The formation of O and H radicals in a pulsed discharge in atmospheric pressure helium with water vapour admixtures

Abstract

The spatio-temporal distribution of O and H radicals in a 90 ns pulsed discharge, generated in a pin-pin geometry with a 2.2 mm gap, in He + H2O (0.1 and 0.25%), is studied both experimentally and by 1D fluid modelling. The density of O and H radicals as well as the effective lifetimes of their excited states are measured using picosecond resolution Two-Photon Absorption Laser Induced Fluorescence (ps-TALIF). Good agreement between experiments and modelling is obtained for the species densities. The density of O and H is found to be homogenous along the discharge axis. Even though the high voltage pulse is 90 ns long, the density of O peaks only about 1 μs after the end of the current pulse, reaching 2x1016cm-3 at 0.1% H2O. It then remains nearly constant over 10 μs before decaying. Modelling indicates that the electron temperature (Te) in the centre of the vessel geometry ranges from 6 to 4 eV during the peak of discharge current, and after 90 ns, drops below 0.5 eV in about 50 ns. Consequently, during the discharge (<100 ns), O is predominantly produced by direct dissociation of O2 by electron impact, and in the early afterglow (from 100ns to 1 μs) O is produced by dissociative recombination of O2 +. The main loss mechanism of O is initially electron impact ionisation and once Te has dropped, it becomes mainly Penning ionisation with He2* and He* as well as 3-body recombination with O+ and He. On time scales of 100-200 μs, O is mainly lost by radial diffusion. The production of H shows a similar behaviour, reaching 0.45x1016 cm-3 at 1 μs, due to direct dissociation of H2O by electron impact (<100ns) followed by electron-ion recombination processes (from 200 ns to 1.5 us). H is dominantly lost through Penning ionisation with He* and He2* and by electron impact ionisation, and by charge exchange with O+. Increasing concentrations of water vapour, from 0.1 to 0.25%, have little effect on the nature of the processes of H formation but trigger a stronger initial production of O, which is not currently reproduced satisfactorily by the modelling. What emerges from this study is that the built up of O and H densities in pulsed discharges continues after electron-impact dissociation processes with additional afterglow processes, not least through the dissociative recombination of O2 + and H2 +.

Pulsed Dielectric Barrier Discharges for Gas-Phase Composition Control: A Simulation Model

“…As a reference, we considered a density of 1.2 × 10 14 cm −3 , based on the typical charge transported by microdischarges, of about 0.7 nC [10,33]. This value is also in the same range of other reported results [34]. A discussion of the effect of the electron density on the chemical kinetics of the discharges can be found in our previous works [8,9].…”

“…This could be better thought of as a determination of the mean electron energy, which, in turn, is linked to the local electric field value through the Boltzmann equation [31]. This value should be used in order to make a direct comparison between this simulation and other ones or with the experimental measurements [34]. So, instead of a specific value of the electric field strength E max in the ionizing wave pulse, we considered an electron temperature value of 4 eV as reference [8].…”

Absolute calibration of the ratio of Xe/O two-photon absorption cross-sections for O-TALIF applications

“…TALIF with Xe-calibration has become a routine method for oxygen density measurements [14][15][16][17][18][19]. In recent years, the same principle of O-TALIF measurements has been extended to picosecond [20] and femtosecond [21] TALIF. For a more detailed analysis of the technique of TALIF diagnostics, including the latest developments, one can refer to the recent review [2].…”

Ab initio investigation of potential energy curves of He2, He2+, and extrapolation by the machine learning method

“…Helium gas has been used widely to produce plasma in various discharges [1–6]. No matter pure helium gas discharge or helium‐oxygen, helium‐air mixture, and so forth gas discharges, it is a common sense that molecular $$$ \mathrm{H}{\mathrm{e}}_2^{+} $$$, $$$ \mathrm{H}{\mathrm{e}}_2 $$$ are compositions in plasmas using the helium gas.…”