Browsing by Subject "Laser wakefield"

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An exploration on electron bunching of ionization induced self-injection in laser wakefield accelerators
(2016-05) Li, Deyun, M.A.; Shvets, G.; Berk, Herbert
Plasma-based wakefield accelerator is attractive for generating quasi-monoenergetic electron beams using the bubble regime. The bubble is formed by an intense driver, which propagates through the plasma and expels all electrons transversely, creating a cavity free of cold plasma electrons that trailing behind the driver. Self-injection is applicable in the bubble regime, which can produce bunches of quasi-monoenergetic electrons. (1) Such electron bunching structure can be diagnosed with coherent transition radiation and may be exploited to generate powerful high frequency radiation [16].This thesis focuses on electron bunching phenomenon through WAKE simulations and theoretical analysis. The simulation is completed under laser-driven field ionization wakefield acceleration. The code is improved by taking into consideration the high frequency property of laser driver in wakefield acceleration. Finer grid size is introduced to the ionization injection part of WAKE, for increasing simulation accuracy without much sacrifice of programming efficiency. Various conditions for optimal bunching in the trapped electrons are explored computationally and analytically.
Coherent optical diagnostics of laser-wakefield-accelerated electron bunches
(2022-11-28) LaBerge, Maxwell; Downer, Michael Coffin; Lumpkin, Alex; Keto, John; Breizman, Boris; Ditmire, Todd; Schramm, Ulrich
This dissertation presents a comprehensive study of coherent optical transition radiation (COTR) as a diagnostic for electron bunches from laser wakefield accelerators (LWFAs). In Chapter 2, I provide an intuitive description of the LWFA process, derive scaling laws, and describe injection regimes used in the experimental results presented later. In chapter 3, I present the theory of COTR with relevant approximations for the context of the experimental work shown later and in chapter 4 I present a COTR-imaging based algorithm to reconstruct portions of electron beams. Chapters 5, 6, 7, and 8 are published or soon to be published works using COTR diagnostics. The first section of each of these chapters is a "context of contribution," which summarizes my input as well as input from fellow coauthors. Chapter 5 presents a COTR interferometry diagnostic used to determine the emittance of a portion of an LWFA accelerated electron beam. It also sheds light on the level of microbunching present in beams from this acceleration method. Chapter 6 develops how multi-spectral COTR imaging allows for better characterization of an electron bunch. It goes on to present two-dimensional reconstructions of the coherent portion of the beam at several wavelengths for a single shot. Finally, it gives evidence that the source of the radiation is the charge population in quasi-monoenergetic peak of the electron spectrum as opposed to the low-energy background electrons or electrons accelerated by laser-foil interactions. Chapter 7 presents the differences between coherent and incoherent OTR for relativistic beams and demonstrates the high level (few percent) of microbunching from laser wakefield accelerated electron beams. This level of microbunching is shown to be injection regime dependent. Finally, chapter 7 further develops on how to glean spatial electron bunch information from multi-spectral COTR images, building up to a reproduction of features seen in particle in cell simulations as well as a candidate three-dimensional reconstruction of the coherent portion of an electron bunch. Chapter 8 is the culmination of the multi-spectral COTR imaging work. Here, I present analysis of COTR patterns from three different injection regimes, starting with phenomenology across many shots and then going in-depth to discuss what three-dimensional features account for the experimentally observed COTR patterns. Finally, chapter 9 provides a summary of this work and the outlook for future experiments.
Pulsed photoneutron source driven by electrons from laser wakefield acceleration
(2017-12) Jiao, Xuejing; Hegelich, Bjorn Manuel; Keto, John
Relativistic electron beams driven by laser wakefield acceleration (LWFA) were utilized to produce ultrashort neutron sources. The experiment was carried out on the 10 TW UT3, 38 f s, ~0.5 J, 800 nm Ti:Sapphire laser at the University of Texas at Austin. The target was a high-density pulsed gas jet composed of 90% Helium and 10% Nitrogen. The laser pulse with a peak intensity of 1.5E¹⁸ W/cm² interacted with the target to create a cylindrical plasma channel of 60 um radius (FWHM) and 1.5 mm length (FWHM). Electron beams of ~80 pC with Gaussian energy distribution centered at 37MeV and a width of 30MeV (FWHM) were produced via laser wakefield acceleration. A 2D particle in cell (PIC) simulation was performed to study the acceleration process. The LWFA was found to be running at a nonlinear broken-wave regime. The electron spectrum acquired from simulation quantitatively agree with the experimental observation. Neutron fluence of ~2.4x10⁶ per shot with ~300 ps temporal length was generated through bremsstrahlung and subsequent photoneutron reactions in a 26.6mm thick tungsten converter. Results were compared with simulations in GEANT4, showing agreement in neutron fluence, neutron angular distribution and conversion ratio.