Measurements of the ionsation current Growth and the ratio of Diffusion coefficient to mobility for electronics in feebly Attaching gases

Abstract

For a proper understanding of the mechanism of breakdown in gases, it is essential to have knowledge of the swarm coefficients, namely, the sparking potentials (V?), Townsend's primary (?/N) and secondary (?) ionization coefficients, electron attachment (?/N) coefficients, and the ratio of the diffusion coefficient to mobility (D/?) for electrons (this is a measure of the electron mean energy). The present investigation is mainly concerned with an accurate determination of these coefficients in nitrogen, air, and other feebly attaching gases such as carbon dioxide, ammonia, water vapour, chlorine, and their mixtures, over a wide range of E/N (electric field/gas number density) and the gas number density N. Although considerable work has been done to measure the swarm coefficients in nitrogen (N?) over a wide range of E/N, only recent studies have clearly demonstrated that the experimental parameters and the evaluation techniques impose significant limitations on the measurements, which become extremely important at higher values of E/N due to the onset of non-equilibrium ionization. Also, no reliable data on ?/N is available over the range 110 ? E/N ? 300 Td in this gas. Hence, extensive investigations have been carried out to measure the values of ?/N and ? over a wide range of E/N (i.e., 100 ? E/N ? 3000 Td) using the pressure variation technique. Similar studies have been carried out in dry air also, in order to investigate the presence of electron attachment in this gas over the lower range (106 ? E/N ? 151.5 Td). Sparking potentials (V?) have also been measured in both these gases up to N values of 212.2 × 10¹? cm?³ in nitrogen and 297 × 10¹? cm?³ in dry air. Investigations were extended to carbon dioxide (CO?) gas because of its importance as an insulating gas and also as a medium in high-power gas lasers. Swarm coefficients were measured over the range 75 ? E/N ? 5000 Td. Because of their importance in gas lasers and since no data is available, it was thought desirable to investigate (CO?–N?) gas mixtures. Hence, detailed investigations were carried out and the swarm coefficients were evaluated in these mixtures over the range 75 ? E/N ? 150 Td. Negative ion formation can be stabilized by the addition of CO? to dry air. Also, it was observed by Moruzzi and Price (1974) that addition of CO? to dry air up to 10 percent increases the breakdown voltage of the mixtures. The ?/N coefficients for CO? evaluated from a study of air–CO? mixtures were found to be substantially lower than the values obtained in pure CO? by earlier workers (e.g., Bhalla and Craggs, 1960). Also, the above investigations have been carried out only at a single E/N value of 106 Td. It was, therefore, decided to critically examine these aspects over a wide range of E/N (75 ? E/N ? 150 Td), with the percentage of CO? in air varying from 0 to 100 percent. There has been very little information available on the attachment properties of ammonia and water vapour. Recently, Parr and Moruzzi (1972) measured (?/N) in ammonia and this is the only data available in this gas. Although a few investigations have been carried out in water vapour (see Parr and Moruzzi, 1972), the data obtained has been very inconsistent. Hence, accurate measurements were made in both the above gases to determine the swarm coefficients. In spite of the large amount of work that has been done in sulphur hexafluoride (SF?) and Freon (CCl?F?), etc., the strongly attaching gases, there is very little information available in chlorine (Cl?). Hence, an attempt was made to determine ?/N and ? in this gas. In all the above gases, measurements have been made over the range 100 ? E/N ? 3000 Td, with gas densities varying from 1.65 × 10¹? cm?³ to 165 × 10¹? cm?³. Extensive calculations have also been made in all these gases to verify the validity of the various semi-empirical equations for ionization (?/N) and sparking potentials (V?) by comparing with the data obtained experimentally by the author. In the earlier work on the evaluation of V?, the effect of ? was completely neglected. In the present study, the equations have been suitably modified to take into consideration the effect of ? on V?. The results so obtained, in gases of interest to this thesis and also in other strongly attaching gases, have been presented (see also Risbud and Naidu, 1978). Similar calculations have also been made for ?/N and (D/?). In general, a very good agreement has been obtained between the theoretically calculated values and experimental values on these parameters over a wide range of E/N. The ratio of the diffusion coefficient to mobility (D/?) for electrons has been measured in nitrogen, dry air, carbon dioxide, and water vapour over the range 75 ? E/N ? 300 Td, with gas densities varying between 3.3 × 10¹? cm?³ and 16.5 × 10¹? cm?³, using the well-known Townsend–Huxley diffusion method. The limitations of this technique over the high range have been discussed. In addition, the ionization and attachment coefficients have also been evaluated from the measured current ratios. These investigations (measurements of D/?) have also been extended to CO?–N? air mixtures for various concentrations of CO? in N?/air over the range 75 ? E/N ? 150 Td. In both CO?–N? and CO?–dry air mixtures, the values of D/? are observed to be above the boundaries set by the pure gases. In the absence of independent data regarding the various inelastic processes and the form of electron energy distribution, it is not possible to interpret these data quantitatively. Most of the results reported in this thesis have been obtained for the first time. Extensive care has been taken to determine the various parameters most accurately. All the results obtained have been compared with the available data, wherever present, and were found to be in very good agreement.