In nitrogen, for pressures below 200 Torr, the anodic glow is characterized by current oscillations, superimposed on a dc 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.

Dc positive point-to-plane discharges in the 1-100 Torr pressure range display a positive slope of dV/dI 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.

The present work is a part of a general study dealing with pure gases at pressures in the 50-760 Torr range. This experimental study concerns discharges occurring in oxygen, between a 100 µm curvature radius rhodium point and a stainless steel plane 1 cm distant. A positive dc high voltage is applied to the point using a 0-30 kV, 0-10 mA power supply. Electrical measurements are performed so that voltage-current characteristic curves can be drawn for the whole pressure range. Current impulses are recorded at the earthed plane in each typical regime of the discharge. These electrical impulses are compared with the simultaneous optical signals obtained from a photomultiplier. A focalization on the photomultiplier of various precise spots in the interelectrode gap allows one to get some information about the propagation of an ionizing front from the high-voltage electrode to the grounded plane. From comparison with previous studies, it may be inferred that the glow regime is more difficult to visualize on the characteristic curves than it is in the case of nitrogen. However, 'streamer-like' impulses are observed for oxygen in the whole pressure range, which is not the case for nitrogen. The continuity between these streamer-like impulses and the 'classical' streamers observed at atmospheric pressure seems to be established and confirms the important role played by electronegativity in the inception of ionizing fronts.

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