Electrical & Computer Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2765
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Item 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.Item 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.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 A Wave-Chaotic Approach To Predicting And Measuring Electromagnetic Field Quantities In Complicated Enclosures(2006-10-24) Hemmady, Sameer Dileep; Anlage, Steven M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The coupling of short-wavelength electromagnetic waves into large complicated enclosures is of great interest in the field of electromagnetic compatibility engineering. The intent is to protect sensitive electronic devices housed within these enclosures from the detrimental effects of high-intensity external electromagnetic radiation penetrating into the enclosure (which acts as a resonant cavity) through various coupling channels (or ports). The Random Coupling Model introduced by Zheng, Antonsen and Ott is a stochastic model where the mechanism of the coupling process is quantified by the non-statistical "radiation impedance" of the coupling-port, and the field variations within the cavity are conjectured to be explained in a statistical sense through Random Matrix Theory- by assuming that the waves possess chaotic ray-dynamics within the cavity. The Random Coupling Model in conjunction with Random Matrix Theory thus makes explicit predictions for the statistical aspect (Probability Density Functions-PDFs) of the impedance, admittance and scattering fluctuations of waves within such wave-chaotic cavities. More importantly, these fluctuations are expected to be universal in that their statistical description depends only upon the value of a single dimensionless cavity loss-parameter. This universality in the impedance, admittance and scattering properties is not restricted to electromagnetic systems, but is equally applicable to analogous quantities in quantum-mechanical or acoustic systems, which also comprise of short-wavelength waves confined within complicated-shaped potential wells or acoustic-resonators. In this dissertation, I will experimentally show the validity of the "radiation impedance" to accurately quantify the port-coupling characteristics. I will experimentally prove the existence of these universal fluctuations in the impedance, admittance and scattering properties of quasi-two-dimensional and three-dimensional wave-chaotic systems driven by one-port or two-ports, and validate that their statistical nature is described through Random Matrix Theory. Finally, I will utilize the Random Coupling Model to formulate a prediction-algorithm to determine the shape and scales of induced voltages PDFs at specific points within complicated enclosures, such as computer boxes, when irradiated by high-intensity, short-wavelength electromagnetic energy. The insight gained from the experimental validation of the Random Coupling Model allows one to conceive of certain design-guidelines for cavity-enclosures that are more resistant to attack from an external short-wavelength electromagnetic source.Item Development of Magnetic Field Sensors Using Bismuth - Substituted Garnets Thin Films with In-Plane Magnetization(2006-04-24) NISTOR, IULIAN; Mayergoyz, Isaak D.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, the use of magnetic single crystal Bismuth-substituted Iron Garnet thin-films with giant magneto-optical effect as optical sensors for measuring low intensity magnetic fields over a high frequency range (up to 1GHz) is discussed. The advantages of these optical sensors are high intrinsic sensitivity and the possibility of tailoring the field range of the sensor. Such sensors could find applications in various industry and research fields where high sensitivity and electric isolation are required, such as power industry, vehicle detection, and read heads for recording magnetic media with high-density and high transfer rates. The thesis has three major components that correspond, in order, to the following topics: garnet growth, characterization and actual device design. First, the liquid phase epitaxy method is discussed for the growth of single crystal epitaxial garnet thin films of high optical quality. Second, the garnet thin films are fully characterized using various magnetic and optical techniques. Novel optical techniques are suggested, that allow the local measurement of properties such as magnetostriction constants and magnetic anisotropy of garnets. The results of these extensive measurements allow for the identification of melt compositions and growth conditions that yield thin garnet films with in-plane magnetization, giant Faraday rotation per unit length, large negative uniaxial anisotropies and small cubic anisotropy, as required for the sensing applications. In the end, the design of magnetic field sensors based on single and multi-layer garnet thin films is demonstrated, and devices are built for measurements of response and noise equivalent fields. Under the category of sensors, another sensing application is included, that utilizes garnet thin films for direct imaging of two-dimensional fringing magnetic fields with sub-micron resolution.Item Domain Wall Engineering of Nanoscale Ferromagnetic Elements and its Application for Memory Devices(2006-04-14) Florez, Sylvia Helena; Gomez, Romel D; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis concerns the interaction of spin polarized electrons with the local magnetic moments in nanopatterned metallic systems. We study novel magnetic phenomena appearing in patterned thin film magnetic wires with length scales in the nanometer regime and in magnetic multilayers. The work has three mayor foci. The first is the interaction between magnetic domain walls and conduction