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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WAVES IN PLASMAS GENERATED BY A ROTATING MAGNETIC FIELD AND IMPLICATIONS TO RADIATION BELTS
(2010) Karavaev, Alexey V.; Papadopoulos,, Konstantinos; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The interaction of rotating magnetic fields (RMF) with magnetized plasmas is a fundamental plasma physics problem with implications to a wide range of areas, including laboratory and space plasma physics. Despite the importance of the topic the basic physics of the phenomenon remains unexplored. An important application of a RMF is its potential use as an efficient radiation source of low frequency waves in space plasmas, including whistler and shear Alfven waves (SAW) for controlled remediation of energetic particles in the Earth's radiation belts. In this dissertation the RMF waves generated in magnetized plasma are studied using numerical simulations with a semi-analytical three-dimensional magnetohydrodynamic (MHD) model and experiments on the generation of whistler and magnetohydrodynamic waves conducted in UCLA's Large Plasma Device. Comparisons of the simulation results with the experimental measurements, namely, measured spatiotemporal wave structures, dispersion relation with finite transverse wave number, wave amplitude dependence on plasma and RMF source parameters, show good agreement in both the whistler and MHD wave regimes. In both the experiments and the 3D MHD simulations a RMF source was found to be very efficient in the generation of MHD and whistler waves with arbitrary polarizations. The RMF source drives significant field aligned plasma currents confined by the ambient magnetic field for both the whistler and MHD wave regimes, resulting in efficient transport of wave energy along the ambient magnetic field. The efficient transfer of the wave energy results in slow decay rates of the wave amplitude along the ambient magnetic field. The circular polarization of the waves generated by the RMF source, slow amplitude decay rate along the ambient magnetic field and nonzero transverse wave number determined by the RMF source size lead to nonlocal gradients of the wave magnetic field in the direction perpendicular to the ambient magnetic field. A RMF can be generated by a system of polyphase alternating currents or by a rotating permanent or superconducting magnet. For the magnetospheric plasma rotating permanent or superconducting magnets are suitable for injection of very low frequency (VLF) shear Alfven and magnetosonic waves. The generation of whistler waves in the magnetosphere plasma requires frequencies of the order of kHz, so in order to inject whistler waves generated by a RMF it is necessary to use an antenna with polyphase alternating currents. The interactions of the waves generated by a RMF source with highly energetic electron population were investigated in LAPD experiment and by test-particle simulations of non-resonant pitch angle scattering of trapped energetic electrons using the electromagnetic fields calculated using the 3D model. It was found in both the experiment and test-particle simulations that waves generated by a RMF source are, indeed, very efficient in pitch angle scattering of trapped hot electrons due to the creation of magnetic field gradients in the direction perpendicular to the ambient magnetic field. Different scenarios for the applications to the precipitation of highly energetic electrons in the magnetosphere are presented.
Characterization of Quantum Efficiency and Robustness of Cesium-Based Photocathodes
(2010) Montgomery, Eric J.; O'Shea, Patrick G.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
High quantum efficiency, robust photocathodes produce picosecond-pulsed, high-current electron beams for photoinjection applications like free electron lasers. In photoinjectors, a pulsed drive laser incident on the photocathode causes photoemission of short, dense bunches of electrons, which are then accelerated into a relativistic, high quality beam. Future free electron lasers demand reliable photocathodes with long-lived quantum efficiency at suitable drive laser wavelengths to maintain high current density. But faced with contamination, heating, and ion back-bombardment, the highest efficiency photocathodes find their delicate cesium-based coatings inexorably lost. In answer, the work herein presents careful, focused studies on cesium-based photocathodes, particularly motivated by the cesium dispenser photocathode. This is a novel device comprised of an efficiently photoemissive, cesium-based coating deposited onto a porous sintered tungsten substrate, beneath which is a reservoir of elemental cesium. Under controlled heating cesium diffuses from the reservoir through the porous substrate and across the surface to replace cesium lost to harsh conditions -- recently shown to significantly extend the lifetime of cesium-coated metal cathodes. This work first reports experiments on coated metals to validate and refine an advanced theory of photoemission already finding application in beam simulation codes. Second, it describes a new theory of photoemission from much higher quantum efficiency cesium-based semiconductors and verifies its predictions with independent experiment. Third, it investigates causes of cesium loss from both coated metal and semiconductor photocathodes and reports remarkable rejuvenation of full quantum efficiency for contaminated cesium-coated surfaces, affirming the dispenser prescription of cesium resupply. And fourth, it details continued advances in cesium dispenser design with much-improved operating characteristics: lower temperature and cleaner operation. Motivated by dispenser integration with semiconductor coatings, initial fabrication of those coatings are reported on dispenser-type substrates with measurement of quantum efficiency and analysis of thermal stability. Detailed investigations are performed on dispenser substrate preparation by ion beam cleaning and on dispenser pore structure by electron microscopy and focused ion beam milling. The dissertation concludes by discussing implications of all results for the demonstration and optimization of the future high quantum efficiency cesium dispenser photocathode.
