Search Results

open access

DESIGN AND ANALYSIS OF AN INTEGRATED PULSE MODULATED S-BAND POWER AMPLIFIER IN GALLIUM NITRIDE PROCESS

Description: The design of power amplifiers in any semi-conductor process is not a trivia exercise and it is often encountered that the simulated solution is qualitatively different than the results obtained. Phenomena such as oscillation occurring either in-band or out of band and sometimes at subharmonic intervals, continuous spectrum noticed in some frequency bands, often referred to as chaos, and jumps and hysteresis effects can all be encountered and render a design useless. All of these problems might have been identified through a more rigorous approach to stability analysis. Designing for stability is probably the one area of amplifier design that receives the least amount of attention but incurs the most catastrophic of effects if it is not performed properly. Other parameters such as gain, power output, frequency response and even matching may suitable mitigation paths. But the lack of stability in an amplifier has no mitigating path. In addition to of loss of the design completely there are the increased production cycle costs, costs involved with investigating and resolving the problem and the costs involved with schedule slips or delays resulting from it. The Linville or Rollett stability criteria that many microwave engineers follow and rely exclusively on is not sufficient by itself to ensure a stable and robust design. It will be shown that the universal belief that unconditional stability is obtained through an analysis of the scattering matrix S to determine if 1 and |{Delta}{sub S}| < 1 is only part of the procedure and other tools must be used to validate the criteria. The research shown contributes to the state of the art by developing a more thorough stability design technique for designing amplifiers of any class, whether that be current mode or switch mode, than is currently undertaken with the goal of obtaining first pass design …
Date: April 4, 2012
Creator: SEDLOCK, STEVE
Partner: UNT Libraries Government Documents Department
open access

Wide Bandgap Extrinsic Photoconductive Switches

Description: Photoconductive semiconductor switches (PCSS) have been investigated since the late 1970s. Some devices have been developed that withstand tens of kilovolts and others that switch hundreds of amperes. However, no single device has been developed that can reliably withstand both high voltage and switch high current. Yet, photoconductive switches still hold the promise of reliable high voltage and high current operation with subnanosecond risetimes. Particularly since good quality, bulk, single crystal, wide bandgap semiconductor materials have recently become available. In this chapter we will review the basic operation of PCSS devices, status of PCSS devices and properties of the wide bandgap semiconductors 4H-SiC, 6H-SiC and 2H-GaN.
Date: January 17, 2012
Creator: Sullivan, J S
Partner: UNT Libraries Government Documents Department
open access

A Proof-of-Principle Echo-enabled Harmonic Generation Free Electron Laser Experiment at SLAC

Description: With the advent of X-ray Free Electron Lasers (FELs), new methods have been developed to extend capabilities at short wavelengths beyond Self-Amplified Spontaneous Emission (SASE). In particular, seeding of a FEL allows for temporal control of the radiation pulse and increases the peak brightness by orders of magnitude. Most recently, Gennady Stupakov and colleagues at SLAC proposed a new technique: Echo-Enabled Harmonic Generation (EEHG). Here a laser microbunches the beam in an undulator and the beam is sheared in a chicane. This process is repeated with a second laser, undulator and chicane. The interplay between these allows a seeding of the X-ray laser up to the 100th harmonic of the first laser. After introducing the physics of FELs and the EEHG seeding technique, we describe contributions to the experimental effort. We will present detailed studies of the experiment including the choice of parameters and their optimization, the emittance effect, spontaneous emission in the undulators, the second laser phase effect, and measurements of the jitter between RF stations. Finally, the status and preliminary results of the Echo-7 experiment will be outlined.
Date: January 6, 2012
Creator: Pernet, Pierre-Louis
Partner: UNT Libraries Government Documents Department
open access

