The present work is devoted to the comparison between numerical and experimental determination of the velocity profile of an ionizing front (primary streamer) in a DC positive point-to-plane corona discharge in dry air at atmospheric pressure. The inception and propagation of the ionizing front is simulated by a one-dimensional model, using finite differences in a flux-corrected transport numerical scheme, including gamma -effects, and using experimental results concerning the swarm parameters. This model provides the spatio-temporal local field and charge density variations as well as the ionization front velocity. An optical measurement of the velocity is performed with the same discharge parameters, using a photomultiplier and a single-slit device. The technique is based on the experimental fact that, for a 1 cm gap in the 7-9 kV voltage range, the successive primary streamers corresponding to a given gap voltage display identical velocity profiles. As a result of the comparison, it appears that a precise coupling between simulation and experiment is possible. There is a voltage range (8-9 kV) within which good agreement is observed. The front velocity in most of the gap is about 2*107 cm s-1 and the profile presents an increase when the streamer leaves the point electrode and when it reaches the cathode. The possible mechanisms of these accelerations are discussed. The model may be applied to a large variation range for various parameters such as the nature of the gas, pressure, inter-electrode gap and curvature radius of the active electrode.

No abstract

The temperature distribution of nitrogen neutral molecules in a secondary streamer discharge in a point-to-plane air gap is determined by using emission spectroscopy. A relatively high neutral gas temperature ( approximately 800 K) was measured in the vicinity (0.5 mm) of the point electrode. For greater distances the temperature decreases and remains practically constant ( approximately 450 K). These results, combined with others obtained by simulation, lead the authors to a quantitative description of the secondary streamer propagation and to a satisfactory approach to electric wind formation. The electric wind velocity evaluated in the middle of the gap ( approximately 30 m s-1) is in a fairly good agreement with the literature.

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.

A two-dimensional numerical model for the filamentary mode of a dielectric barrier discharge (DBD) is presented. Special attention is paid to the arrangement of electrodes with the dielectric on the cathode. The gas is nitrogen at atmospheric pressure and the discharge gap is equal to 1 mm. The model shows that the discharge develops in two phases. There is first the formation and propagation of an ionizing wave across the discharge gap. Then, as a result of the interaction of the filament with the dielectric, a surface streamer is formed on the dielectric. The propagation of this surface streamer is correlated with the further charging of the dielectric and its characteristics are analysed thoroughly.