Pierre Tardiveau

Diffuse mode and diffuse-to-filamentary transition in a high pressure nanosecond scale corona discharge under high voltage

The dynamics of a point-to-plane corona discharge induced in high pressure air under nanosecond scale high overvoltage is investigated. The electrical and optical properties of the discharge can be described in space and time with fast and precise current measurements coupled to gated and intensified imaging. Under atmospheric pressure, the discharge exhibits a diffuse pattern like a multielectron avalanche propagating through a direct field ionization mechanism. The diffuse regime can exist since the voltage rise time is much shorter than the characteristic time of the field screening effects, and as long as the local field is higher than the critical ionization field in air. As one of these conditions is not fulfilled, the discharge turns into a multi-channel regime and the diffuse-to-filamentary transition strongly depends on the overvoltage, the point-to-plane gap length and the pressure. When pressure is increased above atmospheric pressure, the diffuse stage and its transition to streamers seem to satisfy similarity rules as the key parameter is the reduced critical ionization field only. However, above 3 bar, neither diffuse avalanche nor streamer filaments are observed but a kind of streamer–leader regime, due to the fact that mechanisms such as photoionization and heat diffusion are not similar to pressure.

Experimental and numerical study of the propagation of a discharge in a capillary tube in air at atmospheric pressure

This paper presents an experimental and numerical study of a pulsed air plasma discharge at atmospheric pressure propagating in a capillary glass tube. In this work, we have compared the discharge structures and the axial propagation velocities of discharges. First, we have studied a needle-to-plane configuration without tube. For applied voltages in the range 7–18 kV, we have observed in experiments and in simulations that a plasma ball starts to develop around the needle tip. Then, for applied voltages less than 14 kV, in experiments, the discharge rapidly splits into several streamer channels with a main axial streamer. In simulations, we have computed only the main axial discharge. For applied voltages higher than 14 kV, in experiments and in simulations, we have observed that the discharge propagates with a cone shape in the gap. For all studied voltages, a good experiment/modelling agreement is obtained on the axial propagation velocity of the discharge, which increases with the applied voltage. Then, we have studied the propagation of discharges inside capillary tubes with radii in the range 37.5–300 µm. In experiments and simulations, we have observed that for small tube radius, the discharge front is quite homogeneous inside the tube and becomes tubular when the tube radius increases. Experimentally, we have observed that the velocity of the discharge reaches a maximum for a tube radius slightly less than 100 µm. We have noted that for a tube radius of 100 µm, the discharge velocity is three to four times higher than the velocity obtained without tube. This clearly shows the influence of the confinement by a capillary tube on the discharge dynamics. In this work, we have only simulated discharges for tube radii in the range 100–300 µm. We have noted that both in experiments and in simulations, the velocity of the discharge in tubes increases linearly with the applied voltage. As the radius of the tube decreases, the discharge velocity derived from the simulations slightly increases but is less than the experimental one. We have noted that the discrepancy on the discharge velocity between experiments and simulations increases as the voltage increases.

Sub-nanosecond time resolved light emission study for diffuse discharges in air under steep high voltage pulses

Pin-to-plane discharges in centimetre air gaps and standard conditions of pressure and temperature are generated under very high positive nanosecond scale voltage pulses. The experimental study is based on recordings of sub-nanosecond time resolved and Abel-processed light emission profiles and their complete correlation to electrical current waveforms. The effects of the voltage pulse features (amplitude between 20 and 90 kV, rise time between 2 and 5.2 ns, and time rate between 4 and 40 kV ns−1) and the electrode configuration (gap distance between 10 and 30 mm, pin radius between 10 and 200 µm, copper, molybdenum or tungsten pin material) are described. A three time period development can be found: a glow-like structure with monotonic light profiles during the first 1.5 ns whose size depends on time voltage rate, a shell-like structure with bimodal profiles whose duration and extension in space depends on rise time, and either diffuse or multi-channel regime for the connection to the cathode plane according to gap distance. The transition of the light from monotonic to bimodal patterns reveals the relative effects and dynamics of streamer space charge and external laplacian field. A classical 2D-fluid model for streamer propagation has been used and adapted for very high and steep voltage pulses. It shows the formation of a strong space charge (streamer) very close to the pin, but also a continuity of emission between the pin and the streamer, and electric fields higher than the critical ionization field (28 kV cm−1 in air) almost in the whole gap and very early in the discharge propagation.

