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Formation, propagation, and contraction of the plasma bullets emitted by a pulsed plasma jet

Recently non-thermal atmospheric pressure plasma jets have been playing an important role in plasma processing including biomedical applications [1]. This is due to the ability of providing plasmas not confined by electrodes. In this paper we report experimental investigations on the characteristics of the plasma jet emitted by a pulsed plasma generator, the “Plasma Pencil”. To study the formation and the discharge phenomenon of the plasma jet, we investigated the jet initiation point for three different electrodes configurations. The devices were operated by a high voltage pulse generator (up to 10 kV) with variable pulse widths and repetition rate. Using ICCD images we show that the plume is a series of plasma packets/bullets traveling at high 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) [1]. Correlation between the discharge current and ICCD images reveals when and how the bullets are emitted from the device. The ICCD images of the jet formation point and the discharge current waveform showed that the jet generated in a discharge chamber with dielectric barrier method, begun as a surface discharge around the outlet hole of the barrier and propagated as a surface discharge, maintaining a donut shape. If the high voltage electrode is directly exposed to the ambient gas (no barrier) a filamentary discharge was found responsible for the jet formation and propagation. Depending on the input voltage and the configuration of the discharge chamber, the discharge process could be filamentary or glow-like. Even when the bullet first exhibited a donut shape, it eventually contracted and collapsed to a single head as it propagated away from the device.