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.
Browse
4 results
Search Results
Item CONTRIBUTIONS TO RESOLVING ISSUES IMPEDING THE OPERATION OF HIGH POWER MICROWAVE AND SUBMILLIMETER DEVICES(2014) Kashyn, Dmytro; Granatstein, Victor L.; Nusinovich, Gregory S.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis reports an experimental study aimed at extending high power, high efficiency gyrotron operation to submillimeter wavelengths. A series of experiments carried out both at the University of Maryland and the Institute of Applied Physics of the Russian Academy of Science, succeeded in demonstrating output power at 670 GHz of 180 kilowatts with 20% efficiency (gyrotron voltage was 57 kV and beam current was 16 amperes). The maximum output power achieved in the experiments was 210kW at somewhat higher voltage and current (viz. 58kV and 22A). The achieved output power and efficiency are twice as large as achieved in previous experiments in this frequency range with pulse duration in the range of tens of microseconds. These performance parameters are relevant to a previously proposed application of detecting concealed radioactive materials by air breakdown in a focused beam of sub-millimeter radiation. The 670 GHz gyrotron combined features of two lines of previous experiments: (a) to operate at the required frequency, pulsed solenoids producing 28T magnetic were employed and (b) to obtain high efficiency a very high order mode was used in the gyrotron cavity, as in the experiments with gyrotrons for plasma heating. Evidence of multimode beating was observed in submillimeter output envelope. The excitation of spurious modes, especially during the rise of the gyrotron voltage pulse, was analyzed and the method of avoiding this was proposed which also allows to reduce collector loading in gyrotrons operating in modulated regimes. The present study also includes theoretical analysis of the processes that deepens the understanding of microwave breakdown (arcing) in high power microwave devices. The effect of the dust particles microprotrusions on the device operation was analyzed. These microprotrusions were observed and their negative effects were remedied by careful polishing and machining of the resonator surface. Finally, the generated 670 GHz radiation was focused and used to study breakdown both in argon and in the air. This breakdown volume was theoretically analyzed and the effects of the atmospheric turbulence on the air breakdown were included.Item Radiation Transport Measurements in Methanol Pool Fires with Fourier Transform Infrared Spectroscopy(2008) Yilmaz, Aykut; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Pool fires rely on radiation and conduction heat feedback from the combustion process to the liquid surface to vaporize the fuel. This coupled relationship determines the fuel burning rate and thus the fire structure and size. Radiative heat transfer is the dominant heat feedback in large pool fires. Species concentrations and temperatures have large influence on the radiative heat transfer in the fuel rich-core between the flame and the pool surface. To study radiative transport in the fuel-rich core, an experimental method was developed to measure radiative absorption through various pathlengths inside a 30 cm diameter methanol pool fire by using a Fourier Transform Infrared Spectrometer with N2 purged optical probes. The measured spectra are used to estimate species concentration profiles of methanol, CO, and CO2 in the fuel rich core by fitting predictions of a spectrally resolved radiation transport model to the measured spectra. Results show the importance of reliable temperature measurements for fitting the data and the need for further measurements to further understand the structure of fuel rich cores in pool fires.Item Self-consistent simulation of radiation and space-charge in high-brightness relativistic electron beams(2007-06-25) Gillingham, David; O'Shea, Patrick G; Antonsen, Thomas M.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ability to preserve the quality of relativistic electron beams through transport bend elements such as a bunch compressor chicane is increasingly difficult as the current increases because of effects such as coherent synchrotron radiation (CSR) and space-charge. Theoretical CSR models and simulations, in their current state, often make unrealistic assumptions about the beam dynamics and/or structures. Therefore, we have developed a model and simulation that contains as many of these elements as possible for the purpose of making high-fidelity end-to-end simulations. Specifically, we are able to model, in a completely self-consistent, three-dimensional manner, the sustained interaction of radiation and space-charge from a relativistic electron beam in a toroidal waveguide with rectangular cross-section. We have accomplished this by combining a time-domain field solver that integrates a paraxial wave equation valid in a waveguide when the dimensions are small compared to the bending radius with a particle-in-cell dynamics code. The result is shown to agree with theory under a set of constraints, namely thin rigid beams, showing the stimulation resonant modes and including comparisons for waveguides approximating vacuum, and parallel plate shielding. Using a rigid beam, we also develop a scaling for the effect of beam width, comparing both our simulation and numerical integration of the retarded potentials. We further demonstrate the simulation calculates the correct longitudinal space-charge forces to produce the appropriate potential depression for a converging beam in a straight waveguide with constant dimensions. We then run fully three-dimensional, self-consistent end-to-end simulations of two types of bunch compressor designs, illustrating some of the basic scaling properties and perform a detailed analysis of the output phase-space distribution. Lastly, we show the unique ability of our simulation to model the evolution of charge/energy perturbations on a relativistic bunch in a toroidal waveguide.Item Analysis and Mitigation of Electromagnetic Noise in Resonant Cavities and Apertures(2004-08-10) Li, Lin; Ramahi, Omar M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The trend of low voltage in electronics circuits and boards makes them vulnerable to electromagnetic interference (EMI). Furthermore, higher speed (clock rate) leads to faster switching which increases the potential for higher radiation from circuits and boards. These inevitable trends collectively compromise the electromagnetic compatibility of electronic systems by increasing their electromagnetic susceptibility. In this work, radiation from enclosures and apertures is studies and characterized and radiation mitigation techniques are proposed. High-speed circuit radiation within an enclosure leads to cavity resonance that can have critical impact on other electronic components housed within the same enclosure. The amplified electric field in the enclosure can couple to critical circuits leading to either hard or soft failures. One measure to gauge the resonance of an enclosure is through the determination of S-parameters between certain ports connected to the enclosure. In this work, different numerical methods for efficient prediction of S-parameters are proposed and evaluated for their effectiveness and accuracy. Once an efficient procedure is established for calculating S-parameters, novel topological variations within the enclosure can be tested before manufacturing using accurate numerical prototyping. The proposed numerical S-parameters calculation algorithms are validated by comparison to laboratory measurements. Radiation from resonant apertures present in the walls of enclosures represents a second major source for radiation. In this work, a novel analysis of aperture radiation is presented based on the interpretation of the aperture as a transmission line. Once the transmission line analogy is established, a novel aperture resonance mitigation technique is proposed based on the use of material coating that mimics the behavior of matching loads that typically terminate transmission lines. The technique consists of adding resistive sheets in selected places in, or around the aperture. The effectiveness of the proposed method is demonstrated by first using numerical simulation of an aperture present in an infinite perfectly conducting sheet, and then by designing an experiment where the novel technique proposed here is tested on resonant apertures present in a metallic box. Both radiation measurements in an anechoic chamber and S-parameters measurements were conducted to test the validity of the proposed mitigation techniques.