Ion sampling from the negative glow plasma in a cylindrical hollow cathode

Abstract

A new hollow-cathode design is described which is used for the continuous recording of radial density distributions of individual ion species in the negative glow region of a cylindrical hollow-cathode d.c. discharge. The ions effuse through a sampling probe P of 1 mm diameter, the orifice diameter being 20–50 μm. The radial density distributions of individual ion species in the negative glow region may be determined directly from the effusive ion currents i( r) if the probe potential is equal to the space potential. Due to elementary processes in the negative glow region, e.g. ion—molecule reactions, secondary and tertiary ions are formed whose density distributions over the radius differ appreciably from the distribution of primary ions produced by electron impact. The measurement method described in the present paper is the basis of the determination of reaction-rate constants from the radial ion-density distribution in the negative glow region of a cylindrical hollow-cathode d.c. discharge. A first evaluation of this method has already been given elsewhere. Further applications of the present measurement method include the recording of the radial field-strength distribution in the transition zone between negative glow and dark space near the glow edge.