S. Pasquiers

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.

Self‐consistent modeling of surface wave produced discharges at low pressures

A self‐consistent theory of a low‐pressure gas discharge sustained by a surface wave (SW) is presented which provides a complete description of the plasma density (ne) and SW‐field distribution both in the radial (r) and the axial (z) directions. The theory is based on a complete set of equations including Maxwell’s equations and the boundary conditions for the SW‐field, the electron Boltzmann equation which yields local collisional and transport data versus the SW‐electric field E, and the continuity and momentum transfer equations for the electrons and the ions. For given operating frequency, gas density, setup dimensions and total incident power Pi(0), the theory enables the determination of: (i) the SW‐dispersion relation; (ii) ne(r,z) and E(r,z); (iii) Pi(z); and (iv) θ(z), the radially averaged mean absorbed power per electron. It is shown that E(z), the radially averaged field in the plasma, and θ(z) are practically constant along z, in spite of the fact that Pi(z) steadily decreases. Comparison w...

Characterisation of a low-pressure oxygen discharge created by surface waves

Low-pressure oxygen discharges created by surface waves are characterised. Three discharge characteristics, the electron-neutral collision frequency nu , the maintaining electric field Eeff and the power required to maintain one electron theta , are deduced from the measurement of the electron density. They are independent of the microwave power and are studied as functions of the tube diameter (8-52 mm), the pressure (0.05-2 Torr) and the excitation frequency (390 or 210 MHz). The concentrations of ground state and singlet molecular oxygen are measured by VUV absorption and the concentration ratio of singlet to ground state molecular oxygen is about 10%, whatever the microwave power and the pressure. The ground state atomic oxygen concentration is measured downstream from the discharge by VUV absorption. The dissociation rate increases with the microwave power but remains less than 5%. A power balance model in the discharge, i.e. calculation of theta , is performed and yields a relation between theta /N and Eeff/N (n being the total neutral density), which is in good agreement with the experimental results. Finally, similarity laws are shown in the form theta /N and Eeff/N (Nr), where r denotes the tube radius.

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.

Influence of the radial electron density profile on the determination of the characteristics of surface-wave-produced discharges

Until recently, surface wave discharges have been studied, both experimentally and theoretically, using the wave propagation constant calculated in the assumption of a flat electron density profile. This paper deals with determination of wave characteristics (wavenumber, attenuation and fields) taking into account a radial profile of the electron density. The consequences for the experimental diagnostics are presented. In the range of low electron densities, the authors show that some results deduced from experimental measurements are spoilt if the radial electron density profile is not taken into account. They present an argon surface wave discharge (390 MHz) at low pressure (about 1 Torr) in a 76 mm diameter tube. The influence of the radial electron density profile is studied on the following experimental results: electron density, effective electron-neutral collision frequency and the mean power needed to maintain an electron in the discharge.

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.

Influence of water on NO removal by pulsed discharge in N2/H2O/NO mixtures

Kinetic mechanisms of NO removal are studied in N2/NO and N2/H2O/NO gas mixtures. A very short duration (60?ns) photo-triggered discharge is used to create a homogeneous plasma at a total pressure between 230 and 460?mbar. Measurements of the NO density are performed in the afterglow by time-resolved laser-induced fluorescence, for a time scale between 2 and 200??s after the current pulse excitation. Plasma homogeneity allows effective comparison between experimental results and predictions of a fully self-consistent discharge and kinetic modelling. It is shown that the NO removal efficiency is mainly determined through loss mechanisms balance for nitrogen metastable singlet states. In the absence of H2O, NO is in great part dissociated owing to collisions with singlet states. When water vapour is added, these states are destroyed through collisions with H2O with a rate constant k = (3.0?1.5)?10-10?cm3?s-1, and it leads to the decrease of the NO removal efficiency. This reaction is invoked for the first time.

Time-resolved laser-induced fluorescence study of NO removal plasma technology in N2/NO mixtures

The laser-induced fluorescence technique is used to study NO removal in N2/NO mixtures, after excitation by a homogeneous photo-triggered discharge. Time-resolved experiments are performed in the decaying post-discharge in order to measure the NO concentration 200 µs after a 75 ns duration current pulse. For the first time the NO removal is measured for a single current pulse excitation of the polluted mixture near atmospheric pressure. We discuss the influence of both the electrical energy deposited in the discharge and the initially applied reduced electric field on the NO removal efficiency. It is shown that near 100% of the initial NO molecules can be removed with a single shot, depending on experimental conditions. A detailed kinetic analysis is performed, which leads to straightforward analytical approach used to estimate the N atom density produced by the discharge as a function of the electrical parameters values. It strengthens that, with regard to known kinetic processes, the ideal case for 100% NO removal is to create the exact number of N atoms corresponding to the number of NO molecules, at the lowest energetic cost.

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.