Simulation On Mechanism And Characteristics Of Pre-ionized Direct-current Glow Discharge In Atmospheric Helium

Atmospheric-pressure low-temperature plasma has many applications in various fields,such as material treatment,biomedical applications,environmental governance and so on.Due to no expensive vacuum system,moderate areal power density,and uniform discharge,atmospheric-pressure glow discharge has become a common method for generating atmospheric-pressure low-temperature plasma.The atmospheric-pressure glow discharge can be generated by dielectric-barrier discharge（DBD）and direct-current（DC）glow discharge.As for the DBD,the mechanisms of which have not been clearly clarified.Therefore,it is rather difficult to control the operation conditions of plasma reactor working in glow mode at atmospheric pressure.Compared with DBD,DC glow discharge can easily generate uniform plasma with high voltage applied between two naked electrodes.As result of the above advantages,DC glow discharge has followed with interest.Applying high-voltage DC power between two bare electrodes can easily and quickly generate plasma.However,high-voltage DC power supplies can increase energy consumption and reduce operational safety.These defects limit its application in various fields.In recent years,pre-ionization has become a hotspot in the research of optimizing discharge and enhancing discharge performance.To solve the above problems,we use the pre-ionization to enhance DC glow discharge.Due to the fact that the pre-ionization is mainly experimental,and there are difficulties in characterizing relevant parameters in the experiment.In addition,the discharge process involves complex physical and chemical reactions.Therefore,the mechanism of pre-ionization enhancing DC glow discharge has not yet been fully elucidated and parameter control is not yet clear.In order to better elucidate the mechanism,this article used one-dimensional and two-dimensional self-consistent fluid models to theoretically study the discharge characteristics of atmospheric-pressure DC glow discharge at different pre-ionization levels.The models select two discharge lengths of long gap（10 mm）and short gap（1mm）for simulation,and analyze the spatial distributions of electron density,ion density,electric field potential and electron temperature,as well as the change trends of maintaining voltage,discharge current and discharge power in DC glow discharge under normal pressure helium.This work helps us to deeply analyze the formation mechanism of DC glow discharge under the effect of pre-ionization and provides a theoretical basis for optimizing the plasma source.The research content of this thesis is divided into the following two parts:（1）A one-dimensional self-consistent fluid model was employed to comparatively investigate the influence of pre-ionization on the helium DC glow discharge in the large gap and the small gap at atmospheric-pressure.The results show that for the large-gap and small-gap discharges,with the increase of pre-ionization,the electron and ion density distributions shift towards the cathode as a whole,resulting in an extension of the positive column region and a narrowing of the cathode fall layer.As for the electric field,there is also a phenomenon of contraction in the strong electric field region,and the electric field in the cathode fall layer gradually decreases with the enhancement of pre-ionization.Due to pre-ionization,both the sustaining voltage and the discharge power are significantly reduced.In addition,during large-gap discharge,the potential fall gradually shifts from the cathode fall layer to the positive column region,while during small-gap discharge,the potential fall always concentrates in the cathode fall layer.（2）A two-dimensional self-consistent fluid model was employed to comparatively investigate the influence of pre-ionization on the helium DC glow discharge in the large gap and the small gap at atmospheric-pressure.The results show that for the large-gap and small-gap discharges,as the pre-ionization increases,the distribution of charged particles gradually contracts towards the cathode along the discharge direction（x direction）.From the vertical discharge direction（y direction）,the distributions of electron and ion densities are more uniform.As for the electric field,with the enhancement of the pre-ionization,the x-directional component distribution of the electric field in the cathode fall layer gradually contracts toward the cathode,and the electric field near the cathode decreases as a whole and distributes more uniformly.The y-directional component distribution of the electric field gradually decreases,while the electric field area gradually shrinks towards the wall surface.The sustaining voltage and discharge power have significantly decreased;when the pre-ionization level is1×10¹⁵ m^-3,the maintaining voltage and discharge power can be reduced to 35.1%and36.6%of the original one,respectively.The overall simulation results show that pre-ionization can effectively enhance the discharge uniformity while reducing the maintaining voltage and energy consumption of the discharge.This work provides a theoretical reference for further optimization of the electrode configuration and operating parameters of the plasma source in practical applications.