Final Defense

Neil Fazel, UT-Austin

"Quasi-monoenergetic laser-plasma acceleration of electrons beyond 1 GeV at the Texas Petawatt laser"

1:00pm, RLM 9.222

Abstract: Laser-plasma accelerators first produced 1 GeV electrons with few percent energy spread and high beam quality in 2006. The goal of laser-plasma acceleration experiments conducted at the Texas Petawatt (TPW) laser starting in 2011 was to advance this energy frontier significantly while maintaining high electron beam quality. To maximize energy transfer from laser pulse to electrons, we adopted the strategy of lowering the plasma density so that accelerating electrons and the laser-driven plasma accelerating structure remained in phase over several centimeters, instead of only millimeters. This was only possible because pulses from the TPW laser uniquely have the power (~1 PW) and duration (150 fs) required to excite the plasma resonantly and nonlinearly, and thus to achieve the favorable blowout (bubble) regime, at electron densities as low as 10^17 cm-3.

In this presentation, I will describe laser-plasma acceleration experiments driven by the TPW laser that successfully accelerated > 10^9 self-injected electrons (~1 nC) to > 1 GeV (> 10^8 self-injected electrons > 2 GeV) energy while maintaining 5% energy spread and sub-milliradian divergence. These experiments have generated ~2 GeV electron bunches more consistently and in larger numbers than any laser-plasma accelerator in the world. I also describe single-shot diagnostic methods devloped to characterize the divergence and energy spectrum of the electrons, and of the betatron x-rays they produced, despite low-repetition rate, significant pointing fluctuations, and electromagnetic pulse (EMP) background.

Betatron x-ray radiation originates from the transverse wiggling motion of accelerating electrons in the electrostatic field of a plasma bubble. It is useful as a broadband, femtosecond x-ray source and as a diagnostic of transverse electron beam emittance. Experiments at the TPW laser yielded betatron x-rays that were brighter, more collimated and more energetic than in previous experiments. I describe in depth an x-ray spectrometer design and methodology that I developed for single-shot, spatially-resolved measurement of betatron x-ray spectra. X-rays were sampled through K-edge transmission filters distributed strategically on a planar detector, high-fidelity x-ray images were reconstructed, and iso-intensity contours subsequently defined. I demonstrate how x-ray spectra may be calculated on such contours and used to produce a 3D representation of the x-ray spectrum. This represents the first single-shot method for betatron x-ray spectroscopy offering spatial resolution over the entire range of the x-ray profile.