A Numerical Investigation On The Behavior Of Electrons In The Bullet In The Atmospheric-Pressure Ar And He Plasma Jets

Due to their many advantages,the atmospheric-pressure low-temperature plasma jets have been widely applied in biomedical fields such as disinfection,wound healing,and cancer treatment.In these applications,the expected biomedical effects are mainly obtained through the interactions of the reactive species produced in the jets with the biological tissue.To reveal the mechanism of the plasma jets and to explore the effectiveness of plasma applications,the study of plasma-living cell interaction has aroused great interest.The reactive species arrived at the surface of living cells is a result due the synergistic effect between plasma particles.Electrons are important particles in the plasma jets,and they play an active role in the mass transfer of reactive species in the penetration of the plasmas into the aqueous solution.Accordingly,it is of great significance to probe the behavior of electrons in the plasma jets.Based on a typical needle-plate configuration with a positive applied pulsed voltage and using a one-dimensional Particle-in-Cell Monte-Carlo Collision(PIC-MCC)simulation method,a systematic investigation has been carried out on the behaviors of electrons in the bullet in the atmospheric-pressure argon(Ar)and helium(He)plasma jets,including the angle and energy spectra of electrons(ASE and ESE),and the corresponding mechanism is also analyzed.In addition,the effects of the applied voltage amplitude and the gap width on the electron behavior are also explored and the differences between the ASEs in the bullets in the Ar and He plasma jets are compared.The scattering angle of electron in the bullet is described using the angle between its scattering direction and the propagation direction of the bullet.This thesis obtains the following new results:1.On the argon plasma jet(a)ASE in the bullet in the Ar plasma jet is close to a sinusoidal distribution,namely,the peak position of ASE has a displacement in the direction of increasing scattering angle.As the bullet propagates along the jet axis,the peak position of ASE gradually shifts toward small scattering angle,meaning that more and more electrons are scattered towards the target.This is induced due to an increasingly low average electron energy in the bullet in the bullet propagation.But,at any moment when the bullet approaches the target,ASE in the bullet generally remains unchanged,allowing an estimation of electrons to most probably interact with the target.In the two angle ranges above and below 90°,the evolution of ESEs with time are basically the same,but the distribution range of ESEs in the angle range below 900 is smaller than that above 90°,thus presenting a low average electron energy.(b)With the increase of the applied voltage amplitude,the peak value of ASE in the bullet gradually increases,and the peak position shifts in the direction of increasing scattering angle,namely,the electrons scattered towards the target become increasingly less with increasing applied voltage amplitude.This feature can be ascribed to an enhanced electric field induced by increasing the applied voltage amplitude.The increase of the applied voltage amplitude leads to a decrease of the ratio of electrons in the angle range below 90° to the total number of electrons in the bullet,but induces an increase of the electrons above the excitation threshold in this angle range.Thus,a balance is required between these two quantities in the applications of the plasma jets in biomedicine,i.e.,the appropriate electron density and electron energy.The effects of the decrease of the gap width on the behavior of electrons in the bullet are the same as those due to the increase of the applied voltage amplitude,thus,it can induce the same effect to control the density and energy of electrons in the bullet by changing the applied voltage amplitude or the gap width.2.Comparison of electron behaviors in the bullets in the Ar and He plasma jets(a)In the bullet propagation,in comparison with Ar,at a given propagation time,the peak position of ASE in the bullet in the He plasma jet has a large displacement in the direction of decreasing scattering angle,meaning more electrons in the bullet scattered towards the target.The reason for this difference may be ascribed to the different excitation and ionization thresholds as well as the different collision cross-sections between Ar and He.(b)In the angle range below 90° in the bullets in the Ar and He plasma jets:the ratio of electrons above the excitation threshold in He is low,compared to that in Ar,which is due to the excitation threshold of He higher than Ar;there is a high ratio of electrons being able to induce the dissociative electron attachment to water molecules in He,but in spite of this,the total number of these electrons in Ar is higher than in He because of a high electron density of Ar.