electrons in single layer nanowires. We demonstrate the effect of using small constrictions as artificial traps for domain walls and use these structures to measure the contribution of a domain wall to the electrical resistivity. These measurements are correlated with the specific micromagnetic distribution induced by the constriction geometry. Similarly, we demonstrate and characterize the effect of spin current induced magnetization reversal in nanowires. This includes a measurement of the critical current/field phase space boundary between static and moving walls and an estimation of the intrinsic wall mobility. The second is focused on understanding the effects of spin currents on magnetoresistance and domain wall motion, in a multilayer nanostructure device exhibiting giant magnetoresistance (GMR). To demonstrate a potential application, we incorporate the effects of domain wall trapping and spin current induced domain wall motion into a nanometer scale spin-valve device. The device can be fully controlled through current and exhibits significant GMR response. This approach may be useful as a memory element in magnetoresistive random access memory (MRAM) technology, and the device serves as a proof of concept. The third focus is the understanding of the effect strain on the resistance of antiferromagnetically (AF) coupled giant magnetoresistive (GMR) multilayers containing highly magnetostrictive materials. Our measurements reveal that inverse magnetostriction effects lead to enhanced strain sensitivity in comparison to films made of the materials that compose the multilayer. A simple phenomenological model describing the measured field dependence of these effects is used to identify field-biasing values that optimize amplitude, linearity and reversibility of the effect.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.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.Item Indium Phosphide Based Optical Micro-Ring Resonators(2003-10-24) Grover, Rohit; Ho, Ping-Tong; Goldhar, Julius; Ghodssi, Reza; Ritter, Kenneth J; Electrical EngineeringMicro-ring resonators are a strong candidate for the basic building blocks of very-large-scale-integrated optics. They can be used in many applications, such as filters, routers, switches, lasers, and amplifiers. They are simple in design and concept, can be made very small, and do not require exotic materials or fabrication techniques. In this thesis, I describe my work on indium phosphide based active and passive micro-ring resonators. To enable low-loss devices, I develop a dry-etching process for InP using the methane chemistry in a capacitively-coupled reactive-ion-etching machine. Using the etch process, I demonstrate single-mode micro-ring resonators in the vertically- and laterally-coupled geometries, all-optical logic, and a tunable micro-ring notch filter. The best devices in the vertically-coupled geometry have bandwidth as low as $0.24~\nm$, free spectral range of $24~\nm$, $Q = 6200$, and finesse of $100$ while the laterally-coupled micro-rings have bandwidth as low as $0.25~\nm$, free spectral range of $8~\nm$, $Q = 6250$, and finesse of $32$. Some of the laterally-coupled devices have free spectral ranges as high as $28~\nm$, though the corresponding $Q$ is low. Tarek Ibrahim and I demonstrate all-optical logic (AND operation) using carrier-induced refractive index change by two-photon-absorption with switching speed of $100~\ps$, dominated by ambipolar diffusion. Finally, I demonstrate tuning by $100~\GHz$ ($0.8~\nm$) with $8~\volt$ reverse bias of an InP-based micro-ring resonator with a p-i-n structure using the quadratic electro-optic effect, obtaining $1.5~\GHz/\volt^2$ of tuning.Item Analysis of fluctuations in semiconductor devices(2004-04-21) Andrei, Petru; Mayergoyz, Isaak D; Peckerar, Martin C; Ghodssi, Reza; Krishnaprasad, Perinkulam S; Electrical EngineeringThe random nature of ion implantation and diffusion processes as well as inevitable tolerances in fabrication result in random fluctuations of doping concentrations and oxide thickness in semiconductor devices. These fluctuations are especially pronounced in ultrasmall (nanoscale) semiconductor devices when the spatial scale of doping and oxide thickness variations become comparable with the geometric dimensions of devices. In the disseration, the effects of these fluctuations on device characteristics are analyzed by using a new technique for the analysis of random doping and oxide thickness induced fluctuations. This technique is universal in nature in the sense that it is applicable to any transport model (drift-diffusion, semiclassical transport, quantum transport etc.) and it can be naturally extended to take into account random fluctuations of the oxide (trapped) charges and channel length. The technique is based on linearization of the transport equations with respect to the fluctuating quantities. It is computationally much (a few orders of magnitude) more efficient than the traditional Monte-Carlo approach and it yields information on the sensitivity of fluctuations of parameters of interest (e.g. threshold voltage, small-signal parameters, cut-off frequencies, etc.) to the locations of doping and oxide thickness fluctuations. For this reason, it can be very instrumental in the design of fluctuation-resistant structures of semiconductor devices. Quantum mechanical effects are taken into account by using the density-gradient model as well as through self-consistent Poisson-Schrödinger computations. Special attention is paid to the presenting of the technique in a form that is suitable for implementation on commercial device simulators. The numerical implementation of the technique is discussed in detail and numerous computational results are presented and compared with those previously published in literature.