Role of the Cathode Field Emission in the Streamer-Spark Transition

Abstract

In a point-to-plane gap in atmospheric air streamers propagate under a voltage pulse from the point anode into the low-field region at high velocities and under continuous branching. It was found that electron emission from the copper cathode occurs under the effect of the high field between the streamer tips and the cathode just before impact. Photographic evidence by means of paper films placed on the cathode with sensitive emulsion facing it showed that at the points where strong streamer tips approached the back side of the film great concentrations of minute dots appeared. These dots are attributed to electron avalanches produced by those electrons liberated from the cathode and accelerated towards the streamer tip. The liberation of electrons was ascribed to the high field intensities at the tips since radiation cannot penetrate the paper. To confirm this, the back side of the paper was blackened at spots with no change in pictures. To further substantiate this, a thin metallic layer was placed at the back of the paper. No electrons were liberated and no dots appeared opposite the metallic layer because asymmetrical fields cannot build up between the two plane surfaces. The avalanches between film and cathode became diffuse when the air gap was increased to 3 mm since they merged into each other. At a gap of 5 mm, only a few avalanches were observed, of which some formed negative streamers as evidenced by distinct negative Lichtenberg figures on the film.