Role of the species generated by plasma jets in the formation of the plasma bullets

Summary form only given. Prior work on atmospheric pressure plasma jets has shown that the plasma plumes are in fact made of trains of small plasma packets/bullets. These so-called bullets were found to travel at high velocities in ambient air. In this paper optical emission spectroscopy (OES) is used to identify the chemical species that may play a role in the formation and propagation of the plasma bullets. The emission spectra under various operating conditions for a helium plasma plume in air are recorded by an Ocean Optics mini-spectrometer USB4000. It is found that the emission spectra are dominated by N₂ ⁺ and by N₂ excited states. Atomic oxygen, O, and OH lines are also present. To study the spatial variation of the intensity of the main emission lines (N₂, N₂ ⁺, O, OH, and He) an optical fiber is used to collect the localized emission along the axis of the plume. It is found that the N₂ ⁺ density is a crucial parameter influencing the physical characteristics of the plasma plume. Helium emission lines seem to indicate that N₂ ionization by helium metastable states may play an important role in the sustainment and propagation of the plasma bullets.