Open Access. Powered by Scholars. Published by Universities.®

Physical Sciences and Mathematics Commons™

Open Access. Powered by Scholars. Published by Universities.®

1999

Theses and Dissertations

Physics

Articles 1 - 17 of 17

Full-Text Articles in Physical Sciences and Mathematics

A Numerical Simulation Of A Carbon Black Suspension Cell Via A Time-Reversed, Double Layer Compute Algorithm, Gregg T. Anderson Dec 1999

A Numerical Simulation Of A Carbon Black Suspension Cell Via A Time-Reversed, Double Layer Compute Algorithm, Gregg T. Anderson

Theses and Dissertations

A numerical simulation of a carbon black suspension cell is explored which models a laser-induced plasma within a liquid ethanol medium of approximately 1 mm thickness. The simulation model assumes a laser pulse with a pulse width of approximately 9 nsecs propagating in the left-to-right direction striking the front surface of the medium and focusing to a spot within the liquid volume. When the energy density within a given irradiated volume is sufficiently high, it ignites the carbon particles and generates a large number of free electrons, i.e., a plasma. The plasma couples with the in-coming laser energy on a …

Go to article

Application Of The Interaction Picture To Reactive Scattering In One Dimension, Michael J. Maclachlan Dec 1999

Application Of The Interaction Picture To Reactive Scattering In One Dimension, Michael J. Maclachlan

Theses and Dissertations

The interaction picture is used together with the channel-packet method in a new time-dependent approach to compute reactive scattering matrix elements. The channelpacket method enables the use of the interaction picture for computing reactive S-matrix elements by splitting the computational effort into two parts. First, asymptotic reactant and product wavepackets are individually propagated into the interaction region of the potential to form Moller states. The interaction picture, in contrast to the usual Schrödinger picture of quantum mechanics, is so constructed that a wavefunction that experiences no change in potential (that is, a free-particle wavefunction) remains always fixed, with no translation …

Go to article

Implementation Of A Two-Dimensional Hydrodynamic Shock Code Based Upon The Weighted Average Flux Method, Mark P. Wittig Jun 1999

Implementation Of A Two-Dimensional Hydrodynamic Shock Code Based Upon The Weighted Average Flux Method, Mark P. Wittig

Theses and Dissertations

Numerical modeling of shock propagation and reflection is of interest to the Department of Defense (DoD). Propriety state-of-the-art codes based upon E. F. Toro's weighted average flux (WAF) method are being used to investigate complex shock reflection phenomena. Here we develop, test, and validate a one-dimensional hydrodynamic shock code. We apply WAF to Gudonov's first-order upwind method to achieve second-order accuracy. Oscillations, typical of second-order methods, are then removed using adaptive weight limiter functions based upon total variation diminishing (TVD) flux limiters. An adaptive Riemann solver routine is also implemented to improve computational efficiency. This one-dimensional code is then extended …

Go to article

Fan-Beam Multiplexed Compton Scatter Tomography For Single-Sided Noninvasive Inspection, Brian L. Evans Apr 1999

Fan-Beam Multiplexed Compton Scatter Tomography For Single-Sided Noninvasive Inspection, Brian L. Evans

Theses and Dissertations

Multiplexed Compton Scatter Tomography (MCST) is explored as a method of nondestructively generating cross-sectional images of a sample's electron density. MCST is viable when access is available to only one side of the sample because it registers scattered gamma radiation. Multiplexing in scattered photon energy and in detector position allows simultaneous interrogation of many voxels with comparatively wide collimation. Primary components include a radioisotope source, fan beam collimators, and energy-discriminating detectors. The application of MCST to inspecting aluminum airframes for corrosion is considered. This application requires source gammas near 100 keV where the scattered gamma energy is severely broadened by …

Go to article

Inelastic Scattering Matrix Elements For The Collision B (2P1/2) + H2(J) → B (2P3/2) + H2(J'), Thomas A. Niday Mar 1999

Inelastic Scattering Matrix Elements For The Collision B (2P1/2) + H2(J) → B (2P3/2) + H2(J'), Thomas A. Niday

Theses and Dissertations

Initial efforts to characterize the scattering dynamics of B + H2 focus on computing scattering matrix elements for the fine structure transition B (2P1/2) --> B (2P3/2) in collisions with H2, allowing for rotational excitation. Using a new application of the time dependent Channel Packet Method (CPM), reactant and product wave packets are prepared in the asymptotic limit on the B (2P1/2) and B (2P3/2) surfaces. They are propagated using the split operator method together wit a unitary transformation between the …

Go to article

A Continuous Wave Atomic Bromine Laser Produced By Photolysis Of Iodine Monobromide, Brian A. Smith Mar 1999

A Continuous Wave Atomic Bromine Laser Produced By Photolysis Of Iodine Monobromide, Brian A. Smith

Theses and Dissertations

An apparatus to generate continuous wave (CW) lasing at 2.714 µm by Br(2P1/2) atoms produced from photodissociation of IBr was designed, constructed and tested. A frequency doubled Nd:YAG laser firing 8 ns, 850 mJ pulses at 532 nm was used to photodissociate static IBr vapor inside a flow nozzle. The CaF2 windows on either side of the nozzle proved to have a damage threshold below the threshold for lasing. Pulsed lasing in static mode was not observed. Therefore, CW lasing in flow mode, using an Ar+ laser at 488 nm, could not be investigated.

