Theses and Dissertations from UMD
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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 give thesis/dissertation in DRUM
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Item Experimental Study of Solitons on Intense Electron Beams(2012) Mo, Yichao; Kishek, Rami A; O'Shea, Patrick G; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solitons are localized persistent waves that behave like particles, preserving their properties (shape, velocity, etc.) over long distances and through collisions with other solitons. They have practical applications and are of interest to many disciplines such as condensed matter physics, plasma physics, beam physics, optics, biology and medicine. Whereas solitons in electron beams have been predicted on theoretical grounds decades ago, they have been observed experimentally only recently by Thangaraj at the University of Maryland Electron Ring (UMER). In this thesis, I report on the first systematic characterization of solitons in electron beams and confirm the soliton's particle-like behavior. The transient longitudinal space charge force on the beam bunch can launch large-amplitude waves, for example from imperfections in matching the focusing force to the beam bunch. By introducing a pulsed laser beam on a thermionic cathode, an electron beam with a narrow density perturbation is generated. The perturbation then evolves into longitudinal space charge waves that propagate along the beam. For large-amplitude initial perturbations, a soliton wave train is observed. The experimental results are reproduced by simulations with the WARP particle-incell (PIC) code.Item DEVELOPMENT OF AN ADAPTIVE MASKING METHOD TO IMAGE BEAM HALO(2011) Zhang, Hao; O'Shea, Patrick G; Fiorito, Ralph B; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Beam halo is a common phenomenon in most intense particle beams and is associated with many bad effects. Halo is very hard to characterize because of its low intensity, which requires a measurement system with high dynamic range (≥105). Here, we have developed a technique that employs a digital micro-mirror array to produce an image of the halo of an electron beam with an enhanced dynamic range. Light produced by the beam intercepting a phosphor screen is first imaged onto the array; an adaptive mask is created and applied to filter out the beam core; and the result is re-imaged onto a CCD camera. In this thesis, we describe the optics used, the masking operation and preliminary results of experiments we have performed to study beam halo at the University of Maryland Electron Ring.Item LINEAR AND NONLINEAR ANALYSIS OF A GYRO-PENIOTRON OSCILLATOR AND STUDY OF START-UP SCENARIO IN A HIGH ORDER MODE GYROTRON(2006-01-31) Yeddulla, Muralidhar; Antonsen, Thomas; Nusinovich, Gregory; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Cyclotron Resonant Maser (CRM) is a device in which electrons gyrating in an external magnetic field produce coherent EM radiation. A DC electron beam current must be converted to an AC beam current to create RF energy. There are two possible approaches: phase bunching (O-type) and spatial segregation (Mtype). In phase bunching, electrons are either accelerated or decelerated depending on when the electrons enter the interaction region, causing phase bunching. The electron bunches are then slowed down by the RF field for energy extraction. Not all electrons lose energy; some even gain energy. In spatial segregation, electrons entering the interaction region at different times are deflected in different directions. With an appropriate spatially varying RF field, all electrons can lose energy leading to very high conversion efficiency. A CRM with a smooth walled cylindrical waveguide interaction cavity and an annular electron beam passing through it can generate very large amount of RF energy. Depending on the electron beam position a gyrotron (O-type device) and a gyro-peniotron (M-type device) are possible. In this work, first, a nonlinear theory to study CRMs with a smooth walled cylindrical waveguide interaction cavity is presented. The nonlinear set of differential equations are linearized to study the starting conditions of the device. A gyropeniotron operating in the TE0,2 - mode is studied using the theory presented. It is found that a gyro-peniotron operating in a low order mode can be self excited without mode competition from gyrotron modes, leading to the possibility of a very efficient high power RF source. A higher order mode gyro-peniotron experiences severe mode competition from gyrotron modes. The cavity Q required for gyropeniotron operation is very high, which can lead to excessive heat in the cavity walls due to ohmic losses. Hence, a gyro-peniotron operation seems practical only in low order modes and in short pulses. Second, an existing linear theory of gyrotrons is extended to include effects of magnetic field tapering, cavity wall profile, finite beam thickness, velocity spread and axially dependent beam coupling to the fields of competing modes. Starting currents are calculated for the operating and the most dangerous competing mode in a 140 GHz gyrotron, which was developed at Communications and Power Industries (CPI). Start-up scenario of this device is also studied using the non-stationary code MAGY, which is a tool for modeling slow and fast microwave sources.