Thesis--Wisconsin. / Includes bibliographical references (leaf 30).

Au cours de ce travail de thèse, nous avons développé un procédé plasma-catalyse d'élimination de l'acétaldéhyde en utilisant un processus diphasique couplant un catalyseur nano-structuré et a plasma à la pression atmosphérique. L’élaboration du catalyseur nanostructuré a été d'abord étudiée. Puis la performance de dégradation du polluant a été étudiée. Les nanoparticules monodispersées (titane-oxo-alcoxy) sont générées dans le réacteur de sol-gel avec micro-mélange turbulent et déposés sur des plaques de verre ou des billes de verre comme monocouches nanostructurées. Le dépôt de l'argent et de l'or est réalisé par la réduction des ions sous l’irradiation de UV-A. La cinétique de croissance photocatalytiques et de la morphologie des nanoparticules sont étudiés expérimentalement par les méthodes MET, MEB et AFM. Il est également intéressant de discuter du mécanisme de la formation des nanoparticules et d'évaluer son efficacité quantique. Les conclusions expérimentales sont supportées théoriquement par le calcul des spectres d'absorption. Ensuite l'efficacité du processus de couplage d'une décharge à barrière diélectrique et d’un lit fluidisé d'argent et d’or nanostructurés, pour la dégradation d'un polluant modèle (acétaldéhyde CH₃CHO), est étudiée. Dans la première partie, l'efficacité du procédé plasma seul est discutée, en termes de dégradation des polluants et de production de CO et CO₂. Dans la deuxième partie, la dégradation de CH₃CHO ainsi que la production COx sont étudié en fonction du temps de réduction photocatalytique d’Ag+ et d’Au³⁺ ions, qui est liée à la masse d'argent et d’or déposée. Les voies de dégradation des polluants, notamment la chimie homogène dans la phase de plasma et la chimie hétérogène sur la surface, sont discutées. Enfin, la production des sous-produits principaux est présentées et comparées entre les catalyseurs Ag et Au. / During this Phd work, we have developed a plasma-catalytic process of acetaldehyde removal using a diphasic process coupling a nano-structured catalyst and an atmospheric pressure plasma. The elaboration of the nanoparticulate catalyst has been firstly studied. Then its performance coupling with plasma has been investigated. The monodispersed titanium-oxo-alkoxy nanoparticles are generated in the sol-gel reactor with turbulent micromixing and deposited onto glass plates or glass balls as monolayer nanocoatings. The silver and gold deposition is achieved by the ions reduction at UV-A light illumination. The photocatalytic growth kinetics and nanoparticle morphology are studied experimentally by the TEM, SEM and AFM methods. It’s also interesting to discuss the mechanism of the nanoparticles formation and evaluate its quantum efficiency. The drawn conclusions are supported theoretically through the calculation of the absorption spectra. Then the efficiency of the process coupling a dielectric barrier discharge and a fluidized nanostructured silver and gold based bed for the degradation of a model pollutant (acetaldehyde CH₃ CHO) is studied. In the first part, the efficiency of the plasma alone process is discussed, in terms of pollutant removal and CO and CO₂ production. In the second part, CH₃ CHO removal as well as COx production is studied as a function of the photocatalytic reduction time of Ag⁺ and Au³⁺ ions, which is related to the deposited silver and gold mass. The pollutant removal pathways, including homogeneous chemistry in the plasma phase and heterogeneous chemistry on the surface, are discussed. Finally, the production of main by-products is presented and compared between Ag and Au catalysts.

An ion trap has been constructed that creates a potential well suitable for confining ions with the space charge of an electron cloud. The trap uses the concept of artificially structured boundaries, regions of overlapping electric and magnetic fields, to confine particles in a relatively field free volume. Measurements are presented from the build-up of ionized molecular hydrogen over time. Molecular hydrogen is introduced into the confinement volume by direct electron bombardment ionization of neutral background H2 leaked into the trap. Detailed analysis of the data is conducted using particle-in-cell simulations of trap operation and rate mechanics analysis. Pressure dependent estimates of ion lifetimes in the trap are on the order of milliseconds. Along with discussion of the trap a full introduction to the particle-in-cell technique is conducted through an original code implementation.

No description available.

In many natural and laboratory conditions, plasmas are often in the non-equilibrium state due to presence of stationary flows, when one particle species (or a special group, such as group of high energy particles, i.e., beam) is moving with respect to the other plasma components. Such situations are common for a number of different plasma applications such as diagnostics with emissive plasma probes, plasma electronics devices and electric propulsion devices. The presence of plasmas flows often leads to the instabilities in such systems and subsequent development of large amplitude perturbations. The goal of this work is to develop physical insights and numerical tools for studies of ion sound instabilities driven by the ion flow in a system of a finite length. The ion sound waves are modified by the presence of ion beam resulting in negative and positive energy modes. The instability develops due to coupling of negative and positive energy modes mediated by reflections from the boundary. It is shown that the wave dispersion due to deviation from quasi-neutrality is crucial for the stability. In finite length system, the dispersion is characterized by the length of the system measured in units of the Debye length. The instability is studied analytically and the results are compared with direct initial value numerical simulations. The numerical tools to simulate these systems are developed based on Godunov and multiple shooting methods. The initial value simulations show the time dependent evolution from which the growth rates were determined for different parameters of the system. The results of the simulations were benchmarked against the analytical results in some limiting cases. In the pursuit of simulation efficiency, the parallelization of the code was investigated for two basic types of parallel systems: shared and distributed memory. The OpenMP and MPI library were used correspondingly.

