Spatially- and time-resolved measurements of HO2 radicals in a Ns pulse atmospheric pressure plasma jet

The absolute, spatially-resolved, and time-resolved number density of the hydroperoxyl radical is measured in a quasi-two-dimensional, atmospheric pressure “curtain” plasma jet powered by a train of ns discharge pulses. The spatial distribution of HO2 is measured across the shorter dimension of the jet. The measurements are made in two different configurations, (a) H2O-O2-He jet impinging on a copper foil target, and (b) O2-He jet incident on the liquid water surface. In the first configuration, the water vapor is added to the O2-He flow in a bubbler filled with distilled, deionized water. The measurements are made using the previously developed pulsed Cavity Ring Down Spectroscopy (CRDS) diagnostic near 1.5 μm. The ring-down cavity is formed between two high-reflectivity mirrors placed at the ends of the stainless steel “arms” purged with dry air, with the plasma jet placed in the gap between the arms. The objectives of this work are to use the HO2 number density to assess the accuracy of the modeling predictions using a previously developed “global” reaction mechanism, and to estimate the efficiency of hydrogen peroxide generation in the ns pulse discharge plasma. HO2 was detected only in the first configuration, most likely due to the rapid decay of the metastable He atoms and O atoms generated in the plasma, which prevents the generation of H atoms (dominant HO2 precursors) in the evaporation/mixing layer. Both the water vapor in the jet and HO2 generated in the plasma have been measured. The results exhibit a rapid accumulation of HO2 during the ns pulse discharge burst, followed by the decay in the afterglow on a ms time scale. The kinetic model overpredicts the quasi-steady-state HO2 number density, as well as the HO2 decay rate after the discharge is turned off. The relatively slow HO2 decay in the afterglow suggests that it may be affected by diffusion, along with the surface adsorption and desorption of radicals. The present approach demonstrates the utility of a 2-D curtain plasma jet for the line-of-sight absorption spectroscopy measurements of radicals and excited species present in small concentrations in ambient plasma environments.