Effects of residual channel and voltage amplitude on the streamer behavior initiated in a plasma jet simulated by a fluid model

Atmospheric pressure plasma jet (APPJ) has broad application prospects in numerous fields, which is a hot issue in low temperature plasma research. APPJ with noble gas operates in a streamer discharge mechanism. The streamer leaves an electropositive channel in its propagation trail. In this paper, a two-dimensional axisymmetric fluid model is used to study the effects of a residual channel with different density and voltage amplitude (Va) on the streamer propagation behaviors. The simulation results indicate that the streamer head (i.e., plasma bullet) always remains a disk-like structure if the density of the residual channel is pretty low (whether Va is low or high), presenting almost no effect on the propagation behaviors of streamers (such as electron density and electric field). Compared with the low Va case, the enhanced electric field leads to higher electron density and higher ionization rate under high Va, accompanied by an increase in the average propagation velocity of the streamer. In the case of medium density of the residual channel, the maximal electric field appears at the periphery of the residual channel due to the field is weakened by it, triggering a lateral discharge. As a result, the shape of the plasma bullet changes from a disk-like structure to a solid-in-hollow structure, which finally transitions into a disk-like structure. In addition, the influence of the residual channel on streamers is weakened when Va is high enough, accompanied by an increase in electron density and electric field. Under such a condition, the streamer head always looks like a disk-like structure. With the presence of very high density for the residual channel, the plasma bullet is initially disk-like near the needle electrode, which presents an annular shape as the streamer propagates towards the residual channel, and subsequently results in a solid-in-hollow structure. Finally, it turns back to the disk-like structure. For a very high density residual channel, the change of Va may not significantly affect the propagation behaviors of plasma bullets. However, the primary streamer and secondary streamer appear earlier, and the radius of the annular bullet decreases under high Va. These behaviors of the streamers are closely correlated with the electric field, which is significantly affected by the residual channel, resulting in variations of ionization rate and electron density.