Influence Of Operating Parameters On High-pressure Micro-hollow Cathode Discharge With A Cylindrical Hole

Micro-hollow cathode discharge（MHCD）has the ability to generate stable and dense plasma at high pressure with a reasonably low voltage and dissipated power.Using it as a cathode,a large-scale uniform discharge plasma can be generated at high pressure,so MHCD opens up possibilities in various applications,such as material surface modification,pollution treatment and sterilization.Hollow cathode discharge is one of the hot topics in low-temperature plasma research,and its discharge characteristics are usually investigated by fluid simulation.Therefore,MHCD with a cylindrical hole has been numerically simulated at argon pressure higher than 50 Torr by a two-dimensional particle-in-cell Monte Carlo collision（PIC/MCC）method.The main results of the paper are as follows.Results indicate that MHCD is operated in a confined phase at the beginning,which transits to an expanded phase and reaches a steady state at about 160 ns.For the expanded phase,some swells with low electron density appear,which can be attributed to secondary electrons emitted stochastically from the cathode surface.These swells increases with time,which leads to the discharge advancing toward the cathode backside until it reaches steady state.In steady-state MHCD,the majority of electrons have a low energy of several eV,and some electrons have a high energy up to 480 eV.High-energy electrons mainly appear adjacent to the cathode surface,corresponding to the secondary electrons which experience none or few collisions in the sheath.With increasing applied voltage,there is a transition from the confined phase to the expanded phase for steady-state MHCD.Maximal density of charged particles always appears at the axis,which locates in the hole for low voltage,while extends outside for high voltage.Moreover,maximal density of charged particles increases for the expanded phase with increasing applied voltage and argon pressure or decreasing hole diameter.The discharge morphology has been investigated in detail with varying operating parameters.All these results have been compared with those simulated by fluid model.Besides,with a one-dimensional fluid model,influence of the asymmetric degree of applied voltage is investigated on the characteristics of homogeneous dielectric barrier discharge（DBD）in atmospheric pressure helium.For various industrial applications,an adjustable higher electron density is a prerequisite to realize different purposes.Based on the standard sinusoidal waveform,an asymmetric sinusoidal voltage can be obtained by decreasing or increasing the ratio of the rise time in each half voltage period.As a controlling parameter,the asymmetry degree can significantly affect the applied voltage profile and consequently the discharge characteristics.By increasing the absolute value of D_as,the amplitude of the discharge current density can be significantly elevated and the current peak phase gradually approaches the origin of applied voltage.Meanwhile,the discharge transits from the Townsend mode to the glow one.Moreover,the electron density can be elevated and the existence time of relatively high electron density also be prolonged.The dominate positive ion species are changed during this process.The role of the direct ionization becomes more important,and eventually replaces the Penning ionization as the dominate pathway of electron generation,which accounts for the significant elevation in electron density.