DETERMINATION OF BASIC QUANTITIES DURING GLOW-TO-ARC TRANSITION IN A POSITIVE POINT-TO-PLANE DISCHARGE

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 Halphaand Hbeta 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 Ne. The current density j is derived and from total current i a characteristic radius r0 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*103 A cm-2, r0 approximately 20 mu m, Ne approximately 1015 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.