Concentration measurements of atomic nitrogen in an atmospheric-pressure RF plasma jet using a picosecond TALIF.

The absolute concentration and spatial distribution of ground-state atomic nitrogen (N) in an atmospheric pressure plasma jet were measured using the two-photon absorption laser-induced fluorescence (TALIF). The jet was ignited by radio frequency (RF) voltage in argon (or argon with nitrogen admixture) flowing through a silica tube. The spatially resolved measurements of atomic nitrogen concentration were realized in the effluent of the jet. In a pure argon plasma, the N concentration was increased with the distance from the silica tube and reached the maximum value (8*1014 cm-3) at the distance of 15 mm, and then sharply decreased at the end of the plume. On the contrary, plasma ignited in Ar with nitrogen admixture, the maximum N concentration was located directly at the end of the silica tube, where plasma starts to blow out into the ambient air. The highest N concentrations for 0.5 % and 2 % of N2 in the feed gas were 3*1015 cm-3 and 8*1015 cm-3, respectively.