F. Marchal

Quantitative Determination of Density of Ground State Atomic Oxygen from Both TALIF and Emission Spectroscopy in Hot Air Plasma Generated by Microwave Resonant Cavity

Two experimental techniques have been used to quantify the atomic oxygen density in the case of hot air plasma generated by a microwave (MW) resonant cavity. The latter operates at a frequency of 2.45 GHz inside a cell of gas conditioning at a pressure of 600 mbar, an injected air flow of 12 L/min and an input MW power of 1 kW. The first technique is based on the standard two photon absorption laser induced fluorescence (TALIF) using xenon for calibration but applied for the first time in the present post discharge hot air plasma column having a temperature of about 4500 K near the axis of the nozzle. The second diagnostic technique is an actinometry method based on optical emission spectroscopy (OES). In this case, we compared the spectra intensities of a specific atomic oxygen line (844 nm) and the closest wavelength xenon line (823 nm). The two lines need to be collected under absolutely the same spectroscopic parameters. The xenon emission is due to the addition of a small proportion of xenon (1% Xe) of this chemically inert gas inside the air while a further small quantity of H2 (2%) is also added in the mixture in order to collect OH(A-X) and NH(A-X) spectra without noise. The latter molecular spectra are required to estimate gas and excitation temperatures. Optical emission spectroscopy measurements, at for instance the position z=12 mm on the axis plasma column that leads to a gas measured temperature equal to 3500 K, an excitation temperature of about 9500 K and an atomic oxygen density 2.09×1017±0.2×1017 cm−3. This is in very good agreement with the TALIF measurement, which is equal to 2.0×1017 cm−3.

Electric and spectroscopic analysis of a pure nitrogen mono-filamentary dielectric barrier discharge (MF-DBD) at 760?Torr

Mono-filamentary dielectric barrier discharge (MF-DBD), occurring within 1?mm gap of atmospheric pressure pure nitrogen and operating with a sinusoidal electric supply at about 8?kHz, is studied in this paper. A thorough electrical analysis allows experimental determination of the ignition and extinction voltages, respectively (15?750 ? 50)?V and (2097 ? 7)?V, the injected energy (158 ? 2)?J and charge (17.22 ? 0.22) nC in a single filament. The mean axial reduced electric field is equal to (644 ? 2)?Td at ignition. An empirical technique is proposed to evaluate these discharge parameters by avoiding bulky calculations. Optical emission spectroscopic measurements of the vacuum ultraviolet (VUV), ultraviolet (UV), visible and near infrared (IR) emissions are presented and discussed. Two atomic nitrogen lines attributed to the decay of the N[2s2p23s 2P] triplet towards N[2s22p3 2D?] level are observed at 150 and 175?nm, together with the Lyman?Birge?Hopfield system in the VUV range. The second positive system (N2[C?3?u] ? N2[B?3?g]) dominates the UV and visible-blue spectra. The (0?0) transition of the first negative system peaking at 391.4?nm, the first positive system and the Herman IR transitions are also present. Both our VUV and near IR spectra are consistent with recently reported results in hollow cathode and cylindrical DBDs. The electrical and spectroscopic experimental results reported here are useful for ongoing and forthcoming modelling of filamentary nitrogen dielectric barrier discharges.

Temperature dependence of xenon excimer formations using two-photon absorption laser-induced fluorescence

The temporal dependence of VUV emissions of the first continuum of xenon at around 150 nm are investigated for pressures ranging from 10 to 100 Torr and for temperatures ranging from 200 to 300 K, after briefly and selectively populating exclusively the resonant or the metastable atomic states of the 5p56s configuration. A kinetic and spectroscopic study was performed by analyzing the VUV post luminescence decay of xenon operating with such excitation conditions. Our results clearly show the temperature effects on the rate constants α31 and α32 of formation of excimers correlated respectively with the first resonant and the first metastable states by three-body collisions. The resulting reaction rates are obtained as a function of temperature T : α31 = 3.90 × 10−27 T−1.78 cm−6 s−1, α32 = 1.34 × 10−27 T−1.70 cm−6 s−1. For the whole pressure range, low temperatures favour the formation of xenon excimers and the efficiency of the three-body collisions in forming stabilized excimers increases with decreasing temperature.

Electro-Hydrodynamics and Kinetic Modeling of Dry and Humid Air Flows Activated by Corona Discharges

The present work is devoted to the 2D simulation of a point-to-plane Atmospheric Corona Discharge Reactor (ACDR) powered by a DC high voltage supply. The corona reactor is periodically crossed by thin mono filamentary streamers with a natural repetition frequency of some tens of kHz. The study compares the results obtained in dry air and in air mixed with a small amount of water vapour (humid air). The simulation involves the electro-dynamics, chemical kinetics and neutral gas hydrodynamics phenomena that influence the kinetics of the chemical species transformation. Each discharge lasts about one hundred of a nanosecond while the post-discharge occurring between two successive discharges lasts one hundred of a microsecond. The ACDR is crossed by a lateral dry or humid air flow initially polluted with 400 ppm of NO. After 5 ms, the time corresponding to the occurrence of 50 successive discharge/post-discharge phases, a higher NO removal rate and a lower ozone production rate are found in humid air. This change is due to the presence of the HO2 species formed from the H primary radical in the discharge zone.

Particle transport in neutral xenon: 2D modelling approach for the contribution of recombination processes to VUV emissions

This paper constitutes a first step in studying transport phenomena, consecutive to a very brief two-photon excitation of the Xe(3P2) metastable state in neutral xenon, followed by absorption of a further photon leading to ionization, in two-photon absorption laser-induced fluorescence (TALIF) experiments. Here, the laser beam simply defines the column volume in which electrons and atomic ions are initially confined. Performing a numerical approach, we only consider the reactions occurring once electrons and atomic ions are produced by the incident laser pulse. All metastable atoms initially present are discarded, thus allowing evaluation of the contribution of only recombination processes to the vacuum ultraviolet (VUV) emissions. In addition to kinetics of collisional and radiative processes, we also account for diffusion and possible drift phenomena (in the presence of a Laplacian electric field). We provide spatial distributions of the number densities of the various species as well as temporal profiles of both their total number and the different VUV emissions. The influence of the external electric field and xenon pressure is discussed as well.