Three modes of a direct-current plasma jet operated underwater to degrade methylene blue

A direct-current air plasma jet operated underwater presents three stable modes including an intermittently-pulsed discharge, a periodically-pulsed discharge and a continuous discharge with increasing the power voltage. The three discharge modes have different appearances for the plasma plumes. Moreover, gap voltage-current characteristics indicate that the continuous discharge is in a normal glow regime. Spectral lines from reactive species(OH, N_2, N_2~+, H_α,and O) have been revealed in the emission spectrum of the plasma jet operated underwater.Spectral intensities emitted from OH radical and oxygen atom increase with increasing the power voltage or the gas flow rate, indicating that reactive species are abundant. These reactive species cause the degradation of the methylene blue dye in solution. Effects of the experimental parameters such as the power voltage, the gas flow rate and the treatment time are investigated on the degradation efficiency. Results indicate that the degradation efficiency increases with increasing the power voltage, the gas flow rate or the treatment time. Compared with degradation in the intermittently-pulsed mode or the periodically-pulsed one, it is more efficient in the continuous mode, reaching 98% after 21 min treatment.     

Physicochemical properties of the AC-excited helium discharges using a water electrode

In this paper, the AC-excited helium discharges generated between the powered needle electrode enclosed in a conical quartz tube and the grounded de-ionized water electrode are investigated.The current and voltage waveforms exhibit a transition from the glow-like to streamer-like mode discharges, which forms a stable cone-shaped structure at the gas–liquid interface. In this region,the air and water vapor diffusion initiate various physical–chemical processes leading to substantial changes of the primary species emission intensities(e.g., OH, N_2, NO, and O) and the rotational temperatures. The experimentally measured rotational temperature at the gas–liquid interface is 870 K from the N_2(C–B) band with a power input of 26 W. With the prolongation of the discharge time, significant changes in the discharge voltage and current, discharge emission patterns, instantaneous concentrations of the secondary species(e.g., H_2O_2,NO_2~-, and NO_3~-) in the liquid phase, p H values and electrical conductivities of the liquids are observed experimentally. The present study is helpful for deepening the understandings to the basic physical–chemical processes in the discharges in contact with liquids, especially to those occurring in the vicinity of the gas–liquid interface, and also for promoting existing and potential applications of such type of discharges in the fields of environmental protection, biomedicine,agriculture, and so on.     

A Repetitive Nanosecond Pulse Source for Generation of Large Volume Streamer Discharge

Using a unipolar pulse with the rise time and the pulse duration in the order of microsecond as the primary pulse, a nanosecond pulse with the repetitive frequency of several kilohertz is generated by a spark gap switch. By varying both the inter-pulse duration and the pulse frequency, the voltage recovery rate of the spark gap switch is investigated at different working conditions such as the gas pressure, the gas composition as well as the bias voltage. The results reveal that either increase in gas pressure or addition of SF6 to the air can increase the voltage recovery rate. The effect of gas composition on the voltage recovery rate is discussed based on the transferring and distribution of the residual space charges. The repetitive nanosecond pulse source is also applied to the generation of large volume, and the discharge currents are measured to investigate the effect of pulse repetition rate on the large volume streamer discharge.     

The effect of gas additives on reactive species and bacterial inactivation by a helium plasma jet

In this paper,the influences of gas doping(O_2,N_2,Air)on the concentrations of reactive species and bactericidal effects induced by a He plasma jet are studied.Firstly,results show that gas doping causes an increase in voltage and a decrease in current compared with the pure He discharge under the same discharge power,which might be attributed to the different chemical characteristics of O_2 and N_2 and verified by the changes in the gaseous reactive species shown in the optical emission spectroscopy(OES) and Fourier transform infrared(FTIR)spectroscopy.Secondly,the concentrations of aqueous reactive oxygen species(ROS) and reactive nitrogen species(RNS) are tightly related to the addition of O_2 and N_2 into the working gas.The concentrations of aqueous NO_2~- and NO_3~- significantly increase while the concentrations of aqueous ROS decrease with the admixture of N_2.The addition of O_2 has little effect on the concentrations of NO_2~- and NO_3~- and pH values; however,the addition of O_2 increases the concentration of O_2~- and deceases the concentrations of H_2O_2 and OH.Finally,the results of bactericidal experiments demonstrate that the inactivation efficiency of the four types of plasma jets is He?+?O_2??He+AirHeHe+N_2,which is in accordance with the changing trend of the concentration of aqueous O_2~-.Simultaneously to the better understanding of the formation and removal mechanisms of reactive species in the plasma–liquid interaction,these results also prove the effectiveness of regulating the concentrations of aqueous reactive species and the bacteria inactivation effects by gas doping.     

