Light impulses at the dark- to glow discharge transition in a low pressure point-to-plane gap

摘要

The transition from low pressure corona (dark discharge) to glow discharge is studied for nitrogen in the 1-10 mbar pressure range in DC positive point-to-plane geometry for various cathode materials. A systematic study of the spatial distribution of the discharge's light intensity, correlated with the current impulses recorded at the cathode, is performed by focusing on the slit of a photomultiplier the light emitted from small regions along the gap axis. For pressures higher than 2 mbar, the luminous activity of the discharge, amplified by a photomultiplier, is characterized by complex impulse waveforms. A study of these waveforms shows that a weak light peak initiated at the anode, propagates towards the cathode with increasing velocity and amplitude. Travelling slower than a streamer, this impulse may be attributed to a weak ionizing front accelerating in the middle of the gap. A second peak is superimposed in the anode-sided half interval, the interpretation of which, supported also by numerical simulations, could reveal the fast propagation of a new front. The influence of cathode material on this second peak shows that it is in fact linked to the secondary emission of the cathodic region. From the analysis of these results, the transition appears to be a complex physical process beginning by a weak ionizing wave starting at the anode and propagating towards the cathode. The arrival of this wave in the cathode region enhances the local field and contributes to the amplification of the cathodic secondary processes. Arriving in the anode region, the secondary electrons initiate a new ionizing wave, stronger than the first one, which crosses the gap very rapidly and establishes the glow discharge.