Finite element analysis of ring-shaped emission profile in plasma bullet

Several research groups demonstrated that atmospheric pressure plasma jets are not continuum jet but discrete fast- moving plasma packets/bullets. The discharge mechanisms are however not fully understood. In this study, we focus on modeling of ring-shaped emission patterns observed in our experiments. Our model is based on a fluid model with the local field approximation in 1D cylindrical coordinates, corresponding to a cross section of a plasma bullet. An expected concentration gradient of humid air is assumed to be present due to diffusion of air. The current model is almost identical to our previous report. The major difference is that pulse-like electric field is given perpendicular to the simulation domain. The pulse width and repetition rate are determined based on experimental conditions. The magnitude of the electric field is uniform and chosen so that a periodic steady state solution can be obtained. Our simulation results show that helium metastable density is mostly uniform except near the outer boundary where significant amount of air diffuses from the surrounding. Metastables remain at high density for much longer time than the pulse although the density decays before the next pulse. The major ionization reaction is Penning ionization between nitrogen (air) and helium metastables. The density of electrons and positive nitrogen ions show the peaks near the outer boundary due to the higher concentration of air there. Accordingly, the emission peak appears near the outer boundary, corresponding to a ring-shaped emission profile. The ring-shaped profile is consistent with our experimental observation. This mechanism is also similar to plasma needle discharge although the discharge condition are different.