Effects of water addition on OH radical generation and plasma properties in an atmospheric argon microwave plasma jet

Abstract

Water vapor was added to the feeding gas of a continuous atmospheric argon (Ar) microwave plasma jet to study its influence on plasma shape, plasma gas temperature, and OH radical concentrations. The plasma jet was created by a 2.45 GHz microwave plasma source operating at constant power of 104 W with H2O-Ar mixture flow rate of 1.7 standard liter per minute (slm). With an increase in the H2O/Ar ratio from 0.0 to 1.9%, the plasma jet column length decreased from 11 mm to 4 mm, and the plasma jet became unstable when the ratio was higher than 1.9%; elevation of plasma gas temperature up to 330 K was observed in the plasma temperature range of 420-910 K. Optical emission spectroscopy showed that the dominant plasma emissions changed from N2 in the pure Ar plasma jet to OH with the addition of water vapor, and simulations of emission spectra suggested non-Boltzmann distribution of the rotational levels in the OH A-state (v'=0). Spatially resolved absolute OH number densities along the plasma jet axis were measured using UV cavity ringdown spectroscopy of the OH (A-X) (0-0) band in the H2O/Ar ratio range of 0.0–1.9%. The highest OH number density is consistently located in the vicinity of the plasma jet tip, regardless of the H2O/Ar ratio. OH number density in the post-tip region follows approximately an exponential decay along the jet axis with the fastest decay constant of 3.0 mm in the H2O/Ar ratio of 1.5%. Given the low gas temperature of 420-910 K and low electron temperature of 0.5-5 eV along the jet axis, formation of the OH radical is predominantly due to electron impact induced dissociation of H2O and dissociative recombination of H2O+ resulting from the Penning ionization process.