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Zhu, Yonry RApplications and Modeling of Non-Thermal Plasmas
Bachelor of Science (BS), Ohio University, 2018, Engineering Physics
This thesis focuses on validation of a 0D plasma kinetic model and its subsequent use as an explanatory tool to support the results of hot-fire tests of a plasma assisted rotating detonation combustor. The plasma model predictions showed good agreement with experimentally measured values of various ground state species number densities, vibrationally excited N2 number densities, plasma temperatures, and ignition delay times. Once validated, the plasma model was combined with a ZND detonation model and semi-empirical correlation to determine the effects of a non-thermal plasma on the reduction of the detonation cell size for an H2 - air mixture. The modeling results showed that non-thermal plasma significantly reduces the detonation cell size. This effect is most pronounced at lean conditions, where the model predicted a reduction in cell size by a factor of more than 100. For stoichiometric and rich conditions, the cell size reduction was around a factor of 5. An investigation was conducted to determine the viability of using a non-thermal plasma to expand the operating regime of a rotating detonation combustor. The plasma was produced with a nanosecond pulse generator connected to a ceramic and metal centerbody electrode. Hot-fire testing results showed that the plasma causes detonation onset in conditions that would otherwise not support detonation. This effect was most prominent at near-stoichiometric conditions, with a reduced effect for richer or leaner mixtures.

Committee:

David Burnette (Advisor)

Subjects:

Aerospace Engineering; Mechanical Engineering; Plasma Physics

Keywords:

non-thermal plasma; plasma assisted combustion; nanosecond pulsed plasma; plasma modeling; detonation combustion; rotating detonation combustor; detonation;

Nawarange, Amruta V.Optical Emission Spectroscopy during Sputter Deposition of CdTe Solar Cells and CuTe-Based Back Contacts
Doctor of Philosophy, University of Toledo, 2011, Physics

In this dissertation sputtering processes are studied in detail through optical emission spectroscopy. In order to extract plasma parameters, experimental data and simulations were matched together. We could extract excitation temperatures, vibrational temperatures and rotational temperatures of the plasmas. To explain the simulations and to understand the different mechanisms involved in the sputtering plasmas, relevant aspects of atomic spectroscopy and molecular spectroscopy are reviewed here. A mixture of argon and nitrogen gas was used to sputter a CuxTe target by RF magnetron sputtering. The emission data were then studied as a function of deposition pressure and RF power. These data show many non equilibrium aspects of the plasma; however, in most cases the data are consistent with energy distributions of the rotational, vibrational, and electronic systems that can be characterized individually by distinct temperatures.

We have also used sputter deposition of CuxTe thin-film layers instead of our standard Cu/Au metal layers for back contacts to look for an improved back contact. We prepared three different compositions of CuxTe target material and studied the properties of sputtered films using X-Ray Diffraction (XRD), Energy Dispersive X-Ray Spectroscopy (EDS), Scanning Electron Microscopy (SEM) and Hall measurements. At optimized deposition conditions for Cu2Te target sputtered films (2 nm thickness and 20 minutes annealing in vacuum) as determined from the thin-film properties, we sputtered this layer onto the back surface of the CdTe of the cell structure. We achieved efficiencies of 13.1% using Cu2Te target sputtered films followed by Au which is very close to our best efficiency achieved with Cu/Au contacts.

Committee:

Alvin Compaan, PhD (Advisor); Alvin Compaan, PhD (Committee Chair); Brian Bagley, PhD (Committee Member); Randall Ellingson, PhD (Committee Member); Sanjay Khare, PhD (Committee Member); Dean Giolando, PhD (Committee Member)

Subjects:

Alternative Energy; Condensed Matter Physics; Experiments; Materials Science; Molecular Physics; Physics; Plasma Physics; Solid State Physics; Theoretical Physics

Keywords:

OES

Bhardwaj, ShubhenduHybrid Numerical Models for Fast Design of Terahertz Plasmonic Devices
Doctor of Philosophy, The Ohio State University, 2017, Electrical and Computer Engineering
Electron-plasmonic devices are of strong interest for terahertz applications. In this work, we develop rigorous computational tools using finite difference time domain (FDTD) methods for accurate modeling of these devices. Existing full-wave-hydrodynamic models already combine Maxwell's and hydrodynamic electron-transport equation for multiphysical hybrid modeling. However, these multilevel methods are time-consuming as dense mesh is required for plasmonic modeling. Therefore, they are not suited for design and optimization. To address this issue, we propose new iterative ADI-FDTD-hydrodynamic hybrid coupled model. The new implementations provide time-efficient, yet accurate, modeling of these devices. It is demonstrated that for a typical simulation, up to 50% reduction in simulation-time is achieved with a nominal 3% error in calculations. Using the new tool-set, we investigate several devices that operate using the properties of 2D electron gas (2DEG). We provide one of the first multiphysical numerical analyses of these devices, giving accurate estimates of their terahertz performance. The developed tool allows simulation of arbitrary 2DEG based terahertz devices, providing useful and intuitive 2D field information. This has allowed understanding of the operation and radiation principles of these devices. Specifically, we examine the known plasma-wave instability in short-channel high electron mobility transistors (HEMTs) that leads to terahertz emissions at cryogenic temperatures. We also examine terahertz emitters that exploit resonant tunneling induced negative differential resistance (NDR) in HEMTs. Finally, using this tool we numerically demonstrate the existence of acoustic and optical-plasmonic modes within 2DEG bilayer systems in HEMTs. Methods for exciting and controlling these modes are also discussed enabling new physics among bilayer devices.

Committee:

John Volakis (Advisor); Siddharth Rajan (Advisor); Kubilay Sertel (Committee Member); Teixeira Fernando (Committee Member); Niru Nahar (Committee Member); Karin Musier-Forsyth (Committee Member)

Subjects:

Electrical Engineering; Plasma Physics

Keywords:

Plasma-wave, electron plasmonics, 2D electron gas, mutliphysical, computational , FDTD, hydrodynamic, terahertz radiation, terahertz detectors, terahertz sources, graphene modeling, hybrid model, full-wave modeling, HEMT, FET

Roettgen, Andrew MVibrational Energy Distribution, Electron Density and Electron Temperature Behavior in Nanosecond Pulse Discharge Plasmas by Raman and Thomson Scattering
Doctor of Philosophy, The Ohio State University, 2015, Mechanical Engineering
Kinetic processes controlling N2 vibrational distribution, electron temperature and electron density in nanosecond pulse, nonequilibrium plasma, electric discharges are studied through laser scattering diagnostic techniques. The experiments are conducted in high pulse energy (≥4 mJ/pulse), nanosecond pulse gas discharge plasmas at moderate pressures (75-200 torr) in nitrogen, air, helium, H2-He and O2-He mixtures. In electric discharges, local energy loading is a function of the electron number density (ne) and electron temperature (Te). Furthermore, electron temperature, and more specifically, electron energy distribution function (EEDF) control the electron energy partition in nonequilibrium plasmas by controlling the rates of critical kinetic processes including ionization, vibrational and electronic excitation, and recombination of molecules, atoms and electrons in the gas discharge. Thus, obtaining time-resolved, quantitative measurements for these values (ne, Te, and EEDF) is critical in understanding the energy requirements for sustaining these discharges, as well as discerning how electron energy is partitioned among different molecular energy modes, and which excited species and radicals are generated in the plasma. Furthermore, in molecular plasmas, significant electron energy is loaded into vibrational modes. Study of temporally resolved vibrational distribution function (VDF) and vibrational temperature (Tv) is important in quantifying vibrational energy loading and relaxation in these plasmas. This affects the rate of temperature rise in nanosecond pulse discharges and the afterglow, as well as rates of vibrationally stimulated chemical reactions, such as NO formation. Applications of these studies include plasma flow control (PFC), plasma assisted combustion (PAC), electrically excited laser development and various plasma bio-medical applications. Time-resolved N2 vibrational distribution function (VDF) and first-level N2 vibrational temperature have been measured via spontaneous Raman scattering in a nanosecond pulse, nonequilibrium, single-filament gas discharge sustained between two spherical copper electrodes. Gases studied include nitrogen and air (P=100 torr). Highly nonequilibrium N2 VDFs have been observed (vibrational levels up to v=12 significantly populated and detected). Results in nitrogen have been compared with a 0-D, master equation kinetic model. A Thomson scattering diagnostic, including a solid state Nd:YAG laser as the pump source, a custom-made glass test cell, and custom-built triple-grating spectrometer has been developed. Thomson scattering has very low signal intensity, and is therefore highly susceptible to several types of interference. Rayleigh scattering interference has been filtered from the spectra by using a spectral mask in the spectrometer, while a second slit was used to provide critical stray light rejection. Background interference due to plasma emission has been subtracted. Time-resolved electron number density, electron temperature and electron energy distribution function (EEDF) have been measured via the Thomson scattering diagnostic. Studies of two highly nonequilibrium plasma environments have been conducted, including a nanosecond pulse, single-filament discharge sustained between two spherical copper electrodes, as well as a nanosecond pulse near surface discharge (which develops initially as a surface ionization wave). Studies in helium, as well as mixtures of H2 in helium and O2 in helium have been conducted in the single-filament discharge, while a study in helium has been conducted in the near surface discharge. Results in helium for the single-filament discharge have been compared with a 2-D, axisymmetric, kinetic model. Electron density measurements in these experiments ranged from 1013 - 1015 cm-3, while electron temperatures were observed to range from 0.1 - 7.0 eV.