Study of CP Violation in Dalitz-Plot Analyses of B-Meson Decays to Three Kaons

Description: The Standard Model (SM) explains CP violation in terms of the CKM matrix. The BABAR experiment was designed mainly to test the CKM model in B decays. B decays that proceed through b {yields} s loop diagrams, of which B {yields} KKK decays are an example, are sensitive to new physics effects that could lead to deviations from the CKM predictions for CP violation. We present studies of CP violation in the decays B{sup +} {yields} K{sup +}K{sup -}K{sup +}, B{sup +} {yields} K{sub S}{sup 0}K{sub S}{sup 0}K{sup +}, and B{sup 0} {yields} K{sup +}K{sup -}K{sub S}{sup 0}, using a Dalitz plot amplitude analysis. These studies are based on approximately 470 million B{bar B} decays collected by BABAR at the PEP-II collider at SLAC. We perform measurements of time-dependent CP violation in B{sup 0} {yields} K{sup +}K{sup -}K{sub S}{sup 0}, including B{sup 0} {yields} {phi}K{sub S}{sup 0}. We measure a CP-violating phase {beta}{sub eff} ({phi}K{sub S}{sup 0}) = 0.36 {+-} 0.11 {+-} 0.04 rad., in agreement with the SM. This is the world's most precise measurement of this quantity. We also measure direct CP asymmetries in all three decay modes, including the direct CP asymmetry A{sub CP} ({phi}K{sup +}) = (12.8 {+-} 4.4 {+-} 1.3)%, which is 2.8 sigma away from zero. This measurement is in tension with the SM, which predicts an asymmetry of a few percent. We also study the resonant and nonresonant features in the B {yields} KKK Dalitz plots. We find that the hypothetical scalar f{sub X}(1500) resonance, introduced by prior analyses to explain an unknown peak in the m{sub KK} spectrum, cannot adequately describe the data. We conclude instead that the f{sub X}(1500) can be explained as the sum of the f{sub 0}(1500), f'{sub 2}(1525), and f{sub 0}(1710) resonances, removing the need for the hypothetical …
Date: February 15, 2012
Creator: Lindquist, Brian
Partner: UNT Libraries Government Documents Department
open access

SANE's Measurement of the Proton's Virtual Photon Spin Asymmetry, A^p_1, at Large Bjorken x

Description: The experiment SANE (Spin Asymmetries of the Nucleon Experiment) measured inclusive double polarization electron asymmetries on a proton target at the Continuous Electron Beam Accelerator Facility at the Thomas Jefferson National Laboratory in Newport News Virgina. Polarized electrons were scattered from a solid {sup 14}NH{sub 3} polarized target provided by the University of Virginia target group. Measurements were taken with the target polarization oriented at 80 degrees and 180 degrees relative to the beam direction, and beam energies of 4.7 and 5.9 GeV were used. Scattered electrons were detected by a multi-component novel non-magnetic detector package constructed for this experiment. Asymmetries measured at the two target orientations allow for the extraction of the virtual Compton asymmetries A{sub 1}{sup p} and A{sub 2}{sup p} as well as the spin structure functions g{sub 1}{sup p} and g{sub 2}{sup p}. This work addresses the extraction of the virtual Compton asymmetry A{sub 1}{sup p} in the deep inelastic regime. The analysis uses data in the kinematic range from Bjorken x of 0.30 to 0.55, separated into four Q{sup 2} bins from 1.9 to 4.7 GeV{sup 2}.
Date: May 1, 2012
Creator: Mulholland, Jonathan
Partner: UNT Libraries Government Documents Department
open access

Study of charmonium resonances in the gg -> K0SK pi- and gg -> K K-pi pi-pi0 processes

