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published_or_final_version / Physics / Master / Master of Philosophy

Computational simulations of air glow discharge plasma in the presence of supersonic flow are presented. The glow discharge model is based on a self-consistent, multi-species, continuum description of the plasma with finite-rate chemistry effects. The glow discharge model is coupled to a compressible Navier-Stokes solver to study the effect of the plasma on the flow and the counter-effect of the flow on the plasma. A finite-rate air chemistry model is presented and validated against experiments from the literature at a pressure of 600 mTorr. Computational results are compared with experimentally measured V-I characteristics, axial positive ion densities and electron temperature, and reasonably good qualitative and quantitative agreement is observed. The validated air plasma model is then used to study the effect of the surface plasma discharge on M=3 supersonic flow at freestream pressure 18 Torr and the corresponding effects of the flow on the discharge structure in two dimensions. The species concentrations and the gas temperature are examined in the absence and presence of bulk supersonic flow. The peak gas temperature from the computations is found to be 1180 K with the surface plasma alone in the absence of flow, and 830 K in the presence of supersonic flow. Results indicate that O- ions can have comparable densities to electrons in the pressure range 1-20 Torr, and that O2- ion densities are at least two orders of magnitude smaller over the pressure range considered. Different ion species are found to be dominant in the absence and presence of supersonic flow, highlighting the importance of including finite-rate chemistry effects in discharge models for understanding plasma actuator physical phenomena. Electrode polarity effects are investigated, and the cathode upstream actuation is found to be stronger than the actuation strength with the cathode downstream, which is consistent with experimental findings of several groups. A parallel computing implementation of the plasma and flow simulation tools has been developed and is used to study the three-dimensional plasma actuator configuration with circular pin electrodes. / text

Spectroscopic investigations have been carried out on an argon inductively coupled plasma operating at non-atmospheric pressure. The relationship between torch pressure and a number of plasma operating characteristics was explored for torch pressures between 100 and 3000 torr. The plasma operating characteristics examined include observed analyte emission intensities, electron densities, ion to atom ratios, and the deviation of plasma conditions from local thermodynamic equilibrium. The effect of pressure on the observed analyte emission intensities was found to include factors in addition to the change in density of species within the torch. Emission lines originating from ions and atoms with high ionization potentials (greater than 7 eV) increased in intensity with increasing torch pressure, in excess of that predicted by the increase in density of species present. Conversely, emission lines originating from atoms of low ionization potential decreased in intensity with increasing torch pressure despite the increase in density. The results of the spatial determination of electron densities and ion to atom ratios indicate that excitation conditions within the central channel of the plasma are shifted towards conditions of local thermodynamic equilibrium as the pressure within the torch is increased. In addition, it is possible to obtain improved limits of detection by optimizing the torch pressure for the analyte element of interest.

Fundamental investigations have been carried out on an Inductively Coupled Plasma (ICP) operated at 148 MHz, a frequency which is nearly three times higher than any previously reported for analytical ICPs used in spectrochemical analysis. High frequency operation is expected to provide easier sample introduction into the discharge, with a consequence of less energetic conditions in the central channel. Several plasma diagnostic techniques were employed in order to determine the conditions experienced by the analyte species in this source for spectrochemical analysis. Three different torch systems were investigated at 148 MHz and compared to the "standard" 27 MHz configuration. The highest excitation temperatures and electron densities were obtained in the 27 MHz configuration, and the lowest values in the largest torch at 148 MHz. Intermediate values were obtained in the intermediate-size torches at 148 MHz. These observations correlate reasonably well with the ratio of the plasma radius to the skin depth (r/s). The skin depth defines the region in which the majority of the electrical energy is deposited into the discharge, and is smaller at 148 MHz than at 27 MHz. The measurement of electron densities also allows the estimation of how closely a particular discharge approaches Local Thermal Equilibrium (LTE). As may be expected, LTE is most closely approached in the 27 MHz arrangement. The less energetic conditions characterized by lower temperatures and electron densities result in less intense analyte emission from the high frequency ICPs. Signal-to-Background ratios and detection limits reflect this trend, but the linearity of the calibration curves and freedom from vaporization interferences are not degraded. Finally, the introduction of organic solvents is much easier, and better detection limits in an organic matrix are obtained at 148 MHz. These investigations have shown the utility of classifying the effects of changing torch sizes and operating frequencies by means of the r/s ratio. This provides the analyst with a means of selecting the general range of conditions to be employed in a particular analysis.

No description available.

