Numerical Investigation On The Behaviors Of Electrons In The Bullet In Ar/H₂O And He/H₂O Plasma Jets

In the applications of atmospheric-pressure low temperature plasma jets in biomedicine,reactive species plays a key role in inducing biomedical effects.The addition of an oxygen-containing admixture to the working gas is beneficial to the production of reactive species.Electrons are important particles in the plasma jets,and they have a great impact on the propagation of the jets,on the generation of reactive species,and on the mass transfer of the plasmas in the aqueous solution on the surface of the treated matter.Therefore,it is of great significance to have a deep insight into the behavior of electrons in the bullet in plasma jet as well as its effects of the admixture.Using a one-dimensional Particle-in-Cell Monte-Carlo Collision method and based on a needle-plate discharge configuration with an applied positive pulsed voltage,this thesis has performed a systematic investigation on the effects of both the applied voltage and water vapor concentration on the behaviors of electrons in the bullet in the atmospheric-pressure Ar/H2O and He/H2O plasma jets.The behaviors include the angle and energy spectra of electrons(ASE and ESE),the electron density,and the related generation of OH radical.A comparison has also been done between the behaviors of electrons in the bullets in the Ar/H2O and He/H2O plasma jets.The scattering angle,θ,of electron in the bullet is described by the angle between the scattering direction of electron and the propagation direction of the bullet.This thesis obtains the following results:1.The behaviors of electrons in the bullet in the Ar/H2O plasma jet(a)At a fixed water vapor concentration,with the increase of the applied voltage(<7kV),the peak position of ASE in the bullet shifts in the direction of increasing θ above 90°,meaning the decrease of electrons scattered toward the target,the evolution of ESE in the bullet shows an increasing average electron energy,and both the electron density and OH density increase.The electrons in the angle range of θ<90° in the bullet most probably interact with the target,and the evolution of these electrons with the applied voltage is in agreement with that of electrons in the entire bullet.The above-mentioned results indicate that an increase of the applied voltage allows to effectively control the energy and density of electrons as well as the OH density in the bullet.(b)The addition of water vapor to Ar working gas not only can contribute to the considerable OH generation,but also allows the electrons above 3.27eV in the angle range ofθ<90° in the bullet to maintain at a high ratio,which is beneficial to the mass transfer of the plasmas in aqueous solution,producing more reactive species.(c)With the increase in water vapor concentration,in the bullet,there is a relative decrease of electrons scattered into the angle range of θ<90°;the OH density increases dramatically,but the electron density nearly keeps constant at water vapor concentrations below about 0.5%and then decreases,and there is a close correlation between these two densities;in the angle range of θ<90°,the ratio of electrons above 3.27 eV nearly remains unchanged,but the electron density presents a dependence on water vapor concentration similar to that in the entire bullet.Accordingly,from the point of view of the overall consideration of the electron density,the ratio of high energy electrons,and OH generation,a water vapor concentration of 0.5%is a suitable addition to Ar working gas.2.Comparison between the behaviors of electrons in the bullet in the Ar/H2O and He/H2O plasma jets(a)At the water vapor concentration of 0.5%,with the propagation of the bullet,in comparison to the Ar/H2O.more electrons in the bullet in the He/H2O are scattered into the angle range of θ<90°,namely,there are relative more electrons scattered toward the target.(b)Compared to the He/H2O plasma jet,there are more high energy electrons to most probably interact with the target in the Ar/H2O plasma jet,which is beneficial to produce more reactive species in the mass transfer of the plasmas in aqueous solution.(c)As the propagation of the jet,the OH densities in the discharge space in the Ar/H2O and He/H2O plasma jets increase dramatically,but those in the bullets basically remain unchanged.The OH density in the He/H2O plasma jets is much higher than in the Ar/H2O plasma jet.