Investigation On The Discharge Characteristics Of An Atmospheric Pressure Plasma Plume Excited By An Alternating Current Voltage

Atmospheric pressure plasma jet can generate low temperature plasma plume with a variety of reactive species in open space,which is one of the hot topics in low temperature plasma investigation.The scale of the traditional plasma plume is very small,so a large-scale plasma brush excited by alternating current voltage is generated between two bare needle electrodes in the downstream region of the argon flow.The main contents are as follows:The large-scale plasma plume consists of a uniform region at the core and asymmetric brighter zones at the two sides.The left side near the ground electrode is much slimmer than the right side near the powered electrode,and there is a dark region in the vicinity of the outlet.Moreover,the plasma plume turns longer with increasing the dissipated power.Through electrical methods,results indicate that besides the primary oscillations of the applied voltage,a secondary oscillation is observed for the amplitude envelope of the gas voltage.The secondary oscillation is composed of an increasing amplitude region and a fluctuating amplitude one.With increasing the dissipated power,the duration time of the increasing amplitude region increases,and that of the fluctuating amplitude region decreases until the latter one disappears completely.Furthermore,there are one current pulse and one emission pulse per half voltage cycle in the increasing amplitude region.In the fluctuating amplitude region,there is still one emission pulse per half voltage cycle,however,only two current pulses per fluctuation can be found just before and after the abrupt voltage change.The spatio-temporal evolution of the plasma brush is investigated with a high-speed video camera,result shows that the plasma brush is in fact a temporal superposition of the moving micro-discharge filament,which periodically generates in the vicinity of the outlet and disappears at the plume tail.An ICCD is synchronized with the oscilloscope by an external trigger,and the ICCD image and the corresponding waveforms can be recorded simultaneously.Results indicate that the plasma column length and its minimal impedance increase with the plasma column moving away from the outlet in a voltage envelope.That is to say,the amplitude envelope of the gas voltage is attributed to the periodic movement of the plasma column.Based on optical emission spectroscopy,the gas temperature of the plasma brush is investigated in detail.Results shows that the gas temperature of the plasma brush increases with increasing the dissipated power or decreasing the gas velocity.And the AC plasma brush has a slightly lower temperature than the DC plasma brush.