Description: This thesis reports the analysis of the e{sup +}e{sup -} {yields} e{sup +}e{sup -}K{sub S}{sup 0}K{sup {+-}}{pi}{sup {-+}} and e{sup +}e{sup -} {yields} e{sup +}e{sup -}K{sup +}K{sup -}{pi}{sup +}{pi}{sup -}{pi}{sup 0} processes using the final dataset of the BABAR experiment located at the SLAC National Accelerator Laboratory. From previous measurements, the K{sub S}{sup 0}K{sup {+-}}{pi}{sup {-+}} final state is known to show a clear signal from the {eta}{sub c}(2S) particle. This c{bar c} state escaped detection for almost twenty years and its properties are still not well established on the experimental ground, while accurate predictions exist on the theoretical side. The e{sup +}e{sup -} {yields} e{sup +}e{sup -}K{sup +}K{sup -}{pi}{sup +}{pi}{sup -}{pi}{sup 0} process is first studied in this thesis. An accurate determination of the {eta}{sub c}(2S) properties is obtained in the K{sub S}{sup 0}K{sup {+-}}{pi}{sup {-+}} decay mode. We also report the first observation of {eta}{sub c}(2S) and other charmonium states to the K{sup +}K{sup -}{pi}{sup +}{pi}{sup -}{pi}{sup 0} final state. The results of this thesis have been published in Physical Review D, and will be useful to test theoretical models describing the charmonium system. The thesis is organized in four chapters. The first one gives a brief introduction of the theoretical models used to describe the charmonium system. The second one discuss the current status of conventional and exotic charmonium spectroscopy, reporting recent experimental results and their interpretation. The third Chapter is devoted to describe the BABAR experiment. The analysis technique and results are described in Chapter 4. Finally, conclusions from this analysis are drawn.
Date: February 22, 2012
Creator: Biassoni, Pietro & /U. Milan, Dept. Phys.
Partner: UNT Libraries Government Documents Department
open access

Towards a Precision Measurement of Parity-Violating e-p Elastic Scattering at Low Momentum Transfer

Description: The goal of the Q-weak experiment is to make a measurement of the proton's weak charge Q{sub W}{sup p} = 1 - 4 sin{sup 2}({theta}{sub W}) to an accuracy of {approx} 4%. This would represent a {approx} 0.3% determination of the weak mixing angle sin{sup 2}({theta}{sub W}) at low energy. The measurement may be used for a precision test of the Standard Model (SM) prediction on the running of sin{sup 2}({theta}{sub W}) with energy scale. The Q-weak experiment operates at Thomas Jefferson National Accelerator Facility (Jefferson Lab). The experiment determines Q{sub W}{sup p} by measuring the parity violating asymmetry in elastic electron-proton scattering at low momentum transfer Q{sup 2} = 0.026 (GeV/c){sup 2} and forward angles (?8 degrees). The anticipated size of the asymmetry, based on the SM, is about 230 parts per billion (ppb). With the proposed accuracy, the experiment may probe new physics beyond Standard Model at the TeV scale. This thesis focuses on my contributions to the experiment, including track reconstruction for momentum transfer determination of the scattering process, and the focal plane scanner, a detector I designed and built to measure the flux profile of scattered electrons on the focal plane of the Q-weak spectrometer to assist in the extrapolation of low beam current tracking results to high beam current. Preliminary results from the commissioning and the first run period of the Q-weak experiment are reported and discussed.
Date: May 31, 2012
Creator: Pan, Jie
Partner: UNT Libraries Government Documents Department
open access

First observation of the Charmless Decay B to K pi0pi0 and Study of its Dalitz Plot Structure

Description: Results for the first measurement of the inclusive branching and CP asymmetry of the charmless 3-body decay B{sup +} {yields} K{sup +}{pi}{sup 0}{pi}{sup 0} are presented. The analysis uses a data sample with an integrated luminosity of 429.0 fb{sup -1}, recorded by the BABAR detector at the PEP-II asymmetric B Factory. This sample corresponds to 470.9 {+-} 2.8 million B{bar B} pairs. Measurements of the branching fractions (B) and CP asymmetries (A{sub CP}) of some of the intermediate resonances in the K{sup +}{pi}{sup 0}{pi}{sup 0} Dalitz plot are also presented.
Date: March 21, 2012
Creator: Puccio, Eugenia Maria Teresa Irene & /SLAC, /Warwick U.
Partner: UNT Libraries Government Documents Department
open access

A Comparison of "Total Dust" and Inhalable Personal Sampling for Beryllium Exposure.

