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It is critical to measure the electron density and its transient variation in an atmospheric-pressure plasma jet. In the present study, the transient electron density of a pulsed microwave argon plasma jet at atmospheric gas pressure is measured using a homemade microwave Rayleigh scattering device. First, the high-speed photographs of the plasma jet are captured by an intensified charge-coupled device camera. Then, the radius and length of the plasma jet are determined from the high-speed photographs. Finally, the temporal scattering signal measurement is performed and the time-varying electron density is obtained. The electron density increases to a peak value of 4.55 × 1020 m−3 at 45 μs. Then, it decays monotonically until it reaches the detection limit. It is worth pointing out that there is no input microwave power after 40 μs, but the electron density still presents a sharp peak. This indicates that there is a change in the ionization process that further induces a variation in the discharge morphology. In addition, an electromagnetic simulation model is modulated to simulate the scattered electric field distributions produced by two different scatterers (air and plasma). The simulation results indicate that a higher electron density of scatterer leads to the acquisition of more intense scattering signals, which indicate that the microwave scattering process is similar to that associated with dipole radiation. These results verify the accuracy of the transient electron density measured using microwave Rayleigh scattering. [ABSTRACT FROM AUTHOR]

Although plasmas commonly present as electrical neutrality, for plasma jet plume at atmospheric gas pressure, it might appear nonneutral polarity due to plasma bullet being consisted of a large amount of electron avalanches. In this paper, the polarity transition and the ionization enhancement of atmospheric argon plasma jet plumes generated by the repetitive microwave pulses are studied. Three interesting phenomena have been found despite of discharges either with or without the hairpin metal wire, including: 1) a potential voltage pulse with positive polarity invariably arises steeply at each moment of power on; 2) high-speed photograph images show that there are ionization enhancements happened twice in each pulsed periods; and 3) ahead of the confluence point, the polarity transits from negative to positive while behind of this point, its positive polarity changes into the negative one. Moreover, the experiment results and electromagnetic simulations suggest that the theories of guided negative ionization wave and microwave resonant discharge excited by surface plasma polaritons should be used to interpret the spatial–temporal discharge process of the proposed microwave plasma jets. [ABSTRACT FROM AUTHOR]