The production and evolution of atomic oxygen in the afterglow of streamer discharge in atmospheric pressure fuel/air mixtures

In this work two-photon absorption laser-induced fluorescence was used to measure oxygen atom (0) concentrations in streamer discharge afterglow in a variety of fueVair mixtures in order to account for the 0 reaction pathways in transient plasma ignition. It is demonstrated that 0 atoms are generated in high concentration (~5 x 1017 cm-3) directly below the high-voltage anode in a point-to-plane geometry. The corresponding lifetimes in air were on the order of hundreds of microseconds. Fuel chemistry provides consumption pathways via chain branching reactions even without sustained combustion, and the corresponding 0-atom lifetimes were much shorter than in air and dependent on the fuel concentration. At the richest conditions, corresponding to a fuel-air equivalence ratio of 2.4, 0 lifetimes were on the order a few microseconds or less. These experimental results are compared to modelling estimates in order to better understand the role of atomic oxygen in the chemical processes leading to ignition. (Some figures may appear in colour only in the online journal)