PIC-MCC Simulation Study On The Discharge Character Of Microspace And Microtime Plasma

The generation of homogeneous nonequilibrium atmospheric-pressure plasmas through gas discharge has become one of the most attractive researches owing to their high efficiencies, potential economic and technological impact in industrial applications. The character of nonequilibrium enables the use of plasmas in a broad range such as plasma display panel,plasma medicine,nanofrabrication, plasma stealth,etc.Atmospheric pressure discharge tends to develop to thermodynamic equilibrium because of the frequently collosions,the high temperature in cathode may also lead to a arc discharge which is harmful to some uses such as the surface treatment of materials. Some strategies are commonly used for generating large-scale nonequilibrium plasmas such as microdischarge,pulse discharge.In the paper,we address the discharge character of microtime-scale and microspace-scale plasma,given the experimental challenges,computer simulation provides a valueable alternative for the study and a PIC-MCC modeling is setting up, and the main contents and results are summarized as follows.l)Microplasmas generated in Atmospheric radio-frequency (rf) discharges have shown several unique characteristics revealed by experimental and computational investigations. In this paper, a PIC-MCC simulation is performed to study the evolution of discharge structures while varying the current density and the coefficient of secondary electron emission at a given electrode gap of 75 μm. As the current density increases the electron density and electron energy also enhance, as well as the electric field in the sheath region. On the other hand at a given current density by increasing the coefficient of secondary electron emission, the plasma density rises but the electric field in sheath reduces. The numbers of electrons with energy higher than 20 eV decrease and the electrons with energy smaller than 2 eV increases. Its difficult to obtain a sheath leading structure with the low frequency 0.5 MHz. This study is helpful to deepen the understanding of discharge structures in atmospheric rf microplasmas.2) Pulse-modulated rf discharge has attracted lots of attentions recent years, the discharge is operated to generate plasma only in a fraction of rf periods,the power consumption is saved and the stability of the discharge is enhanced. According to the simulation result,when the rf voltage is on immediately after voltage-off time in microwave pulse-modulated microplasma, the corresponding discharge current shows a strong peak, a region of enhanced electric field is formed near the cathode without a change in the field polarity and some high energy electron are formd with the phenomenon and some electron can gain energy over 40 eV.In the microwave pulse-modulation,higher duty cycle,lower modulation frequency,higher rf frequency and smaller electrode sapcing are beneficial to the emerge of high current peak.Higher duty cycle and lower modulation frequency are beneficial to generate more high energy electron during a modulation cycle in microwave pulse-modulated microplasma.3)Pulsed discharge shows lots of advantages to generate stable atmospheric pressure nonequilibrium plasma,according to the simulation result in the paper,the discharge current peak of the nanosecond pulsed discharge without dielectric barrier appears to coincide with the fall of the voltage.The electron density keeps rising during the pulsed voltage and falling in the power-off period.The electron energy rises during the voltage increases and then falls because of the change of space charge distribution,then the electron energy will rise again during the peak value time,and in the end the electron energy falls with the voltage decreases.The electrical field may keep rising with the voltage increases and finally falling with the voltage decreases.