Diffuse and constricted glow discharges in SF6

Abstract

SF6 glow discharges have been investigated for currents up to 20 mA, in the pressure range 0.1 to 10 Torr. As the pressure is increased, the discharge changes from an abnormal glow with a diffuse positive column to a normal glow with a constricted, positive column. The diffuse discharges have positive voltage-current characteristics, while the characteristics of the constricted discharges are negative. From measurements of axial potential distribution and radial profiles of gas temperature in the positive column, radial profiles of E/N (electric field/gas number density) were determined. E/N falls off from the axis to the wall, and the profiles show that the radial limit of the positive column for both diffuse and constricted discharges corresponds closely to the position where E/N=(E/N)c, the value (362 Td) where the rates of ionisation and attachment in SF6 are the same. The E/N measurements have been used, together with established electron and ion transport data for SF6 and a set of continuity equations for the electrons and ions, to determine radial density profiles for electrons, positive ions and negative ions in several SF6 diffuse and constricted positive columns.