Go to article

Optimization Of A Multilayer Photothermal Sensor For Infrared Spectroscopy, Janine O.J. Wiggins Mar 1999

Optimization Of A Multilayer Photothermal Sensor For Infrared Spectroscopy, Janine O.J. Wiggins

Theses and Dissertations

Tri-layer thermal diffusion modeling was applied to the optimization of a multi-layer reed sensor for use in a photothermal infrared detector. The multi-layer reed sensor deflects in response to increased temperature. Deflection, of angstroms or larger, is measured using an atomic force microscope. A newly developed thermal diffusion model for three layer reeds was combined with an existing two-layer cantilever model, in order to explore the effects of length, operating frequency, and layer thickness on signal to noise ratio. Model behavior is presented, and compared to laboratory results.

Go to article

Laser Beam Combining And Cleanup Via Stimulated Brillouin Scattering In Multi-Mode Optical Fibers, Blake C. Rodgers Mar 1999

Laser Beam Combining And Cleanup Via Stimulated Brillouin Scattering In Multi-Mode Optical Fibers, Blake C. Rodgers

Theses and Dissertations

The main objective of this thesis was to demonstrate the feasibility of combining and cleaning up multiple laser beams via Stimulated Brillouin Scattering (SBS) in a multi-mode optical fiber. Beam combining via SBS in an optical fiber is of interest because of the low SBS threshold power in an optical fiber allowing low power diode laser beams to be combined into one high power beam. SBS theory and some of the important historical results are described briefly. The experimental results clearly indicate that SBS in multi-mode optical fiber can indeed combine laser beams and clean up the beams at the …

Go to article

Numerical Solutions To The Two Dimensional Boltzmann Equation, Christopher G. Smithtro Mar 1999

Numerical Solutions To The Two Dimensional Boltzmann Equation, Christopher G. Smithtro

Theses and Dissertations

A strict kinetic, two-dimensional model of the electron kinetics within a glow discharge positive column is developed. The problem is solved in cylindrical geometry using the standard two-term Legendre expansion of the electron velocity distribution function. The model establishes a steady state solution, such that the net ionization rate is exactly balanced by the wall loss. In addition to a thorough analytic development, we present the numerical techniques used to solve the resulting elliptic partial differential equation, including an efficient method to treat sparse banded matrices. The model is validated against published results, local and nonlocal kinetic approximations, and a …

Go to article

Kinetic Model Of A Space-Based, Br (4 2P1/2 → 4 2P3/2) Laser Pumped By Solar Photolysis Of Ibr, Barry N. Behnken Mar 1999

Kinetic Model Of A Space-Based, Br (4 2P1/2 → 4 2P3/2) Laser Pumped By Solar Photolysis Of Ibr, Barry N. Behnken

Theses and Dissertations

A kinetic model of the directly solar-pumped, atomic bromine laser-operating on the Br (4 2P1/2 → 4 2P3/2) transition under IBr photolysis-was developed, executed, and interpreted. In recognition of an evolving national interest in space-based laser development, the model presumed operation on a space station platform. Results indicate that a well-engineered IBr laser is capable of generating 1.2 kilowatts of continuous-wave (CW) power under a pumping concentration of 20,000 solar units. Such performance translates to an efficiency of roughly 0.29%, appreciably better than the 0.1% ascribed to the heretofore leading solar-pumped competitor. An extensive analysis …

Go to article

Double Layer Effects On Shock Wave Propagation, Shannon L. Walker Mar 1999

Double Layer Effects On Shock Wave Propagation, Shannon L. Walker

Theses and Dissertations

An analysis and assessment of two mechanisms in plasma shock interactions was conducted under conditions typically encountered in a weakly ionized glow discharge. The mechanisms of a spatially-dependent electron temperature and additional electron impact ionization at the shock front were examined for effects on shock structure and propagation. These mechanisms were incorporated into an existing one-dimensional, time-dependent, fluid dynamics code that uses the Riemami problem as a basis and numerically solves the Euler equations for two fluids: the neutral gas and the charged component. The spatial variation in electron temperature was modeled as a shock-centered rise in temperature. Additional ionization …

Go to article

Active Multispectral Band Selection And Reflectance Measurement System, Bradley D. Rennich Mar 1999

Active Multispectral Band Selection And Reflectance Measurement System, Bradley D. Rennich

