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

Brisset, Alexandra, Bieniek, Matthew, Invernizzi, Laurent, Hasan, Mohammad ORCID: 0000-0001-6993-933X, Walsh, James ORCID: 0000-0002-6318-0892, Niemi, Kari and Wagenaars, Erik
(2023) The formation of O and H radicals in a pulsed discharge in atmospheric pressure helium with water vapour admixtures. PLASMA SOURCES SCIENCE & TECHNOLOGY, 32 (6). 065004-065004.

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<jats:title>Abstract</jats:title> <jats:p>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 + H<jats:sub>2</jats:sub>O (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. 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 <jats:italic>μ</jats:italic>s after the end of the current pulse, reaching 2 × 10<jats:sup>16</jats:sup> cm<jats:sup>−3</jats:sup> at 0.1% H<jats:sub>2</jats:sub>O. It then remains nearly constant over 10 <jats:italic>μ</jats:italic>s before decaying. Modelling indicates that the electron temperature (<jats:italic>T</jats:italic>e) 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 (&lt;100 ns), O is predominantly produced by direct dissociation of O<jats:sub>2</jats:sub> by electron impact, and in the early afterglow (from 100 ns to 1 <jats:italic>μ</jats:italic>s) O is produced by dissociative recombination of O<jats:sub>2</jats:sub> <jats:sup>+</jats:sup>. The main loss mechanism of O is initially electron impact ionisation and once <jats:italic>T</jats:italic> <jats:sub>e</jats:sub> has dropped, it becomes mainly Penning ionisation with He<jats:sub>2</jats:sub>* and He* as well as three-body recombination with O<jats:sup>+</jats:sup> and He. On time scales of 100–200 <jats:italic>μ</jats:italic>s, O is mainly lost by radial diffusion. The production of H shows a similar behaviour, reaching 0.45 × 10<jats:sup>16</jats:sup> cm<jats:sup>−3</jats:sup> at 1 <jats:italic>μ</jats:italic>s, due to direct dissociation of H<jats:sub>2</jats:sub>O by electron impact (&lt;100 ns) followed by electron–ion recombination processes (from 200 ns to 1.5 us). H is dominantly lost through Penning ionisation with He* and He<jats:sub>2</jats:sub>* and by electron impact ionisation, and by charge exchange with O<jats:sup>+</jats:sup>. 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 O<jats:sub>2</jats:sub> <jats:sup>+</jats:sup> and H<jats:sub>2</jats:sub> <jats:sup>+</jats:sup>.</jats:p>

Item Type: Article
Uncontrolled Keywords: two-photon absorption laser induced fluorescence, nanosecond discharge, radical density, distribution, 1D fluid modelling
Divisions: Faculty of Science and Engineering > School of Electrical Engineering, Electronics and Computer Science
Depositing User: Symplectic Admin
Date Deposited: 26 Sep 2023 15:45
Last Modified: 26 Sep 2023 15:45
DOI: 10.1088/1361-6595/acd57f
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