Fig. 3: Physical appearances of the plasma jet.

a Typical images of the plasma jet acquired by a digital camera with the gas flowrate fixed at 0.5 l min⁻¹ at four different moments. The plasma jet extends along the z-axis, with its length of 18 mm. The exposure time is 10 ms. b Same as a, but with the gas flowrate fixed at 2.2 l min⁻¹ at four different moments. c Same as a, but with the gas flowrate fixed at 7.8 l min⁻¹ at four different moments. d Typical images of the plasma jet captured by a fast intensified charge-coupled device (ICCD) camera with the gas flowrate fixed at 0.5 l min⁻¹ at four random current pulses for both the positive alternation (PA) and negative alternation (NA), as well as the tube profile without discharge. Images in PA and NA are separated by the red dashed line. The gate width is 4 μs. e Same as d, but with the gas flowrate fixed at 2.2 l min⁻¹ at six random current pulses for both PA and NA. f Same as d, but with the gas flowrate fixed at 7.8 l min⁻¹ at four random current pulses for both PA and NA. g Plots (purple solid circle) of the plasma jet length as a function of the gas flowrate. Within the flowrate range from 2.2 to 7.8 l min⁻¹, the plots show an average of five experimental measurements, and the measurement uncertainty (one standard deviation) is indicated by the error bars and is estimated to be 5–7%. The shaded regions are introduced in order to focus the attention on the variation of discharge mode with the gas flowrate, i.e., gray shadow for the filamentary discharge, green shadow for the diffuse discharge, orange shadow for the mixing mode of filamentary and diffuse discharge, and red shadow for the discharge interruption. h Spatial distributions of light intensity of discharge along the transverse direction at the distance of 1 mm away from the nozzle for the gas flowrates of 0.5 l min⁻¹ (black dotted line), 2.2 l min⁻¹ (red solid line), and 7.8 l min⁻¹ (blue dashed line). The origin is set at the center of the quartz tube.