Theses and Dissertations

Due to system design requirements, an active multispectral laser radar system may be limited in the number of spectral bands that can be integrated into the system. To aid in the selection of these bands, a novel multispectral band selection technique is presented based on the cross-correlation of the material class reflectance spectra over a wavelength range of 1 - 5 microns. The algorithm uses directional hemispherical reflectance data from the Nonconventional Exploitation Factors database to select a number of spectral bands for classification purposes. Because the target material spectral reflectance is so important to the performance of an active …

Go to article

Use Of Diffuse Reflectance Spectroscopy To Determine Desorption Coefficients Of Trichioroethylene From Powdered Soils, Jay H. Foil Mar 1999

Use Of Diffuse Reflectance Spectroscopy To Determine Desorption Coefficients Of Trichioroethylene From Powdered Soils, Jay H. Foil

Theses and Dissertations

This laboratory thesis investigated the applicability of reflectance spectroscopy as a tool to measure desorption rates by directly observing the change in trichloroethylene bound to surface sites. Diffuse reflectance spectroscopy holds the promise of being a faster technique with minimal sample preparation time. Reflectance spectroscopy's dependence on the surface of the sample is its greatest advantage, as well as its greatest disadvantage. Both high and low resolution scans were made of different soil samples contaminated by trichloroethylene. After the technique of spectral subtraction was used, potential TCE signals were observed from the dolomitic limestone sample. Unfortunately, due to the low …

Go to article

Characterization Of The Double Scatter Spectrum In Multiplexed Compton Scatter Tomography, David W. Gerts Mar 1999

Characterization Of The Double Scatter Spectrum In Multiplexed Compton Scatter Tomography, David W. Gerts

Theses and Dissertations

The Multiplexed Compton Scatter Tomograph (MCST) uses single back-scattered photons to image electron density in aluminum. A source of error in this imaging technique is the presence of multiple scatters. This thesis studies the double scatter spectrum as an approximation of the multiple scatter spectrum. A deterministic code called Monte Carlo Double Scatter (MOCADS) was developed to investigate the double scatter spectrum. The code includes calculations of the Rayleigh scatter, Compton scatter, Doppler broadening effects of the spectrum, and polarization effects following the Compton scatter. The Doppler broadening portion of the code was validated by a deterministic code called Scatgram. …

Go to article

Electrical Characterization Of Ion-Implanted 4h-Silicon Carbide, Christian P. Morath Mar 1999

Electrical Characterization Of Ion-Implanted 4h-Silicon Carbide, Christian P. Morath

Theses and Dissertations

Electrical characterization has been performed on ion-implanted p-type 4H-SiC to assess the activation efficiency and implantation-related damage recrystallization with the intention of developing an implantation/annealing scheme. Low doped (Na - Nd = 5x10(exp 15)/cu cm) epitaxial p-type layers grown by MOCVD were implanted with Al or B at doses ranging from 1x10(exp 13) to 1x10(exp 14)/sq cm at room temperature or 500 deg. C. The electrical technique of Temperature Dependent Hall Effect (TDHE) indicated that Al and B act as shallow acceptors 4H-SiC with ionization energies of ^252 and ^285 meV, respectively. The highest activation efficiency for Al and B …

Go to article

Band Structure Anisotropy In Semiconductor Quantum Wells, Steven J. Novotny Mar 1999

Band Structure Anisotropy In Semiconductor Quantum Wells, Steven J. Novotny

Theses and Dissertations

The focus of this research is an investigation of energy band anisotropy in simple quantum well structures. This anisotropy results from the asymmetry of the periodic potential within the crystal lattice. For sufficiently high doping levels, band structure anisotropy is expected to play an important role in the evaluation of the electronic and optical properties of the quantum well structures. The analysis uses a model based on a 6x6 Luttinger-Kohn k.p approach for bulk material valence band structure together with the Envelope Function Approximation. The model is used to analyze Si/SiGe, AlGaAs/GaAs, and GaAs/InGaAs quantum wells for the 001 and …

Go to article

Space Charge Structure Of A Glow Discharge In The Presence Of A Longitudinal Inhomogeneity, Frank A. Tersigni Mar 1999

Space Charge Structure Of A Glow Discharge In The Presence Of A Longitudinal Inhomogeneity, Frank A. Tersigni

Theses and Dissertations

A survey of space charge structures arising due to inhomogeneities in glow discharges was conducted. Space charge structures associated with tube geometries, the cathode sheath, striations, and shockwaves were examined. Space charge effects on the Electron Energy Distribution Function (EEDF) were explored for a geometric inhomogeneity using an approximate nonlocal solution to the one dimensional Boltzmann equation after Godyak. The approximate solution partially captured qualitative aspects of space charge effects on the EEDF. Simplification of collisional effects and adaptation of an approximate electric field restricted quantitative comparisons with experimental data. It is recommended that any future analysis of space charge …

Go to article