Modeling study on different discharge characteristics in pulsed discharges with and without barriers on electrodes

High-pressure nanosecond pulsed discharges(NPDs)have attracted increasing attention in recent years due to their wide potential applications.In this study,a barrier-free NPD in pure helium plasma at 120 Torr was numerically investigated by a one-dimensional self-consistent fluid model,and its current–voltage characteristics show very different behaviors from those in pulsed dielectric barrier discharges(DBDs),indicating an entirely distinctive discharge evolution in pulsed discharges with or without barriers on electrodes.Without the control of barriers,the computational data suggest that the discharge current increases very sharply during the plateau phase of the pulsed voltage and reaches its peak value at approximately the instant when the pulsed voltage starts to drop,together with a gradual reduction in the sheath thickness and an increase in electric field in the sheath region,which is in good agreement with experimental observations.By increasing the voltage plateau width and repetition frequency,the discharge current density from the simulation can be substantially enhanced,which cannot be observed in conventional pulsed DBDs,and the spatial distributions of the electric field and charged particles are given to unravel the underlying physics.From the computational data,the distinctive discharge characteristics in barrier-free NPDs are deeply understood,and could be further optimized by tailoring the waveform of the pulsed voltage to obtain desirable plasmas for applications.

A Study on Numerical Calculation Method of Small Cluster Density in Percolation Model

Percolation theory deals with the numbers and properties of the clusters formed in the different occupation probability. In this Paper, we study the calculation method of small clusters. We calcu-lated the small cluster density of 1, 2 and 3 in the percolation model with the exact method and the numerical method. The results of the two methods are very close, which can be verified by each other. We find that the cluster density of all three kinds of small clusters reaches the highest value when the occupation probability is between 0.1 and 0.2. It is very difficult to get the analytical formula for the exact method when the cluster area is relatively large (such as the area is more than 50), so we can get the density value of the cluster by numerical method. We find that the time required calculating the cluster density is proportional to the percolation area, which is indepen-dent of the cluster size and the occupation probability.

Numerical study on peak current in pulse-modulated radio-frequency discharges with atmospheric helium–oxygen admixtures

Atmospheric pressure pulse-modulated radio-frequency(rf) plasmas have drawn growing attention due to their potential in applications.By selecting appropriate modulation parameters,the diffused and large-volume plasma can be generated in the pulse-modulated rf plasma with plenty of reactive oxygen species,which is essential for the biomedical application of helium–oxygen plasmas.In this paper,by means of a fluid model,the formation of the peak current in the first period(PCFP) in a pulse-modulated rf helium–oxygen discharge driven by a sinusoidal voltage is discussed,the existence of a reverse field near the anode caused by the negative and positive charges contributes greatly to the mechanism of PCFP.In the simulation,as oxygen admixture increases,the negative ions of O~- and O~-become dominative anions in the sheath region,which can’t be driven to the anode very quickly to build a reverse field,thus the PCFP eventually disappears.This study can effectively enhance the understanding of different transportation behavior of heavy negative ions and electrons,and further optimize pulsemodulated rf discharges with helium–oxygen mixtures in various applications.