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.

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.

Compared with the diffuse mode of helium plasma jets, a filamentary mode is normally produced in the plasma plume with less expensive inert gas, such as argon, which is undesirable because intense discharge in the filamentary mode may cause damage to fragile samples. Many efforts have been attempted to realize the diffuse mode of an argon plasma jet. In this paper, the diffuse mode is realized in an atmospheric pressure argon plasma jet with increasing biased voltage (V _b) applied to a downstream plate electrode. Results show that the diffuse mode transits from the filamentary mode with the increase of V _b. Waveforms of voltage, current and integrated light emission reveal that the filament originates from a positive discharge with a needle anode, while the diffuse mode results from negative discharges with a needle cathode, which consist of a Trichel-pulse phase and a slower-varying phase. By fast photography, the positive discharge is attributed to a streamer regime, whereas the negative discharges belong to a Townsend-discharge regime. From optical emission spectroscopy, electron density, electron excitation temperature, vibrational temperature, and rotational temperature are studied as functions of V _b.

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.