UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    COMPACT LASER DRIVEN ELECTRON AND PROTON ACCELERATION WITH LOW ENERGY LASERS
    (2017) Hine, George Albert; Milchberg, Howard M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Laser-driven particle accelerators offer many advantages over conventional particle accelerators. The most significant of these is the magnitude of the accelerating gradient and, consequently, the compactness of the accelerating structure. In this dissertation, experimental and computational advances in laser-based particle acceleration in three intensity regimes are presented. All mechanisms investigated herein are accessible by “tabletop” ultrashort terawatt-class laser systems found in many university labs, with the intention of making them available to more compact and high repetition rate laser systems. The first mechanism considered is the acceleration of electrons in a preformed plasma “slow-wave” guiding structure. Experimental advances in the generation of these plasma guiding structures are presented. The second mechanism is the laser-wakefield acceleration of electrons in the self-modulated regime. A high-density gas target is implemented experimentally leading to electron acceleration at low laser pulse energy. Consequences of operating in this regime are investigated numerically. The third mechanism is the acceleration of protons by a laser-generated magnetic structure. A numerical investigation is performed identifying operating regimes for experimental realizations of this mechanism. The key advances presented in this dissertation are:  The development and demonstration of modulated plasma waveguide generation using both mechanical obstruction and an interferometric laser patterning method  The acceleration of electrons to MeV energy scales in a high-density hydrogen target with sub-terawatt laser pulses and the generation of bright, ultra-broadband optical pules from the interaction region  3D particle-in-cell (PIC) simulations of self-modulated laser wakefield acceleration in a plasma, showing the generation of broadband radiation, and the role of “direct laser acceleration” in this regime  3D PIC simulations of laser wakefield acceleration in the resonant regime, identifying spatio-temporal optical vortices in a laser-plasma system  3D PIC simulations of proton acceleration by magnetic vortex acceleration using TW-class laser pulses
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    ELECTRON ACCELERATION IN MAGNETIC RECONNECTION
    (2015) Dahlin, Joel Timothy; Drake, James F; Swisdak, Michael M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Magnetic reconnection is a ubiquitous plasma physics process responsible for the explosive release of magnetic energy. It is thought to play a fundamental role in the production of non-thermal particles in many astrophysical systems. Though MHD models have had some success in modeling particle acceleration through the test particle approach, they do not capture the vital feedback from the energetic particles on the reconnection process. We use two and three-dimensional kinetic particle-in-cell (PIC) simulations to self-consistently model the physics of electron acceleration in magnetic reconnection. Using a simple guiding-center approxima- tion, we examine the roles of three fundamental electron acceleration mechanisms: parallel electric fields, betatron acceleration, and Fermi reflection due to the re- laxation of curved field lines. In the systems explored, betatron acceleration is an energy sink since reconnection reduces the strength of the magnetic field and hence the perpendicular energy through the conservation of the magnetic moment. The 2D simulations show that acceleration by parallel electric fields occurs near the mag- netic X-line and the separatrices while the acceleration due to Fermi reflection fills the reconnection exhaust. While both are important, especially for the case of a strong guide field, Fermi reflection is the dominant accelerator of the most energetic electrons. In a 3D systems the energetic component of the electron spectra shows a dramatic enhancement when compared to a 2D system. Whereas the magnetic topology in the 2D simulations is characterized by closed flux surfaces which trap electrons, the turbulent magnetic field in 3D becomes stochastic, so that electrons wander over a large region by following field lines. This enables the most energetic particles to quickly access large numbers of sites where magnetic energy is being released.