The determination of basic quantities during glow-to-arc transition in a positive point-to-plane discharge

Abstract

The discharge is operated in oxygen with a 1 cm gap between a positive point and a plane at a pressure near 300 Torr. The discharge has been found to have a behaviour similar to that in atmospheric air. By introducing traces of hydrogen it has been possible to study the H alpha and H beta lines broadening which is mainly connected to the Stark effect. However, due to the presence of a macroscopic field the usual theory does not apply, i.e. it is shown that the Stark profile is related to the electron current density j rather than to the electron density N e . The current density j is derived and from total current i a characteristic radius r 0 of the filamentary discharge is obtained. A value of the macroscopic field F e is then determined from a numerical simulation of the discharge, leading in turn to the electron density. All these measurements are time resolved and are given as a function of time during the glow-to-arc transition. Typical values are j approximately 3*10 3 A cm -2 , r 0 approximately 20 mu m, N e approximately 10 15 cm -3 . Some indications concerning the primary streamer, the nature of the discharge plasma and the glow to arc transition are derived from these measurements.