On the Influence of Metastables on the Formation and Propagation of Guided Ionization Waves

Non-equilibrium atmospheric pressure plasma jets have been under investigation for more than a decade¹. There is general agreement that these plasma jets are sustained by guided ionization waves (known as plasma bullets)². Many experimental measurements reported on the magnitude and axial distribution of the electric field at the wave front. However, the mechanisms of formation of the plasma bullets and how the ionization ahead of the propagating front occurs is not well understood. The presence of helium metastable plays an important role in propagation of the jet and radical generation. In this paper we estimate the density of metastable in the channel and investigate the influence of small admixtures of air to a helium plasma jet on the physical and chemical makeup of the plasma bullet. We report on the importance of metastable helium in ionizing the trace amounts of nitrogen and oxygen molecules. In addition, we investigate how the presence of small amounts of humidity (H₂O) in the gas flow influences the ionization process and the distribution of species generated in the afterglow.

¹ M. Laroussi and T. Akan “Arc-free Atmospheric Pressure Cold Plasma Jets: A Review”, Plasma Process. Polym. 4, 777, (2007).

² X. Lu, G. Naidis, M. Laroussi, and K. Ostrikov, “Guided Ionization Waves: Theory and Experiments”, Physics Reports 540, 123, (2014).