Study of He + 0.2% O2 plasma jet impinging on liquid surface from He(23S1) metastable atoms density measurements

Abstract

This work presents spatial variations of the helium He(23S1) metastable atom absolute densities in a plasma jet at atmospheric pressure in He + 0.2% O2 gas mixture impinging on a liquid target. The plasma jet is ignited with 6 kV pulses of 2.5 μs duration and 8 ns rise and fall times at 20 kHz repetition rate. Streamers are created in both rising and falling fronts of the high voltage (HV). From Abel inversion of the line averaged absorbance of 1083 nm helium line, measured by laser absorption spectroscopy, the radial profiles of He(23S1) density along the plasma jet are deduced in two sources with different residence time of the gas within the discharge volume, having three different outlet tube diameters and with two gas flow rates. The metastable densities up to ∼3 × 1013 cm−3 have been measured. The radial profile remained always axially centered for the 1.5 mm outlet tube and with 0.5 slm for the larger diameter tubes. But in this later tubes and with 2 slm gas flow rate, the profile changed from a ring shape at the tube exit to axially centered when increasing the distance from the tube edge. In the rising front of the HV, the ring profile of He(23S1) density in the tube exit is attributed to the shape of the electric field, produced by the applied HV to the electrode wrapped around the dielectric tube. In the falling front of the HV, the electric field results from electrons deposited on the inner surface of the dielectric tube and positive ions accumulated on the surface of liquid during the 2.5 μs duration of the HV pulse. The influence of oxygen by-products formed in the plasma volume (O, O2(Δ), O3, positive and negative oxygen ions) on the dynamic of the plasma jet are analyzed.