Longfei Ji

Plasma inactivation of candida albicans by an atmospheric cold plasma brush composed of hollow fibers

It remains a challenge to generate the uniform and large-area cold plasma at atmospheric pressure. In this paper, we report a method to generate a reliable and homogeneous brush-shaped plasma plume running at atmospheric pressure. The plasma brush (110 mm in length and 10 mm in width) is mainly composed of well-aligned and hollow optical fibers. Current-voltage measurements indicate that the brush-shaped plasma plume consists of glowlike pulsed discharge with their pulsewidths of several microseconds. The generation of the He/O2 glowlike discharge is further proved by the uniform distribution of O atoms along the transverse location of the plasma brush. This brush-shaped plasma plume is used for the application in the plasma inactivation of Candida albicans cells. About 97% of Candida albicans cells with a population of 105 spores are killed when exposed to the O2/He (volume ratio: 5/95) brush-shaped plasma plume for 150 s. Analysis indicates that plasma-activated species, such as O radicals, may play a crucial role in killing the resistant Candida albicans cells.

Atmospheric pressure dielectric barrier microplasmas inside hollow-core optical fibers

An atmospheric glow microplasma is generated in the hollow core of microns-thick optical fibers (>1 m long) when the sinusoidal voltage with a peak voltage of 5 kV and a frequency of 5.0 kHz is applied to these microelectrodes along the outside of optical fibers. Measurements show that the atmospheric glow microdischarge consists of current pulses with amplitudes of tens of amperes and pulse widths of several microseconds. Atmospheric surface barrier discharges are formed along the inner surface of hollow optical fibers between adjacent microelectrodes, which results in the pulsed glow microdischarges. By flowing octafluorocyclobutane (c-C4F8)/helium (He) mixtures through the hollow-core optical fiber, fluorocarbon polymer (FCP) coatings are deposited on the inner surface of the > 1 m long optical fiber. Analysis indicates that the glow microdischarge contributes to the uniform deposition of FCP coatings on the inner surface of hollow fibers. The in situ optical emission measurements show that various car...

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.

An Atmospheric-Pressure Large-Area Diffuse Surface Dielectric Barrier Discharge Used for Disinfection Application

A large-area diffuse surface dielectric barrier discharge (SDBD) produced using ambient air at atmospheric pressure is investigated with respect to its electrical and optical characteristics. Current-voltage measurements and intensified charge coupled device images show that the multiple-groove SDBD device is able to generate a kind of visible large-area diffuse discharge at the applied voltage more than 22 kV with the repetition frequency of 9 kHz, which is associated with the accumulation of electric charges. Moreover, the multiple-groove SDBD is also found to be very efficient in the inactivation of Escherichia coli and Candida albicans on the surface of chopsticks. It is found that the electrostatic tensile generated by accumulated charges may play a key role on the cell inactivation.

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.

The effect of target materials on the propagation of atmospheric-pressure plasma jets

The current study is focused on the effect of target materials (quartz plate, copper sheet, and quartz plate with a grounded copper sheet on the back) on the propagation of atmospheric-pressure helium plasma jets. The dynamics of ionization waves (IWs) and the relative amount of reactive oxygen species (OH and O) in the IW front were compared by using spatial and temporal images and relative optical emission spectroscopy. Our measurements show that the targets can significantly affect the propagation and intensity of the IWs. In addition, strong OH emission lines were detected when the IWs impinged upon the damp surface. Numerical simulations have been carried out to explain the experimental observation. The propagation velocity of IWs predicted by the simulation was in good agreement with the experimental results. Simulation results suggest that the density and velocity of IWs mainly depend on the electric field between the high voltage electrode tip and the target. Analysis indicates that the targets co...

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.