C. Postel

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.

Production and reactivity of the hydroxyl radical in homogeneous high pressure plasmas of atmospheric gases containing traces of light olefins

A photo-triggered discharge has been used to study the production kinetic mechanisms and the reactivity of the hydroxyl radical in a N2/O2 mixture (5% oxygen) containing ethane or ethene for hydrocarbon concentration values in the range 1000?5000?ppm, at 460?mbar total pressure. The discharge (current pulse duration of 60?ns) has allowed the generation of a transient homogeneous non-equilibrium plasma, and the time evolution of the OH density has been measured (relative value) in the afterglow (up to 200??s) by laser induced fluorescence (LIF). Experimental results have been explained using predictions of a self-consistent 0D discharge and plasma reactivity modelling, and reduced kinetic schemes for OH have been validated. It has been shown that recombination of H- and O-atoms, as well as reaction of O with the hydroperoxy radical HO2, plays a very important role in the production of OH radicals in the mixture with ethane. H is a key species for production of OH and HO2 radicals. As for ethane, O, H and HO2 are key species for the production of OH in the case of ethene, but carbonated radicals, following the partial oxidation of the hydrocarbon molecule by O, also play a non-negligible role. The rate constant for O- and H-atom recombination has been estimated to be 3 ? 10?30?cm6?s?1 at near ambient temperature, consistent with LIF measurements on OH for both mixtures with ethane and ethene.

Removal of formaldehyde in nitrogen and in dry air by a DBD: importance of temperature and role of nitrogen metastable states

The removal of traces of formaldehyde (150?ppm) in nitrogen and in dry air, at atmospheric pressure, by the filamentary plasma of a dielectric barrier discharge (in a cylindrical geometry) energized by a HV pulse is experimentally studied, at ambient temperature (20??C) and at 300??C. It is found that the pollutant molecule is more efficiently removed in nitrogen than in air at 20??C, whereas it is the opposite at 300??C. In air, the removal of CH2O strongly increases when the temperature increases. This effect also occurs in nitrogen, but it is less important. A qualitative explanation for these results can be found in the competitive influence of quenching collisions of the nitrogen metastable states by formaldehyde and oxidation reactions of this molecule.

Detailed Characterization of 2-Heptanone Conversion by Dielectric Barrier Discharge in N2 and N2/O2 Mixtures

The products of 2-heptanone conversion by dielectric barrier discharge plasma are analyzed under different conditions: alternating current (ac) or pulsed mode of excitation, variable energy, variable composition of the carrier gas. The efficiency of the conversion is higher using a pulse excitation mode than an ac mode. With a small oxygen percentage (about 2-3%) added to nitrogen, 2-heptanone is about 30% more efficiently removed than in pure nitrogen, while the 2-heptanone removal decreases with an oxygen percentage higher than 3%. A new analysis method, based on chemical ionization mass spectrometry, is used for volatile organic compound detection along with chromatography. Several products issued from 2-heptanone conversion with ac excitation are identified in nitrogen and in air, and a chemical scheme is proposed to explain their formation and their treatment by the discharge. It appears that byproducts are issued not only from oxidation reactions, but also from C-C bond cleavage by collisions with electrons or nitrogen excited states.

OH kinetic in high-pressure plasmas of atmospheric gases containing C2H6 studied by absolute measurement of the radical density in a pulsed homogeneous discharge

