Numerical Simulations Of The Mode Transition In Atmospheric Helium Dielectric Barrier Uniform Discharge

Atmospheric pressure dielectric barrier discharge plasma has good uniformity,moderate plasma density,temperature,and moderate energy.It is widely used in materials surface treatment,environmental protection,plasma etching and ozone synthesis.Under certain conditions,the atmospheric pressure dielectric barrier discharge can have different discharge modes.In view of this,this article studies the discharge mode of the atmospheric pressure dielectric barrier uniform discharge.The influence of nitrogen impurities on the atmospheric pressure helium discharge in Townsend mode has been investigated based on a 1D fluid model.The level of nitrogen impurities was varied in the range 0.1–10000ppm,in which the discharge exhibits three notable discharge modes,namely multi-pulse Townsend discharge,single-pulse Townsend discharge,and multi-pulse glow discharge with the increase in the impurity level.It is found that discharge characteristics are different under different nitrogen contents.Under 1 ppm nitrogen content,the dominant ion isHe₂⁺.Meanwhile both the breakdown voltage and discharge current remain unchanged,but the number of pulse decreases.As the N₂ content increases,The dominant ion is stillHe₂⁺,but nitrogen ions?N₂⁺andN₄⁺?are increased rapidly.At this time,the breakdown voltage still remains unchanged but the discharge current and the number of pulse decrease.As the N₂ content exceeds 350ppm,The dominant ion transits fromHe₂⁺toN₄⁺.And the discharge transits from the Townsend mode into the glow mode.Meanwhile the breakdown voltage decreases dramatically while the current density and the number of pulse increase.The symmetry of the atmospheric pressure helium discharges with modulation voltage has been investigated based on a 1D fluid model.It is found that the discharge transits from a symmetrical discharge into an asymmetrical discharge by increasing the duty ratio of modulation voltage.By analyzing the space-averaged electron density,ion density,space charge density and electric field at breakdown in the same cycle of two discharges?positive discharge and negative discharge?under symmetric and asymmetric discharge with the variation of the duty ratio,it is demonstrated that the change of electron multiplication at breakdown is the main reason for the symmetric-to-asymmetric transition.The simulation shows that when the duty ratio of the modulation voltage is less than 40%,the space-averaged charge density and spatial distributions of electric field at breakdown and electron multiplication in the same cycle of two discharges?positive discharge and negative discharge?is the same.And the discharge is symmetric discharge.With increase of the duty ratio,the electron multiplication changes firstly.When the duty ratio increases to 43.47%,the difference of discharge current density,the space-averaged electron density,ion density,space charge density and electric field between positive and negative discharges at breakdown appears.Due to the memory effecting of space charge,the space charge density at breakdown shows the different value in the same cycle of two discharges?positive discharge and negative discharge?,and spatial distributions of electric field of two discharges are different too.It causes the electron multiplication at the time of breakdown of two discharges in the same cycle of two discharges?positive discharge and negative discharge?is different,and the discharge is asymmetric discharge.The study of the mode transition of the atmospheric pressure helium dielectric barrier asymmetric discharge is beneficial to obtain a more uniform plasma,and it has a great significance for the application of plasmas.