Committee:

Igor Adamovich, Professor (Advisor); Jeffrey Sutton, Professor (Committee Member); Vishwanath Subramaniam, Professor (Committee Member); J. William Rich, Professor Emeritus (Committee Member)

Subjects:

Aerospace Engineering; Chemistry; Engineering; Mechanical Engineering; Molecular Chemistry; Molecular Physics; Optics; Physical Chemistry; Plasma Physics

Keywords:

Plasma; scattering; Thomson scattering; Raman scattering; electron density; electron temperature; EEDF; electron energy distribution function; vibrational distribution function; VDF; electric discharge; filament discharge; discharge; kinetics

Chalasani, DheerajFEASIBILITY OF A PLASMA CONTACT FOR FARADAY GENERATORS
Master of Science, The Ohio State University, 2013, Mechanical Engineering
A Faraday disc generator is a direct current power source that works on the principle of electromagnetic induction. Michael Faraday discovered this generator effect in 1831. Since then a lot of improvements were made upon the original design but the Faraday disc generator has not enjoyed wide spread success. One of the chief reasons for this is the use of contact brushes. The wear of the contact brushes significantly affects the current carrying capacity of the disc. The current distribution in a Faraday disc as a result of energy extraction through a brush contact results in Eddy currents in the Faraday disc which significantly reduces the energy efficiency of the disc. As an alternative, the use of ionized gas or plasma as an electrical contact is considered. The use of plasma as an electrical contact in Faraday disc generators is a new and a novel idea as such an idea has not been suggested by anyone until now. The current work examines the feasibility of using plasma as an electrical contact instead of the conventional brush contacts for energy extraction from a Faraday disc generator from the perspective of plasma attachment and current-voltage characteristics. An experimental apparatus has been designed to simulate the conditions of a Faraday disc generator envisioned as receiving power from a wind or a hydraulic turbine. The setup consists of co-axial electrodes with an air gap of 1mm with the inner electrode being the anode that can be rotated about its axis as required. Experiments were performed at 9.1 torr, 9.55 torr and 10.6 torr to determine the current-voltage characteristics for the cases of rotating anode and stationary anode using a DC power supply. It was observed that the current-voltage characteristics in both the cases of rotating anode and non-rotating anode are qualitatively similar. The various operating regimes of DC plasma were identified qualitatively from the plotted current –voltage characteristics and by visual inspection. As far as plasma attachment is concerned, it seemed to be diffuse and looked to follow the characteristics of a glow regime with by gradually covering the annular gap as the current was increased and stayed diffuse after covering the entire air gap. From the experimental results and observations it was determined that using plasma as an electrical contact is feasible but there are problems that have to be addressed. Based upon the observations and results of the experimental work, two possible self-sustaining configurations for energy extraction were proposed. Finally, a plasma de-coupler design was provided as a solution to the problem of high current densities in the energy extraction area of the plasma.

Committee:

Vish Subramaniam (Advisor); Anthony Luscher (Advisor)

Subjects:

Electromagnetism; Energy; Engineering; Plasma Physics

Keywords:

Faraday disc; plasma;homopolar generator;plasma brush

Ghosh, SouvikATMOSPHERIC-PRESSURE in situ PLASMA REDUCTION AND PATTERNING OF METAL-ION CONTAINING POLYMERS
Doctor of Philosophy, Case Western Reserve University, 2017, Chemical Engineering
In this dissertation, we are describing a plasma based approach to fabricate electrical conductors on the surface of thin polymer films. We incorporated a direct-write approach derived from additive manufacturing techniques that minimizes wastage; and a post-patterning thin film removal and transfer protocol derived from subtractive manufacturing techniques. Using such a hybrid protocol, we made electrically conducting patterns embedded at the surface of polymeric thin films or deposited on affordable non-rigid substrates such as paper without incorporating the complexities of making a stabilized nanoparticle ink or high temperature annealing. In this first section of this dissertation, we describe the fabrication of thin films of polyacrylic acid after mixing silver nitrate in solution and blade casting them as thin films. The DC and AC argon microplasma based direct-write patterning was performed after mounting these films on a programmable x-y stage. Microplasma exposure lead to the formation of electrically conductive patterns of reduced and percolated silver nanoparticles. Further, by incorporating an elastomer as the support structure for the thin films enabled us to fabricate stretchable electrical conductors. We discovered an electrodiffusion phenomena whereby the plasma can drive the silver ions from the bulk of the film to the surface leading to percolation of reduced silver nanostructures. In the later part of the dissertation, we describe our efforts to understand the plasma reduction process by exposing these thin films to a controlled atmospheric-pressure and low-pressure plasma. It was found that photons from the plasma alone cannot reduce the nanoparticles. Instead, it was found that nanoparticle agglomeration and percolation depend on both, the properties of the thin film such as concentration and thickness, and the plasma operating parameters such as pressure, exposure time, and period and duty cycle of the driving pulse. We conclude that the harmonious effect of all these parameters contribute to controlling the particle size, number density and distribution of the nanoparticles at the surface of the polymer.

Committee:

R. Mohan Sankaran, Ph.D. (Advisor); Daniel Lacks, Ph.D. (Committee Member); Rohan Akolkar, Ph.D. (Committee Member); Christian Zorman, Ph.D. (Committee Member); Philip X-L. Feng, Ph.D. (Committee Member)

Subjects:

Chemical Engineering; Materials Science; Nanoscience; Plasma Physics

Keywords:

Microplasma, Electrodiffusion, Silver, Percolation, Electrical Conduction, Stretchable Conductor

Zechar, Nathan E.Experimental Investigation of a Parametric Excitation of Whistler Waves
Master of Science (MS), Wright State University, 2017, Physics
Previous theoretical work has shown that a parametric interaction between electrostatic lower oblique resonance (LOR) and ion acoustic waves (IAW) can produce electromagnetic whistler waves in a cold magnetized plasma. It was also demonstrated theoretically that this interaction can more efficiently generate electromagnetic whistler waves than by direct excitation using a conventional loop antenna. For the purpose of experimentally validating the above result, an experimental facility was designed and constructed utilizing a vacuum chamber, electromagnets, and a helicon plasma source array capable of producing a volume of plasma with high density and spatial uniformity. Additionally, positioning equipment and plasma diagnostics such as an RF compensated Langmuir probe and electrostatically shielded Bdot probes were fabricated to capture plasma parameter and time varying magnetic field data. The ability to experimentally excite the LOR, whistler wave, and IAW was demonstrated by utilizing the fabricated diagnostics. This data was then arranged to display spatial wave topologies which agreed with each wave's respective dispersion relation. Finally the parametric antenna was implemented to produce whistler waves through the interaction of LOW and IAW waves. The spatial and temporal information of the frequency components of these waves were analyzed by applying a band pass filter via Fourier analysis.

