Numerical Investigation On Energy Spectrum Of Electrons In The Plasma Bullet And Its Applied Voltage Effect In Ar And He Plasma Jets

In recent years,the non-equilibrium atmospheric-pressure plasma jets have been widely applied in biomedicine due to their advantages.A large number of reactive species are produced in the plasma jets,and these species are of essential importance for the applications of the plasma jets in biomedicine where the expected biomedical effects can be obtained mainly through the interactions of the reactive species with the biological tissue.On the other hand,as the important particles in the plasma jet,electrons not only are of high concentration but also play an important role in the interactions of the plasma jets with the aqueous solutions on the surface of living cells.These two aspects closely correlate with electron energy,thus it is of great significance to investigate the spatiotemporal evolution of the energy spectrum of electrons in the plasma jet and its parameter effects.This thesis presents a systematical numerical investigation on the energy spectrum of electrons(ESE)in the atmospheric pressure argon(Ar)and helium(He)plasma jets,based on a typical needle-plate discharge configuration with a positive applied pulsed voltage and using a one-dimensional Particle-in-cell Monte-Carlo collision model.For the Ar plasma jet,the spatiotemporal evolutions of the ESEs in the entire discharge space and three characteristic areas have been investigated.These three characteristic areas refer to the plasma bullet(the bullet),the dark channel behind the bullet,and the dim area in front of the bullet along the propagation direction of the bullet.The effects of the applied voltage amplitude on the ESEs have also been explored,and a systematical comparison between the ESEs of both Ar plasma jet and He plasma jet has been carried out,revealing their differences.This thesis obtains the following new results:1.On argon plasma jet(1)In the temporal evolution of ESE in the jet,there is a characteristic time,before which the peak value of ESE decreases,and its distribution becomes wider and wider,but after which the reverse is true.(2)As the bullet propagates along the jet axis,the ESEs in the bullet and in the dark channel present an increasingly high peak and a gradually narrowing distribution,showing a decreasing average energy,Eave,of electrons,and in the dim area the evolution of the ESE is opposite to those in the others.In contrast,the bullet has a dominative contribution to energetic electrons above the excitation threshold of argon.(3)In the three characteristic areas,ESEs present an increasingly low peak and an increasingly wide distribution with the increase in the applied voltage amplitude,meaning an increasing Eave.Compared to the other two areas,the Eave in the bullet is evidently high.2.Comparison between ESEs of the argon and helium plasma jets(1)As the bullet propagates along the jet axis,the temporal evolution of the velocity and electron density of bullet in Ar are similar to those in He.These two quantities present a gradual decrease with time t,but in He,the bullet velocity is higher and the bullet electron density is lower than in Ar.In He,the ESE of the plasma bullet is of an evidently low peak and an obvious wide distribution,and thus has an evidently high Eave,when compared to Ar.(2)From area to area in Ar plasma jet,the variety of ESE with the area is similar to that in He plasma jet,but in He,the ESE in each area is of an evidently low peak and a obvious wide distribution in comparison with Ar.In addition,as the plasma bullet propagates along the jet axis,in He plasma bullet,the proportion of high electrons with energies higher than the excitation threshold of He in the total electrons and the electron density with energies above the lowest excitation threshold of inducing the dissociative electron attachment to water molecules,these two quantities are evidently low,compared to those in Ar plasma bullet.(3)In increasing the applied voltage amplitude,the ESEs in Ar and He plasma bullets present a similar evolution behavior,but that in Ar plasma bullet is more sensitive to the change of the applied voltage amplitude.