Beam halo creation and propagation in the University of Maryland Electron Ring
(2009) Papadopoulos, Christos F.; O'Shea, Patrick G; Kishek, Rami A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In this thesis we discuss the phenomenon of halo creation in charged particle beams. For this, we combine analytical, numerical and experimental work, which focuses on the University of Maryland Electron Ring, but is applicable to a wide range of accelerators in the same intensity regime. We find that the details of the beam distribution do not affect the structure of the halo, but are nonetheless important as they determine the number of particles in the halo and whether the latter can be regenerated. Furthermore, we show that the halo in configuration and velocity space comprises of the same particles, a prediction that has great importance for halo removal and diagnostics. In particular, we show that even in the case of ideal halo removal in phase space, the complicated internal dynamics of the beam core lead to halo regeneration. Following on previous work, we also construct a theoretical particle-core model for a skew quadrupole focusing channel, and compare the results to PIC simulations as well as measurements on UMER. The agreement between these three approaches is satisfactory, within the constraints of each case.
Scattering from chaotic cavities: Exploring the random coupling model in the time and frequency domains
(2009) Hart, James Aamodt; Ott, Edward; Antonsen, Thomas M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Scattering waves off resonant structures, with the waves coupling into and out of the structure at a finite number of locations (`ports'), is an extremely common problem both in theory and in real-world applications. In practice, solving for the scattering properties of a particular complex structure is extremely difficult and, in real-world applications, often impractical. In particular, if the wavelength of the incident wave is short compared to the structure size, and the dynamics of the ray trajectories within the scattering region are chaotic, the scattering properties of the cavity will be extremely sensitive to small perturbations. Thus, mathematical models have been developed which attempt to determine the statistical, rather than specific, properties of such systems. One such model is the Random Coupling Model. The Random Coupling Model was developed primarily in the frequency domain. In the first part of this dissertation, we explore the implications of the Random Coupling Model in the time domain, with emphasis on the time-domain behavior of the power radiated from a single-port lossless cavity after the cavity has been excited by a short initial external pulse. In particular, we find that for times much larger than the cavity's Heisenberg time (the inverse of the average spacing between cavity resonant frequencies), the power from a single cavity decays as a power law in time, following the decay rate of the ensemble average, but eventually transitions into an exponential decay as a single mode in the cavity dominates the decay. We find that this transition from power-law to exponential decay depends only on the shape of the incident pulse and a normalized time. In the second part of this dissertation, we extend the Random Coupling Model to include a broader range of situations. Previously, the Random Coupling Model applied only to ensembles of scattering data obtained over a sufficiently large spread in frequency or sufficiently different ensemble of configurations. We find that by using the Poisson Kernel, it is possible to obtain meaningful results applicable to situations which vary much less radically in configuration and frequency. We find that it is possible to obtain universal statistics by redefining the radiation impedance parameter of the previously developed Random Coupling Model to include the average effects of certain classical trajectories within the resonant structure. We test these results numerically and find good agreement between theory and simulation.