Committee:

Amit Sharma, Ph.D. (Advisor); Jerry Clark, Ph.D. (Committee Member); Vladimir Sotnikov, Ph.D. (Committee Member)

Subjects:

Plasma Physics

Keywords:

Plasma Physics; Whistler Wave; Ion Acoustic Wave; Langmuir Probe; Bdot Probe

Burnette, David DeanNitric Oxide Studies in Low Temperature Plasmas Generated with a Nanosecond Pulse Sphere Gap Electrical Discharge
Doctor of Philosophy, The Ohio State University, 2014, Mechanical Engineering
This dissertation presents studies of NO kinetics in a plasma afterglow using various nanosecond pulse discharges across a sphere gap. The discharge platform is developed to produce a diffuse plasma volume large enough to allow for laser diagnostics in a plasma that is rich in vibrationally-excited molecules. This plasma is characterized by current and voltage traces as well as ICCD and NO PLIF images that are used to monitor the plasma dimensions and uniformity. Temperature and vibrational loading measurements are performed via coherent anti-Stokes Raman spectroscopy (CARS). Absolute NO concentrations are obtained by laser-induce fluorescence (LIF) measurements, and N and O densities are found using two photon absorption laser-induced fluorescence (TALIF). For all dry air conditions studied, the NO behavior is characterized by a rapid rate of formation consistent with an enhanced Zeldovich process involving electronically-excited nitrogen species that are generated within the plasma. After several microseconds, the NO evolution is entirely controlled by the reverse Zeldovich process. These results show that under the chosen range of conditions and even in extreme instances of vibrational loading, there is no formation channel beyond ~2 µsec. Both the NO formation and consumption mechanisms are strongly affected by the addition of fuel species, producing much greater NO concentrations in the afterglow.

Committee:

Walter Lempert (Advisor)

Subjects:

Aerospace Engineering; Mechanical Engineering; Plasma Physics

Ovchinnikov, Vladimir MikhailovichDetermining the Properties of Laser Induced Fast Electrons from Experiments and Simulations
Doctor of Philosophy, The Ohio State University, 2011, Physics
We live in the era of high energy demand. Sooner or later we will exhaust all of our natural resources that are currently being used to make energy most of which are non-renewable. Therefore the quest to find another source of energy is an important one and should be completed within the next few decades. Thermonuclear fusion could become the solution to our energy problem. Currently, there are two main approaches to fusion: Magnetic Confinement Fusion (MCF) and Inertial Confinement Fusion (ICF). This thesis explores the variation of the ICF concept known as the Fast Ignition (FI). The FI concept relies on fast electrons with energies above 1 MeV created from the laser-matter interaction to deposit their energy into the compressed target core and start the fusion burn. Divergence of those electrons is one of the most crucial parameters in FI. If the divergence is found to be too large, then virtually all the anticipated advantages of FI over the conventional hot spot ignition will be lost. Spatially resolved, time-integrated Kα x-ray imagers are frequently used to infer the spatial distribution, and hence the divergence, of fast electrons by looking at the Kα radiation created by those energetic electrons passing through the target. Since any electron with energy above some threshold can produce a Kα photon, the Kα emission distribution can be quite different from that of the fast electrons. This thesis describes the physics behind the formation of Kα images and settles the question of how well those images represent the spatial distributions of the hot electrons that created them. A computational study using the Particle-In-Cell code LSP is presented that shows that a Kα image is not solely determined by the initial population of forward directed hot electrons, but rather also depends upon electrons refluxing off the front, the back and the sides of the target. All these effects create significant features in the Kα time-integrated images making them hard to interpret. To make the Kα image be more representative of the first pass hot electron spatial distribution one needs to eliminate multiple electron refluxing. Attaching a thick layer of a low-Z material to the back of the target (Get-Lost-Layer or GLL) can greatly reduce electron refluxing by slowing them down without significantly attenuating the Kα signal. Two experiments using buried cone targets with a GLL designed to measure the true fast electron divergence were conducted at Titan laser at Lawrence Livermore National Laboratory and are described in this thesis. The experimental results are supported by LSP simulations that prove that the presence of a GLL indeed suppresses electron refluxing. Finally, benchmarking between the experiment and the simulations allows extraction of the correct electron divergence.

Committee:

Linn Van Woerkom (Advisor); Richard Freeman (Committee Co-Chair); Richard Furnstahl (Committee Member); Fengyuan Yang (Committee Member)

Subjects:

Physics; Plasma Physics

Keywords:

Fast Ignition; fast electron divergence; K-alpha imaging; plasma simulations

Karki Gautam, LaxmiSpectroscopic Ellipsometry Studies of Thin Film Si:H Materials in Photovoltaic Applications from Infrared to Ultraviolet
Doctor of Philosophy, University of Toledo, 2016, Physics
Optimization of thin film photovoltaics (PV) relies on the capability for characterizing the optoelectronic and structural properties of each layer in the device over large areas and correlating these properties with device performance. This work builds heavily upon that done previously by us, our collaborators, and other researchers. It provides the next step in data analyses, particularly that involving study of films in device configurations maintaining the utmost sensitivity within those same device structures. In this Dissertation, the component layers of thin film hydrogenated silicon (Si:H) solar cells on rigid substrate materials have been studied by real time spectroscopic ellipsometry (RTSE) and ex situ spectroscopic ellipsometry (SE). Growth evolution diagrams has been used to guide deposition of materials with good optoelectronic properties in the actual hydrogenated amorphous silicon (a-Si:H) PV device configuration. The nucleation and evolution of crystallites forming from the amorphous phase were studied using near infrared to ultraviolet spectroscopic ellipsometry in situ, during growth for films prepared as a function of hydrogen to reactive gas flow ratio R = [H2] /{[SiH4] + [Si2H6]. Furthermore, the major challenge in Si:H manufacturing is that quantitative analysis, characterization, and control of the relative nanocrystalline and amorphous volume fractions within mixed-phase films were covered during these studies. In conjunction with higher photon energy measurements, the presence and relative absorption strength of silicon-hydrogen infrared modes were measured by infrared extended ellipsometry measurements to gain some insight into chemical bonding. Structural and optical models have been developed for the back reflector (BR) structure consisting of sputtered undoped zinc oxide (ZnO) on top of silver (Ag) coated glass substrates. Characterization of the free-carrier absorption properties in Ag and the interface formed when Ag is over-coated with ZnO were also studied by infrared extended spectroscopic ellipsometry. Measurements ranging from 0.04 to 5 eV were used to extract layer thicknesses, composition, and optical response in the form of complex dielectric function spectra (e = e1 + ie2) for undoped a-Si:H layers in a substrate n-i-p a-Si:H based PV device structure and on TCO coated glass for p-i-n configurations.

