Numerical Simulations Of AC Driven And DC Driven Atmospheric Pressure Helium-oxygen Discharge

As a commonly used method for generating active oxygen particles,atmospheric pressure helium-oxygen discharges have been one of the research hotspot in the field of low-temperature plasma.Previous studies had focused on radio-frequency capacitively coupled discharges and atmospheric pressure plasma jets,as well as the plused dielectric barrier discharges.Seldom has been done for the ac-driven dielectric barrier helium-oxygen discharges and the DC driven helium-oxygen microdischarges.The understandings of the discharge mechanismson in these issues are still insufficient,and further detailed researchs are needed.The basic discharge characteristics of the ac-driven dielectric barrier helium-oxygen discharges at atmospheric pressure has been investigated based on the one-dimensional fluid model.It is found that the discharge operates in the glow mode and there are two discharge current pulses in each half cycle.The discharge exhibits an asymmetric feature.The main positive and negative ions are O₂⁺and O₂^-,respectively.The main neutral particles are ground state atomic oxygen,single delta oxygen and ozone.As the oxygen concentration increases,the discharge current peak increases,the maximum spatial-averaged electron temperature decreases,the maximum time-averaged electron temperature first drops and then increases,the maximum spatial-averaged electron density increases,the maximum spatial-and time-averaged densities of ground state atomic oxygen,single delta oxygen and ozone all increase.As the frequency increases,the discharge waveform experiences a transition from double-pulse discharge to single-pulse discharge.The positive half-cycle maximum current peak first increases,then decreases,and finally increases,and the negative half-cycle maximum current peak has been wavy.The maximum spatial-averaged electron temperature first decreases and then rises.The change tendency of the maximum spatial-averaged electron density is the same as that of the positive half-cycle maximum current peak.Based on the one-dimensional fluid model,the basic discharge characteristics of the DC driven helium-oxygen microdischarges at atmospheric pressure and the effects of the discharge conditions on the spatial-averaged particle density were simulated.It is observed that the main positive and negative ions are O₂⁺and O^-,respectively in the Townsend mode.The main neutral particles are ground state atomic oxygen,single delta oxygen and ozone.The selected discharge conditions are oxygen concentration,gas gap,and applied voltage.As the oxygen concentration increases,the reactions of electron attachment by oxygen molecules and O₂⁺loss is promoted,the spatial-averaged densities of electrons and O₂⁺decrease accordingly,the spatial-averaged densities of ground state atomic oxygen,single delta oxygen and ozone increase.As the gas gap increases,the spatial-averaged densities of electron,O₂⁺,ground state atomic oxygen,single delta oxygen and ozone all increase first and then decrease.The increase in the amplitude of the applied voltage leads to an increase in the rate of direct ionization reaction,so more electrons and O₂⁺are generated.The spatial-averaged densities of ground state atomic oxygen,single delta oxygen and ozone also increases because the main generation reactions of ground state atomic oxygen,single delta oxygen and ozone are directly and indirectly related to electrons.