The dissertation examines topics related to the formation of supercapacitors using plasma technology and their analysis. Plasma spray technology was used to form supercapacitors electrodes. Carbon was deposited on stainless steel surface using the atmospheric pressure argon-acetylene plasma. The deposition of nickel oxide on the surface of carbon electrodes was made using magnetron sputtering method. The influence of acetylene amount to the supercapacitors electrodes and the electrical characteristics of the structure were estimated. The nickel oxide influence to the electrical parameters of supercapacitor carbon electrodes, structure and microrelief was assessed too. The etching of carbon electrodes surface with oxygen plasma was performed and its impact on the capacitors electrical parameters and carbon electrode structure was evaluated. Mathematical modeling was used to theoretically estimate the influence of oxygen plasma to the supercapacitor carbon electrode surface microrelief. The anisotropic growth of carbon surface relief and isotropic surface erosion mechanism was theoretically explained. The impact mechanism of the acetylene content and plasmatron power to the carbon electrode structure and electrical parameters was qualitatively explained. The mechanism for understanding the influence of NiO content on the carbon electrodes to the supercapacitor electrical parameters and electrode structure was proposed. / Disertacijoje nagrinėjama tematika yra susijusi su superkondensatorių formavimu ir jų tyrimais naudojant plazmines technologijas. Superkondensatorių elektrodams formuoti panaudota plazminio purškimo technologija. Anglis ant nerūdijančio plieno paviršiaus užnešta panaudojant atmosferinio slėgio argono-acetileno plazmą. Nikelio oksidui nusodinti ant anglies elektrodų paviršiaus panaudotas magnetroninio garinimo metodas. Įvertinta acetileno kiekio įtaka superkondensatorių elektrinėms charakteristikoms ir elektrodų struktūrai. Tai pat įvertinta nikelio oksido kiekio įtaka superkondensatorių anglies elektrodų elektriniams parametrams, struktūrai bei mikroreljefui. Atliktas anglies elektrodų paviršiaus ėsdinimas deguonies plazma ir įvertintas jo poveikis kondensatorių elektriniams parametrams ir anglies elektrodų struktūrai. Matematiniu modeliavimu teoriškai įvertinta deguonies plazmos įtaka superkondensatorių anglies elektrodų paviršiaus mikroreljefui. Teoriškai pagrįstas anglies dangos reljefo anizotropinis augimas ir izotropinės paviršiaus erozijos mechanizmas. Kokybiškai paaiškintas acetileno kiekio ir plazmotrono galios įtakos mechanizmas anglies elektrodų struktūrai ir elektriniams parametrams. Pasiūlytas mechanizmas, aiškinantis NiO kiekio, esančio ant anglies elektrodų, įtaką superkondensatorių elektriniams parametrams ir elektrodų struktūrai.

Criteria for the confinement of plasmas consisting of a positive and negative component in Penning type traps with nested electric potential wells are presented. Computational techniques for the self-consistent calculation of potential and plasma density distributions are developed. Analyses are presented of the use of nested well Penning traps for several applications. The analyses include: calculations of timescales relevant to the applications, e.g. reaction, confinement and relaxation timescales, self-consistent computations, and consideration of other physical phenomenon important to the applications. Possible applications of a nested well penning trap include production of high charge state ions, studies of high charge state ions, and production of antihydrogen. In addition the properties of a modified Penning trap consisting of an electric potential well applied along a radial magnetic field are explored.

No description available.

A thesis submitted to the faculty of science, university of the Witwatersrand, Johannesburg,in fulfilment of the requirements for the degree of Doctor of Philosophy, 1994 / Alternative sample manipulation and sample introduction methods for inductively coupled plasma-atomic emission spectrometry (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS) have been investigated. The broad objectives of the study were to broaden the scope and improve the analytical performance of the techniques. The methods which were investigated were: 1) Laser ablation. This technique was used only in combination with ICP-MS. The technique was not applied to ICP-MS because the laser ablation system is dedicated to the ICP-MS instrument. The laser ablation system was applied to the direct analysis of solid refractory materials, without dissolution of the sample. The main advantages of avoiding the dissolution step are that the sample preparation is rapid, there is no dilution or contamination of the sample, and no loss of volatile analyte elements. The problems which were encountered with the use of laser ablation were firstly, poor precision of measurement relative to solution analysis due to sample particles of widely varying size entering the plasma, and secondly, memory effects when changing from one sample type to another. 2) Flow injection. This sample manipulation method was used in combination with pneumatic nebulization for ICP-AES and ICP-MS. The technique was applied to a number of different analytical problems, with the objectives of speeding up analysis times, increasing the matrix tolerance of the instruments, and automating a variety of sample preparation processes. 3) Hydride generation. This technique was applied to the determination of arsenic and selenium by ICP-AES and ICP-MS, using a novel type of gas, liquid separator. The advantages of the technique were the separation of the analyte elements from interfering matrix species, and increased analyte sensitivity due to the excellent transport efficiency of hydride generation relative to solution nebulization. Numerous advantages have been obtained from the application of these sample manipulation and sample introduction techniques. Methods have been developed for the analysis of materials which are not suited to conventional solution nebulization, and the techniques have been used to improve the efficiency of analysis, to achieve lower detection eliminate, and to eliminate interferences. / GR2017