Current and light waveforms associated with the dark- to glow-discharge transition in medium- and low-pressure point-to-plane gaps

In nitrogen, for pressures below 200 Torr; the anodic glow is characterized by current oscillations, superimposed on a de component. These oscillations may be attributed to fluctuations of the space charge, structured as a double sign sheath (double layer). The transition from a dark to a glow discharge can occur either directly from the oscillating state or, in the 20-200 Torr pressure range, through the appearance of a particular regime characterized by recurrent impulses. Both cases are considered here in the case of pure nitrogen, and the influence of pressure and gap length on the current and light waveforms is studied. An analysis of the light emitted by the discharge shows that a luminous structure, formed in the anode region, propagates towards the cathode, and travels a few millimetres before being absorbed by the sheath structure. This may be interpreted as the beginning of an ionizing front, which cannot propagate and is choked by the double layer. For higher applied voltages, the double layer cannot be maintained, and the current limiting effect seems to be suppressed during the time for charge evacuation; the current and light impulses corresponding to this latter case are then integrated into the much larger ones which characterize the glow discharge.