Open AccessThe spatial distribution of HO2 in an atmospheric pressure plasma jet investigated by cavity ring-down spectroscopy
Author(s) -
Sarah-Johanna Klose,
Katherine Manfred,
Helen C. Norman,
Grant A. D. Ritchie,
J. H. van Helden
Publication year - 2020
Publication title -
plasma sources science and technology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 108
eISSN - 1361-6595
pISSN - 0963-0252
DOI - 10.1088/1361-6595/aba206
Subject(s) - spectroscopy , nozzle , plasma , atmospheric pressure plasma , absorption spectroscopy , cavity ring down spectroscopy , atmospheric pressure , jet (fluid) , chemistry , absorption (acoustics) , number density , atomic physics , spectral line , optics , analytical chemistry (journal) , molecular physics , physics , meteorology , astrophysics , mechanics , chromatography , quantum mechanics , thermodynamics , astronomy
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 HO 2 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 HO 2 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 HO 2 that is presented along the symmetry axis of the effluent. Based on that localised density, which was (4.8 ± 0.6) ⋅ 10 14 cm −3 at the maximum, we will discuss the importance of the plasma zone for the production of HO 2 .