Spatio-temporal distribution of absolute densities of argon metastable 1s5 state in the diffuse area of an atmospheric pressure nanosecond pulsed argon microplasma jet propagating into ambient air

Abstract

Atmospheric microplasma jets (MPJs) sustained in rare gases have gained increased attention due to their potential to generate highly reactive species. In this paper, we present space- and time-resolved argon metastable densities, Ar(1s5), measured in an atmospheric pressure MPJ operated in Ar and propagating into ambient air using tunable diode laser absorption spectroscopy (1s5 → 2p9 optical transition). The MPJ was produced using a dielectric barrier discharge energized by short duration (230 ns) high-voltage positive pulses (4.2–6.2 kV) at a repetition frequency of 20 kHz. The spectral absorption line profile was recorded and allowed measurements of the absolute metastable Ar(1s5) density integrated in the line-of-sight of the laser beam under various operating conditions of the MPJ. The results reveal a sensitive dependence of the Ar(1s5) density on spatial coordinates, i.e., distance from the exit of the capillary tube of the discharge and from the axis of the argon jet. The highest Ar(1s5) densities of about 3 × 1013 cm−3 were measured at the axis of the argon jet at longitudinal distances between 4 and 5.5 mm downstream from the nozzle of the tube. The temporal distribution of the Ar(1s5) density, which presents three maxima, is thoroughly discussed in this paper. The spatial distribution of the effective Ar(1s5) lifetime (<250 ns) is also reported, giving some insight into the surrounding environment of the argon metastable atoms. The determined spatiotemporal distributions of the Ar(1s5) density can be useful for the optimization of argon MPJs for different applications like surface or biomedical processes.