A study of ion oscillations in the annular regions of a strontium plasma is reported. An ultracold neutral plasma is formed by photoionizing the 1 P 1 electrons using a pulsed dye laser' and absorption spectroscopy is done on the 2 S ½ - 2 P ½ transition of the Sr+ ion. The kinetic energy of the ions is then calculated using Doppler broadening of the spectrum. The variation of temperature with time is fit to a theoretical model of kinetic energy oscillation. The result of the fitting is presented in this thesis. The importance of an annular analysis of the absorption spectrum is demonstrated and the mathematical procedures employed to calculate the kinetic energy are developed. The oscillations are observed to be damped which is a characteristic of strongly coupled plasmas.

Thesis (M.S.)--Rochester Institute of Technology, 1994. / Typescript. Includes bibliographical references (leaves 98-99).

Thesis (Ph.D.)--University of California, Berkeley, 1961. / "Controlled Thermonuclear Processes, UC-20" -t.p. "TID-4500 (16th Ed.)" -t.p. Includes bibliographical references (p. 68).

Thesis--University of Wisconsin-Madison. / Includes bibliographical references.

At high winds, turbulence in the ocean surface mixed layer is dominated by organized coherent structures in the form of counterrotating helical vortices known as Langmuir cells. While the dynamics of the ocean surface layer has been studied rather extensively at lower wind speeds, the detailed physics at higher winds has remained largely inaccessible because of limited sea-going operations and difficulty conducting in situ measurements at high sea states. In the present thesis new measurement techniques, based on acoustical remote sensing, are described. A freely drifting imaging sonar was employed, which allowed us to follow time-evolving features for an extended period of time. This imaging sonar extends the acoustical approach beyond fixed orientation sonars and covers a full 360° circle on the surface. The full circle capability turns out to be a key addition to the measurements: it allowed quantitative evaluation of the directional properties of Langmuir circulation surface structure. These new methods allow us to sample near-surface circulation and bubble distributions even in extreme conditions, and contribute to our understanding of small scale dynamics in the wind driven surface layer. Using vertical velocity measurements in the convergent regions of Langmuir circulation and a model scaling, we infer the effective viscosity relevant to cell generation. Matching velocity- and temperature-inferred turbulent viscosities we estimate the depth scale over which the wind-wave forcing is of most importance. The velocity-inferred viscosity compares favorably with the mean model viscosity values evaluated at approximately two significant wave heights below the surface. Combining the effective viscosity calculated at different depths with the observed Stokes drift and friction velocity we estimate Langmuir numbers La between 0.015 and 0.1. We observe evolving cell patterns at larger La (between 0.02 and 0.05), which indicates that higher viscosity values than previously assumed in the models may be relevant for Langmuir circulation dynamics. Acoustical observations of the orientation of surface bubble clouds and the directional wave field during several deployments provided an opportunity for comparison of the directional properties of Langmuir circulation with a model that takes into account effects associated with misalignment of the Stokes drift and wind forcing. Model results imply that the growth rate is maximal overall when wind and waves are aligned. For a given angle between the Stokes drift and the wind (the misalignment angle) the direction of the cell axis for maximal growth lies between the Stokes drift and the wind and is mainly determined by (i) the misalignment angle and (ii) the ratio of the Stokes drift shear and mean Eulerian shear. Our ocean observations showed Langmuir cells responding to the changes in wind direction within 15 to 20 min. On two occasions, when the wind changed direction and waves lagged behind, the cells were observed to form in an intermediate direction (between wind and waves) consistent with model predictions. Observations of the near-surface circulation and thermal structure during a storm motivate analysis in terms of the Froude number derived from the measured vertical density gradient, the turbulent diffusivity which is inferred from the measured temperature distributions, and velocity and spatial structure of the circulation. The results demonstrate inhibition of Langmuir circulation by the presence of warm surface water at the beginning of a storm and provide a test of model description of the balance between wind-driven stirring and buoyant resistance. To better understand our measurements and the limitations of the approach, based on the acoustical backscatter, a technique for scatter location estimation is proposed. By comparing velocity magnitudes, independently measured with side-looking and upward-looking sonars, we estimate an effective scattering depth. These results show that the backscatter measured with side-looking sonars originates not right at the surface but at some depth below. / Graduate