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.
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Item VIBRATIONS OF MULTIPLE INTERCONNECTED BEAMS BY DIRAC-LAPLACE-HEAVISIDE METHOD(2021) Zhao, Xingzhuang; Chang, Peter; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Multiple Inter-Connected Beams (MICB) system is widely used in civil and mechanical engineering. Adopting Euler-Bernoulli beam theory, this thesis presents novel exact and closed-form solutions for free and forced vibrations of the MICB system with intermediate connections and masses using a continuum approach. Recently, Roncevic et al. (2019) adopted Green's function method to study the free vibrations of a beam system with arbitrary intermediate supports. A total of 49 cases of Green's functions were derived and tabulated for each combination of the boundary conditions, i.e., fixed, pinned, sliding, free, translational spring-supported, rotational spring-supported, and combined translational-rotational spring-supported. Han et al. (2021) adopted the dynamic stiffness matrix method to study the free vibrations of a double-beam system with intermediate supports. With this method, one must discretize the beams at the connections and then apply the continuity conditions. This thesis is intended to treat MICB with arbitrary boundary conditions in a unified manner and avoid the need to discretize the beams. In this work, the Dirac-Laplace-Heaviside (DLH) method is proposed to investigate vibrations of the MICB with arbitrary intermediate connections and concentrated masses under the arbitrary boundary conditions subjected to arbitrary external excitation. The Dirac's delta function is adopted to formulate the mathematical model; the Laplace transform is utilized to solve the model; and the Heaviside function is used to implement the solution. Euler-Bernoulli beam is assumed to have a uniform cross-section. Axial loads, arbitrary external exciting forces, and arbitrary boundary conditions are also incorporated in the studied model. The exact mode shape solutions are developed by the proposed DLH method. The arbitrary boundary conditions are handled in a unified form. The solutions are validated by numerical results using a Finite Element Method (FEM). They are also compared to the results of specific cases and Green's function method. This study contributes to a continuum approach to handle beams with interconnections and concentrated masses. It is a general model that could be reduced to the specific models investigated in the literature. The method and model reported in this thesis may be useful for vibration analysis, dynamic control, vibration attenuation, design optimization of the MICB.Item TOMOGRAPHIC MEASUREMENT OF THE PHASE-SPACE DISTRIBUTION OF A SPACE-CHARGE-DOMINATED BEAM(2008-04-24) Stratakis, Diktys; O'Shea, Patrick G; Kishek, Rami A; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Many applications of accelerators, such as free electron lasers, pulsed neutron sources, and heavy ion fusion, require a good quality beam with high intensity. In practice, the achievable intensity is often limited by the dynamics at the low-energy, space-charge dominated end of the machine. Because low-energy beams can have complex distribution functions, a good understanding of their detailed evolution is needed. To address this issue, we have developed a simple and accurate tomographic method to map the beam phase using quadrupole magnets, which includes the effects from space charge. We extend this technique to use also solenoidal magnets which are commonly used at low energies, especially in photoinjectors, thus making the diagnostic applicable to most machines. We simulate our technique using a particle in cell code (PIC), to ascertain accuracy of the reconstruction. Using this diagnostic we report a number of experiments to study and optimize injection, transport and acceleration of intense space charge dominated beams. We examine phase mixing, by studying the phase-space evolution of an intense beam with a transversely nonuniform initial density distribution. Experimental measurements, theoretical predictions and PIC simulations are in good agreement each other. Finally, we generate a parabolic beam pulse to model those beams from photoinjectors, and combine tomography with fast imaging techniques to investigate the time-sliced parameters of beam current, size, energy spread and transverse emittance. We found significant differences between the slice emittance profiles and slice orientation as the beam propagates downstream. The combined effect of longitudinal nonuniform profiles and fast imaging of the transverse phase space provided us with information about correlations between longitudinal and transverse dynamics that we report within this dissertation.Item Longitudinal Dynamics of an Intense Electron Beam(2005-07-29) Harris, John Richardson; O'Shea, Patrick G.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The dynamics of charged particle beams are governed by the particles' thermal velocities, external focusing forces, and Coulomb forces. Beams in which Coulomb forces play the dominant role are known as space charge dominated, or intense. Intense beams are of great interest for heavy ion fusion, spallation neutron sources, free-electron lasers, and other applications. In addition, all beams of interest are dominated by space charge forces when they are first created, so an understanding of space charge effects is critical to explain the later evolution of any beam. Historically, more attention has been paid to the transverse dynamics of beams. However, many interesting and important effects in beams occur along their length. These longitudinal effects can be limiting factors in many systems. For example, modulation or structure applied to the beam at low energy will evolve under space charge forces. Depending on the intended use of the beam and the nature of the modulation, this may result in improved or degraded performance. To study longitudinal dynamics in intense beams, experiments were conducted using the University of Maryland Electron Ring, a 10 keV, 100 mA electron transport system. These experiments concentrated on space charge driven changes in beam length in parabolic and rectangular beams, beam density and velocity modulation, and space charge wave propagation. Coupling between the transverse and longitudinal dynamics was also investigated. These experiments involved operating the UMER gun in space charge limited, temperature limited, triode amplification, photon limited, and hybrid modes. Results of these experiments are presented here, along with a theoretical framework for understanding the longitudinal dynamics of intense beams.