Fig. 1. Plasma jet generated by atmospheric pressure air spark discharge: (a) Structure diagram of atmospheric pressure air spark discharge device; (b) installation photo of spark discharge device; (c) photo of air spark shock-wave plasma jet.

Fig. 2. Spark discharge voltage waveform.

Fig. 3. Microwave Rayleigh scattering device: (a) Experimental schematic diagram; (b) photo of experimental device.

Fig. 4. Calibration on a constant of microwave Rayleigh scattering device.

Fig. 5. Scattering signal measured by microwave Rayleigh scattering device.

Fig. 6. Optical images of air plasma jet.

Fig. 7. The radius r(t) and length l(t) of air plasma shock-wave jet.

Fig. 8. Time-varying electron density of plasma jet with the different equivalent scales: (a) r = 1.2 mm, l = 10 mm; (b) r = 1.2 mm, l = l(t); (c) r = r(t), l = 10 mm; (d) r = r(t), l = l(t).