Numerical Study of Pulsed Dielectric Barrier Discharge at Atmospheric Pressure Under the Needle-Plate Electrode Configuration

In this paper,we study the characteristics of atmospheric-pressure pulsed dielectric barrier discharge (DBD) under the needle-plate electrode configuration using a one-dimensional self-consistent fluid model.The results show that,the DBDs driven by positive pulse,negative pulse and bipolar pulse possess different behaviors.Moreover,the two discharges appearing at the rising and the falling phases of per voltage pulse also have different discharge regimes.For the case of the positive pulse,the breakdown field is much lower than that of the negative pulse,and its propagation characteristic is different from the negative pulse DBD.When the DBD is driven by a bipolar pulse voltage,there exists the interaction between the positive and negative pulses,resulting in the decrease of the breakdown field of the negative pulse DBD and causing the change of the discharge behaviors.In addition,the effects of the discharge parameters on the behaviors of pulsed DBD in the needle-plate electrode configuration are also studied.     

Analysis of Power in an Argon Filled Pulsed Dielectric Barrier Discharge

In this paper an argon filled coaxial dielectric barrier discharge (DBD) has been studied to understand the detail of power transfer from a unipolar square pulse to plasma during discharge. A dielectric barrier discharge based diffuse pulse discharge and its electrical characteristics are investigated. A quartz coaxial DBD tube filled at different pressures is used in the experiment. A unipolar pulse voltage of different peak voltages and frequencies has been applied to the discharge electrodes for the generation of microdischarges. Two current pulses are used for two consecutive discharges per applied voltage pulse. The second discharge, which occurs at the falling flank of the voltage pulse, is induced by the charges stored on the dielectric barrier during the first discharge. It has been deduced that the power supplied to ignite the first discharge is partly stored to ignite the second discharge when the applied voltage decays. This process ultimately leads to much improved power transfer to the plasma. The knowledge obtained from dynamic processes of the DBDs in the discharge gap explains quantitatively the mechanism of ignition, development and extinction of the DBDs.     

Experiment and Analysis on the Treatment of Gaseous Benzene Using Pulsed Corona Discharge Plasma

An experiment and analysis on removal of gaseous benzene by pulse corona inducedplasma is presented in this article. Important parameters effecting removal efficiency have been investigated, such as pulse peak voltage, pulse frequency, gas inlet concentration, gas flow rate and reactor temperature. The result shows that the removal efficiency increases with the increase in pulse peak voltage, pulse frequency and reactor temperature, but decreases in the rise of gas inlet concentration and gas flow rate. On the condition of Vp = 36 kV, f = 80 Hz, C = 1440 mg/m^3 and Q = 640 ml/min, the largest removal efficiency is 98%. Finally, the reacted products are qualitatively analysed and the reaction processes are deduced in combination with plasma-chemistry theory.     

Effects of Xe Gas Content and Total Gas Pressure on the Discharge Characteristics of Colour Plasma Display Panels

The effects of the Xe gas content and total gas pressure on the discharge characteristics of colour plasma display panels including the sustaining voltage margin, white-field chromaticity, discharge time lag （DTL）, discharge current peak, and full-width-at-half-maximum （FWHM） of the discharge current pulse, are experimentally studied. The results indicate that as the Xe gas content in the He-Ne-Xe gas mixture or total pressure increases, the sustaining voltage margin increases, the white-field chromaticity improves, and the discharge current peak has a maximum value, while DTL and FWHM have a minimum value. The mean electron energy in the gas mixture discharge is also calculated through a numerical solution of Boltzmann equation. The experimental results are explained from a view of the mean electron energy variations with the Xe gas content and total gas pressure.     

