Numerical Investigation On Mechanisms And Characteristics Of Energy Spectra Of Electrons In Non-Thermal Atmospheric-Pressure Plasma Jet

The non-thermal atmospheric-pressure plasma jets,due to their advantages including great reactivity,friendly environment,simple technique,and economical feature,have shown their wide application prospects in the fields of biomedicine,material processing,and environmental protection.In the applications of the non-thermal atmospheric-pressure plasma jet in biomedicine,such as sterilization,wound healing,and cancer therapy,it has become clear that the plasmas can affect prokaryotic and eukaryotic cells through the reactive species.Electrons are important particles in the plasma jets,and they are non-monoenergetic,presenting a spectrum distribution.It has been shown that by increasing the number density and energy of electrons,more reactive species can be produced and can also have a deeper penetration into the layer of liquid on the surface of cells.Accordingly,the investigation on the energy spectrum of electrons in the plasma jets as well as its evolution and parameter effects are of importance for a deep knowledge of the propagation of the plasma jets and corresponding mechanisms,for the mass transfer of reactive species in the penetration of the plasmas into the aqueous solution,and for the synergistic effects of the particles passing through the aqueous solution on biological cells,in the applications of the plasma jets in biomedicine.In this dissertation,a model used to simulate the needle-to-plate non-thermal atmospheric-pressure plasma jets has been established,based on a one-space-dimension and three-velocity-dimension(1 D3v)Particle-in-Cell Monte-Carlo collision(PIC-MCC)method.Using this model,the evolution characteristics and parameter effects of energy spectrum of electrons(ESE)and the corresponding mechanisms have been investigated,systematically and numerically.This dissertation includes the following contents and results:1.The conventional one-dimension PIC-MCC model has been described in detail,including modeling structure,modeling particles,governing equations,and numerical method.Based on this model and using the velocity-Verlet method to solve Newton's motion equations,a 1D3v PIC-MCC model has been presented for the simulation of the needle-to-plate construction non-thermal atmospheric-pressure plasma jet.2.The spatiotemporal evolution of ESE in the non-thermal atmospheric-pressure argon plasma jet has been investigated,and the corresponding evolution characteristics are revealed.An energy conservation equation of electrons during each time step in the development of discharge is suggested for analyzing the mechanisms governing the evolution characteristics.The main conclusions are obtained as follows:(1)The temporal evolution of ESE in whole discharge space in the non-thermal atmospheric-pressure argon plasma jet(zero dimension ESE,called the ESE)is of the following characteristic.There is a characteristic time for the temporal evolution of the ESE.Before the characteristic time and as time t goes on,the peak value of the ESE decreases,the peak position shifts toward high energy,and the distribution of the ESE becomes wide,but the reverse is true after the characteristic time.The mechanism of inducing this characteristic can be mainly attributed to the transport of electrons toward a low electric field in the discharge and a balance between the energy gained from the total electric field(resulting from the external electric field and space charge field)and the energy loss due to the inelastic collisions.(2)The behavior of the average energy of electrons(AEE)versus time t corresponds to the evolution of the ESE.As time t goes on,the AEE increases before the characteristic time and then decreases,namely the AEE reaches its maximum at the characteristic time.(3)The ESE in a space volume with a unit length at a fixed position along the jet axis is defined as one dimension ESE(1-D ESE).It is shown that the evolution characteristic of 1-D ESE with time t is the same as that of the ESE.The 1-D ESE near the needle tip has a small characteristic time,compared to that far away from the needle tip.At a fixed time t,with the increase in the distance of the space point at the jet axis from the needle tip,the peak value of 1-D ESE increases,the peak position shifts toward low energy,its distribution gets narrow,and the AEE decreases.3.The effects of the discharge conditions on the ESE in the non-thermal atmospheric-pressure argon plasma jets have been systematically investigated using 1D3v PIC-MCC model.The discharge conditions include the amplitude of the applied voltage,gap distance,radius of the needle tip,dielectric thickness,and relative dielectric constant of the dielectric.The main conclusions are obtained as follows:(1)For the different amplitudes of the applied voltage,the ESEs are of the evolution characteristics similar to each other.The characteristic time decays exponentially with the voltage amplitude,namely the large voltage amplitude leads to a small characteristic time.On the other hand,at a fixed time t,with the increase in the voltage amplitude,the peak value of the ESE decreases,the peak position shifts toward high energy,and the distribution of the ESE gets wide.Therefore,electron energy in the plasma jet can be controlled by way of the voltage amplitude.