With increasing sinusoidal frequency or its voltage amplitude, a central filament emerges in a diffuse plume downstream of an argon jet with a single-electrode geometry. The plume with a co-existing filament and diffuse part, also referred to as a diffuse-and-filamentary (DAF) plume, is formed after a sudden length increment from a diffuse plume. Only one negative discharge appears per voltage cycle for the diffuse plume. Besides the negative discharge, there is one positive discharge per voltage cycle for the DAF plume. By fast photography, negative discharge behaves as plump plasma bullets propagating along the argon flow, which results in diffuse plume or the diffuse part of the DAF plume. Comparatively, positive discharge behaves as slim bullets, leading to the filament formation. Based on optical emission spectroscopy, it is found that both electron density and electron temperature increase with increasing sinusoidal frequency or voltage amplitude for negative and positive discharges. Moreover, there are abrupt increments in them for negative discharge during the morphology transition. Compared with negative discharge, the positive one has higher electron density and electron temperature. All of the experimental phenomena mentioned above are qualitatively explained through considering the discharge enhancement by residual active species.

With increasing peak voltage applied to a plasma jet, surface discharge patterns are formed on the dielectric target, which include diffuse spot, single ring, and concentric triple rings. During the evolvement of these patterns, the number of positive current pulses increases and there is only one negative pulse per voltage cycle. Fast photography reveals that all of these patterns originate from temporal superposition of negative and positive surface discharges. The negative surface discharge is always diffused for each pattern. However, the positive surface discharges in a streamer regime are more complicated. The first positive streamer per voltage cycle is short, which propagates along radial spokes that are distributed symmetrically, resulting in a central spot of each pattern. The last positive streamer per voltage cycle is long, which propagates along a straight line at a lower peak voltage and bifurcates randomly at a higher peak voltage, resulting in diffuse background of each pattern. Other positive streamers also bifurcate after traveling certain distance from the center, and then propagate along an arc, leading to the formation of ring in patterns. The propagation behaviors of these positive streamers are discussed qualitatively by analyzing the influence of applied electric field, residual charges, and air diffusion.

Far different from conical plumes, regularly-swelling plumes are generated in an atmospheric pressure argon plasma jet excited by a biased sinusoidal voltage at low frequency. Depending on the polarity of bias voltage, the plumes with periodic swells can be solid or hollow. Results indicate that discharge initiates once per voltage cycle, which appears in the negative and positive half cycles for the solid and hollow plumes, respectively. By fast photography, it is found that streamers are involved propagating downstream of the argon flow for the two plumes. Transversely, the streamer diffuses for the solid plume, while it propagates only in the flow periphery for the hollow plume. Due to a high field, intense discharges near the nozzle provide the following swell positions with active species, which can remarkably enhance the discharges there and induce the swells. This formation mechanism of periodic swells is verified through investigating the distance of two adjacent swells as a function of gas flow rate and driving frequency. Moreover, spatial distributions of excited electron temperature and streamer propagation velocity are qualitatively explained based on the mechanism.

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.

A needle-plate geometry is used to generate two barrier-discharge filaments composed of volume discharge and surface discharge in atmospheric pressure argon, interactions of which are investigated for the first time on the nanosecond timescale using an intensified charge-coupled device. The results indicate that the onset of volume discharges for the two filaments have a periodical discharge sequence, which implies interactions of the two filaments. Moreover, strong interactions of the two filaments are controlled through surface discharges, one of which is induced by that of the other filament during the positive discharge. Different from repulsive streamers, counter-propagating streamers are attractive between the two filaments.