Zhenhua Bi

Atmospheric-pressure microplasmas with high current density confined inside helium-filled hollow-core fibers

We report on the generation and confinement of atmospheric-pressure microplasmas inside the 100-to-2000-μm-inner-diameter (i.d.) hollow-core fibers (HCFs) filled with helium gas. The microplasma propagation inside these 10-cm-long HCFs results from the anode-driven pulse discharges with their durations of 30 ns–15 μs. The pulse current density generated at an i.d. of 100 μm is about four magnitudes higher than the glow-like one at an i.d. of 2000 μm. Analysis shows that the generation of the microplasmas with high current density can be explained by the confinement mechanism of HCFs with small i.d. values and low surface recombination rate.

Inactivation of Escherichia coli Cells in Aqueous Solution by Atmospheric-Pressure N2, He, Air, and O2 Microplasmas

ABSTRACT Atmospheric-pressure N2, He, air, and O2 microplasma arrays have been used to inactivate Escherichia coli cells suspended in aqueous solution. Measurements show that the efficiency of inactivation of E. coli cells is strongly dependent on the feed gases used, the plasma treatment time, and the discharge power. Compared to atmospheric-pressure N2 and He microplasma arrays, air and O2 microplasma arrays may be utilized to more efficiently kill E. coli cells in aqueous solution. The efficiencies of inactivation of E. coli cells in water can be well described by using the chemical reaction rate model, where reactive oxygen species play a crucial role in the inactivation process. Analysis indicates that plasma-generated reactive species can react with E. coli cells in water by direct or indirect interactions.

Effects of previous ionization and excitation on the ionization wave propagation along the dielectric tube

In this paper, by using a high precision synchronization system, the ignition time, velocity, and propagation properties of the ionization waves (IWs) have been investigated in detail from the 1st high voltage (HV) pulse to the sequential ones over a large range of the pulse-off time. In order to clarify the effects of previous ionization and excitation on the IW propagation, the density of the residual charges are controlled by varying the pulse-off time from 199 μs to 15 μs. The results show that the formation and propagation of IWs can be strongly affected by previous discharge. For a longer pulse-off time (100 μs–190 μs), the propagation velocity of plasma bullets are decreased from the 1st to the 10th HV pulse, then increased after the 10th pulse, and finally become stable after about 500 pulses. When the pulse-off time is reduced to 15 μs, the propagation velocity of plasma bullets will rapidly increase and become stable after the 1st HV pulse. The ignition voltage is significantly reduced after the 1st HV pulse with the decrease in pulse-off time. Consequently, the generation and propagation of IWs in the tube are strongly affected by the accumulation of long-lived metastable helium (He) species and residual charges from previous discharges, which is important for understanding the plasma bullet behavior.

The transfer of atmospheric-pressure ionization waves via a metal wire

Our study has shown that the atmospheric-pressure He ionization waves (IWs) may be transferred from one dielectric tube (tube 1) to the other one (tube 2) via a floating metal wire. The propagation of IWs along the two tubes is not affected by the diameter of a floating metal wire, however, their propagation is strongly dependent on the length of a floating metal wire. The propagation of one IW along the tube 1 may result in the second IW propagating reversely inside the tube in vicinity of a floating metal wire, which keeps from their further propagation through the tube 1. After they merge together as one conduction channel inside the tube 1, the transferred plasma bullet starts to propagate along the tube 2. The propagation of transferred plasma bullets along the tube 2 is mainly determined by the capacitance and inductance effects, and their velocity and density can be controlled by the length of a floating metal wire.

Generation of large-area and glow-like surface discharge in atmospheric pressure air

A large-area (6 cm × 6 cm) air surface dielectric barrier discharge has been generated at atmospheric pressure by using well-aligned and micron-sized dielectric tubes with tungsten wire electrodes. Intensified CCD images with an exposure time of 5 ns show that the uniform surface air discharge can be generated during the rising and falling time of pulsed DC voltage. Current and voltage and optical measurements confirm the formation of glow-like air discharges on the surface of micron-sized dielectric tubes. Simulation results indicate that the microelectrode configuration contributes to the formation of strong surface electric field and plays an important role in the generation of uniform surface air discharge.

Effect of helium pressure and flow rate on microplasma propagation along hollow-core fibers

The effect of helium pressure and flow rate on the propagation of microplasmas obtained inside hollow-core fibers (HCFs) with their inner diameters of 100–2000 μm has been reported in this study. Measurements show that microplasma length, breakdown voltage, pulse durations, and pulse current are strongly dependent on He flow rate and He pressure ranging from 50 Pa to 1.0 atm. Comparison between experimental results and analytical mode shows that Townsends model is valid for understanding the discharge process of microplasmas inside HCFs, where the wall effect has to be considered. Analysis indicates that long-lived metastable species (Hem*) from one previous discharge period can greatly contribute to the propagation of microplasmas along the microns-thick HCFs. An avalanche discharge model combined with the surface recombination of charged species has been used to explain the propagation of microplasma inside HCFs.

Propagation of Brush-Shaped He/O 2 Plasma Plumes in Ambient Air

In this paper, we report on the propagation behavior of the atmospheric-pressure and brush-shaped He/O2 plasma plumes generated by a specially designed plasma device. The measurements show that the frequency of applied voltage (f) and the O2 fraction and flow rate of the feed gas significantly affect the discharge propagation of the atmospheric-pressure plasma plume in ambient air. The uniformity of the brush-shaped plasma plume is greatly improved at a relatively high f or He/O2 flow rate. Addition of a trace amount of O2 into the He gas can also contribute to the formation of uniform and brush-shaped plasma plumes in ambient air. This paper confirms that the plasma device composed of well-aligned and micrometer-thick fibers can be utilized to generate large-scale plasma plumes for potential applications. The propagation of the He/O2 plasma plume in ambient air results from the applied electric field across the small gas spacing of a barrier discharge electrode configuration, where the He atoms are easily ionized or excited for generating the brush-shaped plasma.