Three-dimensional density distributions of NO in the effluent of the COST reference microplasma jet operated in He/N2/O2

Abstract

Laser induced fluorescence spectroscopy (LIF) is used to measure absolute ground state densities of nitric oxide (NO) in the effluent of the COST reference microplasma jet (COST-jet) with three-dimensional spatial resolution. The jet is operated in helium with a nitrogen/oxygen admixture. The experiments are performed with the jet expanding into open air and into a controlled He/synthetic air atmosphere. The most efficient production of NO is found at a 0.5% admixture of N2/O2 at a ratio of 4/1, that is considered to be synthetic air. Maximum NO densities of 3.25 × 1014 cm-3 and 4.5 × 1014 cm-3 are measured in the air and He/synthetic air atmosphere, respectively, at an axial distance of 2 mm from the nozzle. The distribution patterns are found to transit into a turbulent regime for air atmosphere at greater axial distances, while in the He/synthetic air atmosphere this effect is not observed. It is found that the expansion of the region of high NO density in the effluent is strongly coupled to the helium flow. Furthermore, the NO density is found to depend on the absolute feed gas flow, i.e. its maximum decreases as a function of the gas flow. This is a result of the longer residence time of the gas in the active plasma volume at lower gas flows and higher energy densities. For very high values of the applied radio frequency power the NO density is saturated. From time resolved measurements of the LIF signals the quenching coefficient for the NO(A 2Σ+) state by air is found to be k u,air = 4.2(±0.5) × 10-11 cm3 s-1, while quenching by He is negligible, k u,He 1 × 10-14 cm3 s-1. The amount of ambient air intruding the helium effluent is determined as well.