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

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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. 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 2 × 1016 cm−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 (2 by electron impact, and in the early afterglow (from 100 ns to 1 μs) O is produced by dissociative recombination of O2 +. The main loss mechanism of O is initially electron impact ionisation and once T e has dropped, it becomes mainly Penning ionisation with He2* and He* as well as three-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 0.45 × 1016 cm−3 at 1 μs, due to direct dissociation of H2O by electron impact (2* 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 +.
Funding Information
  • Engineering and Physical Science Research Council (EP/S025790/1)

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