Description: In 2009, the American Conference of Governmental Industrial Hygienists (ACGIH) reduced the Beryllium (Be) 8-hr Time Weighted Average Threshold Limit Value (TLV-TWA) from 2.0 {micro}g/m{sup 3} to 0.05 {micro}g/m{sup 3} with an inhalable 'I' designation in accordance with ACGIH's particle size-selective criterion for inhalable mass. Currently, per the Department of Energy (DOE) requirements, the Lawrence Livermore National Laboratory (LLNL) is following the Occupational Health and Safety Administration (OSHA) Permissible Exposure Limit (PEL) of 2.0 {micro}g/m{sup 3} as an 8-hr TWA, which is also the 2005 ACGIH TLV-TWA, and an Action Level (AL) of 0.2 {micro}g/m{sup 3} and sampling is performed using the 37mm (total dust) sampling method. Since DOE is considering adopting the newer 2009 TLV guidelines, the goal of this study was to determine if the current method of sampling using the 37mm (total dust) sampler would produce results that are comparable to what would be measured using the IOM (inhalable) sampler specific to the application of high energy explosive work at LLNL's remote experimental test facility at Site 300. Side-by-side personal sampling using the two samplers was performed over an approximately two-week period during chamber re-entry and cleanup procedures following detonation of an explosive assembly containing Beryllium (Be). The average ratio of personal sampling results for the IOM (inhalable) vs. 37-mm (total dust) sampler was 1.1:1 with a P-value of 0.62, indicating that there was no statistically significant difference in the performance of the two samplers. Therefore, for the type of activity monitored during this study, the 37-mm sampling cassette would be considered a suitable alternative to the IOM sampler for collecting inhalable particulate matter, which is important given the many practical and economic advantages that it presents. However, similar comparison studies would be necessary for this conclusion to be applied to other types of activities, where earlier …
Date: April 25, 2012
Creator: Carter, C. M.
Partner: UNT Libraries Government Documents Department
open access

Measurement of the D_s Decay Constant f_Ds and Observation of New Charm Resonances Decaying to D^(*)\pi

Description: The absolute branching fractions for the decays D{sub s}{sup -} {yields} {ell}{sup -}{bar {nu}}{sub {ell}} ({ell} = e, {mu}, or {tau}) are measured using a data sample corresponding to an integrated luminosity of 521 fb{sup -1} collected at center of mass energies near 10.58 GeV with the BABAR detector at the PEPII e{sup +}e{sup -} collider at SLAC. The number of D{sub s}{sup -} mesons is determined by reconstructing the recoiling system DKX{gamma} in events of the type e{sup +}e{sup -} {yields} DKXD*{sub s}{sup -}, where D*{sub s}{sup -} {yields} D{sub s}{sup -} {gamma} and X represents additional pions from fragmentation. The D{sub s}{sup -} {yields} {ell}{sup -}{nu}{sub {ell}} events are detected by full or partial reconstruction of the recoiling system DKX{gamma}{ell}. The following results are obtained: {Beta}(D{sub s}{sup -} {yields} {mu}{sup -}{nu}) = (6.02 {+-} 0.38 {+-} 0.34) x 10{sup -3}, {Beta}(D{sub s}{sup -} {yields} {tau}{sup -}{nu}) = (5.00 {+-} 0.35 {+-} 0.49) x 10{sup -2}, and B(D{sub s}{sup -} {yields} e{sup -}{nu}) < 2.8 x 10{sup -4} at 90% C.L., where the first uncertainty is statistical and the second is systematic. The branching fraction measurements are combined to determine the D{sub s}{sup -} decay constant f{sub D{sub s}} = (258.6 {+-} 6.4 {+-} 7.5) MeV. In addition, a study has been performed of the D{sup +}{pi}{sup -}, D{sup 0}{pi}{sup +}, and D*{sup +}{pi}{sup -} systems in inclusive e{sup +}e{sup -} {yields} c{bar c} interactions in a search for excited D meson states. The dataset used consists of {approx}454 fb{sup -1}. The mass spectra for these systems show, for the first time, candidates for the radial excitations of the D{sup 0}, D*{sup 0}, and D*{sup +}, as well as the L = 2 excited states of the D{sup 0} and D{sup +}, where L is the orbital angular momentum …
Date: March 15, 2012
Creator: Benitez, Jose
Partner: UNT Libraries Government Documents Department
open access

