Anodic glow and current oscillations in medium- and low-pressure dark discharges

Dc positive point-to-plane discharges in the 1-100 Torr pressure range display a positive slope of d V /d I in the low-current part of the voltage-current characteristic curves. Whereas the visual aspect is a tiny luminous region close to the point (anodic glow) the gap remains dark. The anodic glow is still present at higher currents, and is superimposed in the `classical glow' regime with whole gap illumination. In nitrogen, and also in dry air, the current in the dark discharge regime is characterized by a low dc component upon which oscillations are superimposed. These oscillations have been carefully studied, varying parameters such as pressure, applied voltage, gap length and the tip curvature radius of the stressed electrode. From the analysis of the current (amplitude, frequency and waveform) and of the optical signals coming from precise places of the interelectrode gap (inside or outside the anodic glow), it can be deduced that these oscillations should not be attributed only to the classical ion drift mechanism which dominates the high-pressure corona glow. A mechanism involving double-layer formation and fluctuation is proposed to explain the discharge behaviour in this low-current domain where the applied voltage is not sufficient to cause the transition to a classical glow discharge.