The spatial distribution of HO₂ in an atmospheric pressure plasma jet investigated by cavity ring-down spectroscopy

Abstract

Cold atmospheric pressure plasma jets make important contributions to a range of fields, such as materials processing and plasma medicine. In order to optimise the effect of those plasma sources, a detailed understanding of the chemical reaction networks is pivotal. However, the small diameter of plasma jets makes diagnostics challenging. A promising approach to obtain absolute number densities is the utilisation of cavity-enhanced absorption spectroscopy methods, by which line-of-sight averaged densities are determined. Here, we present first measurements on how the spatial distribution of HO2 in the effluent of a cold atmospheric pressure plasma jet can be obtained by cavity ring-down spectroscopy in an efficient way. Instead of recording fully wavelength resolved spectra, we will demonstrate that it is sufficient to measure the absorption coefficient at two wavelengths, corresponding to the laser being on and off the molecular resonance. By sampling the effluent from the 1.6 mm diameter nozzle in the radial direction at various axial positions, we determined that the distances over which the HO2 density was distributed were (3.9 ± 0.5) mm and (6.7 ± 0.1) mm at a distance of 2 mm and 10 mm below the nozzle of the plasma jet, respectively. We performed an Abel inversion in order to obtain the spatial distribution of HO2 that is presented along the symmetry axis of the effluent. Based on that localised density, which was (4.8 ± 0.6) ⋅ 1014 cm−3 at the maximum, we will discuss the importance of the plasma zone for the production of HO2.

The spatial distribution of hydrogen and oxygen atoms in a cold atmospheric pressure plasma jet

“…Within the plasma zone, one direction for the filament could be preferred due to inhomogeneities at the electrodes, which would result in an asymmetric production of H and O atoms due to the dissociation of H 2 O and other molecular species. A similar, asymmetric distribution has also been reported previously for the density distribution of HO 2 [69].…”

“…In Figure 5 A similar, asymmetric distribution has also been reported previously for the density distribution of HO 2 [63].…”

Formation and consumption of HO2 radicals in ns pulse O2–He plasmas over a liquid water surface

“…Previously, HO 2 and H 2 O 2 have been measured ex situ in a low-pressure continuous wave cavity ring down spectroscopy (cw-CRDS) cell used to sample the products from an atmospheric pressure, heated, jet-stirred reactor during the thermal oxidation of hydrocarbons, such as n-pentane [6], n-butane [7][8][9], and dimethyl ether [10]. More recently, HO 2 was also measured in situ by the cw-CRDS, in an atmospheric pressure Ar-O 2 -H 2 O plasma jet [4,11], and by pulsed CRDS in a heated plasma flow reactor in H 2 -O 2 -Ar, CH 4 -O 2 -Ar, and C 2 H 4 -O 2 -Ar mixtures [12]. In the present work, we extend the approach developed in [12] to the time-resolved, in situ HO 2 measurements in a plasma generated over a liquid water surface.…”

“…This greatly simplifies the experimental data reduction and the kinetic modeling predictions. In contrast, the analysis of CRDS measurements in circular plasma jets is rather more complex [4,11], and their quantitative interpretation would require the use of a two-dimensional kinetic model. Basically, the present approach is designed to obtain the experimental results and modeling predictions that would be straightforward to analyze, compare, and interpret.…”

Cavity Ring-Down Spectroscopy: Recent Technological Advancements, Techniques, and Applications

“…They reported a NEA of 3.25 × 10 −5 cm −1 corresponding to a detection limit of 74 ppm of H 2 O 2 at the combustion temperature of 1200 K and pressure of 1 atm. In a recent study, Klose et al 225 investigated the spatial distribution of HO 2 in an atmospheric pressure plasma jet utilizing the CRDS technique. They employed a distributed feedback diode laser operating at 1506.4 nm and demonstrated an on/off resonance method to acquire absorption at the different wavelength points, instead of acquiring full spectra, for the measurement of HO 2 density.…”