Numerical Investigatiorn On The Control Of Electron Ernergy And Related Reactive Species In The Low-Temperature Atmospheric-Pressure Plasma Jets

Low-temperature atmospheric-pressure plasma jets(hereafter,called the plasma jets)have received considerable interest for the past few decades due to their remarkable advantages including simple operation,excellent safety,and friendly environment.In recent years,the applications of the plasma jets in biomedicine have become increasingly active.In these applications,the target to be treated by plasmas is often affected by the ambient aqueous solution.Electrons,as important particles in the plasma jets,actively participate in the interactions of the plasmas with the aqueous solution and in the mass transfer of the reactive species in the aqueous solution,and play an important role in the above processes.Therefore,to have a deep insight into behaviors of electrons in the plasma jets and to effectively control their energies are of significance for the applications of the plasma jets in biomedicine.In this dissertation,based on a needle-plane discharge system and using a two-dimensional axisymmetric fluid model,the effects of the operating conditions on electron energy in the plasma jets have been systematically investigated,and the corresponding mechanisms governing the parameter effects of electron energy are also analyzed in detail.In addition,the reactive species production as well as its correlation with electron energy has been explored.The considered operating conditions include the power supply parameters,the structure parameters,and the gas compositions.The electron energy refers to the averaged electron energies in.the three areas along the jet axis,i.e.,in the entire plasma jet,in the bullet,and near the cathode plane.This dissertation includes the following four contents and results.1.The effects of the voltage amplitude,the rising time t,of voltage pulse,and their synergy on electron energy in the atmospheric-pressure helium plasma jets have been systematically investigated,the corresponding mechanisms have also been analyzed in detail,indicating the dominant role of the spatial electric field in the control of electron energy.The main conclusions are obtained as follows;(1)The effects of the voltage amplitude on the averaged electron energies become much more evident for smaller tr.Fixing the voltage amplitude,the averaged electron energies in the three areas decrease with the increase in the tr.The dependence of electron energy on the tr is evident for a high voltage amplitude.For the synergy between the voltage amplitude and rising time tr,both ways,to change the voltage amplitude under a fixed small tr or to change the rising time t,for a given voltage amplitude,can control electron energy sensitively and effectively.(2)The plasma bullet has a substantial contribution to high-energy electrons in contrast with the other region in the plasma jet.Especially,an averaged electron energy of about 10 eV,which can be obtained due to the synergy between the voltage amplitude and the tr,covers the energy thresholds inducing all the processes of dissociative electron attachment to water molecule.This is of essential importance for the study on the mass transfer of the reactive species in the aqueous solutions in plasma biomedicine.2.The effects of the structure parameters of the discharge system on electron energy in the atmospheric-pressure helium plasma jets have been systematically investigated,and the corresponding mechanisms have also been analyzed in detail.The structure parameters include the needle radius,the gap width,and both the inner diameter and the relative permittivity of the dielectric tube.The main conclusions are obtained as follows:(1)The needle radius has only a slight effect on the averaged electron energies in the three areas.The increase of the gap width induces an evident decrease of the averaged electron energies in the three areas.The effect of the dielectric tube on electron energy becomes evident only when its inner diameter is smaller.The relative permittivity of the dielectric tube slight affects the electron energy.(2)Compared to the other areas in the plasma jet,the plasma bullet still has a considerable contribution to high-energy electrons no matter which structure parameter is changed.3.The effects of the gas compositions on electron energy in the atmospheric-pressure helium plasma jets have been systematically investigated,and the corresponding mechanisms have also been analyzed in detail.The gas compositions include He/H2O,Ar/H2O,He/air,and Ar/air.The main conclusions are obtained as follows:(1)For a fixed impurity concentration,the averaged electron energies in the three areas are much higher in the helium plasma jets than in the argon plasma jets.(2)For the He/H2O plasma jet,the averaged electron energies in the three areas keep basically a constant at water vapor concentrations below about 0.1%,and then they evidently decrease.For the Ar/H2O plasma jet,the averaged electron energies in the three areas present only a slight decrease when water vapor concentration is above 1%.This difference from that in He/H2O is mainly due to the different energy thresholds for the electron collision reactions related to helium and to argon.(3)The averaged electron energies in the three areas in the He/air and Ar/air plasma jets basically remain constant at air concentrations below about 0.5%,but they are in varying degrees of decrease above the concentration of 0.5%.4.For the He/humid air,He/H2O,and Ar/H20 plasma jets,the reactive species production and its correlation with electron energy in the entire plasma jet have been systematically investigated,and the corresponding mechanisms have also been analyzed in detail.The main conclusions are obtained as follows:(1)For the He/humid air plasma jets,increasing the averaged electron energy by changing the voltage amplitude,the averaged densities of OH,O3,O(,D),and O-increase correspondingly,but that of H2O2 decreases.When increasing the humid air concentration,the averaged densities of the five reactive species increase in varying degrees,but the averaged H2O2 density increases more evidently than OH,O3,O(1D),and O-.(2)For the He/H2O and Ar/H2O plasma jets,increasing the averaged electron energy by decreasing the gap width,the averaged OH densities in the two working gases also increase,but this variation trend in He/H2O becomes more obviously than in Ar/H2O.In the He/H2O plasma jets,as the water vapor concentration increases,the averaged electron energy approximately keeps a constant value at the concentrations below about 0.5%,and basically,the averaged OH density increases linearly.But,when the water vapor concentration is above 0.5%,the averaged electron energy decreases evidently and the increase in the averaged OH density gets slow.For the Ar/H2O plasma jet,with the increase in the water vapor concentration,the averaged electron energy hardly changes,and the averaged OH density presents a linear growth.(3)From the point of view of practical application in biomedicine,the impurity of a proper high concentration in the plasma jets may allow an expected electron energy and also generate abundant reactive species.