Committee:

Nikolas J. Podraza (Committee Chair); Robert W. Collins (Committee Member); Randall Ellingson (Committee Member); Song Cheng (Committee Member); Rashmi Jha (Committee Member)

Subjects:

Physics; Plasma Physics; Solid State Physics

Schmidt, Jacob BrianUltrashort Two-Photon-Absorption Laser-Induced Fluorescence in Nanosecond-Duration, Repetitively Pulsed Discharges
Doctor of Philosophy, The Ohio State University, 2015, Aero/Astro Engineering
Absolute number densities of atomic species produced by nanosecond duration, repetitively pulsed electric discharges are measured by two-photon absorption laser-induced fluorescence (TALIF). Relatively high plasma discharge pulse energies (=1 mJ/pulse) are used to generate atomic hydrogen, oxygen, and nitrogen in a variety of discharge conditions and geometries. Unique to this work is the development of femtosecond-laser-based TALIF (fs-TALIF). Fs-TALIF offers a number of advantages compared to more conventional ns-pulse-duration laser systems, including better accuracy of direct quenching measurements in challenging environments, significantly reduced photolytic interference including photo-dissociation and photo-ionization, higher signal and increased laser-pulse bandwidth, the ability to collect two-dimensional images of atomic species number densities with far greater spatial resolution compared with more conventional diagnostics, and much higher laser repetition rates allowing for more efficient and accurate measurements of atomic species number densities. In order to fully characterize the fs-TALIF diagnostic and compare it with conventional ns-TALIF, low pressure (100 Torr) ns-duration pulsed discharges are operated in mixtures of H2, O2, and N2 with different buffer gases including argon, helium, and nitrogen. These discharge conditions are used to demonstrate the capability for two-dimensional imaging measurements. The images produced are the first of their kind and offer quantitative insight into spatially and temporally resolved kinetics and transport in ns-pulsed discharge plasmas. The two-dimensional images make possible comparison with high-fidelity plasma kinetics models of the presented data. The comparison with the quasi-one-dimensional kinetic model show good spatial and temporal agreement. The same diagnostics are used at atmospheric pressure, when atomic oxygen fs-TALIF is performed in an atmospheric-pressure plasma jet (APPJ). Here, the accuracy and spatial resolution of the fs-TALIF diagnostic illustrate the effect of air entrainment into the plasma jet and is able to provide accurate quantitative results in regimes where quenching is prohibitive for traditional ns-TALIF measurements. The APPJ geometry is used for a side-by-side comparison between the newly developed fs-TALIF diagnostic and the more conventional ns-TALIF diagnostic. While, in general, the imaging results are similar, numerous advantages of the fs-TALIF diagnostic are illustrated. Finally, the fs-TALIF diagnostic is used in an atmospheric-pressure combustion environment, where plasma-assisted combustion (PAC) is used to augment flame characteristics. Fs-TALIF measurements of atomic hydrogen and oxygen are compared with hydroxyl radical number densities obtained with ns laser-induced fluorescence (LIF) measurements, to understand OH production mechanisms in the discharge occurring in the hot, downstream products of a premixed hydrocarbon/air flame. The fs-TALIF diagnostic provides fluorescence measurements more accurately from the PAC-produced atomic species, due to simultaneous measurement of quenching rates and reduced photolytic production in the hydrocarbon flames.

Committee:

Igor Adamovich, Dr. (Advisor); Jeffrey Suttor, Dr. (Committee Member); Terry Gustafson, Dr. (Committee Member); James Gord, Dr. (Committee Member)

Subjects:

Aerospace Engineering; Chemistry; Engineering; Mechanical Engineering; Optics; Physics; Plasma Physics

Keywords:

Plasma; fluorescence; two-photon-absorption laser-induced fluorescnece, TALIF; femtosecond; atomic species; quencing; nanosecond; filament discharge; discharge; kinetics, APPJ, atmospheric-pressure plasma jet

Poole, PatrickLiquid crystals as high repetition rate targets for ultra intense laser systems
Doctor of Philosophy, The Ohio State University, 2015, Physics
This thesis presents the development of and first experiments on freely suspended liquid crystal film targets for intense laser-matter experimentation. Liquid crystals exhibit additional phases between solid and liquid which are characterized by different levels of molecular ordering. One of these, the smectic phase, entails molecular orientational order and positional order such that the constituent molecules arrange into layers of set thickness. The surface tension inherent to this smectic phase allows a liquid crystal film to be formed within an aperture in a rigid frame; control over the parameters of film formation (temperature, volume, wiper speed, etc.) allows the number of layers comprising this freely suspended film to be modified on-demand. The result is a variable thickness target with planar geometry that is robust to target chamber vacuum environments and is also inexpensive due to the low volume used per film. Initial ion acceleration experiments will be discussed, where the variable thickness of liquid crystal films is uniquely capable of characterizing the regions of dominance of various acceleration mechanisms. These experiments were performed on the Scarlet laser facility at the Ohio State University, which is a 400 TW, 12 J, 30 fs Ti:sapphire laser system that produces focused intensities in excess of 1021 W/cm2. The upgrade of this facility to these specifications including damage testing measurements on the new optics is also discussed. Additionally, a device for the rapid insertion in-vacuo of these liquid crystal films into the target plane will be presented. This apparatus takes advantage of techniques developed for film formation in a single shot capacity but increases the repetition rate possible to the 0.1 Hz scale. Films formed with the device are done so within 2 μm of the same position each time, which is critical for high repetition rate insertion where no time for target alignment is available.

Committee:

Douglass Schumacher, PhD (Advisor); Lou DiMauro, PhD (Committee Member); Ulrich Heinz, PhD (Committee Member); Ezekiel Johnston-Halperin, PhD (Committee Member)

Subjects:

Optics; Physics; Plasma Physics

Keywords:

liquid crystals; intense lasers; short pulse lasers; plasma physics; ion acceleration

Kemp, Gregory ElijahSpecular Reflectivity and Hot-Electron Generation in High-Contrast Relativistic Laser-Plasma Interactions
Doctor of Philosophy, The Ohio State University, 2013, Physics
Ultra-intense laser (>10^18 W/cm^2) interactions with matter are capable of producing relativistic electrons which have a variety of applications in state-of-the-art scientific and medical research conducted at universities and national laboratories across the world. Control of various aspects of these hot-electron distributions is highly desired to optimize a particular outcome. Hot-electron generation in low-contrast interactions, where significant amounts of under-dense pre-plasma are present, can be plagued by highly non-linear relativistic laser-plasma instabilities and quasi-static magnetic field generation, often resulting in less than desirable and predictable electron source characteristics. High-contrast interactions offer more controlled interactions but often at the cost of overall lower coupling and increased sensitivity to initial target conditions. An experiment studying the differences in hot-electron generation between high and low-contrast pulse interactions with solid density targets was performed on the Titan laser platform at the Jupiter Laser Facility at Lawrence Livermore National Laboratory in Livermore, CA. To date, these hot-electrons generated in the laboratory are not directly observable at the source of the interaction. Instead, indirect studies are performed using state-of-the-art simulations, constrained by the various experimental measurements. These measurements, more-often-than-not, rely on secondary processes generated by the transport of these electrons through the solid density materials which can susceptible to a variety instabilities and target material/geometry effects. Although often neglected in these types of studies, the specularly reflected light can provide invaluable insight as it is directly influenced by the interaction. In this thesis, I address the use of (personally obtained) experimental specular reflectivity measurements to indirectly study hot-electron generation in the context of high-contrast, relativistic laser-plasma interactions. Spatial, temporal and spectral properties of the incident and specular pulses, both near and far away from the interaction region where experimental measurements are obtained, are used to benchmark simulations designed to infer dominant hot-electron acceleration mechanisms and their corresponding energy/angular distributions. To handle this highly coupled interaction, I employed particle-in-cell modeling using a wide variety of algorithms (verified to be numerically stable and consistent with analytic expressions) and physical models (validated by experimental results) to reasonably model the interaction's sweeping range of plasma densities, temporal and spatial scales, electromagnetic wave propagation and its interaction with solid density matter. Due to the fluctuations in the experimental conditions and limited computational resources, only a limited number of full-scale simulations were performed under typical experimental conditions to infer the relevant physical phenomena in the interactions. I show the usefulness of the often overlooked specular reflectivity measurements in constraining both high and low-contrast simulations, as well as limitations of their experimental interpretations. Using these experimental measurements to reasonably constrain the simulation results, I discuss the sensitivity of relativistic electron generation in ultra-intense laser plasma interactions to initial target conditions and the dynamic evolution of the interaction region. This work was performed under DOE contract DE-AC52-07NA27344 with support from the Lawrence Scholar Program, OFES-NNSA Joint Program in High-Energy-Density Laboratory Plasmas and an allocation of computing time from the LLNL Grand Challenge and the Ohio Supercomputing Center.

