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 disp lay a positive slope of dV/dI in the low-current part of the voltage-curren t characteristic curves. Whereas the visual aspect is a tiny luminous regio n 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 gl ow' regime with whole gap illumination. In nitrogen, and also in dry air, the current in the dark discharge regime is characterized by a low de component upon which oscillations are superimp osed. These oscillations have been carefully studied, varying parameters su ch 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 oscillatio ns should not be attributed only to the classical ion drift mechanism which dominates the high-pressure corona glow. A mechanism involving double-laye r formation and fluctuation is proposed to explain the discharge behaviour in this low-current domain where the applied voltage is not sufficient to c ause the transition to a classical glow discharge.