The Study of Active Atoms in High-Voltage Pulsed Coronal Discharge by Optical Diagnostics

In this study, the emission spectra of active atoms O (3p^5p → 3s^5S^02 777.4 nm),Ha (3P→2S 656.3 nm) and N (3p^4P→ 3s^4S^0 742.3 nm, 744.2 nm, 746.8 nm) produced by thepositive high-voltage pulsed corona discharge (HVPCD) of N2 and H2O mixture in a needle-plate reactor have successfully been recorded against a severe electromagnetic interference coming from the HVPCD at one atmosphere. The effects of the peak voltage, the repetition rate of pulsed discharge and the flow rate of oxygen on the production of those active atoms are investigated. It is found that when the peak voltage and the repetition rate of the pulsed discharge are increased,the emission intensities of those active atoms rise correspondingly. And the emission intensities of O (3p^5P→3s^5S^0 777.4 nm), Hα (3P→2S 656.3 nm) and N (3p^4P→3s^4S^0 742.3 nm, 744.2 nm,746.8 nm) increase with the flow rate of oxygen (from 0 to 25 ml/min) and achieve a maximum value at a flow rate of 25 ml/min. When the flow rate of oxygen is increased further, the emission intensities of those atoms visibly decrease correspondingly. The main physicochemical processes of interaction involved between electrons, neutrals and ions are also discussed.     

Generation of reactive atomic species of positive pulsed corona discharges in wetted atmospheric flows of nitrogen and oxygen

The emission spectra of excited radicals(OH(A~2E),O(3p~3 P),H_α(3P)) and emissive species(N_2~+(B~2∑_u~+),N_2(C_3Π_u)) produced by positive pulsed high-voltage needle-plane corona discharges in atmospheric N_2 and O_2 flows wetted with 10%H_2O at 80 ℃ are used to investigate the relative concentrations of the produced radicals.The results indicate that the tendencies of the concentrations of radicals with discharge conditions are similar to each other due to their similar excitation processes by electron collision.The influence of oxygen flow mixed with the nitrogen flow on the emission intensities of O(3p~5P → 3s~5S_2~.),H_α(3P → 2S),N_2_+(B~2∑_u~+ → X~2∑_g+0-0),and N_2(C~3Π_u → B~3Π_g 1-0) is presented.When the flow rate of oxygen addition is varied from 0-30 ml min~(-1),the emission intensities of O(3p~5P → 3s~5S_2~0.),H_α(3P → 2s),and N_2~+(B~2∑_u~+ →X~2∑_g 0-0) increase and reach a maximum.Then,if the oxygen flow rate increases further,the emission intensities tend to decrease.However,the intensity of N_2(C~3Π_u → B~3Π_g1-0) decreases monotonously with the increasing oxygen flow,which indicates that the electron density decreases with the increasing oxygen flow.By the tendencies of the relative intensities to N_2(C~3Π_u → B~3Π_g 1-0),the concentrations of the total produced O,H,and N_2~+ are shown to increase with the oxygen flow.Based on the reactions for the production of H and O without and with the addition of O_2,the analytic solutions for H and O production are derived in accordance with the experimental results.     

Degradation of phenol using a combination of granular activated carbon adsorption and bipolar pulse dielectric barrier discharge plasma regeneration

A combined method of granular activated carbon (GAC) adsorption and bipolar pulse dielectric barrier discharge (DBD) plasma regeneration was employed to degrade phenol in water. After being saturated with phenol, the GAC was filled into the DBD reactor driven by bipolar pulse power for regeneration under various operating parameters. The results showed that different peak voltages, air flow rates, and GAC content can affect phenol decomposition and its major degradation intermediates, such as catechol, hydroquinone, and benzoquinone. The higher voltage and air support were conducive to the removal of phenol, and the proper water moisture of the GAC was 20％. The amount of H2O2 on the GAC was quantitatively determined, and its laws of production were similar to phenol elimination. Under the optimized conditions, the elimination of phenol on the GAC was confirmed by Fourier transform infrared spectroscopy,and the total removal of organic carbons achieved 50.4％. Also, a possible degradation mechanism was proposed based on the HPLC analysis. Meanwhile, the regeneration efficiency of the GAC was improved with the discharge treatment time, which attained 88.5％ after 100 min of DBD processing.     

Temporal evolution of atmospheric cascade glow discharge with pulsed discharge and radio frequency discharge

Atmospheric cascade discharges with pulsed discharge and radio frequency(RF)discharge were experimentally investigated by the temporal evolution of discharge spatial profile and intensity.The indium tin oxide(ITO)coated glass was employed as the transparent electrode to capture the discharge distribution above the electrode surface.It is demonstrated that in the pulsed discharge with dielectric barrier,the first discharge at the rising edge of pulse voltage is uniformly ignited and then forms an expanding plasma ring on the ITO electrode surface,which shrinks to the same diameter as that of bare stainless steel electrode with the generation of second discharge at the falling edge of pulse voltage.The discharge profiles along the electrode surface and discharge gap of the successive RF discharge are dependent on the intensity and spatial distribution of residual plasma species generated by the pulsed discharge,which is determined by the time interval between the pulsed discharge and RF discharge.It is demonstrated that the residual plasma species before the RF discharge ignition help to achieve the stable operation of RF discharge with elevated intensity.     