Committee:

Richard Freeman (Advisor); Linn Van Woerkom (Committee Member); Richard Furnstahl (Committee Member); Richard Hughes (Committee Member)

Subjects:

Electromagnetism; Optics; Physics; Plasma Physics

Bowman, Sherrie S.Atomic and Molecular Oxygen Kinetics Involved in Low Temperature Repetitively Pulsed Nonequilibrium Plasmas
Doctor of Philosophy, The Ohio State University, 2013, Chemistry
This dissertation presents novel results in the study of nanosecond pulsed, non-equilibrium plasmas. Specifically, an in-depth experimental study of the role of atomic oxygen on the kinetic mechanisms involved in three distinct discharge geometries was conducted. First, a low temperature (~300 K) and low pressure (<100 Torr) pulsed plasma in a plane-to-plane dielectric barrier discharge was studied using a high repetition rate (40 kHz) high voltage pulsed discharge. Second, a higher temperature (~1000 K) and low pressure (<100 Torr) pulsed plasma in a bare metal, spherical electrode geometry was studied using a 60 Hz repetition rate high voltage pulsed discharge. Third, a high temperature (~1200 K) and high pressure (~760 Torr) pulsed plasma in a pin-to-plane geometry was studied using a 10 Hz repetition rate high voltage pulsed discharge. Additionally, a study of the role of electronically excited molecular oxygen, SDO, on the kinetics of a low temperature (~300 K) and low pressure ( <100 Torr) nonequilibrium plasma in a plane-to-plane dielectric barrier discharge was conducted. Kinetic modeling results were compared to all the experimental results. UV ICCD camera imaging was used to confirm the stable and diffuse nature of the plasma under all of the conditions that were studied. Current and voltage traces were measured using commercially available probes to determine the energy coupled to the plasma. All of these results were used for modeling of experimental results. Two photon Absorption Laser Induced Fluorescence (TALIF) measurements were used for determining atomic oxygen concentration.. Calibration by comparison with xenon gas gave absolute O atom concentration in a variety of gas mixtures and discharge geometries. IR emission spectroscopy was used for electronically excited molecular oxygen, SDO, measurements. Calibration by comparison with a blackbody source was used for absolute scale results. The effect of SDO on ignition delay time was measured spontaneous OH A¿X(0,0) emission spectroscopy was used. Ignition delay was defined as the onset of continuous OH emission between discharge pulses. It was found that while, in general, the mechanism for atomic oxygen formation and decay in each of the plasmas studied can be compared there are significant differences in quantitative values in each case. Initial conditions, such as the coupled energy and number density of electrons, play a strong role in determining how the chemistry propagates in time. The role of SDO was found to be complicated by concurrent NOx chemistry happening in the discharge and significantly higher concentrations would be needed to differentiate these effects.

Committee:

Walter Lempert (Advisor); Heather Allen (Committee Member); Anne McCoy (Committee Member); Frank DeLucia (Committee Member)

Subjects:

Atoms and Subatomic Particles; Chemistry; Experiments; Gases; Molecular Chemistry; Molecular Physics; Optics; Plasma Physics

Keywords:

Lasers; Laser Diagnostics; Plasmas; Plasma Assisted Combustion; Kinetics; Nonequilibrum Thermodynamics

Cross, Lee WDesign of Microwave Front-End Narrowband Filter and Limiter Components
Doctor of Philosophy in Engineering, University of Toledo, 2013, College of Engineering
This dissertation proposes three novel bandpass filter structures to protect systems exposed to damaging levels of electromagnetic (EM) radiation from intentional and unintentional high-power microwave (HPM) sources. This is of interest because many commercial microwave communications and sensor systems are unprotected from high power levels. Novel technologies to harden front-end components must maintain existing system performance and cost. The proposed concepts all use low-cost printed circuit board (PCB) fabrication to create compact solutions that support high integration. The first proposed filter achieves size reduction of 46% using a technology that is suitable for low-loss, narrowband filters that can handle high power levels. This is accomplished by reducing a substrate-integrated waveguide (SIW) loaded evanescent-mode bandpass filter to a half-mode SIW (HMSIW) structure. Demonstrated third-order SIW and HMSIW filters have 1.7 GHz center frequency and 0.2 GHz bandwidth. Simulation and measurements of the filters utilizing combline resonators prove the underlying principles. The second proposed device combines a traditional microstrip bent hairpin filter with encapsulated gas plasma elements to create a filter-limiter: a novel narrowband filter with integral HPM limiter behavior. An equivalent circuit model is presented for the ac-coupled plasma-shell components used in this dissertation, and parameter values were extracted from measured results and EM simulation. The theory of operation of the proposed filter-limiter was experimentally validated and key predictions were demonstrated including two modes of operation in the on state: a constant output power mode and constant attenuation mode at high power. A third-order filter-limiter with center frequency of 870 MHz was demonstrated. It operates passively from incident microwave energy, and can be primed with an external voltage source to reduce both limiter turn-on threshold power and output power variation during limiting. Limiter functionality has minimal impact on filter size, weight, performance, and cost. The third proposed device demonstrates a large-area, light-weight plasma device that interacts with propagating X-band (8–12 GHz) microwave energy. The structure acts as a switchable EM aperture that can be integrated into a radome structure that shields enclosed antenna(s) from incident energy. Active elements are plasma-shells that are electrically excited by frequency selective surfaces (FSS) that are transparent to the frequency band of interest. The result is equivalent to large-area free-space plasma confined in a discrete layer. A novel structure was designed with the aid of full-wave simulation and was fabricated as a 76.2 mm square array. Transmission performance was tested across different drive voltages and incidence angles. Switchable attenuation of 7 dB was measured across the passband when driven with 1400 Vpp at 1 MHz. Plasma electron density was estimated to be 3.6 × 1012 cm–3 from theory and full-wave simulation. The proposed structure has potential for use on mobile platforms.

Committee:

Vijay Devabhaktuni, Ph.D. (Advisor); Mansoor Alam, Ph.D. (Committee Member); Mohammad Almalkawi, Ph.D. (Committee Member); Matthew Franchetti, Ph.D. (Committee Member); Daniel Georgiev, Ph.D. (Committee Member); Telesphor Kamgaing, Ph.D. (Committee Member); Roger King, Ph.D. (Committee Member)

Subjects:

Electrical Engineering; Electromagnetics; Plasma Physics

Keywords:

microwave; bandpass filter; narrowband; half-mode substrate integrated waveguide; bent hairpin; microstrip; plasma limiter; frequency selective surface; plasma absorber

Prabhakar, TejasStudy of Earth Abundant TCO and Absorber Materials for Photovoltaic Applications
Master of Science, University of Toledo, 2013, Physics
In order to make photovoltaic power generation a sustainable venture, it is necessary to use cost-effective materials in the manufacture of solar cells. In this regard, AZO (Aluminum doped Zinc Oxide) and CZTS (Copper Zinc Tin Sulfide) have been studied for their application in thin film solar cells. While AZO is a transparent conducting oxide, CZTS is a photovoltaic absorber. Both AZO and CZTS consist of earth abundant elements and are non-toxic in nature. Highly transparent and conductive AZO thin films were grown using RF sputtering. The influence of deposition parameters such as working pressure, RF power, substrate temperature and flow rate on the film characteristics was investigated. The as-grown films had a high degree of preferred orientation along the (002) direction which enhanced at lower working pressures, higher RF powers and lower substrate temperatures. Williamson-Hall analysis on the films revealed that as the working pressure was increased, the nature of stress and strain gradually changed from being compressive to tensile. The fall in optical transmission of the films was a consequence of free carrier absorption resulting from enhanced carrier density due to incorporation of Al atoms or oxygen vacancies. The optical and electrical properties of the films were described well by the Burstein-Moss effect. CZTS absorber layers were grown using ultrasonic spray pyrolysis at a deposition temperature of 350 C and subsequently annealed in a sulfurization furnace. Measurements from XRD and Raman spectra confirmed the presence of pure single phase Cu2ZnSnS4 . Texture analysis of as-deposited and annealed CZTS films indicated that the (112) plane which is characteristic of the kesterite phase was preferred. The grain size increased from 50 nm to 100 nm on conducting post-deposition annealing. CZTS films with stoichiometric composition yielded a band gap of 1.5 eV, which is optimal for solar energy conversion. The variation of tin in the film changed its resistivity by several orders of magnitude and subsequently the tin free ternary chalcogenide Cu2ZnS2 having very low resistivity was obtained. By carefully optimization of concentrations of tin, zinc and copper, a zinc-rich/tin-rich/copper-poor composition was found to be most suitable for solar cell applications. Etching of CZTS films using KCN solution reduced their resistivity, possibly due to the elimination of binary copper sulfide phases. CZTS solar cells were fabricated both in the substrate and superstrate configurations.

