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.

A laser perturbation of a positive corona discharge is performed in ambient air. This latter is a self-sustained discharge composed of recurring current pulses resulting from propagation of an ionizing potential wave through a streamer process. The period T of the regime is about 100-120 mu s. The laser shot occurs between two successive recurring pulses at a given time tau , referred to the first recurring pulse, and at a given position X on the gap axis, referred to the plane. The laser is an excimer one (XeCl at 308 nm). The power density at the focal point is about 2.4*103 MW cm-2, so that no optical breakdown occurs. Nevertheless, we observe that the laser shot induces a streamer type pulse: depending on values of both tau and X, this streamer travels through the whole gap or only part of it, and the next recurring current pulses of the discharge occur later, their regime being undisturbed. Then, critical values tau c and Xc are defined. We observe that laser perturbation first propagates from X towards the rod, and then the cathode-directed streamer develops from the rod. Assuming that the laser shot creates primary electrons through a multiphoton effect, these photoelectrons drift from the X position towards the anode until their density is sufficient to allow propagation of the streamer.