Swarm coefficients and the ratio of diffusion coefficients of mobility for electronic in Gases

Abstract

The use of ever-increasing voltages for transmission of electrical power and nuclear particle accelerators continues to stimulate research into the fundamental aspects of electrical breakdown in gases. A knowledge of primary and secondary ionization coefficients (? and ?), attachment and detachment coefficients (? and ?), and electron mean energies (??/M) is required for a proper understanding of the various breakdown processes. This thesis is mainly concerned with the accurate determination of these coefficients in gases using high vacuum techniques and controlled experimental conditions.

1. Measurement of Sparking Potentials and Ionization Coefficients in Dry Air at High Values of E/N In recent years, considerable data have become available on the growth of ionization currents in dry air at low values of E/N (E is the electric field strength and N the gas number density) ? 19.7 × 10?¹? V·cm². A survey of literature shows that reliable data are not available in dry air for the coefficients ?/N and ? over the range 14.1 × 10?¹? ? E/N ? 282.5 × 10?¹? V·cm². The earlier results of Sanders, Masch, and Townsend which covered this range of E/N were obtained in air which was contaminated with mercury vapour. Hence, the measurements of ?/N and ? were carried out in dry air over a wide range of pressure (0.3 to 40 torr) and E/N (mentioned above) using an ionization chamber completely free from even small traces of mercury. The ionization currents were measured with increase of pressure at various constant values of E/N at constant electrode separation (d) in a uniform field. Measurements of the sparking potentials showed that Paschen's law is not obeyed in dry air at values of Nd near and below the Paschen minimum. Townsend's primary ionization coefficient ?/N was found to depend on E/N only, and a general increase of ? with E/N was observed. ? was evaluated for aluminium and gold-plated cathodes at various values of E/N.

2. Measurement of Sparking Potentials, Ionization and Attachment Coefficients in Freon and Mixture of Freon and Air Ionization and attachment coefficients were also determined in dichlorodifluoromethane (Freon-12) and mixture of Freon and air over a range of E/N: 28.3 × 10?¹? ? E/N ? 310 × 10?¹? V·cm². Measurement of sparking potentials were made in Freon and Freon-air mixtures over an Nd range: 38 × 10¹? ? Nd ? 128 × 10¹? cm?². Freon was found to withstand much higher voltage (more than double) than air. Further, a small percentage of Freon in air produced a large rise in electrical strength of the gas, indicating practical advantages of such mixtures. Attachment coefficients were found to increase in air with the addition of increasing percentages of Freon. Both ionization and attachment coefficients were found to depend on E/N only. The critical E/N (where ? = ? and below which breakdown cannot occur irrespective of Nd) was found to increase from 10.8 × 10?¹? V·cm² at 100% air to 37.8 × 10?¹? V·cm² at 100% Freon.

3. Measurement of the Ratio of Diffusion Coefficient (D) to Mobility (?), D/?, for Slow Electrons in Gases Using Huxley's solution of the diffusion equation for electron-attaching gases, the ratio of diffusion coefficient to mobility (D/?) for electrons in nitrogen and dry air was measured over the range 3.06 × 10?¹? ? E/N ? 76.6 × 10?¹? V·cm². The current ratios R in the above equation were computed for various values of ? and ? (? = W/D, where W is the drift velocity of electrons and D the diffusion coefficient). These values can be used for the determination of D/? and ? in any electron-attaching gas. The experimental apparatus used was similar to that used originally by Townsend and modified by Huxley and his collaborators. The results in nitrogen and dry air agreed with those of earlier workers who used the diffusion equation for non-attaching gases. Factors influencing the accurate determination of D/? were also discussed.