Committee:

Yanfa Yan (Committee Chair); Victor Karpov (Committee Co-Chair); Alvin Compaan (Committee Co-Chair)

Subjects:

Condensed Matter Physics; Electrical Engineering; Energy; Engineering; Experiments; Materials Science; Morphology; Nanotechnology; Physics; Plasma Physics; Solid State Physics; Sustainability; Technology; Theoretical Physics

Keywords:

Solar cells; Photovoltaic; CZTS; Thin films; Materials Science; TCO; Transparent conducting oxide; ZnO; AZO; Zinc oxide; Sputtering; Spray pyrolysis; Williamson-Hall; Stress; lattice strain; Argon flow rate; Working pressure; Earth abundant

Kelly, Danielle KSimulation of Uniform Heating of Wires Attached to Reduced Mass Targets
Master of Science, The Ohio State University, 2014, Physics
A simulation of the temperature changes of wires connected to reduced mass targets is presented. This simulation looks at thin copper wires and determines the temperature changes due to thermal diffusion, electrical heating, and losses from radiation using material properties from SESAME equation of state and Lee-More-Desjarlais models. The effects of changing the current, and therefore the electrical heating, were simulated. This simulation finds that for a gaussian current with an exponential fall-off tail the copper wire is heated to a uniform temperature in 0.5 ps and stays at that temperature for several picoseconds before thermal diffusion and radiation effects add a noticable temperature gradient to the wire.

Committee:

Richard Freeman, PhD. (Advisor); C. David Andereck, PhD. (Committee Member); Kramer Akli, PhD. (Committee Member)

Subjects:

Physics; Plasma Physics

Keywords:

Laser Plasma Interaction; High Energy Density Physics

Wheeler, Jonathan AllenThe Scaling of High Harmonics with Mid-Infrared Driving Fields and a Method for the Spatial Isolation of Individual Subfemtosecond Pulses
Doctor of Philosophy, The Ohio State University, 2012, Physics
The XUV bursts generated from the strong field processes in both gas and solid targets are increasingly becoming important coherent light sources within laboratories as the nature of their generation and control is better understood and are driving the push into studies of attosecond-scale processes in nature. This work discusses experimental studies on the characterization and application of the XUV pulses generated using both infrared and mid-infrared laser sources with femtosecond pulse durations and kHz repetition rates. The methods employed involve the manipulation of the driving fields to gain insight on the nature of the interactions between the material and field that lead to the modulation of the original waveform responsible for the observed high harmonics. Three different experiments are presented that seek to characterize the trend of the harmonics with driving wavelength, reconstruct the orbital structure of molecular targets, and spatially isolate the individual XUV bursts from each optical cycle of the driving pulse.

Committee:

Louis DiMauro, PhD (Advisor); Pierre Agostini, PhD (Committee Member); David Stroud, PhD (Committee Member); Jay Gupta, PhD (Committee Member); Allan Litsky, PhD (Committee Member)

Subjects:

Atoms and Subatomic Particles; Molecular Physics; Optics; Physics; Plasma Physics

Keywords:

High Harmonic Generation; Wavelength-Scaling; Attosecond; Lighthouse Effect; Tomography; Mid-Infrared

Penkal, Bryan JamesSteps in the Development of a Full Particle-in-Cell, Monte Carlo Simulation of the Plasma in the Discharge Chamber of an Ion Engine
Master of Science in Engineering (MSEgr), Wright State University, 2013, Mechanical Engineering

The design and development of ion engines is a difficult and expensive process. In order to alleviate these costs and speed ion engine development, it is proposed to further develop a particle-in-cell (PIC), Monte-Carlo collision (MCC) model of an ion engine discharge chamber, which has previously been worked on by the Wright State Ion Engine Modeling Group. Performing detailed and accurate simulations of ion engines can lead to millions of dollars in savings in development costs.

In order to recognize these savings more work must be done on the present day models used to simulate ion engine performance. The work presented in this thesis is an effort to do this with a computer model of the plasma in the discharge chamber of an ion engine. In particular, this thesis presents a few steps in the process of moving a Wright State developed PIC-MCC computer code, developed specifically for the plasma in the discharge chamber, to include detailed electric field calculations. This is a rather difficult process in that the electric fields present in the discharge chamber are strongly dependent on the location of the charged particles in the plasma. This means there is a strong and unstable connection between the particle position calculation and the electric field calculation. Other difficulties are the relatively large computational domain and the relatively large plasma density present. Because of the computational times involved,PIC-MCC techniques are generally not applied to large computational domains with high particle number densities, but this is the precise physical model that is required to obtain accurate results for the plasma in the discharge chamber of an ion engine.

This thesis presents a few steps taken to get such a program to converge and to run in a stable fashion. Not only is getting the program to converge an issue, but getting convergence times that are less than one week is difficult. By no means is the work in this thesis a complete solution to these problems; the work done here is just a few steps in this process. There are many problems and issues that still need to be addressed.

In addition to discussing the work done to move detailed PIC-MCC calculations with a fully coupled electric field and particle position calculation forward, a good deal of discussion about the physics of ion engines and the computational tools used in this work will be presented. This is done to familiarize the reader with ion engines and so they will understand how difficult it is to develop a model that will accurately predict the performance of an ion engine.

The baseline computer code used in this research is reviewed. The baseline code is called VORPAL, which the Tech-X Corporation developed. VORPAL itself is an outgrowth of a computer program called OOPIC PRO. This project started using OOPIC PRO, but switched to VORPAL, an object orientated, relativistic, plasma simulation code, because of the many benefits it provides.

Following the discussion of VORPAL, techniques used to decrease run time that were undertaken by the ion engine group at Wright State and the Tech-X Corporation are given. These include particle fragmenting and merging, scaling of the discharge chamber, and two-dimensional domain decomposition. Programming issues that were discovered in VORPAL and in an earlier version of VORPAL called OOPIC PRO are discussed.

Due to the sensitivity that PIC-MCC codes have to the time step used and the desire to implement a time throttling technique to reduce computational times, a time step survey is conducted. PIC-MCC codes are extremely sensitive to time step size. It is found that a time step size of 10-12 seconds is the largest time step that can be used.

Committee:

James Menart, Ph.D. (Advisor); Allen Jackson, Ph.D. (Committee Member); Scott Thomas, Ph.D. (Committee Member)

Subjects:

Aerospace Engineering; Electrical Engineering; Engineering; Plasma Physics

Keywords:

plasma; vorpal; ion; thruster; nasa; pic; mcc; magnetic; field

Santana, JulioInvestigating Ionospheric Parameters Using the Plasma Line Measurements From Incoherent Scatter Radar
Master of Science, Miami University, 2012, Computational Science and Engineering
Because of deficiencies in sampling resolution and storage space, the plasma line frequency component of the incoherent scatter radar (ISR) spectrum has been neglected in experimentally verifying ionospheric parameters. Several incoherent scatter theories were independently developed with confirmation from low resolution data in the 1960s that used the plasma line resonant frequency and plasma line peak intensity to derive ionospheric parameters. Now that higher resolution measurement techniques exist, this thesis investigates three methods for obtaining plasma line resonant frequency, peak intensity, and spectral width. Following this study, several salient features endemic to the ISR experiment performed on January 15-17th, and January 22nd of 2010 are presented and analyzed.

