Generation, measurement and application of x-rays from laser-plasma electron accelerators




Hannasch, Andrea Jeanne

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This dissertation presents a comprehensive study of the generation mechanisms, diagnostic techniques and possible applications of few keV to 100 MeV x-rays generated by laser wakefield electron accelerators. Chapters 1-3 review the principles of x-ray science and laser wakefield acceleration, and 3 mechanisms by which laser wakefield accelerators produce x-rays: 1) betatron oscillations of the electrons while still accelerating; 2) inverse Compton scatter (ICS) x-rays involving electron oscillations induced when electrons collide with a counter-propagating laser pulse after exiting the accelerator; 3) bremsstrahlung from the impact of accelerated electrons with a solid target. Chapters 4-6 then present original, recently published work, starting in chapter 4 with experiments that characterized secondary x-rays from a laser wakefield accelerator at Helmholtz-Zentrum Dresden Rossendorf. In this work, a laser wakefield accelerator was driven by the 150 TW DRACO laser system and produced electrons tunable in energy from 250 to 350 MeV. I co-designed and built a compact calorimeter consisting of a stack of x-ray absorbers alternating with imaging plates. This single device enabled me to unfold spectra of all three major types of x-rays, both individually and in mixtures: 1) few-keV betatron x-rays, 2) ICS x-rays that were spectrally peaked at ~1 MeV photon energy, and 3) broadband bremsstrahlung with an average energy of ~30 MeV and a high energy tail extending beyond 100 MeV photon energy. Chapter 5 presents results obtained at The University of Texas in which I extended the work in chapter 4 and used a redesigned compact calorimeter to characterize secondary x-rays generated from a GeV-class accelerator. In this work, the accelerator was driven by the 1 PW Texas Petawatt Laser (TPW) which accelerated electrons to energies ranging from 500 MeV to 2 GeV. The compact calorimeter was redesigned for improved sensitivity to photons from 1 MeV to >100 MeV and enabled me to unfold ICS x-rays that were peaked at ~10 MeV photon energy, and broadband bremsstrahlung with average energies ~80 MeV. Chapter 6 then presents additional results obtained on the DRACO laser system in which I characterized the capabilities of a LPA and plasma mirror to generate ICS x-rays in both a linear and nonlinear regime. I used a CsI(Tl) scintillator to characterize the strength and divergence of ICS x-rays generated by retro-reflecting the accelerator’s spent drive laser pulse back onto the accelerated electrons using a plasma mirror. These measurements showed that the laser-electron interaction ranged from sub-relativistic to relativistic, depending on the plasma mirror distance from the accelerator exit. Finally, chapter 7 presents unpublished results from the TPW and presents unfolded spectra from a bremsstrahlung target scan in which a series of targets ranging from 25 μm-thick Kapton to 7.6 mm-thick Pb were used to produce Bremsstrahlung with average energies ranging from 60 MeV to >100 MeV. Chapter 7 also presents preliminary results from the application of bremsstrahlung x-rays to nuclear activation of copper. This dissertation concludes with a summary of the presented results and a discussion of future directions for laser plasma produced x-ray science.



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