Investigations of rapid plasma chemistry generated by nanosecond discharges in air at atmospheric pressure using advanced optical diagnostics

Nanosecond repetitively pulsed discharges at atmospheric pressure are currently used to stabilize lean flames, which are have the advantage of emitting reduced levels of nitric oxides. We have investigated the production of atomic oxygen, often considered as one of the key species in the flame stabilization process. Using Two-photon Absorption Laser Induced Fluorescence (TALIF), we have conducted time-resolved measurements of the density of atomic oxygen produced in preheated air by a nanosecond repetitively pulsed discharge preheated at 1000 K. In addition, we have investigated the mechanism for formation of atomic oxygen using time-resolved Optical Emission Spectroscopy (OES) and Cavity Ring-Down Spectroscopy (CRDS) to measure the densities of N₂(B), N₂(C), and N₂(A), respectively. These species have been suggested to play a key role in the formation of atomic oxygen. The present measurements show that the filamentary discharge produces a very high atomic oxygen density on the order of 10¹⁸18 cm ^-3, which corresponds to an oxygen dissociation fraction of about 40%. Our measurements also prove for the first time that the main channel of formation of atomic oxygen is through the dissociative quenching of molecular oxygen by N₂(B).