Committee:

Qihou Zhou, PhD (Advisor); Jade Morton, PhD (Committee Member); Chi Hao Cheng, PhD (Committee Member)

Subjects:

Aeronomy; Electrical Engineering; Plasma Physics

Keywords:

Plasma line; ionosphere; incoherent scatter radar; aeronomy; Arecibo Observatory; space weather

Ongkodjojo Ong, AndojoElectrohydrodynamic Microfabricated Ionic Wind Pumps for Electronics Cooling Applications
Doctor of Philosophy, Case Western Reserve University, 2013, EECS - Electrical Engineering
This work demonstrates an innovative microfabricated air cooling technology that employs an electrohydrodynamic (EHD) corona discharge or ionic wind pump that has the potential to meet industry requirements as a next generation solution for thermal management applications. A single ionic wind pump element consists of two parallel collecting electrodes between which a single emitting tip is positioned. A grid structure on the collector electrodes enhances the overall heat transfer coefficient and facilitates an IC compatible and batch process. The main purpose of the work presented here is thus to investigate whether an optimized ionic wind pump employed in an array configuration might exhibit performance comparable to a conventional CPU fan. The manufacturing procedure developed for the device uses a glass wafer, a single mask-based photolithography process, a low cost copper-based electroplating method, and explores the effect of employing a palladium coating on the device. Various design configurations and optimization processes were explored and modeled computationally to investigate their influence on the cooling phenomenon. The optimized single element device provides a convection heat transfer coefficient of up to 3200 W/m2-K and a COP of up to 46.7 (a maximum COP of 51.5 exhibited by the 6-element array) exhibiting an overall area of 5.35 mm x 3.61 mm, an emitter-to-collector gap of 500 ¿¿m, and an emitter radius curvature of 12.5 ¿¿m. When compared with other ionic wind pumps, the device developed for this work is superior in terms of heat transfer coefficient and COP. However, the overall performance of the array does not compare favorably to a conventional CPU fan except in terms of COP. Additionally, the lifetime experiments conducted demonstrate that additional work may be required to extend the operation of the device, and some form of non-porous coating may be required to protect the underlying copper material. Nonetheless, the device described herein exhibits a flexible and small form factor, low noise generation, high efficiency, large heat removal over a small dimension, relatively simple technology, high reliability (no moving parts), lower power consumptions, and low cost; these are characteristics required by the semiconductor industry for next generation thermal management solutions.

Committee:

Alexis Abramson, PhD (Advisor); Norman Tien, PhD (Committee Member); Christian Zorman, PhD (Committee Member); Jaikrishnan Kadambi, PhD (Committee Member)

Subjects:

Design; Electrical Engineering; Electromagnetics; Energy; Engineering; Experiments; Fluid Dynamics; Materials Science; Mechanical Engineering; Physics; Plasma Physics; Solid State Physics; Systems Design; Theoretical Physics

Keywords:

ionic wind pumps; thermal management; MEMS; electronics cooling; corona discharge; electrohydrodynamics; heat transfer coefficients; coefficient of performance (COP); FEM/FEA; optimization; lifetime test; microfabrication process

Burwell, Edwin DudleyA MICROPLASMA-BASED SPUTTERING SYSTEM FOR DIRECT-WRITE, MICROSCALE FABRICATION OF THIN-FILM METAL STRUCTURES
Master of Sciences (Engineering), Case Western Reserve University, 2015, EECS - Electrical Engineering
This thesis reports the development of a direct write, microplasma-based sputtering instrument and associated process for the fabrication of metallic microstructures on rigid and flexible substrates. The process capitalizes on a physical vapor that is generated within a small capillary by Ar ion bombardment of a small diameter metal wire. Forced Ar flow ejects the sputtered vapor through the orifice and onto the substrate. Integration with an x-y-z stage enables the direct patterning of structures without the need for masks. As-deposited, 40 nm-thick Au structures fabricated on glass substrates exhibit a resistivity that is only 20% higher than that of bulk Au. Deposition has also been demonstrated on liquid crystal polymer and PDMS substrates. The process is performed at atmospheric pressure, thereby addressing one of the most significant limitations associated with conventional magnetron sputtering. The direct-write capability and room temperature deposition make this process a potential alternative to ink-jet printing.

Committee:

Christian Zorman, Dr. (Advisor); Mohan Sankaran, Dr. (Committee Member); Philip Feng, Dr. (Committee Member)

Subjects:

Electrical Engineering; Engineering; Materials Science; Plasma Physics

Keywords:

microplasma; sputtering; additive manufacturing

Middendorf, John RaymondNovel Devices and Components for THz Systems
Doctor of Philosophy (PhD), Wright State University, 2014, Engineering PhD
Since the first demonstration of the generation of terahertz (THz) pulses from photoconductive (PC) antennas, research has pushed toward the development of smaller, cost efficient, and faster THz systems. This dissertation presents the work accomplished in order to realize these more practical terahertz (THz) photoconductive (PC) systems. First, this work will present a novel ErAs:GaAs photoconductive switch used to make a THz source excited by 1550 nm laser pulses. It will be shown that the excitation process taking place in the material relies on extrinsic (rather than intrinsic) photoconductivity. Then, several experiments will be presented that aim to improve the efficiency of the device and further the understanding of the underlying physical mechanisms. The erbium composition of the photoconductive layer will be varied and the effects of these variations on THz generation will be investigated. Then the wavelength of the drive laser used to excite the extrinsic photoconductive mechanism will be varied, while recording the photocurrent responsivity. This wavelength study will be used to find the optimal drive wavelength for maximum THz power. In conclusion, the results of these experiments will show that extrinsic PC THz generation is practical, cost effective, and capable of producing an average THz power of more than 100 μ W. Coinciding with this high power level, the bandwidth of this new source was found to be ~350 GHz, corresponding to a photocarrier recombination time of 450 fs. The work presented in this section will provide a path to develop superior THz PC sources that have a higher THz-power-to-cost ratio than the current state of the art. Photoconductive antennas are mostly used to conduct spectroscopy measurements, either in time domain systems (TDS) or in frequency domain systems (FDS). Currently, both techniques can reach high-frequencies (>1 THz) but struggle to do so while making fast, high-resolution measurements (<2 GHz). In addition, both methods can be time consuming to set up and perform. A superior spectrum analysis technique would greatly facilitate THz application development by making results easier and less expensive to obtain. Therefore, the second part of this dissertation addresses the need for quicker and more precise THz spectrum analysis by demonstrating a new type of THz spectrum analyzer based on a high-speed, tunable, Fabry-Perot interferometer. This new and unique spectrum analyzer reduces the time required to obtain a THz spectrum (a few seconds), while producing a more precise result (<2 GHz resolution). After the presentation of this concept, the various experimental design iterations will be shown, while explaining the improvements gained from each. Then experimental demonstrations of the new spectrum analyzer will be presented, and possible future improvements will be discussed. While the Fabry-Perot based spectrum analyzer is an improvement for THz spectroscopy, it can suffer from two issues: mirror reflectivity that changes with frequency, and the inability to easily tune the mirror reflectivity to optimize the system for different applications. These issues make it challenging to obtain an accurate and useful THz spectrum. Therefore the third part of this dissertation is motivated by these problems and presents a solution; the use of structured-surface-plasmon (SSP) enhanced polarizers as Fabry-Perot mirrors. The SSP polarizers used in this work are composed of metal wire-grids with sub-wavelength feature sizes and high metal fill-factors. It will be shown that high fill-factor SSP polarizers can achieve superior THz performance, compared to traditional THz polarizers, with an extinction ratio exceeding 60 dB. With the use of these polarizers as mirrors, the Fabry-Perot can achieve variable mirror reflectivity by changing the polarizer orientation angle. This will allow the spectrum analyzer to compensate for any reflectivity-vs.-frequency changes that occur on the Fabry-Perot mirrors during a spectral scan. Changing the polarizer orientation makes it possible to optimize the spectrum analyzer for different applications; by choosing maximum frequency selectivity (with low power transmission), maximum power transmission (with low frequency selectivity), or somewhere in between. The SSP enhanced THz polarizers are inexpensive, can provide a significant upgrade to the Fabry-Perot spectrum analyzer, and help to achieve a better physical understanding of plasmonic design in the THz field. After the new extrinsic ErAs:GaAs PC sources, Fabry-Perot spectrum analyzer, and SSP polarizers have been presented, this dissertation will finish with a demonstration of a new polarizing Fabry-Perot spectrum analyzer and then suggestions for future research.

