Pulse Technique for Probe Measurements in Gas Discharges

Abstract

A pulse technique for making Langmuir probe measurements in gas discharges has been developed. The basic procedure consists in operating the probe 40–50 v negative with respect to the discharge, and pulsing it positive at a repetition rate of 60 cps. The probe current-voltage characteristic during the positive-going pulse is presented on an oscilloscope and the resulting trace is photographed. The phase of the pulse may be varied, so that the method is useful for studying 60 cps ac discharges as well as dc discharges. Positive-ion bombardment while the probe is strongly negative tends to sputter away contamination and make it possible to keep the probe clean while making measurements within one mm of an oxide cathode at 1000°C, despite rapid evolution of contaminants from the cathode. Together with the small size of the probes that can be used, this permits meaningful measurements to be made in the negative glow of an oxide-cathode low-pressure discharge of the fluorescent-lamp type. Measurements on such lamps are described which demonstrate the presence of energetic primary electrons from the cathode as well as secondary electrons in the negative glow.