(2)For the considered gap distances,the ESEs present the evolution characteristics similar to each other.The characteristic time increases exponentially with increasing gap distance,namely small gap distance leads to a fast development of the discharge and thus a small characteristic time as well as a large AEE.Accordingly,electron energy in the plasma jet can also be controlled through the gap distance.On the other hand,at a fixed time t,the peak value of the ESE increases,the peak position shifts toward low energy,and the distribution of the ESE becomes narrow,as the gap distance increases.(3)The needle tip radius dependence of the ESE presents the different behaviors in three phases of the discharge.In the first phases,i.e.,the initial phase of the discharge,increasing the radius of the needle tip,the peak of the ESE decreases,the peak position shifts toward high energy,and its distribution becomes wide,and then,i.e.,in the second phase,there is a characteristic value of the needle tip radius,with the increase in the radius of the needle tip below this value,the peak value of the ESE increases,the peak position shifts toward low energy,and the distribution of the ESE becomes narrow,but the reverse is true above this characteristic value.Actually,in the second phase,the temporal evolution of the ESE presents a transition to the third phase where the peak of the ESE increases and peak position shifts toward low energy,but its distribution gets narrow.(4)For the considered thickness of the dielectric plate,with the increase in the thickness of the dielectric plate,the peak of the ESE increases,peak position shifts toward low energy,and its distribution becomes narrow,but these effects are slight.(5)For the considered relative dielectric constant of the dielectric plate,their effects on the ESE are ignorable.4.The effects of gas compositions on the ESE in the non-thermal atmospheric-pressure plasma jet and the corresponding mechanisms have been investigated.The gas compositions include pure argon,pure helium,helium/oxygen mixture,argon/oxygen mixture,helium/air mixture,and argon/air mixture.The main conclusions are obtained as follows:(1)At a fixed time t,for helium,the ESE is of low peak value and wide distribution,and the AEE is obviously high,compared to argon.These are mainly due to the obvious differences between the thresholds of inelastic collisions and between the frequencies of inelastic collisions in argon and in helium.(2)For helium/oxygen mixture,the oxygen concentration dependence of the ESE presents the different behaviors in three phases of the discharge.In the first phase,i.e.the initial phase of the discharge,increasing oxygen concentration,the peak of the ESE decreases,and its distribution becomes wide.In the second phase,the effect of oxygen concentration on the ESE gets weak and presents a transition to the third phase of the discharge.In the third phase,the peak of the ESE increases,and its distribution gets narrow,as oxygen concentration increases.(3)For argon/oxygen mixture,the oxygen concentration dependence of the ESE is in similarity from the concentration to concentration at different time ts in the development of the discharge,i.e.,the rising peak and the distribution narrowing down with the increase in oxygen concentration,but the dependence is weak in comparison with helium/oxygen mixture.(4)The air concentration dependence of the ESE in helium/air mixture is similar to the oxygen concentration dependence of the ESE in helium/oxygen mixture,namely it presents the different behaviors in three phases of the discharge.In the first phase,i.e.,the initial phase of the discharge,increasing air concentration,the peak of the ESE decreases,and its distribution becomes wide.In the second phase,as the discharge develops,the electrons move toward the low electric field,therefore,in this phase the air concentration dependence of the ESE is weak and presents a transition to the third phase of the discharge.In the third phase,with the increase in air concentration,the AEE decreases,and the temporal evolution of the ESE is contrary to that in the first phase.(5)For argon/air mixture,the effect of the air concentration on the ESE is almost negligible under the considered air concentrations.