The electro-dynamical properties and the mutual influence of several simultaneously propagating streamers are investigated in a point-to-plane configuration in air at atmospheric pressure under impulse voltage. When a fast and short high voltage pulse is applied to the point, a bunch of streamer branches develops into the interval. Since no previous ionized path has been traced before in the gap, streamers spread in the whole gap, branching while propagating. This behaviour is quite different from streamers under DC voltage conditions, which successively extend on the point–plane axis. The aim of this work is an attempt to distinguish the properties of an individual streamer simultaneously propagating with the others, by means of fast techniques based on simultaneous streak and still photographs and space-time decoupled currents measurements. Especially, their mean velocity can be estimated. In most cases, an axial streamer is observed, propagating faster than the others. This result can be interpreted in terms of the electric field space distribution by means of a three-dimensional electrostatic solver. This analysis shows that a mutual electro-dynamical influence is exerted from one streamer on another.

Discharges at several atmospheres are widely used for triggering combustion in cars. However, their properties are not well understood since the size and duration of the discharges become very small at high pressures, which makes the experimental approach more difficult. One possibility should be to infer the discharge properties at high pressures from those at atmospheric pressure, provided that the so-called discharge similarity rules work correctly. Thus, an attempt to check this validity is provided in this paper. Then, after establishing the exact conditions which preserve pressure similarity rules in non-uniform gaps, an example is given of two discharges under similar conditions. A departure from similarity is quite clearly observed. At several atmospheres, the streamer induced plasma channel, instead of developing the usual streamer filaments seen at atmospheric pressure, demonstrates a leader structure, i.e. the formation of a hot channel before the whole gap is bridged. Finally, reasons are given explaining that such a fast rise in temperature with pressure is indeed to be expected.

Experimental and modelling studies of ion formation during combustion of propane/air mixtures are presented. The positive and negative ions mass/charge spectra in propane/air stoichiometric flame at atmospheric pressure are recorded in the range from 0 to 512 atomic mass units. The C 2 H 3 O + and HCO − 2 ions are found to be the most abundant ionic species in the flame front region. By increasing the distance from the flame front the ion composition changes significantly. In the burnt gas region the H 3 O + , NO + , CO − 3 , HCO − 3 ions are found to be the major charged species. To explain the experimental results the extended kinetic model describing the ion formation in flame and in the extraction system of the mass-spectrometer as well as ion-soot interaction is developed. It is shown that the ionic clusters, which are observed experimentally, form during the adiabatic expansion in the extraction system, and the presence of soot particles may change the total positive and negative ion concentrations in the gas phase.

Electrical and thermal properties of a single electrode configuration corona discharge generated under radiofrequency high voltage inside an open air gap at pressures above 1 bar is investigated. Time-resolved imaging of the discharge shows a four-step development of the discharge at atmospheric pressure starting by streamers' inception and propagation, evolving in heating waves and stabilizing in a stationary regime until the power supply is switched off. The mean gas temperature reaches about 1700 K in tens of microseconds with electrical energy release around tens of millijoules. Heating has been attributed to ion collisions and excited species relaxation, promoted by the successive time periods of the power supply. At higher pressures, beyond 3 bar, this behaviour changes and heating occurs at the same time as the discharge propagates. It leads to hot channels which constrict near the electrode as long as the voltage pulse is applied. Temperature gets higher and saturates at 2600 K whatever the voltage and the pressure. Considering the change in the electrical energy density released within the plasma channels with pressure and voltage, temperature saturation seems to be an effect of heat confining within the channels due to pressure. The large and non-thermal plasma generated by the RF corona discharge is a very good candidate for car engine lean mixtures ignition issues.

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