Argon plume transition from a hollow swell to a diffuse swell with increasing amplitude of a trapezoidal voltage

Atmospheric pressure plasma jets can generate a remote plasma plume, which usually presents a conical or cylindrical morphology. Despite a few morphologies being observed, efforts should be made to obtain more plume structures because streamer dynamics may be revealed from them. For this purpose, an argon plasma plume excited by a trapezoidal voltage is investigated, which presents two kinds of swells (a hollow swell and a diffuse swell) with increasing voltage amplitude (V (p)). The results indicate that there are two positive discharges (D-p1 and D-p2) and one negative discharge (D-n) per voltage cycle for both of the swells. With increasing V (p), the inception voltage and discharge intensity increase for every positive discharge, while they decrease for the negative discharge. Fast photography reveals that the positive streamer (D-p2) leaves different tracks in the two swells, which are curved in the hollow swell and randomly branched in the diffuse swell. The different tracks of D-p2 are explained with the consideration of applied field strength and residual positive ions of D-p1. The existence of residual positive ions is finally verified from optical emission spectra.