Much Ado about Microbunching: Coherent Bunching in High Brightness Electron Beams

Description: The push to provide ever brighter coherent radiation sources has led to the creation of correspondingly bright electron beams. With billions of electrons packed into normalized emittances (phase space) below one micron, collective effects may dominate both the preservation and use of such ultra-bright beams. An important class of collective effects is due to density modulations within the bunch, or microbunching. Microbunching may be deleterious, as in the case of the Microbunching Instability (MBI), or it may drive radiation sources of unprecedented intensity, as in the case of Free Electron Lasers (FELs). In this work we begin by describing models of microbunching due to inherent beam shot noise, which sparks both the MBI as well as SLAC's Linac Coherent Light Source, the world's first hard X-ray laser. We first use this model to propose a mechanism for reducing the inherent beam shot noise as well as for predicting MBI effects. We then describe experimental measurements of the resulting microbunching at LCLS, including optical radiation from the MBI, as well as the first gain length and harmonic measurements from a hard X-ray FEL. In the final chapters, we describe schemes that use external laser modulations to microbunch light sources of the future. In these sections we describe coherent light source schemes for both both linacs and storage rings.
Date: May 25, 2012
Creator: Ratner, Daniel & /SLAC, /Stanford U.
Partner: UNT Libraries Government Documents Department
open access

Deciphering the details of RNA aminoglycoside interactions: from atomistic models to biotechnological applications

Description: A detailed study was done of the neomycin-B RNA aptamer for determining its selectivity and binding ability to both neomycin– and kanamycin-class aminoglycosides. A novel method to increase drug concentrations in cells for more efficiently killing is described. To test the method, a bacterial model system was adopted and several small RNA molecules interacting with aminoglycosides were cloned downstream of T7 RNA polymerase promoter in an expression vector. Then, the growth analysis of E. coli expressing aptamers was observed for 12-hour period. Our analysis indicated that aptamers helped to increase the intracellular concentration of aminoglycosides thereby increasing their efficacy.
Date: July 23, 2012
Creator: Ilgu, Muslum
Partner: UNT Libraries Government Documents Department
open access

Cellulose and cellobiose: adventures of a wandering organic chemist in theoretical chemistry

Description: The energies arising from the rotation of free hydroxyl groups in the central glucose residue of a cellulose crystalline assembly, calculated using RHF, DFT, and FMO2/MP2 methods, will be presented. In addition, interactions of this central glucose residue with some of the surrounding residues (selected on the basis of the interaction strengths) are analyzed. The mechanism of acid-catalyzed hydrolysis of cellobiose, which is the repeating unit of cellulose. Energies corresponding to the different steps of this mechanism calculated using RHF and DFT are compared with those previously reported using molecular dynamics calculations and with experimental data.
Date: April 3, 2012
Creator: Baluyut, John
Partner: UNT Libraries Government Documents Department
open access

Feet on the potential energy surface, head in the pie clouds

Description: This work presents explorations of the potential energy surface of clusters of atoms and of the interactions between molecules. First, structures of small aluminum clusters are examined and classified as ground states, transition states, or higher-order saddle points. Subsequently, the focus shifts to dispersion-dominated π-π interactions when the potential energy surfaces of benzene, substituted benzene, and pyridine dimers are explored. Because DNA nucleotide bases can be thought of as substituted heterocycles, a natural extension of the substituted benzene and pyridine investigations is to model paired nucleotide bases. Finally, the success of the dispersion studies inspires the development of an extension to the computational method used, which will enable the dispersion energy to be modeled – and the potential energy surface explored – in additional chemical systems. The effective fragment potential (EFP) method is described, as well as various quantum mechanical methods. An ab inito quantum mechanical study of 13-atom aluminum clusters is described. EFP studies of aromatic dimers are reported in which dispersion energy makes a significant contribution to the attraction between monomers. Theory and code development toward a means of computing dispersion energy in mixed ab inito-EFP systems are described.
Date: July 12, 2012
Creator: Smith, Quentin
Partner: UNT Libraries Government Documents Department
open access

