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_alphaand H_betalines 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 r₀of the filamentary discharge is obtained. A value of the macroscopic field F_eis 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*10³A cm^-2, r₀approximately 20 mu m, N_eapproximately 10¹⁵cm^-3. Some indications concerning the primary streamer, the nature of the discharge plasma and the glow to arc transition are derived from these measurements.