PIC/MCC Simulation Study On Discharge Characteristics Of Atmospheric Discontinuous Discharges

In recent years,the research on gas discharge under atmospheric pressure has attracted more and more attention.Because it is easy to be generated in an open environment,it is of great significance for the research and application of atmospheric-pressure plasma.The generation of homogeneous nonequilibrium atmospheric-pressure plasma has important application value in many aspects,such as biomedicine,material surface treatment and so on.However,The condition to generate a homogeneous nonequilibrium plasma at atmospheric pressure is relatively severe,for the reason that gas discharge tends to convert from non-equilibrium to thermodynamic equilibrium with frequent collision in atmospheric pressure,and this conversation is difficult to control.A variety of discharge strategies have been widely used to control this conversion to generate a stable atmospheric pressure non-equilibrium plasma,which is mainly realized by discont:inuous discharge,including pulse modulation RF discharge,pulse discharge,etc.This not only reduces the power loss,but also reduces the gas temperature and strengthen the discharge stability.This paper mainly introduces the characteristics of discontinuous discharge under atmospheric pressure.Due to its high-pressure and micro-dimension,the electron energy probability function(EEPF)changes very sharply with time,exhibiting long high-energy electron tails,which indicates generating a large number of energetic electrons.In this form of discharge,the electron energy is far from Maxwell distribution.Therefore,the fluid simulation method is no longer adaptable.The research on the mechanism of microplasma discharge is limited,and the corresponding experimental diagnosis and numerical simulation has been facing many challenges.In order to get the kinetic characteristics of non-equilibrium plasma under atmospheric pressure accurately,this paper uses particle simulation to research Pulse modulated RF discharge and pulse discharge under atmospheric pressure.The main conclusions were drawn:(1)Pulse-modulated RF discharge enable to generate stable high-density plasma in large volume under certain conditions,which can effectively heat electrons and reduce power loss.If the RF frequency is increased to very high frequency or even the microwave frequency band,a peak current will appear in the first RF period during power-on phase in pulse modulation conditions.and a large amount of high-energy electrons are generated,whose energy is even larger than 50eV.It is important to study the mechanism of energetic electrons and peak current generation,and necessary to understand how discharging parameters influence on plasma discharge characteristics during power-off phase.By varying duty cycle and the voltage modulation ratio during power-off period,the evolution on energetic electron density and peak current has been investigated.Simulation results show that as the duty cycle increases,the energetic electron density becomes smaller,and the amplitude of peak current firstly increases and then decreases.As the voltage modulation ratio increases during power-off phase,the energetic electron density decreases gradually,and the amplitude of peak current decreases firstly and then increases.This study is of great significance for optimizing the generation of peak current and energeric electrons in pulse-modulated discharge and the conversion of discontinuous discharge to continuous discharge.(2)Pulse driving discharge is another important method to generate non-equilibrium plasma,with high electron generation rate and energetic electron density.Although pulse discharge has been studied by the fluid simulation method,the research on nanosecond-scale pulse discharge,which is under atmospheric pressure in micro-gap,is relatively lack.Therefore.considering the effect of microcosmic electron energy on the discharge,it's necessary to adopt a more accurate PIC/MCC model.In the fourth chapter,the PIC/MCC model has been used to study the nanosecond-scale pulse discharge under atmospheric pressure,including the research on discharge characteristics and its inner mechanism,as well as exploring how the discharge parameters,such as the length of the flat and the voltage rise rate.influents the discharge characteristics.The results show the evolution of electron and ion density distribution.spatial electric field distribution and EEPF,and the inner mechanism has been revealed Besides that,it has been investigated that how the discharge characteristic evolves with different length of pulse flat and voltage rise rate.It's shown that the amplitude of large current at the end of pulse flat increases gradually with the length of pulse flat and voltage rising rate and the peak current in rising period increases with voltage rising rate.