Measurements of Absolute Atomic Oxygen Density by Two-photon Absorption Laser-induced Fluorescence Spectroscopy in Hot Air Plasma Generated by Microwave Resonant Cavity

Abstract

Hot air plasma is generated inside a 2.45GHz microwave resonant cavity and is ejected towards a gas conditioning cell at 600 mbar. A flow of 12 liter/min of dry air with a small amount of H2, in order to better detect OH (A-X) spectra for gas temperature measurements, is injected at the entrance of the resonant cavity using an input power of 1 kW. The measurement of absolute density of atomic ground state oxygen is determined using two-photon absorption laser-induced fluorescence (TALIF) spectroscopy for several radial and axial positions of the hot air plasma column which is a post discharge propagating inside the cell. The gas temperature corresponding to the radial and axial positions, where atomic oxygen density is measured, varies between about 3500K in the column axis down to about 2500K in the plasma border. The measured absolute oxygen density using xenon for calibration varies between about 2.1x1017cm-3 in the axis down to 0.05x1017cm-3. This leads to a dissociation degree of oxygen inside the post discharge air plasma column lower than the case where only local thermodynamic equilibrium is assumed. This deviation is due to non-thermal equilibrium effects that can be associated to the not yet achieved relaxation of the long-lived excited species during such post discharge stage.