Experimental investigations of ring-shaped plasma bullets emitted by a pulsed plasma jet

Non-thermal atmospheric pressure plasma jets have recently been playing an increasingly important role in plasma processing. These devices are able to provide plasma plumes/jets in ambient air and not confined by electrodes. In this paper we report experimental investigations on the characteristics of the plasma jet emitted by a pulsed plasma source, the "plasma pencil". The plasma pencil is driven by high voltage pulses (up to 10 kV) with variable pulse widths (from nanoseconds to milliseconds) and repetition rates (up to 10 kHz). Using ICCD images we show that the plume is a series of plasma packets/bullets traveling at supersonic velocities. The plasma bullet phenomenon was first observed by Teschke and co-workers for an RF jet (2005) and Laroussi and co-workers in the case of a nanoseconds pulsed jet (2006). The ICCD images revealed that the plasma bullets are hollow and assume a ring (or donut) shape. Based on these observations we propose that surface ionization waves are behind the formation and propagation of the plasma bullets. Along with these results we also show that the applied voltage magnitude, the pulse length, and the gas flow rate are the major parameters affecting the characteristics of the plume/jet.