Influence of ambient air on the flowing afterglow of an atmospheric pressure Ar/O2 radiofrequency plasma

TL;DR

This study investigates how ambient air affects the composition and behavior of an atmospheric pressure Ar/O2 plasma afterglow, revealing air intrusion effects, species concentration gradients, and ozone formation depending on distance and flow conditions.

Contribution

It provides experimental insights into air intrusion effects on plasma afterglow and quantifies species variations and ozone formation at different separation distances.

Findings

Air intrusion negligible below 5 mm separation

Species concentrations vary with distance, showing gradients

Ozone formation depends on O2 flow and separation distance

Abstract

The influence of ambient air on the flowing afterglow of an atmospheric pressure Ar/O2 radiofrequency plasma has been investigated experimentally. Spatially resolved mass spectrometry and laser induced fluorescence on OH radicals were used to estimate the intrusion of air in between the plasma torch and the substrate as a function of the torch-to-substrate separation distance. No air is detected, within the limits of measurement uncertainties, for separation distances smaller than 5 mm. For larger distances, the effect of ambient air can no longer be neglected, and radial gradients in the concentrations of species appear. The Ar 4p population, determined through absolute optical emission spectroscopy, is seen to decrease with separation distance, whereas a rise in emission from the N2(C--B) system is measured. The observed decay in Ar 4p and N2(C) populations for separation distances greater than 9mm is partly assigned to the increasing collisional quenching rate by N2 and O2 molecules from the entrained air. Absorption measurements also point to the formation of ozone at concentrations from 10^14 to 10^15 cm-3, depending both on the injected O2 flow rate and the torch-to-substrate separation distance.