Virtual tool mark generation for efficient striation analysis in forensic science

Description: In 2009, a National Academy of Sciences report called for investigation into the scienti#12;c basis behind tool mark comparisons (National Academy of Sciences, 2009). Answering this call, Chumbley et al. (2010) attempted to prove or disprove the hypothesis that tool marks are unique to a single tool. They developed a statistical algorithm that could, in most cases, discern matching and non-matching tool marks made at di#11;erent angles by sequentially numbered screwdriver tips. Moreover, in the cases where the algorithm misinterpreted a pair of marks, an experienced forensics examiner could discern the correct outcome. While this research served to con#12;rm the basic assumptions behind tool mark analysis, it also suggested that statistical analysis software could help to reduce the examiner's workload. This led to a new tool mark analysis approach, introduced in this thesis, that relies on 3D scans of screwdriver tip and marked plate surfaces at the micrometer scale from an optical microscope. These scans are carefully cleaned to remove noise from the data acquisition process and assigned a coordinate system that mathematically de#12;nes angles and twists in a natural way. The marking process is then simulated by using a 3D graphics software package to impart rotations to the tip and take the projection of the tip's geometry in the direction of tool travel. The edge of this projection, retrieved from the 3D graphics software, becomes a virtual tool mark. Using this method, virtual marks are made at increments of 5#14; and compared to a scan of the evidence mark. The previously developed statistical package from Chumbley et al. (2010) performs the comparison, comparing the similarity of the geometry of both marks to the similarity that would occur due to random chance. The resulting statistical measure of the likelihood of the match informs the examiner of the angle of the …
Date: November 16, 2012
Creator: Ekstrand, Laura
Partner: UNT Libraries Government Documents Department
open access

Surfaces of Intermetallics: Quasicrystals and Beyond

Description: The goal of this work is to characterize surfaces of intermetallics, including quasicrystals. In this work, surface characterization is primarily focused on composition and structure using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) performed under ultrahigh vacuum (UHV) conditions.
Date: October 26, 2012
Creator: Yuen, Chad
Partner: UNT Libraries Government Documents Department
open access

An investigation of exploitation versus exploration in GBEA optimization of PORS 15 and 16 Problems

Description: It was hypothesized that the variations in time to solution are driven by the competing mechanisms of exploration and exploitation.This thesis explores this hypothesis by examining two contrasting problems that embody the hypothesized tradeoff between exploration and exploitation. Plus one recall store (PORS) is an optimization problem based on the idea of a simple calculator with four buttons: plus, one, store, and recall. Integer addition and store are classified as operations, and one and memory recall are classified as terminals. The goal is to arrange a fixed number of keystrokes in a way that maximizes the numerical result. PORS 15 (15 keystrokes) represents the subset of difficult PORS problems and PORS 16 (16 keystrokes) represents the subset of PORS problems that are easiest to optimize. The goal of this work is to examine the tradeoff between exploitation and exploration in graph based evolutionary algorithm (GBEA) optimization. To do this, computational experiments are used to examine how solutions evolve in PORS 15 and 16 problems when solved using GBEAs. The experiment is comprised of three components; the graphs and the population, the evolutionary algorithm rule set, and the example problems. The complete, hypercube, and cycle graphs were used for this experiment. A fixed population size was used.
Date: May 8, 2012
Creator: Koch, Kaelynn
Partner: UNT Libraries Government Documents Department
open access