ICCD Camera Imaging of Discharges in Porous Ceramics

The microdischarges generated inside porous ceramics represent a novel way to create stable atmospheric pressure plasmas. Hybrid plasma-catalytic system utilizing ceramics loaded with catalysts may be very effective for flue gas treatment. This paper presents the images of the discharges in porous ceramics generated by an ac high voltage and visualized by intensified charge-couples device (ICCD) camera system.

Large Conical Discharge Structure of an Air Discharge at Atmospheric Pressure in a Point-to-Plane Geometry

The experimental and simulated optical emissions of an air discharge at atmospheric pressure propagating in a point-to-plane geometry with a sharp point and a high overvoltage are compared. A conical discharge structure is observed. A good agreement on the maximum diameter of the discharge and its propagation velocity is obtained.

Nanosecond Scale Discharge Dynamics in High Pressure Air

The use of pulsed nanosecond scale discharges is promising for automotive engine ignition because air-hydrocarbon mixtures can be chemically activated with a nonthermal plasma. Very short high voltage pulses are a good way to control the energy which is transferred into the gas at pressures above atmospheric. The development of such a discharge, in a point-to-plane configuration, under a short and high overvoltage, is investigated in air for different pressures through fast imaging and electrical records.

Propagation of an Air Discharge at Atmospheric Pressure in a Capillary Glass Tube: Influence of the Tube Radius on the Discharge Structure

The experimental and simulated optical emissions of an air discharge at atmospheric pressure propagating in a capillary glass tube are compared for two tube radii: 100 and 300 μm. For 300 μm, both experimental and calculated optical emissions show a tubular structure of the discharge. For 100 μm, the emission of the discharge appears to be more homogeneous radially.

Towards a kinetic understanding of the ignition of air-propane mixture by a non-equilibrium discharge: the decomposition mechanisms of propane

The decomposition of propane in non-thermal plasmas of N2/C3H8 and N2/O2/C3H8 mixtures (oxygen percentage up to 20%) at low temperature is studied in a photo-triggered discharge. Quenching of nitrogen metastable states dissociate C3H8 to produce propene and hydrogen. Oxidation reactions are growing in importance when the O2 concentration increases, but the dissociation quenching reactions still occurs for the air-based mixture. Even for a low concentration of oxygen, OH is an important specie involved in the conversion of the hydrocarbon. A kinetic analysis emphasises that OH comes in great part from the production of H, in which the methyl radical plays a role, strengthening the role of the dissociation processes of propane and propene in the medium reactivity. Results of PLIF measurements performed on OH during the diffuse afterglow of a nanosecond corona discharge correlate with results obtained on the photo-triggered discharge.

Filamentation of a Nanosecond Pulse Corona Discharge in Air–Propane Mixtures at Atmospheric Pressure

The space and time development of a single nanosecond pulsed corona discharge is studied in atmospheric air-propane mixtures with different concentrations of propane up to 8%. Time-resolved imaging of the discharge shows a complete diffuse pattern in pure dry air, which becomes more and more filamentary when propane is added. Filaments are thinner with higher concentrations of propane, and the whole discharge energy increases and gets saturated.

Optical and electrical characterization of pulsed dielectric barrier discharges in heterogeneous structures

In the context of expanding plasma-catalytic cleaning techniques where discharges develop inside heterogeneous catalytic supports like packed beds or honeycomb monoliths, the understanding of streamers propagation physics in such nonhomogeneous media is of great interest. For that purpose, have been compared pulsed discharges inside two identical plane-to-plane dielectric barrier devices, one of them being coupled with a honeycomb monolith of cordierite. Electrical records and corresponding images of plasma properties in both these reactors are presented.