Browsing by Subject "Laser wakefield acceleration"

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Laser spatiotemporal effects on self-modulated laser wakefield acceleration
(2024-05) Richmond, Christopher Grant ; Ditmire, Todd; Hegelich, Bjorn Manuel; Bank, Seth R; Paban, Sonia; Downer, Michael W.
We present experimental data comparing the effects of beam apodization and two different laser pulse contrast profiles on self-modulated laser wakefield acceleration using the Texas Petawatt Laser (TPW). 2D particle-in-cell simulations and experimental shadowgraphic imaging indicate that with poor contrast, self-focusing of the laser prepulse results in direct laser acceleration becoming a dominant acceleration mechanism. This results in lower charge yields and a lower quality electron beam. Three different apertures were used for beam apodization. Each aperture showed an increase in total charge generated. Low power focal spot measurements show an increase in beam quality, implying improved wakefield formation. This is supported by using TPW beam information and Gerchberg-Saxton phase retrieval to simulate the high power apodized focus on shot. Greatest improvements in charge and focal spot quality were seen using a serrated aperture. The combined effect of these two methods showed nearly an order of magnitude increase in charge when comparing a low contrast, unapodized beam to a high contrast beam using a serrated aperture.
Laser wakefield and direct acceleration in the plasma bubble regime
(2017-08) Zhang, Xi, Ph. D.; Mahajan, Swadesh M.; Shvets, G.; Downer, Michael; Morrison, Philip; Milosavljevic, Milos
Laser wakefield acceleration (LWFA) and direct laser acceleration (DLA) are two different kinds of laser plasma electron acceleration mechanisms. LWFA relies on the laser-driven plasma wave to accelerate electrons. The interaction of ultra-short ultra-intensive laser pulses with underdense plasma leads the LWFA into a highly nonlinear regime (“plasma bubble regime”) that attracts particular interest nowadays. DLA accelerates electrons by laser electromagnetic wave in the ion channel or the plasma bubble through the Betatron resonance. This dissertation presents a hybrid laser plasma electron acceleration mechanism. We investigate its features through particle-in-cell (PIC) simulations and the single particle model. The hybrid laser plasma electron acceleration is the merging concept between the LWFA and the DLA, so called laser wakefield and direct acceleration (LWDA). The requirements of the initial conditions of the electron to undergo the LWDA are determined. The electron must have a large initial transverse energy. Two electron injection mechanisms that are suitable for the LWDA, density bump injection and ionization induced injection, are studied in detail. The features of electron beam phase space and electron dynamics are explored. Electron beam phase space appears several unique features such as spatially separated two groups, the correlation between the transverse energy and the relativistic factor and the double-peak spectrum. Electrons are synergistically accelerated by the wakefield as well as by the laser electromagnetic field in the laser-driven plasma bubble. LWDA are also investigated in the moderate power regime (10 TW) in regarding the effects of laser color and polarization. It is found that the frequency upshift laser pulse has better performance on avoiding time-jitter of electron energy spectra, electron final energy and electron charge yield. Some basic characters that related to the LWDA such as the effects of the subluminal laser wave, the effects of the longitudinal accelerating field, the electron beam emittance, the electron charge yield and potentially applications as radiation source are discussed.
Using machine learning to measure ultrahigh-flux multi-MeV gamma rays from a laser accelerator
(2020-05-13) Lisi, Luc Amram; Downer, Michael Coffin
In this thesis we present a novel computational method capable of measuring the energy distribution of ultrahigh-flux and high-energy photons ranging from 1-300MeV produced via a Thomson Backscatter process at the University of Texas at Austin Petawatt Laser facility. Due to the large and complex particle showers these kinds of photons produce when interacting with matter, energy measurements of these kinds of sources is notoriously difficult. In our method however, we make use of the complex particle showers these sources produce to extract information about the energy profile by interacting the photons with a compact inorganic scintillator. Then, using predictive simulations in Geant4 and regression analysis techniques, we analyze the raw scintillator response resulting from the incident photon shower, and compute the most likely photon energy spectrum with confidence intervals. In the following thesis, we will cover the methodology of this analysis as well as look at how it performs when applied to a recent experimental shot. Finally, we will compare the result to theoretical predictions in order to gauge the feasibility of this diagnostic method