Polarized {sup 3}He(e,e'n) Asymmetries in Three Orthogonal Measurements

Description: Asymmetry measurements were conducted in Jefferson Lab's experimental Hall A through electron scattering from a polarized {sup 3}He target in the quasi-elastic polarized-{sup 3}He(e,e'n) reaction. Measurements were made with the target polarized in the longitudinal direction with respect to the incoming electrons A_L, in a transverse direction that was orthogonal to the beam-line and parallel to the q-vector A_T, and in a vertical direction that was orthogonal to both the beam-line and the q-vector (A_y^0). The experiment measured $A_y^0$ at four-momentum transfer squared Q^2 of 0.127 (GeV/c)^2, 0.456 (GeV/c)^2, and 0.953 (GeV/c)^2. The A_T and A_L asymmetries were both measured at Q^2 of 0.505 (GeV/c)^2 and 0.953 (GeV/c)^2. This is the first time that three orthogonal asymmetries have been measured simultaneously. Results from this experiment are compared with the plane wave impulse approximation (PWIA) and Faddeev calculations. These results provide important tests of models that use 3He as an effective neutron target and show that the PWIA holds above Q^2 of 0.953 (GeV/c)^2.
Date: September 1, 2012
Creator: Long, Elena
Partner: UNT Libraries Government Documents Department
open access

Increasing the efficiency of organic solar cells by photonic and electrostatic-field enhancements

Description: Organic photovoltaic (OPV) technology is an attractive solar-electric conversion paradigm due to the promise of low cost roll-to-roll production and amenability to flexible substrates. Power conversion efficiency (PCE) exceeding 7% has recently been achieved. OPV cells suffer from low charge carrier mobilities of polymers, leading to recombination losses, higher series resistances and lower fill-factors. Thus, it is imperative to develop fabrication methodologies that can enable efficient optical absorption in films thinner than optical absorption length. Active layers conformally deposited on light-trapping, microscale textured, grating-type surfaces is one possible approach to achieve this objective. In this study, 40% theoretical increase in photonic absorption over flat OPVs is shown for devices with textured geometry by the simulation results. For verifying this theoretical result and improving the efficiency of OPVs by light trapping, OPVs were fabricated on grating-type textured substrates possessing t pitch and -coat PV active-layer on these textured substrates led to over filling of the valleys and shunts at the crest, which severely affected the performance of the resultant PV devices. Thus, it is established that although the optical design is important for OPV performance but the potential of light trapping can only be effectively tapped if the textures are amenable for realizing a conformal active layer. It is discovered that if the height of the underlying topographical features is reduced to sub-micron regime (e.g. 300 nm) and the pitch is increased to more than a micron (e.g. 2 μm), the textured surface becomes amenable to coating a conformal PV active-layer. The resultant PV cells showed 100% increase in average light absorption near the band edge due to trapping of higher wavelength photons, and 20% improvement in power conversion efficiency as compared with the flat PV cell. Another factor that severely limits the performance of OPVs is recombination of charge carriers. …
Date: November 3, 2012
Creator: Nalwa, Kanwar
Partner: UNT Libraries Government Documents Department
open access

Determination of the Azimuthal Asymmetry of Deuteron Photodisintegration in the Energy Region E{sub {gamma}} = 1.1 - 2.3 GeV

Description: Deuteron photodisintegration is a benchmark process for the investigation of the role of quarks and gluons in nuclei. Existing theoretical models of this process describe the available cross sections with the same degree of success. Therefore, spin-dependent observables are crucial for a better understanding of the underlying dynamical mechanisms. However, data on the induced polarization (P{sub y}), along with the polarization transfers (C{sub x'} and C{sub z'} ), have been shown to be insensitive to differences between theoretical models. On the other hand, the beam-spin asymmetry {Sigma} is predicted to have a large sensitivity and is expected to help in identifying the energy at which the transition from the hadronic to the quark-gluon picture of the deuteron takes place. Here, the work done to determine the experimental values of the beam-spin asymmetry in deuteron photodisintegration for photon energies between 1.1 – 2.3 GeV is presented. The data were taken with the CLAS at the Thomas Jefferson National Accelerator Facility during the g13 experiment. Photons with linear polarization of ~80% were produced using the coherent bremsstrahlung facility in Hall B. The work done by the author to calibrate a specific detector system, select deuteron photodisintegration events, study the degree of photon polarization, and finally determine the azimuthal asymmetry and any systematic uncertainties associate with it, is comprehensively explained. This work shows that the collected data provide the kinematic coverage and statistics to test the available QCD-based models. The results of this study show that the available theoretical models in their current state do not adequately predict the azimuthal asymmetry in the energy region 1.1 – 2.3 GeV.
Date: May 20, 2012
Creator: Zachariou, Nicholas
Partner: UNT Libraries Government Documents Department
open access