Treatment of Dye Wastewater by Using a Hybrid Gas/Liquid Pulsed Discharge Plasma Reactor

A hybrid gas/liquid pulsed discharge plasma reactor using a porous ceramic tube is proposed for dye wastewater treatment.High voltage pulsed discharge plasma was generated in the gas phase and simultaneously the plasma channel was permeated through the tiny holes of the ceramic tube into the water phase accompanied by gas bubbles.The porous ceramic tube not only separated the gas phase and liquid phase but also offered an effective plasma spreading channel.The effects of the peak pulse voltage,additive gas varieties,gas bubbling rate,solution conductivity and TiO 2 addition were investigated.The results showed that this reactor was effective for dye wastewater treatment.The decoloration efficiency of Acid Orange II was enhanced with an increase in the power supplied.Under the studied conditions,97% of Acid Orange Ⅱ in aqueous solution was effectively decolored with additive oxygen gas,which was 51% higher than that with argon gas,and the increasing O 2 bubbling rate also benefited the decoloration of dye wastewater.Water conductivity had a small effect on the level of decoloration.Catalysis of TiO 2 could be induced by the pulsed discharge plasma and addition of TiO 2 aided the decoloration of Acid Orange Ⅱ.     

Time-resolved measurements of NO2 concentration in pulsed discharges by high-sensitivity cavity ring-down spectroscopy

To describe the complex kinetics of formation and destruction mechanism of nitrogen dioxide(NO_2), there is an increasing demand for real-time and in situ analysis of NO_2 in the discharge region. Pulsed cavity ring-down spectroscopy(CRDS) provides an excellent diagnostic approach. In the present paper, CRDS has been applied in situ for time evolution measurement of NO_2 concentration which is rarely investigated in gas discharges. In pulsed direct current discharge of NO_2/Ar mixture at a pressure of 500 Pa, a peak voltage of -1300 V and a frequency of 30 Hz, for higher initial NO_2 concentration(3.05?×?10~(14)cm~(-3), 8.88?×?10~(13)cm~(-3)),the NO_2 concentration sharply decreases at the beginning of the discharge afterglow and then becomes almost constant, and the pace of decline increases with pulse duration; however, for lower initial NO_2 concentration of 1.69?×?10~(13)cm~(-3), the NO_2 concentration also decreases at the beginning of the discharge afterglow for 200 ns and 1 μs pulse durations, while it slightly increases and then declines for 2 μs pulse duration. Thus, the removal of low-level NO_2 could not be promoted by a higher mean energy input.     

Spectroscopic study of bipolar nanosecond pulse gas-liquid discharge in atmospheric argon

Atmospheric gas-liquid discharge with argon as a working gas is presented by employed nanosecond pulse power. The discharge is presented in a glow-like mode. The discharge powers are determined to be less than 1 W, and remains almost constant when the discharge duration time increases. Bountiful active species are determined by capturing optical emission spectra,and their main generation processes are also discussed. The plasma gas temperature is calculated as 350 K by comparing the experimental spectra and the simulated ones of N_2(C~3Ⅱ_g→B~3Ⅱ_g, Δv =-2). The time resolved vibrational and rotational temperature is researched to present the stability of discharge when pulse voltage and discharge duration vary.The electron density is determined to be 10~(16) cm~(-3) according to the Stark broadening effect of the H_α line.     

Study on SO2 Removal Efficiency by Nanosecond Rising Edge Pulse DBD Under Different Environmental Conditions

In this paper, an experimental study on SO2 removal by nanosecond rising edge pulse dielectric barrier discharge （DBD） plasma, generated by multi-needle-to-plane electrodes, is carried out. The mechanism of the effect of various factors, such as gap size between dielectric barrier and discharge needles, environmental humidity, and inlet speed of gas flow upon the removal efficiency of air purification is analyzed. The studies show that SOs removal efficiency improves with the increase in the gap size between dielectric barrier and discharge needles in the case of a fixed space between two electrodes, and also improves with the increase in the environmental humidity. For a mixed gas with a fixed concentration, there is an optimal inlet speed of gas flow, which leads to the best removal efficiency.     

