Study On The Propagation Of Helium Plasma Jet And The Electric Field Characteristics Of The Interacting Target Based On Nanosecond Temporal And Spatial Resolved Images

Atmospheric pressure plasma jets（APPJs）have attracted wide attention due to their broad applications.The generation of APPJs is accompanied by strong electric fields.Therefore,the study on the distribution of the target electric field may explain the related issues in the propagation of APPJs and the target interactions.In this thesis,APPJ is fed with helium as the working gas,and the APPJ is generated with a pulsed high voltage（H.V.）power supply as the power source.By using nanosecond temporal and spatial resolved images,the propagation of ionization waves（IWs）in the APPJ is studied with various applied voltage,pulse width and gas flow.In addition,the establishment and distribution of the target electric field is studied based on the Pockels effect of the bismosilicate(Bi₁₂Si O₂₀,BSO)crystal.The main results are as follows:The effects of the discharge parameters（applied voltage,pulse width of the voltage and gas flow rate）on the propagation of the APPJ in the ambient air is studied.The APPJ is a dual barrier discharge structure.The discharge is generated between H.V.electrode and grounded electrode.Two discharges are obtained in the rising and falling phase of the applied voltage,respectively.The propagation velocity of the IW and the length of the APPJ are significantly increased as the applied voltage increases from 7 k V to 10 k V.When the applied voltage is 10k V,the maximum velocity of the IW is obtained in the vicinity of tube nozzle.The maximum velocity reach approximately 1.4×10~5m/s.The analysis shows that when the IW is propagated to the vicinity of the nozzle,the nozzle is polarized,resulting in the enhancement of the electric field in IW front.With the pulse width of the applied voltage is increased from 0.9μs to 150μs,the propagation velocity and the emission intensity of the IW are reduced,meanwhile,the length of the APPJ is reduced.The maximum velocity of the IW can be reached a maximum value of about 0.9×10~5m/s when the voltage pulse width is 0.9μs.As the pulse width increase to 150μs,the maximum value of the velocity decreased to 0.5×10~5m/s.The effect of the pulse width on the propagation of the IW may be related to the concentration of residual species in the discharge channel.With a longer discharge interval,the electric field in IW front is enhanced due to the reduction of the residual species,resulting in the increasement of the propagation velocity and the length of the APPJ.As the gas flow increases from 400 sccm to 2000 sccm,the propagation velocity and the emission intensity of the IW are increased,and the length of the APPJ is significantly increased.When the gas flow is 800 sccm,the maximum propagation velocity of IW is about 0.9×10~5m/s.As the gas flow rate of helium is increased to 2000 sccm,the maximum propagation velocity of IW is reduced to about 0.75×10~5m/s.Due to the existence of the nitrogen（N₂）in the ambient air,the spatial distribution of air particles in the plasma plume is redistributed.The diffusion of N₂enhance the Penning ionization and charge transfer reaction.Therefore,the propagation velocity and emission intensity of the IW are relatively increased when the gas flow rate decreases from2000 sccm to 800 sccm.The effects of the discharge parameters on the distribution of the interacting target are studied.The measurements show that the development of the electric field on the target surface can be classified as the establishment phase,the continuous phase and the attenuation phase caused by the"composite effect"in the falling phase of the applied voltage.As the applied voltage is increased from 7 k V to 10 k V,the maximum value of the electric field gradually increases.The maximum value during the rising phase of the applied voltage is increased from about 4.4 k V/cm to 6.0 k V/cm.The maximum value of the electric field during the falling phase is increased from about 4.2 k V/cm to 5.8 k V/cm.The increase of the applied voltage could increase the electron density and transfer area of the discharge.On the other hand,the increase in the external voltage could also supply additional electric fields to increase the maximum electric field value on the surface.When the pulse width is increased from 0.9μs to 150μs,the maximum electric field is increased and then decreased.During the rising phase of the applied voltage,the maximum value of electric field is slightly increased from 4.5 k V/cm to 5.4 k V/cm,and then decreased to 5.0 k V/cm.During the falling phase,the maximum value of electric field is slightly increased from 4.5 k V/cm to 4.8 k V/cm and then decreased to 4.5 k V/cm.The distribution area of the electric field,the value of electric field,and the deposition charge are affected by the combined effects of the discharge of the APPJ and the residual charge on the surface,resulting in electric field and amount of charge decreases after an increase.As the gas flow increases from 400 sccm to 2000 sccm,the maximum value of electric field is decreased.During the rising phase,the maximum value is decreased from 6.0 k V/cm to 4.0 k V/cm.During the falling phase,the maximum value is decreased from 5.7 k V/cm to 4.2 k V/cm.In addition,the total charge deposition is also decreased as the increase of the gas flow rate.The effects of the gas flow rate may be related to the increase of Penning ionization due to air diffusion.