Formation mechanism of helium plasma jet at atmospheric pressure under medium-frequency sinusoidal voltage

In order to discuss the formation mechanism of He atmospheric pressure plasma jet (APPJ), the intensified CCD (ICCD) is employed to shot the longitudinal and axial section of He APPJ. It is found that, the He APPJ is not symmetrical under positive and negative cycles of applied voltage of 17 kHz Sin wave. Under positive cycle, the He APPJ which is formed as space streamer at the edge of an active electrode propagates to the nozzle of quartz tube, and then it transits to the surface discharge of which the propagation appears along the interface between He and Air in the ambient. However, under negative cycle of applied voltage, the propagation length of the He APPJ is much shorter than that under positive cycle, which presents as a classic corona discharge in He flow. Furthermore, the relationship of APPJ length and discharge modes of dielectric barrier discharge (DBD) inside tube is also investigated. With the increment of applied voltage, 6 kinds of discharge modes such as the "period-doubling", "chaotic", "streamer transit to glow", "unsymmetrical glow", "symmetrical glow", and "glow and filament", appear successively, possessing different characteristics and properties in discharge current waveforms. The length of plasma plume is not only caused by the applied voltage alone but actually determined by the excited He particles density during the discharge and disturbance of gas flow caused by discharge.