Plume transition from solid to hollow with increasing the bias value of a sinusoidal voltage applied to an argon plasma jet

In this paper, a detailed study is carried out on an argon plasma jet excited by a biased sinusoidal voltage. Results indicate that, with increasing the biased voltage from negative polarity to positive one, the resultant plume transits from solid to hollow. Through implementing fast photography, a plump guided bullet is found to propagate in the solid plume, in which a negative streamer mechanism is involved. However, a positive streamer mechanism is involved in the hollow plume. It behaves as a thin guided streamer starting out from the driven electrode end, which subsequently evolves into branching streamers propagating in the flow periphery. To investigate the formation mechanism of the two plumes, the intensity ratio of spectral lines is investigated.