| Nanosecond pulse gas-liquid discharge has been widely applied to a variety of application areas,including biomedical engineering,environmental remediation,and material synthesis and processing,due to excellent stability and abundant reactive oxygen species and reactive nitrogen species.In this paper,stable nanosecond pulse gas-liquid discharges are achieved with needle-to-water and multiple needle-to-water electrode configuration in air,nitrogen,helium and argon at atmospheric pressure.Discharge images,waveforms of discharge voltage and current,and optical emission spectra are measured to investigate the characteristics of discharge plasma.The main contents of this paper are summarized as follows:1.A diffuse gas-liquid discharge is generated by a bipolar nanosecond pulse power with a needle-to-water configuration in atmospheric air.In particular,the diffuse discharge can be achieved in quartz containers with different bottom structures.Discharge images,waveforms of discharge voltage and current,optical emission spectra are measured to present the characteristics of the discharge plasma.Waveforms of discharge voltage and current show that only one discharge with a duration time of 60 ns occurs in each voltage pulse.The emission intensities of OH(A2??X2?),N2(C3?u?B3?g,337.1 nm)and N2+(B2?u+?X2?g+,391.4nm).increase with the increasing pulse peak voltage,and decrease with the increasing electrode gap distance.The vibrational and rotational temperatures are determined as 3700 K and 390 K,respectively under the pulse peak voltage of 30 kV,pulse repetition rate of 150 Hz and electrode gap distance of 3.5 mm.The plasma gas temperature presents an upward trend with increasing the pulse peak voltage,and behaves adversely when the electrode gap distance increases.The plasma gas temperature shows a good stability with the discharge operation time.The pH value and the concentrations of NO2-,,NO3-;and H2O2 in the solution are also measured,and their variations with discharge operation time are researched.The pH value decreases when the discharge operation time increases.The concentrations of NO2-,NO3-and H2O2 increase with the discharge operation time increasing.2.A nanosecond pulse power is employed to excite the gas-liquid discharge with a needle-to-water configuration in a nitrogen flow at atmospheric pressure,which shows a transient spark mode.The electrical properties of transient spark are discussed according to the waveforms of discharge voltage and current.The transient spark is a streamer-to-spark transition mode.which contains three phases.namely the streamer phase.the streamer-to-spark transition,and the spark phase.The pulse peak voltage has great influences on the characteristic of transient spark.especially the streamer-to spark transition time and the park current.As the pulse peak voltage increases.the transition time decreases obviously.and the current of spark discharge increases.meanwhile its duration time become shorter.In the transient spark discharge,the energy consumption occurs mainly in the spark discharge stage,and the spark discharge power accounts for about 90%of the total power.The optical emission spectra are measured to identify active species generated in the discharge,and the generation processes of active species are also discussed.The effects of pulse peak voltage and gas flow rate on the emission intensities of active species are researched.When the pulse peak voltage increases,the emission intensities of N2(C3?u?B3?g,337.1 nm)and N(3p 4So?3s 4P,746 nm)increase,the emission intensities of OH(A2??X2?)and N+2(B2?u+?X2?g+,391.4 nm)almost remain unchanged,and the emission intensities of O(3p 5P?,3s 5So,777 nm)and H? increase first then decrease.When the gas flow rate increases,the emission intensities of N2(C3?u?B3?g,337.1 nm)and N+2(B2?u+?X2?g+,391.4 nm)increase,the emission intensities of O(3p 5P?3s 5So,777 nm),N(3p4So?3S 4P,746 nm)and H? decrease,and the emission intensity of OH(A2??X2?)decreases first then increases.The spectra of N2(C3?u?B3?g,Au=-2)are employed to determine the vibrational and rotational temperatures,which are 2510 K and 380 K respectively with the pulse peak voltage of 30 kV,pulse repetition rate of 120 Hz,and the gas flow rate of 30 mL/min.The vibrational temperature and rotational temperature increase with the pulse peak voltage increasing and gas flow rate decreasing.The electron density is calculated by the Stark broadening of H? line and the Stark parameter ofO(844 nm)and N(746 nm)as about 1017 cm-3,which increases with the increase of pulse peak voltage.3.The bipolar nanosecond pulse power is employed to drive the gas-liquid discharge with a multiple needle-water electrode configuration in helium or argon at atmospheric pressure.The discharge presents a diffuse mode in helium,and a glow-like mode in argon.The discharge areas are approximately 790 mm2 and 690 mm2 with the gas flow rate of 5 mL/min in helium and argon,respectively.Waveforms of discharge voltage and current indicate that only one discharge occurs in each pulse,which lasts only 40 ns.The optical emission spectra show that a large amount of active species generated both in helium and argon discharge.In the helium discharge,the emission intensities of He(3s 3S?2p 3P,706.5 nm)and H? increase when the helium flow rate increases,while the emission intensities of OH(A2??X2?)and O(3jp SP?3s 5S°,777 nm)increase first and then decrease.In argon discharge,the emission intensities of Ar(4p?4s),O(3p 5P?3s5S°,777 nm),Ha and OH(A2??X2?)increase with the pulse peak voltage increasing.As the argon flow rate increase,the emission intensities increase and then keep constant when the argon flow rate reach to 30 mL/min.The spectra of N2(C3?u-B3?g,??=-2)are employed to determine the vibrational temperature and rotational temperature.which are 2600 K and 310 K respectively in helium discharge,while 2800 K and 350 K respectively in argon discharge.4.The diffuse gas-liquid discharge excited by a nanosecond pulse power in atmospheric air is utilized to sterilize Escherichia coli,Candida albicans and Actinomycetes,and the effects of plasma treatment time on the death rate are researched.A death rate of 90%for Escherichia coli can be achieved after the plasma treatment time of 60 s.When the samples are treated by discharge plasma for 5 minutes,the death rates of Escherichia coli and Candida albicans are close to 100%,while the death rate of Actinomycetes is only 90%.The diffuse gas-liquid discharge with multiple needle-water electrode configuration excited by a nanosecond pulse power in atmospheric helium is used for sterilization in flowing water.The effects of plasma treatment time,helium flow rate and liquid flow rate on the concentrations of bacteria and fungus are investigated.The concentration of bacteria decreases sharply under 20 s plasma treatment,and is close to zero under 90 s plasma treatment.The concentration of fungi can reach to zero with 150 s plasma treatment.The flow rates of helium and liquid have obvious effects on the sterilization of bacteria and fungi.When the flow rate of treated water sample is lower,the inactivation efficiency of bacteria and fungi is higher.When the treatment time is 150 s,the higher the helium flow rate is,the higher the treatment efficiency is.When the helium flow rate is over 10 mL/min,the total concentration of bacteria and fungi is close to zero. |
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