Plasma Bullets
ByMounir Laroussi
Pages 9

Nonthermal atmospheric pressure plasma jets have been playing an important role in plasma processing including biomedical applications. This is due to the ability of providing plasmas with ambient air, not confined by electrodes. To study the ignition, formation, and propagation of plasma jets, investigators used intensified charge-coupled device (ICCD) imaging, optical emission spectroscopy, and electrical probes. It was shown that plasma jets are a series of plasma packets/bullets traveling at high velocities. The plasma bullet phenomenon was first independently reported by Engemann and coworkers for a radio-frequency jet and by Laroussi and coworkers in the case of a nanoseconds-pulsed DC jet. Photoionization was first proposed as a necessary mechanism to explain the propagation of the plasma bullets. There is consensus that plasma bullets are guided ionization waves where, along with photoionization, the electric field at the head of the propagating plasma plays an important role. Experimental work shows a correlation between the discharge current and ICCD images which reveal when and how the bullets are emitted from the device. The ICCD images along with Abel inverted emission spectra show that at the jet formation point the bullet exhibits a hollow structure (donut/ring shape). The same diagnostics also reveal that as the plasma bullet propagates away from the source, its shape and size change and eventually collapse on itself. In this entry, plasma bullets, their dynamics and structure, and some of their biomedical applications including the inactivation of bacteria and the destruction of cancer cells are described.