Electrical characters and optical emission spectra of VBD coupled SBD excited by sine AC voltage in atmospheric air

In this paper,volume coupled surface barrier discharge(V-SBD) with three structures possessing different volumes is excited by sine AC power in atmospheric air.Discharge images,waveforms of applied voltage and discharge current,and optical emission spectra simulating rotational and vibrational temperatures are recorded and analyzed.The effects of applied voltage on emission intensities of N_2(C~3Π_u→ B~3Π_g) and N_2~+(B~2∑_u~+ → X~2E_g~+),and rotational and vibrational temperatures are investigated.The results show that as applied voltage rises,emission intensities and rotational temperatures increase while vibrational temperatures decrease.In addition it is found that,as applied voltage varies,the rotational temperature of surface discharge changes faster than that of volume discharge.     

Regulation characteristics of oxide generation and formaldehyde removal by using volume DBD reactor

Discharge plasmas in air can be accompanied by ultraviolet(UV) radiation and electron impact,which can produce large numbers of reactive species such as hydroxyl radical(OH·),oxygen radical(O·),ozone(O3),and nitrogen oxides(NOx),etc.The composition and dosage of reactive species usually play an important role in the case of volatile organic compounds(VOCs) treatment with the discharge plasmas.In this paper,we propose a volume discharge setup used to purify formaldehyde in air,which is configured by a plate-to-plate dielectric barrier discharge(DBD) channel and excited by an AC high voltage source.The results show that the relative spectral-intensity from DBD cell without formaldehyde is stronger than the case with formaldehyde.The energy efficiency ratios(EERs) of both oxides yield and formaldehyde removal can be regulated by the gas flow velocity in DBD channel,and the most desirable processing effect is the gas flow velocity within the range from2.50 to 3.33 m s-1.Moreover,the EERs of both the generated dosages of oxides(O3 and NO2) and the amount of removed formaldehyde can also be regulated by both of the applied voltage and power density loaded on the DBD cell.Additionally,the EERs of both oxides generation and formaldehyde removal present as a function of normal distribution with increasing the applied power density,and the peak of the function is appeared in the range from 273.5 to 400.0 W l-1.This work clearly demonstrates the regulation characteristic of both the formaldehyde removal and oxides yield by using volume DBD,and it is helpful in the applications of VOCs removal by using discharge plasma.     

OH and O radicals production in atmospheric pressure air/Ar/H2O gliding arc discharge plasma jet

Atmospheric pressure air/Ar/H_2O gliding arc discharge plasma is produced by a pulsed dc power supply. An optical emission spectroscopic(OES) diagnostic technique is used for the characterization of plasmas and for identifications of OH and O radicals along with other species in the plasmas. The OES diagnostic technique reveals the excitation Tx?≈?5550–9000 K, rotational Tr?≈?1350–2700 K and gas Tg?≈?850–1600 K temperatures, and electron density n?(1.1-1.9) ′101 4 cm~(-3) e under different experimental conditions. The production and destruction of OH and O radicals are investigated as functions of applied voltage and air flow rate. Relative intensities of OH and O radicals indicate that their production rates are increased with increasing Ar content in the gas mixture and applied voltage. nereveals that the higher densities of OH and O radicals are produced in the discharge due to more effective electron impact dissociation of H_2O and O_2 molecules caused by higher kinetic energies as gained by electrons from the enhanced electric field as well as by enhanced n e.The productions of OH and O are decreasing with increasing air flow rate due to removal of Joule heat from the discharge region but enhanced air flow rate significantly modifies discharge maintenance properties. Besides, Tgsignificantly reduces with the enhanced air flow rate. This investigation reveals that Ar plays a significant role in the production of OH and O radicals.     

Excited Electron Temperature and Molecular Vibration Temperature During Transition from Filamentary Discharge to Glow Discharge in Surface Discharge in Air

A dielectric barrier surface discharge device was used to investigate the transition from a filamentary discharge to a glow discharge in air at different gas pressures. Discharge images and waveforms of the applied voltage and discharge current were recorded simultaneously, and it was found that the discharge could transit from filamentary to glow with the decrease in pressure. Optical emission spectra during the transition from a filamentary discharge to a glow one were recorded. Excited electron temperature can be determined from the ratios of the relative intensities of spectral lines while molecular vibration temperature can be measured by analysing spectral lines of the N2 second positive band system. The results show that both the excited temperature and molecular vibration temperature increase with the decrease in the gas pressure. Qualitative explanations are given.