Study On The Measurement Of Time-varying Electron Density Of Atmospheric Plasma Jet By Microwave Rayleigh Scattering Method

In recent years,with the rapid development of plasma science,atmospheric pressure gas discharge plasma has been widely concerned.Many kinds of atmospheric pressure plasma have been produced by gas discharge,which have been successfully used in material processing,plasma combustion,energy chemical industry and biomedicine.Since atmospheric pressure gas discharge is a typical nonlinear process and atmospheric pressure plasma usually contains multiple physical fields and complex chemical reactions.Therefore,it is necessary to diagnose the parameters of plasma.The electron density,as an important parameter of plasma,has aroused the interest of many researchers.In order to study the temporal and spatial distribution and transient variation of electron density in plasma,the electron density of atmospheric plasma is measured by microwave Rayleigh scattering.The research contents of this paper are as follows.Firstly,based on the theory of electromagnetic scattering,the principle of electric dipole radiation is deduced from the far region approximation and the small field source approximation.According to the electric dipole radiation of the plasma under the action of the incident microwave,the measurement principle of the microwave Rayleigh scattering method is derived.In order to verify the effectiveness of the microwave Rayleigh scattering method,the electromagnetic simulation model was established by using the electromagnetic simulation software(HFSS)to simulate the scattering field intensity distribution of four kinds of scatterers.The simulation results indicate that a higher electron density of scatterer leads to acquisition of more intense scattering signals,which indicate that microwave scattering process is similar to that associated with dipole radiation.The experimental platform of microwave Rayleigh scattering device is built.The measurement steps are as follows:(a)Before measurement,the system parameter Amust be calibrated.Ten dielectric rods and the differential fitting calibration method are used to determine the system parameter.(b)A trigger signal is used to trigger a microwave power supply,ICCD camera,and oscilloscope.A high-speed photograph and Rayleigh scattering signal are captured simultaneously.The transient images captured by the ICCD camera are used to determine the radius and length of the plasma jet plume.(c)The electron density and transient process of plasma jet are calculated by processing the data with the software of microwave Rayleigh scattering method.The time-varying electron density of atmospheric piezoelectric spark plasma jet is measured and studied.Due to the random nature of spark discharge,we use the falling edge of the spark gap voltage as the trigger signal to synchronously trigger the ICCD camera and oscilloscope.According to the transient discharge image and the scattered voltage signal,we obtained four time-varying electron density results with different equivalent radius and equivalent lengths.The maximum value of the electron density is 4.46×10²⁰m^-3.By analyzing the discharge image and laser schlieren images,we discovered the photoionization phenomenon in spark discharge,and discussed the influencing factors of electron density changes.The time-varying electron density of the atmospheric pressure pulsed microwave argon plasma jet is measured and studied.The experimental system is composed of pulsed microwave source,microwave Rayleigh scattering device,ICCD camera and oscilloscope.The experimental results show that the peak value of the electron density is 4.55×10²⁰m^-3.The instantaneous variation of electron density is closely related to the radius and length of plasma jet.Finally,the time-varying evolution process of electron density is discussed by analyzing the physical mechanism of pulsed microwave argon plasma discharge.