in this thesis would never have been possible without the hard work and determination of everyone mentioned above. v Abstract Experimental Studies on High-Energy Radiation Sources from Laser Wakefield Accelerators Joseph Michael Shaw, M.A. The University of Texas at Austin, 2019 Supervisor: Michael C. Downer In this work I discuss a series of experiments on generating and characterizing a compact, ultrashort-duration source of Thomson backscatter γ-rays at the University of Texas, Austin. The γ-rays are created in a three-step process that begins with the Texas Petawatt laser-plasma accelerator producing GeV- scale electron beams. At the exit of the accelerator, the leading edge of the TPW laser pulse ionizes the surface of a glass or plastic substrate to form a plasma mirror. The plasma mirror retro-reflects a majority of the remaining laser energy back into the accelerated electrons to act as an optical undula- tor, which stimulates the production of γ-rays. By adjusting the separation between the plasma mirror and exit of the accelerator, we were able to simulta- neously confirm that the inherently self-aligning quality of the plasma mirror is maintained over a wide range of intensities and observe the transition from linear to nonlinear Thomson backscatter. Linear Thomson backscatter cal- culations inferred from accelerated electron spectra imply γ-ray spectra with peaked components ranging from 5 - 85 MeV. vi Table of Contents List of Tables x List of Figures xi Chapter 1 Introduction 1 1.1 Laser Plasma Accelerators . 1 1.2 Radiation Sources . 2 1.3 Thomson Scattering . 3 1.4 Laser Strength Parameter . 5 1.5 Thomson Backscatter of Relativistic Electrons . 5 Chapter 2 Relevant Laser-Plasma Dynamics 8 2.1 Light Propagation in Plasma . 9 2.2 Laser Wakefield Acceleration . 10 2.3 Plasma Mirrors . 12 2.4 Thomson Backscatter Experiments . 13 Chapter 3 Thomson Backscatter Experiments with the TPW 15 3.1 LPA Experimental Setup . 15 3.1.1 Plasma Mirror Performance . 16 3.1.2 Radiation Diagnostics . 18 3.2 Linear Thomson Backscatter . 20 3.2.1 Laser Intensity Approximation . 20 3.2.2 Bremsstrahlung Contributions . 22 vii 3.2.3 Linear Thomson Spectra . 25 3.3 Nonlinear Thomson Backscatter . 27 3.4 Future Work and Conclusions . 30 3.4.1 γ-ray Spectrometer Measurements . 30 3.4.2 Conclusions . 32 Bibliography 33 viii List of Tables 2.1 Neutral helium number densities by orders of magnitude and the approximate electron and He2+ ion oscillation periods for a fully-ionized plasma, respectively. 9 3.1 Scaling of the γ-beam divergence (FWHM) along the laser po- larization axis with PM z-position. 30 ix List of Figures 1.1 (a) A dipole radiation pattern in the rest frame of an electron (i:e: γe =1). (b) A dipole radiation pattern for an electron moving upward with total energy twice its rest energy (i:e: γe =2). 6 2.1 A computational simulation of electron injection and acceleration in the bubble regime for the TPW LPA, where the color scaling represents the electron density [cm−3]. (a) A laser wakefield bubble near the beginning of its formation and corresponds to the propagation distance z = 0:14 cm. (b) Corresponds to z = 0:336 cm. (c) Corresponds to z = 1:04 cm. [Image and caption are modified versions of a figure courtesy of Stefan Bedacht, University of Texas at Austin] [Original simulations/ figure courtesy of Serguei Kalmykov and Arnaud Beck] . 11 2.2 The Texas Petawatt laser pulse temporal contrast measured via third-order autocorrelation [29]. 13 3.1 A top-down schematic of the LWFA 5.5 experimental setup used for generating and characterizing the GeV-scale laser wakefield accelerator and PM-based Thomson γ-ray source at the Texas Petawatt. 15 x 3.2 Probe beam reflectivity calibration: (a) The (null) probe beam profile imaged from the coverslip surface after scaling the gray values, accounting for reflection and transmission losses of the imaging system, to the initial incident intensity (I0). (b),(c) A localized region of the coverslip surface is activated by the transmitted LPA-driving pulse, enabling a greater proportion of probe light to be reflected from the surface. The gray values are then normalized to I0 to approximate the percent reflected. 17 3.3 (a) Electron spectrum (left) with peak at 2.2 GeV and corresponding betatron x-ray profile (upper right) recorded on IP. Secondary particles from γ-ray conversion produced a bright spot near center of metal disk (lower right) on a separate shot. (b) Scintillator signals with PM in place (top), showing Thom- son γ-ray profile, and with no PM (bottom). 18 3.4 Shot-to-shot pointing fluctuations. (a) Electron spectra (left), γ-ray profiles (right) for two shots showing equal but opposite vertical displacements and differing horizontal γ-ray displacements, with respect to the alignment axis. (b) Plot of vertical γ-ray vs. electron displacements. 19 3.5 Side-scatter emission of the laser-induced plasma channel seen through a glass window in the helium gas cell. The laser prop- agates from left to right. A logarithmic function was applied to the image to dampen the strong scattering near the beginning of the gas cell and enhance the visibility of the channel near the end. The bright edge on the right is the exit aperture of the gas cell. 21 3.6 Scaling of scintillator signal with position z and thickness L of PM: (a) z = 3:3 cm, L = 100µm; (b) z = 5:5 cm, L = 180 µm. Nearly identical laser pulses drove both shots; both yielded electrons with energy peaked at 0.92 GeV and corresponding charge (a) 50 or (b) 125 pC. 23 xi 3.7 Scaling of the integrated fluence with fbrem at zPM = 3:3 cm. The blue, dashed curve has a slope of unity and illustrates the expected scaling if bremsstrahlung radiation were the only con- tributor of signal. The grey, dotted curve represents the best-fit for a linear relationship. 25 3.8 Quasi-monochromatic Thomson γ-ray spectra generated as Ee tuned from 0.5 to 2.2 GeV. Spectra are labeled with multipliers that normalize true peak heights to the height of the two lowest energy curves. 26 3.9 A typical electron dN/dE for LWFA 6.0 and 7.0. 27 3.10 (a) The first three harmonics of a TBS spectrum calculated from the above dN/dE, assuming a0 = 0:25. (b) The first three harmonics of a TBS spectrum calculated from the above dN/dE, assuming a0 = 0:5. 28 3.11 Scaling of the integrated fluence with the PM's z-position. The blue trendline is a second-order polynomial fit for a total of 39 shots, represented by the light-orange boxes. The vertical red line represents the nominal exit plane of the gas cell. A statistical average and standard deviation is represented by the singular data point with error bars at each respective PM position. 29 3.12 Simulation of e− / e+ energy-angle distributions produced by a monoenergetic, 10 MeV photon beam in 2 cm of carbon. Image and simulations courtesy of Luc Lisi. 31 3.13 (Left) A top-down view of the Compton & pair-production spectrometer design. Higher energy electrons or positrons will de- posit their signal further down the length of the spectrometer. (Right) The design and specifications of the magnet housed within the spectrometer. Magnet drawing and specifications courtesy of Ganesh Tiwari. 31 xii Chapter 1 Introduction 1.1 Laser Plasma Accelerators Laser-plasma accelerators (LPAs) harness intense, ultrashort light pulses to drive charge-density waves in a plasma, ranging in electron density from 0.01 - 0.1 atm, to capture and accelerate electron bunches up to relativistic energies.

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