The mechanism of spark breakdown in air at atmospheric pressure between a positive point and a plane. I. Experimental: Nature of the streamer track

The behavior of a point to plane discharge has been analyzed electrically and optically for centimetric gaps in air at atmospheric pressure. This shows that for both steady and pulsed applied potentials the evolution always consists of the sequence: primary streamer, secondary streamer, dark space (more or less pronounced), transient arc. Experiments are presented that are directed towards obtaining an indication of the physical meaning of these different phases. A time-resolved spectroscopic analysis leads to an evaluation of the electron energy within the secondary streamer of ∼1.4 eV. This indicates that the secondary streamer does not correspond to a very strong ionizing region. Three significant observations have been obtained using a device that is able to distinguish between conduction and displacement currents: (i) the secondary streamer occurs within a conducting filament built up by the primary streamer, (ii) when the primary streamer arrives at the plane, a cathode region appears giving continuity between the collected current at the plane and that flowing in the conducting filament, and (iii) there is a strong injection of negative charge into the gap from this cathode region. Finally, by varying the oxygen partial pressure, it is shown that the current within the discharge is mainly due to electrons. These results lead to a model of the discharge evolution in which the filament track of the primary streamer can be considered as a transient positive column of a glow discharge, whose cathode region is formed when the primary streamer arrives at the cathode. The secondary streamer thus appears to be the luminosity of a positive column.