STUDY OF LONGITUDINAL SPACE CHARGE WAVES IN SPACE-CHARGE DOMINATED BEAMS
(2009) Thangaraj, Jayakar Charles Tobin; O'Shea, Patrick; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Future x-ray free electron lasers will probe matter at the atomic scale with femtosecond time resolution. Such x-ray sources require a high current electron beam with very low emittance and energy spread. Any density fluctuation in an intense beam can launch space charge waves that lead to energy modulation. The energy modulations may cause further density modulations in any dispersive element and can, for example, excite the microbunching instability in x-ray free electron lasers. Hence, it is important to understand and control the evolution of density modulations on an intense beam. This dissertation focuses on long path-length experimental study of intense beams with density perturbations. The experimental results are compared with theory and computer simulation. We took advantage of the multi-turn operation of the University of Maryland Electron Ring (UMER), to carry out long path-length (100 m) experimental studies of space-charge-dominated beams with density perturbations. First, a single density perturbation is introduced on a space-charge dominated electron beam using photoemission from a laser. The perturbation splits and propagates as a fast and a slow wave on the beam. The speed of the space charge waves is measured experimentally as a function of beam current and perturbation strength. The results are in good agreement with Particle-in-cell (PIC) simulation and 1-D cold fluid theory in the linear regime. We then show that linear space-charge waves can be used as non-interceptive transverse beam diagnostics in UMER. Using time-resolved imaging techniques, we report the transverse effects of a longitudinal perturbation in a circular machine. We introduce multiple perturbations on the beam and show that the fast and the slow waves superpose and cross each other. We then present experimental results on the beam response from introducing a controlled energy modulation on the density modulated beam and compare them with the theory. In the non-linear regime, where the strength of the perturbation is large (>25% compared to the beam current), we report, for the first time, a wave train formation of the space charge waves. Finally, experimental observation of a photo-emitted beam pulse splitting into sub-pulses under high laser power is presented and compared with 1-D virtual cathode theory. From this work, we conclude that density modulations on an intense beam produce fast and slow waves, which, in the linear regime at least, can be controlled through energy modulation. Moreover, a large amplitude density modulation, when allowed to propagate, can break into sub-pulses, causing energy modulation. Hence, a density modulation should not be allowed to grow and must be controlled as soon as possible.
Low Power Smartdust Receiver with Novel Applications and Improvements of an RF Power Harvesting Circuit
(2009) Salter, Thomas Steven; Goldsman, Neil; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Smartdust is the evolution of wireless sensor networks to cubic centimeter dimensions or less. Smartdust systems have advantages in cost, flexibility, and rapid deployment that make them ideal for many military, medical, and industrial applications. This work addresses the limitations of prior works of research to provide sufficient lifetime and performance for Smartdust sensor networks through the design, fabrication and testing of a novel low power receiver for use in a Smartdust transceiver. Through the novel optimization of a multi-stage LNA design and novel application of a power matched Villard voltage doubler circuit, a 1.0 V, 1.6 mW low power On-Off Key (OOK) receiver operating at 2.2 GHz is fabricated using 0.13 um CMOS technology. To facilitate data transfer in adverse RF propagation environments (1/r^3 loss), the chip receives a 1 Mbps data signal with a sensitivity of -90 dBm while consuming just 1.6 nJ/bit. The receiver operates without the addition of any external passives facilitating its application in Smartdust scale (mm^3) wireless sensor networks. This represents an order of magnitude decrease in power consumption over receiver designs of comparable sensitivity. In an effort to further extend the lifetime of the Smartdust transceiver, RF power harvesting is explored as a power source. The small scale of Smartdust sensor networks poses unique challenges in the design of RF power scavenging systems. To meet these challenges, novel design improvements to an RF power scavenging circuit integrated directly onto CMOS are presented. These improvements include a reduction in the threshold voltage of diode connected MOSFET and sources of circuit parasitics that are unique to integrated circuits. Utilizing these improvements, the voltage necessary to drive Smartdust circuitry (1 V) with a greater than 20% RF to DC conversion efficiency was generated from RF energy levels measured in the environment (66 uW). This represents better than double the RF to DC conversion efficiency of the conventional power matched RF energy harvesting circuit. The circuit is integrated directly onto a 130 nm CMOS process with no external passives and measures only 300 um by 600 um, meeting the strict form factor requirement of Smartdust systems.
