Xuechen Li

本文利用大气压空气针-板介质阻挡放电系统,采用频率为20 kHz的正弦交流电源激励,获得了稳定的轮辐斑图.采用电学和光学手段对其时空特性及形成机理进行了研究分析.结果表明,轮辐斑图由电压正半周期上升沿的两次放电和下降沿的一次负放电叠加而成,对其形貌产生关键作用的是第二次正放电.利用增强型电荷耦合设备(intensified charge-coupled device,ICCD)对第二次正放电进行时空演化研究,发现该放电在介质表面的演化过程由轮毂-轮辐-轮辋-分叉四个阶段组成.利用发射光谱对轮辐斑图的电子密度、电子温度、振动温度和转动温度进行了空间分辨测量.结果表明,轮辐斑图从中心到边缘,电子密度、电子温度、振动温度和转动温度均呈下降趋势,这与电场强度由中心向外衰减的空间规律一致,分析发现该斑图的形成与流光机制和表面电荷密切相关.本研究揭示了针-板介质阻挡放电系统中轮辐斑图的多阶段演化行为及其等离子体参数空间分布特征,为理解针-板结构中斑图形成机理提供了理论和实验依据.

A fascinating phenomenon in atmospheric pressure liquid-electrode discharge is the formation of self-organized patterns. Up to now, the self-organized patterns are excited predominantly by a direct-current (DC) power source, while only a few studies are implemented with an alternate-current (AC) power supply since the patterns will be destroyed when the dissipated power is high. To enrich the self-organized patterns, two power supplies (DC and AC) are simultaneously employed to excite liquid-electrode discharge in atmospheric pressure air in this work. With varying peak voltage (Vp) of AC power supply, the discharge pattern evolves from a pentagon, a flower-like shape, a gear structure, to a single ring, in which the pentagon and the flower-like patterns have not been reported in the literature. Waveforms of AC voltage, discharge current, and integrated emission signal indicate that the negative discharge is stronger than the positive discharge, whose amplitude increases with increasing Vp. Fast photography reveals that during the positive discharge, there are five spots in the pentagon, while the positive discharge is diffuse in the other patterns. Except the five spots of the pentagon, the patterns are formed during the negative discharge. The emission intensities of N2+, N2, and OH radicals increase continuously with Vp. Based on optical emission spectrum, plasma parameters of the patterns including vibrational temperature, gas temperature, electron density, and electron energy are estimated as well as reduced field, which increase with increasing Vp. Furthermore, as Vp increases, solution conductivity rises while pH decreases.

Atmospheric pressure plasma jet (APPJ) offers extensive potential applications and has become a hot topic in low-temperature plasma research. The streamer dynamics are crucial to the distribution of reactive species in APPJ. To shed light on the streamer dynamics, the behaviors of splitting and merging are reported in this work for the positive streamer propagating in an argon APPJ. Results indicate that two discharges (the negative and the positive streamers) occur at the voltage rising edge and the falling edge, respectively. Their discharge intensities vary with changing pulse amplitude (Va) and bias voltage (Vb). High-speed photography reveals that the track of the positive streamer is filamentary, whose structure evolves with increasing Va from a cone, a fork, and to a fork and void. Additionally, the fork-and-void structure becomes a two-void shape with decreasing Vb. Moreover, the positive streamer demonstrates various behaviors, including linear propagation, splitting, merging, double splitting, and double merging. Spatial distributions of streamer velocity, electron excitation temperature, electron density, and molecular vibration temperature are also investigated. It is deduced that splitting and re-splitting are attributed separately to the negative ions and the Penning effect in the interfacial layer. If the negative ions in the interfacial layer are insufficient or the Penning effect at the axis is strong enough, merging and re-merging occur, respectively. The results mentioned above are of great significance for improving the performance of APPJ.

The considered atmospheric pressure plasma jets (APPJs) actually operate in the regime of a positive or negative streamer. Compared with the positive streamer, the negative streamer has been rarely studied. In this article, the behavior of the negative streamer with an external transverse electric field ( $E_{\mathrm {ext}}$ ) is investigated in an argon APPJ. The results indicate that a diffuse plume elongates and its brightness increases with increasing $E_{\mathrm {ext}}$ . The discharge initiates once per voltage cycle at the rising edge of voltage. Fast photography reveals that the discharge operates in the negative streamer regime. During its propagation, the negative streamer distorts in the opposite direction of $E_{\mathrm {ext}}$ . Using optical diagnosis, it is established that electron temperature, electron density, electron excitation temperature, reduced field ( $E$ / $N$ ), and OH density increase for the negative streamer with the increase of $E_{\mathrm {ext}}$ .

Atmospheric pressure plasma jet normally behaves as a positive streamer, whose head is honored as a powerful chemical reactor that can produce active species for various applications. Revealing and understanding the streamer dynamics are of great significance to optimize the plasma for various applications. In this Letter, the dynamic transition from one-step propagation to stepwise propagation of the positive streamer is reported through varying duration (Δt) of a pulse voltage. Results indicate that per voltage cycle, there are two discharges occurring at the falling edge and the rising edge of the pulse voltage, respectively, which are separately referred to as Df and Dr. Comparatively, Dr has a higher intensity than Df. Moreover, the integral intensity for Df and Dr increases with increasing Δt. Fast photography reveals that the positive streamer fulfills the propagation in one step with wider Δt, while it undergoes the stepwise propagation with narrower Δt. The stepwise propagation is attributed to the negative ions left by the previous negative streamer. In addition, spatial distributions of streamer velocity and excited electron temperature are also analyzed from the role of negative ions. Moreover, electron density, electron temperature, and gas temperature are compared for the different behaviors of the positive streamer. These results provide a new approach to manipulate the streamer dynamics behaviors.

