Gas discharges confined by metallic walls

Abstract

This thesis describes experimental and theoretical aspects of low pressure gas discharges confined by metallic walls. Simple considerations predict that the current between the anode and the cathode of a discharge tube which contains isolated cylindrical metal segments is conducted through the metal. Under certain conditions, however, a glow discharge may form along the axis of the tubes conduction occurs along the path which results in the lowest sustaining VO 1tage. The energy losses at insulating and conducting walls in the region of the positive column are investigated theoretically for rare gas discharges at high current densities. The energy losses are found to be greater when the walls are conducting, resulting in a higher axial electric field. A possible criterion for the maximum length of a metal segment is proposed. The maximum length may be defined as the length at which the radial electric field in the positive ion sheath at one end of a metal segment is equal to the breakdown field. A relationship between the maximum length of a metal segment and the tube radius and pressure has been derived. The form of this relationship agrees qualitatively with experimental results. Measurements of the anode-cathode breakdown voltage of a gas in a tube containing metal segments of small bore are presented. The anode-cathode breakdown voltage is found to be approximately equal to the sum of the breakdown voltages of the inter-segment and electrode-segment gaps. When the system is operating on the left of the Paschen minimum, the anode-cathode breakdown voltage may be increased by the addition of metal segments in such away as to reduce the effective gap. Individually the product's of pressure and inter-segment gap (pd) are less than the product (pd) min at the Paschen minimum. Hence, the anode-cathode breakdown voltage will increase as the number of gaps (and segments) increases and pd decreases. Discharges in dual-bore (alternate narrow and wide bore) metal tubes have been investigated. The anode-cathode breakdown voltages and sustaining voltages of low current density discharges, confined by dual-bore tubes of various lengths, are shown to depend upon the system geometry. A laser system using dual-bore metal discharge tubes, has been designed and tested for producing high current densities in rare gas-metal vapour mixtures.