The absolute value of the hydroxyl radical was measured in the afterglow of an homogeneous photo-triggered discharge generated in N2/O2/H2O/C2H6 mixtures, using a UV absorption diagnostic synchronized with the discharge current pulse. Measurements show that OH is efficiently produced even in the absence of water vapour in the mixture, and that the radical production is closely linked to the degradation kinetic of the hydrocarbon. Experimental results for dry mixtures, both for OH and for the removal of ethane in the discharge volume, are compared with predictions of a self-consistent 0D discharge and the kinetic model. It appears that the oxidation reaction of the ethane molecule by O(3P) atoms plays a minor role. Dissociation of the hydrocarbon through quenching collisions of the nitrogen metastable states are of great importance for a low oxygen concentration value. Also, the oxidation of ethane by O(1D) cannot be neglected at high oxygen concentration. The most probable exit channel for N2 states quenching collisions by ethane is the production of ethene and hydrogen molecules. Afterwards C2H4 should be dissociated to produce H and H2. As previously suggested from the study of the OH density time evolution in relative value, the recombination of H and O atoms appears as a main process for the production of OH in transient low temperature plasmas generated in atmospheric gases at high pressure. Another important reaction is the reduction of the HO2 radical by O, this radical coming from the addition of H on the oxygen molecule. H atoms come from numerous kinetic processes, amongst which is the dissociation of ethene.

Corona-plasma triggered pseudospark discharges

This paper reports on a novel pseudospark triggering technique using a corona-plasma electrode to feed initial electrons into one of the pseudospark hollow electrodes. This corona-plasma trigger can be used either at the cathode or anode side of the pseudospark, and it can be connected as well to the high voltage electrode as to the grounded one. With this trigger scheme, a pseudospark switch working at pulse repetition frequencies (PRF) up to 100 Hz has been demonstrated in a continuous mode of operation, while PRF as high as 1 kHz has been achieved in a burst mode. The cumulative number of pseudospark discharges triggered with this technique is already above 5*10/sup 7/, and this corona-plasma triggered pseudospark is still in operation without performance degradation. The main advantage of this pseudospark triggering technique lies in its simultaneous simplicity and reliability allowing high repetition rate, long lifetime, and the complete absence of keep-alive electrode and standby power consumption. >

NO removal in a photo-triggered discharge reactor

This paper presents the first study of a new type of reactor, the so-called photo-triggered reactor, for gas cleaning by non-thermal plasmas. Compared to the classical discharges used for this application, it allows the production of large volumes of uniformly excited plasmas without filamentary structures and glow to arc transition. Experimental and modellization studies of a 50 cm3 photo-triggered reactor have been performed. Chemical efficiency and specific energy consumption for NO removal in N2/O2/NO mixtures have been determined. It is shown that the NO removal efficiency is a sensitive function of the input energy but does not depend on the initial reduced electric field, (E/N)0, for values higher than 200 Td. Depending on mixture composition, NO destruction efficiencies between 30 and 100% are achieved in only one shot with energetic costs in the range 15 to 50 eV per removed NO molecule (22-75 g [NO] kW-1 h-1) for input specific energy lower than 150 J l-1. Moreover, in dry air, an ozone production efficiency as high as 64 g [O]3 kW-1 h-1 (28 eV mol-1) is obtained, demonstrating the excellent capability of the photo-triggered discharges to produce large amounts of atoms and radicals which in turn can reduce or oxidize the pollutant molecules. All these results point out the effectiveness of the photo-triggered discharges to homogeneously produce large volumes of plasma which could be efficiently used for gas cleaning applications.

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.

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.

Investigations of a photo-triggered HF laser

X-ray photo-triggered discharges have been used to energetize Ne/SF6/H2 and Ne/SF6/C2H6 gas mixtures. For a discharge volume of 312 cm3 an output energy as high as 3.0 J has been obtained in the ethane mixture, while the best laser performance achieved in the hydrogen mixture is only 1.8 J. The physical reasons which could explain these differences, i.e. different kinetic pathways leading to the formation of the HF molecule, or the onset of discharge instabilities, have been investigated. It is shown, experimentally and theoretically, that the production of atomic fluorine weakly depends on the gas mixture type. A time resolved imagery of the interelectrodes space emphasized that the Ne/SF6/H2 active medium degenerates into a spatially inhomogeneous plasma, whereas a very stable homogeneous discharge is obtained in the Ne/SF6/C2H6 mixture. As a result the onset of a discharge instability is responsible for the laser emission collapse at low hydrogen partial pressure and high initial applied electric field values.