The morphology of the plasma plume in atmospheric pressure plasma jet (APPJ) is a mapping of the spatial distribution of reactive species, which is actually determined by the dynamic behavior of streamers. Different to a simple striated plume (SSP) reported previously, a complex striated plume (CSP) is generated for the first time in an argon APPJ under higher peak voltage ( V _p ), with which peculiar streamer dynamics are explored. Results show that the CSP evolves from the SSP with increasing V _p . Besides, raising driving frequency ( f ) and decreasing argon flow rate ( Q ) also make the SSP transition to the CSP. For both the SSP and the CSP, the distance between two adjacent ridges (or grooves) decreases with decreasing Q or increasing f . For the SSP, the discharges only appear at the negative voltage polarity. Besides these negative discharges, a discharge initiates at the positive voltage polarity for the CSP. Fast photography reveals that the SSP is solid with ridges and grooves alternately distributed along the flow direction, which originates from the propagation of a negative streamer. In addition to the solid SSP, a hollow SSP originating from a positive streamer is formed during the positive discharge of the CSP, suggesting that the CSP is a superposition of the solid SSP and the hollow SSP. The formation mechanism of the solid SSP and the hollow SSP is discussed through analyzing the negative-ion rings in the mixing layer between the argon stream and ambient air. By optical emission spectroscopy, spatial distribution of active species including OH (A ² Σ ⁺ ) and N ₂ (C ³ Π _u ) is obtained. Moreover, temporal evolutions of intensity ratios for spectral lines are studied to reflect electron density and electron temperature for the negative and positive streamers. In addition, line intensity ratios and gas temperature are investigated as functions of V _p , f , and Q .

From a practical application viewpoint, the generation of a large-diameter plume is of great importance for the atmospheric pressure plasma jet. In this Letter, a large-diameter argon plume is generated downstream of a single-tube plasma jet with a simple needle-plate geometry. Results indicate that the diffuse plume is composed of a semi-spherical discharge close to the needle tip, a trumpet-like discharge on the top of the plate, and a dark region between them. Photoelectric measurement reveals that there is a broad hump with a duration time of >100 μs in the light signal per negative voltage half cycle. By fast photography, spatiotemporal evolution of the diffuse plume is investigated, which indicates that the semi-spherical discharge corresponds to a corona near the needle tip, and the trumpet-like discharge operates in a Townsend discharge regime. Optical emission spectroscopy is used to characterize electron excitation temperature and electron density, both of which decrease in the semi-spherical corona discharge and increase in the trumpet-like Townsend discharge with increasing distance from the needle tip.

In this paper, a large‐diameter diffuse argon plume is generated through being excited by a negatively‐biased sinusoidal voltage. The diffuse plume increases in length with increasing peak voltage and argon flow rate, which transitions to a nonuniform plume when voltage amplitude exceeds a threshold. The diffuse plume initiates at the rising voltage edge, while the nonuniform plume initiates at the rising and falling edges. From fast photography, the diffuse plume originating from the discharges at the rising voltage edge is speculated to operate in a Townsend discharge regime, while the discharge at the falling voltage edge is filamentary in the nonuniform plume. By optical emission spectroscopy, plasma parameters including electron temperature, electron density, and atomic oxygen concentration have been investigated.

Atmospheric pressure two-dimensional (2D) plasma jet has demonstrated its efficient performance in large-area target processing. In this study, a large-area 2D plasma jet is generated using a particularly designed device with a negative polarity pulsed power supply. Electrical and optical methods are utilized to investigate the 2D plasma jet. Both optical emission (brightness) and spatial characteristics (length/area) exhibit positive correlations with peak voltage and gas flow rate, demonstrating parameter-dependent plume expansion. The experimental results show that there exist two discharges in each voltage cycle, namely negative discharge and positive discharge, which correspond to the falling and rising edges of the voltage pulse signal, respectively. The negative discharge exhibits diffuse characteristics, and its origin and propagation are closely related to the negative streamer. The positive discharge is formed by positive streamers and propagates downstream in the form of 2D “bullets.” The differences in the morphology of the two discharges are related not only to the discharge mechanisms of positive and negative streamers but also to the active particles remaining on the surface of the dielectric layer. The plasma parameters of the 2D plasma plume are diagnosed by optical emission spectroscopy. The results show that plasma parameters such as the electron density and electron excitation temperature of the positive streamer are higher than those of the negative streamer. The gas temperature of the discharge is close to room temperature. It provides an important theoretical and experimental foundation for achieving stable and high-efficiency plasma jet operation in large-area target processing applications.

Dielectric barrier discharge (DBD) in open air usually operates in a filamentary mode, which limits the widespread application of DBD technology in the industrial field. In this Letter, a large-scale homogeneous DBD is obtained by using a wire mesh electrode in open air with a relatively high argon flow velocity. Polyethylene terephthalate (PET) is covered on a quartz plate, and they work together as dielectric materials. There is only one wide pulse of optical signal in each half voltage cycle for the homogeneous discharge. Fast photography reveals that the discharge is uniformly distributed over the entire electrode surface during the discharge period. When argon flow velocity is relatively low, the discharge is filamentous. There are multiple narrow optical signal pulses in each half voltage cycle for the filamentary discharge. Optical emission spectroscopy indicates that electron density (ne), electron temperature (Te), molecular vibrational temperature (Tvib), and gas temperature (Tg) decline in both homogeneous discharge and filamentary discharge with increasing argon flow velocity. Polyvinyl chloride and polypropylene dielectrics behave similar to PET for generating homogeneous discharge, which is of great significance for the uniform surface treatment of various materials.