Ar(1s5) density in a co-axial argon plasma jet with N2-O2 shielding

Atmospheric-pressure microplasma jets (μAPPJs) are versatile sources of reactive species with diverse applications. However, understanding the plasma chemistry in these jets is challenging due to plasma-flow interactions in heterogeneous gas mixtures. Spatial metastable density profiles help to understand these physical and chemical mechanisms. This work focuses on controlling the shielding gas around a μAPPJ. We use a dielectric barrier discharge co-axial reactor where a co-flow shields the pure argon jet with different N2-O2 gas mixtures. A voltage pulse (4 kV, 1 μs, 20 kHz) generates a first discharge at the pulse’s rising edge and a second discharge at the falling edge. Tunable diode laser absorption spectroscopy measures the local Ar(1s5) density. A pure N2 (100%N2-0%O2) co-flow leads to less reproducible and lower peak Ar(1s5) density (5.8 × 1013 cm−3). Increasing the O2 admixture in the co-flow yields narrower Ar(1s5) absorbance profiles and increases the Ar(1s5) density (6.9 × 1013 - 9.1 × 1013 cm−3). The position of the peak density is closer to the reactor for higher O2 fractions. Absence of N2 results in comparable Ar(1s5) densities between the first and second discharges (maxima of 9.1 × 1013 and 9.3 × 1013 cm−3, respectively). Local Ar(1s5) density profiles from pure N2 to pure O2 shielding provide insights into physical and chemical processes. The spatially-resolved data may contribute to optimising argon μAPPJ reactors across the various applications and to validate numerical models.