Measurement of double polarized asymmetries in quasi-elastic processes ${}^3\vec{He}(\vec{e},e' d)$ and ${}^3\vec{He}(\vec{e},e' p)$

Description: This thesis is dedicated to a study of a spin-isospin structure of the polarized {sup 3}He. First, an introduction to the spin structure of {sup 3}He is given, followed by a brief overview of past experiments. The main focus of the thesis is the E05-102 experiment at Jefferson Lab, in which the reactions {sup 3}{ovr He} ({rvec e}, e'd) and {sup 3}{ovr He} ({rvec e}, e'p) in the quasi-elastic region were studied. The purpose of this experiment was to better understand the effects of the S'- and D-state contributions to the {sup 3}He ground-state wave-functions by a precise measurement of beam-target asymmetries A{sub x} and A{sub z} in the range of recoil momenta from 0 to about 300 MeV/c. The experimental equipment utilized in these measurements is described, with special attention devoted to the calibration of the hadron spectrometer, BigBite. Results on the measured asymmetries are presented, together with first attempts at their comparison to the state-of-the art Faddeev calculations. The remaining open problems and challenges for future work are also discussed.
Date: August 1, 2012
Creator: Mihovilovic, Miha
Partner: UNT Libraries Government Documents Department
open access

Energy Spread Reduction of Electron Beams Produced via Laser Wake

Description: Laser wakefield acceleration of electrons holds great promise for producing ultra-compact stages of GeV scale, high quality electron beams for applications such as x-ray free electron lasers and high energy colliders. Ultra-high intensity laser pulses can be self-guided by relativistic plasma waves over tens of vacuum diffraction lengths, to give >1 GeV energy in cm-scale low density plasma using ionization-induced injection to inject charge into the wake at low densities. This thesis describes a series of experiments which investigates the physics of LWFA in the self-guided blowout regime. Beginning with high density gas jet experiments the scaling of the LWFA-produced electron beam energy with plasma electron density is found to be in excellent agreement with both phenomenological theory and with 3-D PIC simulations. It is also determined that self-trapping of background electrons into the wake exhibits a threshold as a function of the electron density, and at the densities required to produce electron beams with energies exceeding 1 GeV a different mechanism is required to trap charge into low density wakes. By introducing small concentrations of high-Z gas to the nominal He background the ionization-induced injection mechanism is enabled. Electron trapping is observed at densities as low as 1.3 x 10{sup 18} cm{sup -3} in a gas cell target, and 1.45 GeV electrons are demonstrated for the first time from LWFA. This is currently the highest electron energy ever produced from LWFA. The ionization-induced trapping mechanism is also shown to generate quasi-continuous electron beam energies, which is undesirable for accelerator applications. By limiting the region over which ionization-induced trapping occurs, the energy spread of the electron beams can be controlled. The development of a novel two-stage gas cell target provides the capability to tailor the gas composition in the longitudinal direction, and confine the trapping process to occur only in …
Date: March 19, 2012
Creator: Pollock, B.
Partner: UNT Libraries Government Documents Department
open access

Unorthodox theoretical methods

Description: The use of the ReaxFF force field to correlate with NMR mobilities of amine catalytic substituents on a mesoporous silica nanosphere surface is considered. The interfacing of the ReaxFF force field within the Surface Integrated Molecular Orbital/Molecular Mechanics (SIMOMM) method, in order to replicate earlier SIMOMM published data and to compare with the ReaxFF data, is discussed. The development of a new correlation consistent Composite Approach (ccCA) is presented, which incorporates the completely renormalized coupled cluster method with singles, doubles and non-iterative triples corrections towards the determination of heats of formations and reaction pathways which contain biradical species.
Date: June 20, 2012
Creator: Nedd, Sean
Partner: UNT Libraries Government Documents Department
Back to Top of Screen