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.
Design and Characterization of an Electrohydrodynamic (EHD) Micropump for Cryogenic Spot Cooling Applications
(2008-04-21) Foroughi, Parisa; Ohadi, Michael M.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
High-temperature superconducting (HTSC) components are being incorporated into communication and monitoring electronic devices to increase their signal-to-noise ratio or their channel capacity. Those devices must be maintained at cryogenic temperatures to prevent the loss of their superconducting properties and retain their performance superiority. They are conventionally cooled via direct heat conduction, which leads to undesirable temperature differences among the various components being cooled. Compact micropumps capable of pumping liquid nitrogen at 77 K into liquid-cooling circuits would enable a much more compact and lightweight method of maintaining a uniform temperature across the cooling circuit. These pumps can also address the demand for delivering small doses of LN2 to particular spots in bioengineering applications.
One of the main objectives of the present study was to develop an electrohydrodynamic (EHD) ion-drag micropump with LN2 as the working liquid. EHD ion-drag pumping phenomenon refers to liquid motion caused by an interaction between electric and hydrodynamic fields in a dielectric liquid.
To investigate the effect of each design parameter on the performance of the micropump, several prototypes with four distinct designs were fabricated and packaged. The designs included a variety of emitter shapes, inter-electrode spacings, electrode-pair spacings, and channel heights. The micropumps were tested at different DC voltages ranging from 0 to 2.5 kV. Two test rigs with novel measurement techniques were also designed, built, and calibrated to measure the generated static pressure head, electric current, and flow rate with an acceptable level of accuracy.
The relationships between pressure/current (P-I) and pressure/voltage (P-V) for various designs were investigated experimentally. The results showed good agreement with the general analytical trends reported for EHD pumping in the literature. The experimental results also demonstrated that electrode geometry and gaps are effective in determining the pressure onset voltage. The results also show that a maximum static pressure head of 160 Pa at 1400 V is achievable for a design with a combination of a 50-μm emitter-collector gap, a 200-μm electrode-pair gap, and a saw-tooth shaped emitter/flat collector.
Calculation of Realistic Charged-Particle Transfer Maps
(2007-10-28) Mitchell, Chad Eugene; Dragt, Alex; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The study and computation of nonlinear charged-particle transfer maps is fundamental to understanding single-particle beam dynamics in accelerator devices. Transfer maps for individual elements of the beamline can in general depend sensitively on nonlinear fringe-field and high-multipole effects. The inclusion of these effects requires a detailed and realistic model of the interior and fringe magnetic fields, including knowledge of high spatial derivatives. Current methods for computing such maps often rely on idealized models of beamline elements. This Dissertation describes the development and implementation of a collection of techniques for computing realistic (as opposed to idealized) charged-particle transfer maps for general beamline elements, together with corresponding estimates of numerical error. Each of these techniques makes use of 3-dimensional measured or numerical field data on a grid as provided, for example, by various 3-dimensional finite element field codes. The required high derivatives of the corresponding vector potential A, required to compute transfer maps, cannot be reliably computed directly from this data by numerical differentiation due to numerical noise whose effect becomes progressively worse with the order of derivative desired. The effect of this noise, and its amplification by numerical differentiation, can be overcome by fitting on a bounding surface far from the axis and then interpolating inward using the Maxwell equations. The key ingredients are the use of surface data and the smoothing property of the inverse Laplacian operator. We explore the advantages of map computation using realistic field data on surfaces of various geometry. Maps obtained using these techniques can then be used to compute realistically all derived linear and nonlinear properties of both single pass and circular machines. Although the methods of this Dissertation have been applied primarily to magnetic beamline elements, they can also be applied to electric and radio-frequency beamline elements.
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.

Theses and Dissertations from UMD

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