The role of photo-ionization and residual electrons in atmospheric pressure non-equilibrium plasma jets

Summary form only given. Atmospheric pressure non-equilibrium plasma jets (APNP-Js) generate plasmas in open space rather than in confined discharge gaps and are promising for applications in materials processing, nanotechnology, and plasma medicine. High-speed photographs have shown that the plasma plumes are in fact composed of bullet-like plasma volumes travelling at speeds of 104-105 m/s. It is now generally agreed that the plasma bullets are similar to cathode-directed streamers1. For the propagation of cathode-directed streamers, free electrons ahead of the streamer front are needed; Photo-ionization is one of the key mechanisms to produce these free electrons. The existing theoretical models suggest possible critical roles of the local electric field induced by space charges, as well as photo-ionization and residual electron effects. However, we are not aware of experimental reports on the effect of photo-ionization on the plasma bullet propagation.Another critical factor that affects APNP-Js is the density of residual electrons left from previous repetitive discharges. The density of these electrons could be much higher compared to the seed electrons produced by natural radioactivity; this makes it even more difficult to quantify the contribution of photo-ionization. However, no investigations into the APNP-J ignition phase are presently available, especially to verify the common assumption that the residual electron density within the streamer propagation channel is high. In this work, complementary experiments and numerical modeling reveal the important role of photo-ionization in the propagation of plasma bullets. It is shown that the minimum electron concentration ~108 cm-3 is required for the regular, repeated propagation of the plasma bullets, while the streamers propagate in the stochastic mode below this threshold. The stochastic-to-regular mode transition is related to the higher background electron density in front of the propagating bullets. These findings help improving control of plasma bullets propagation in applications from health care to nanotechnology and improve understanding of generic prebreakdown phenomena.