Committee:

Elliott Brown, Ph.D. (Advisor); Jason Deibel, Ph.D. (Committee Member); Doug Petkie, Ph.D. (Committee Member); Daniel LeMaster, Ph.D. (Committee Member); Julie Jackson, Ph.D. (Committee Member)

Subjects:

Electrical Engineering; Electromagnetics; Engineering; Materials Science; Optics; Physics; Plasma Physics; Radiation

Keywords:

photoconductive switch; ultrafast photoconductivity; Gallium Arsenide; THz; extrinsic photoconductivity; Fabry-Perot; THz spectrum analyzer; THz polarizer; Structured-surface plasmons; spoof-surface plasmons; high fill factor

Godar, Trenton J.Testing of Two Novel Semi-Implicit Particle-In-Cell Techniques
Master of Science in Engineering (MSEgr), Wright State University, 2014, Renewable and Clean Energy
PIC (Particle-in-cell) modeling is a computational technique which functions by advancing computer particles through a spatial grid consisting of cells, on which can be placed electric and magnetic fields. This method has proven useful for simulating a wide range of plasmas and excels at yielding accurate and detailed results such as particle number densities, particle energies, particle currents, and electric potentials. However, the detailed results of a PIC simulation come at a substantial cost of computational requirement and the algorithm can be susceptible to numerical instabilities. As processors become faster and contain more cores, the computational expense of PIC simulations is somewhat addressed, but this is not enough. Improvements must be made in the numerical algorithms as well. Unfortunately, a physical limit exists for how fast a silicon processor can operate, and increasing the number of processing cores increases the overhead of passing information between processors. Essentially, the solution for decreasing the computational time required by a PIC simulation is improving the solution algorithms and not through increasing the hardware capacity of the machine performing the simulation. In order to decrease the computational time and increase the stability of a PIC algorithm, it must be altered to circumvent the current limitations. The goal of the work presented in this thesis is twofold. The first objective is to develop a three-dimensional PIC simulation code that can be used to study different numerical algorithms. This computer code focuses on the solution of the equation of motion for charged particles moving in an electromagnetic field (Newton-Lorentz equation), the solution of the electric potentials caused by boundary conditions and charged particles (Poisson's Equation), and the coupling of these two equations. The numerical solution of these two equations, their coupling, which is the primary cause of instabilities, and the severe computational requirements for PIC codes make writing this code a difficult task. Solving the Newton-Lorentz equation for large numbers of charged particles and Poisson's equation is complex. This is the focus of this newly developed computer code. The second objective of the work presented in this thesis is to use the developed computer code to study two ideas for improving the numerical algorithm used in PIC codes. The two techniques investigated are: 1) implementing a fourth order electric field approximation in the equation of motion and 2) solving for the electric field, i.e. solving Poisson's equation, multiple times within a single time step. The first of these methods uses the electric fields of many cells that a charged particle may pass through in one time step. This is opposed to using only the cell of origin electric field for the particle's entire path during one time step. The idea here is to allow PIC codes to use larger time steps while remaining stable and avoiding numerical heating; thus reducing the overall computer time required. The second technique studied is utilizing multiple Poisson equation solves during a single time step. Typically, an explicit PIC model will solve the electric field only once during a time step; however, solving the field multiple times during the particle push allows particles to distribute themselves in a more electrically neutral manner within a single time step. The idea here is to allow larger time steps to be used without obtaining unrealistic electric potentials due to an artificial degree of charge separation. This eliminates instabilities and numerical heating. Explicit PIC codes have limits on how large the numerical time step can be before the electric potentials blow up. This work has shown that neither of these techniques, in their current state, are practical options to increase the time step of the PIC algorithm while maintaining the correct solution. However, stability improvements are observed which warrant further investigation into alternative implementations of these techniques. The current fourth order electric field technique seems to have little effect on the solution, but the multiple solves technique does show some improvement in stability over the explicit routine. At time steps where the explicit routine begins to oscillate and become unstable, the multiple solves routine remains stable. These techniques are not quite as developed as they could be, meaning some of the future work suggested in this report could lead to one or both of these techniques being successful in the future.

Committee:

James Menart, Ph.D. (Advisor); Zifeng Yang, Ph.D. (Committee Member); Amir Farajian, Ph.D. (Committee Member)

Subjects:

Electromagnetism; Physics; Plasma Physics

Keywords:

particle in cell; PIC; semi-implicit; semi implicit; computational plasma; plasma modelling

Bharadwaja, SakethMolecular Dynamics Simulations of Si binding and diffusion on the native and thermal Silicon Oxide surfaces
Master of Science in Engineering, University of Toledo, 2012, Engineering (Computer Science)
Amorphous silicon (a-Si) thin-film solar cells grown via plasma-enhanced chemical vapor deposition (PECVD) are of significant technological interest. As a result, there is significant interest in understanding the physical processes which control the a-Si thin-film structure and morphology. In particular, since the early stages of a-Si growth on the silicon oxide substrate play a key role in determining the subsequent evolution, it is important to obtain a better understanding of this stage of a-Si growth. The key objectives of the work presented in this thesis are to obtain a better understanding of the structure and morphology of the silicon-oxide substrate used in a-Si growth via PECVD as well as of the key processes of Si diffusion on the substrate which control the nucleation of a-Si islands. In particular, motivated by experimental and simulation results, we have carried out molecular dynamics simulations of the formation of a thermal silicon oxide substrate (corresponding to oxide formation at high-temperature) as well as of the room-temperature oxidation of “native” silicon oxide thin-films. In addition, for the case of a native silicon oxide surface, we have studied the binding energies, binding sites, and diffusion barriers for Si diffusion in order to gain insight into the critical length-scales for a-Si island formation. In the case of thermal silicon oxide formed at high temperature, our molecular dynamics simulations were carried out using an effective Munetoh potential which takes into account the “average” charge transfer as well as bond angles and energies. In this case, due to the relatively high temperature the surface was found to be extremely rough and highly disordered, while the thin-film structure was found to be amorphous. In contrast, in our simulations of the formation of native silicon oxide thin-films at room temperature, a more sophisticated ReaxFF potential was used which properly takes into account the effects of O2 molecular dissociation and rebinding at the surface, as well as the long-range Coulomb interaction and local charge-transfer. We have also studied the binding and diffusion of Si atoms for this case in order to try to explain recent experiments and simulations in which it was shown that 3D a-Si islands with a typical island diameter of approximately 30 A are formed in the early stages of growth. For the case of native silicon-oxide our results for the oxygen penetration profile and surface roughness were found to be in good qualitative agreement with experiments. Our results also indicate that while the typical binding energies for Si adatoms on the SiO2 surface are significantly lower than for Si/Si(100), due to the disordered structure of the surface the barriers for diffusion are typically significantly higher. As a result, at the deposition temperature of 200oC used in low-temperature PECVD, these sites may act like “trapping sites” for deposited Si atoms. We note that these results are consistent with recent experiments on the relaxation of SiO2 microstructures at high temperatures. However, they also imply that the characteristic length-scale for 3D islands in the early stages of a-Si growth via PECVD cannot be explained by a combination of homogenous diffusion and a critical island-size, as is typically found in epitaxial growth.

Committee:

Jacques Amar (Committee Chair); Mohammed Niamat (Committee Co-Chair); Mansoor Alam (Committee Member)

Subjects:

Engineering; Particle Physics; Physical Chemistry; Physics; Plasma Physics

Keywords:

Amorphous; morphology; chemical vapor deposition; Coulomb interactions; critical island size; epitaxial growth.

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