A study of the light emitted from the initial stages of a spark discharge

Abstract
Measurements have been made, by means of submicrosecond impulses, on the threshold voltage at which the first visible discharge occurs in a homogeneous field electrode gap. Various gases have been used at different values of gap and gas pressure. In most gases there is some indication of a change in the form of the initial discharge as the gap is varied. At a certain value of electrode gap, depending upon the impulse length and gap pressure, there is a change in slope of the threshold voltage-gap curve. Above this point the discharges are generally filamentary whereas at smaller gaps they are broad and diffuse. In air, nitrogen and oxygen the filamentary discharges are often constricted at a point in the gap. The position of this constriction depends on the impulse and gap length. It is suggested that the filamentary discharges correspond to a streamer type of discharge and are similar to the ones observed in the previous cloud chamber studies. The broad diffuse discharge which occurs at smaller gaps is probably the Townsend type of discharge In argon and hydrogen the evidence for the existence of two breakdown mechanisms is not very strong. High-speed streak photography has revealed that the filamentary glow discharges in air appear to grow towards the constriction. A spark channel then begins to form at the constriction and grows towards the electrodes. Spectroscopic examination of the various types of discharge shows the prominent second positive N2 bands for discharges in air and nitrogen but with a continuum, N+2 bands and atomic lines in the constriction. An estimate of the peak electron energy has been made for various discharges in air and nitrogen and found to be about 14eV.