Fig. 1. Schematic diagram of the experimental setup

Fig. 2. Images of the downstream plasma plume at different parameters [(a), (b), (c)] (Tube1 and the plate electrode are marked by dashed lines, exposure time of the camera is 10 s) and Distribution of discharge luminance along the X-axis (d) and Y-axis (e)

Fig. 3. Waveforms of applied voltage, light signal from upstream and downstream under (a) V_p= 4.2 kV and (b) V_p=7.6 kV

Fig. 4. Optical spectra emitted from the upstream (a) and the downstream (b) plumes with V_p of 7.6 kV

Fig. 5. The electron density of upstream and downstream plumes varies with V_p and Q

Fig. 6. (a) Spatial distribution of oxygen atom concentration along the argon flow; (b) oxygen atom concentration as a function of experimental parameters