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.

In this Letter, a uniform air plasma plume with a fairly large scale is generated downstream of an air flow through interactions between a pair of argon plasma jets excited by a direct current power supply. With increasing power voltage, two separated plumes are first generated downstream of the pair of jets, which transit to the uniform planar plume with a fairly large scale. The planar plume elongates with the increasing air flow rate or power voltage. Moreover, the temperature of the planar plume is comparatively low with a maximum of about 80 °C, which is symmetrically distributed perpendicular to the pair of jets. For the separated plumes, discharge current is intermittently pulsed under a time-invariant gap voltage. However, both gap voltage and discharge current are periodically pulsed for the planar plume. High-speed video images reveal that the planar plasma plume stems from two discrete plumes flushing out of the pair of jets, which are soon merged together by a cathode-directed streamer due to interactions between them, leading to the generation of an arc-shaped filament. Due to the air flow, the arc-shaped filament keeps moving away from the jet vicinity until it quenches at last, which belongs to a glow discharge regime. In addition, surface modification of polyethylene terephthalate (PET) is performed with our planar plasma plume, which reveals that the hydrophilicity of the PET surface can be improved greatly.

An atmospheric pressure argon plasma brush with air addition is employed to treat polyethylene terephthalate (PET) surface in order to improve its hydrophilicity. Results indicate that the plasma plume generated by the plasma brush presents periodically pulsed current despite a direct current voltage is applied. Voltage−current curve reveals that there is a transition from a Townsend discharge regime to a glow one during one discharge period. Optical emission spectrum indicates that more oxygen atoms are produced in the plume with increasing air content, which leads to the better hydrophilicity of PET surface after plasma treatment. Besides, an aging behavior is also observed. The hydrophilicity improvement is attributed to the production of oxygen functional groups, which increase in number with increasing air content. Moreover, some grain-like structures are observed on the treated PET surface, and its mean roughness increases with increasing air content. These results are of great importance for the hydrophilicity improvement of PET surface with a large scale.

Being able to generate a remote plasma plume, the atmospheric pressure plasma jet has become an indispensable tool for extensive application fields. A plasma plume usually has a straight column morphology, which results from straight-line or stochastic snake-like propagations of streamers. The snake-like propagation of streamers is unclear in the mechanism. In this paper, a meandering plume is generated for the first time downstream of an argon plasma jet excited by a positively biased sinusoidal voltage, which transits from a straight column plume with an increasing bias voltage (Vb). Results indicate that the wavelength of the meandering plume is inversely proportional to voltage frequency and directly proportional to the gas flow rate. For every voltage cycle, integrated light emission from the meandering plume presents one pulse with low Vb and two pulses with high Vb. Fast photography reveals that the meandering plume results from repeated snake-like propagations of positive streamers due to the influence of ion clouds. To confirm the existence of ion clouds, an opposite argon flow is introduced. With wiping off ion clouds in the argon stream, the meandering plume gradually turns back to a straight column with an increasing opposite flow rate. All these phenomena mentioned above have been explained qualitatively on the basis of ion clouds. These results may shed light on the streamers' dynamics in the atmospheric pressure plasma jet.

A uniform plume with pulsed discharges is generated through using a plasma brush excited by a direct current power supply. The results indicate that the plume length increases with the increasing gas flow rate or dissipated power. The optical emission spectrum from the plasma brush reveals that active species are abundant in the plasma plume. Based on the spectrum, an electron density on the order of 1014 cm−3 is obtained, which increases with the increasing dissipated power and gas flow rate. After a single scan of the plasma brush on the polyethylene terephthalate surface, a uniform surface modification is achieved with an improved hydrophilic width of about 24 mm. The water contact angle of the surface decreases with the decreasing scanning velocity and nozzle-sample distance or the increasing dissipated power and gas flow rate. Moreover, the treated surface shows an aging behavior in 6 days. Raman spectra indicate that oxygen-containing polar groups are generated on the treated polyethylene terephthalate surface. The polar groups are contained in oxidized materials, which are observed by scanning electron microscopy.

Self-deformation has been observed for an atmospheric pressure diffuse discharge initiated in the air gap between a needle anode and a water cathode, which undergoes a morphology scenario from a cone, an axially symmetric horn, to a rectangular horn as time elapses. During the self-deformation process, pH value and electric conductivity of the water cathode vary with time. Moreover, electrical measurements indicate that applied voltage and discharge current are time-invariant for the conical discharge, while periodically pulsed for the other two discharges. This may suggest different formation mechanisms of them. Hence, fast photography is implemented by a high-speed video camera and an intensified charge coupled device. Results indicate that the diffuse discharges with different morphologies belong to a glow discharge regime. Among them, the conical discharge is composed of stationary micro-discharge, and the horn-shaped discharges consist of moving micro-discharge. Moreover, the axially symmetric horn is a self-rotating version of the rectangular horn. Furthermore, the rectangular horn originates from the swing of a micro-discharge filament like a pendulum motion. Finally, qualitative explanations are given for the experimental phenomena mentioned above.

Excited by a modulated sinusoidal voltage, self-organized filaments are generated in a parallel plate dielectric barrier discharge with a flowing mixture of argon and nitrogen at atmospheric pressure. With increasing off time of the modulated voltage, a single filament transits into a pair of filaments. Then, the self-organized filaments undergo a scenario from triangle, quadrilateral, pentagon, hexagon, and finally, to a ring composed of rotating filaments. During the transition process, the discharge current always presents a single pulse per half voltage cycle, whose amplitude increases for both positive and negative discharges. However, discharge current symmetry deteriorates. Moreover, with increasing off time, the inception voltage increases for the positive discharge, while it decreases for the negative discharge. For the hexagonal arranged filaments, temporal evolutions are implemented for the positive and negative discharges. The results reveal that the initiation in one current pulse seems to propagate opposite to the gas flow direction in the positive discharge, while advances along it in the negative discharge. By optical emission spectroscopy, the electron temperature and electron density are investigated via Boltzmann plotting and a line ratio from 738 nm to 750 nm, respectively. With increasing off time, both of them increase for the positive discharge, while they decrease for the negative discharge. What is more, both electron temperature and electron density increase as the inception voltage increases.

Through using a direct-current driven plasma jet operated underwater, degradation of methylene blue (MB) is investigated with air and oxygen used as working gases. With a low power voltage, a plasma plume extends from the needle electrode, which is purple in air. It turns pink after it bridges the two electrodes. During the process, oxygen plasma remains white. Discharge operates in a pulsed mode or a continuous one, which depends on the magnitude of power voltage. For the pulsed mode, oxygen discharge has a shorter plume and a higher pulse frequency than air discharge under the same power voltage. For the same current of the continuous mode, both power and gap voltages of oxygen discharge are higher than those of air discharge. Moreover, MB degradation efficiency increases with increasing power voltage or initial concentration of MB solution. Compared with air discharge, oxygen discharge has a higher degradation efficiency with the same power voltage and treatment time. The pulsed oxygen discharge with power voltage of about 6.5 kV has the highest efficiency in degrading MB dye, reaching approximately 85.8% after 10 min treatment. As a comparison, after 10 min treatment in air discharge, the highest degradation efficiency is 63.7%, which appears in the continuous mode at a power voltage of 10.6 kV. Besides, optical spectra from the discharges are also compared for the two types of working gases.