Ionization process and distinctive characteristic of atmospheric pressure cold plasma jet driven resonantly by microwave pulses

Lingli HONG (洪伶俐),Zhaoquan CHEN (陈兆权),∗,Jie YANG (杨洁),Tao CHENG (程涛),Sile CHEN (陈思乐),Yuming ZHOU (周郁明),Bing WANG (王兵) and Xinpei LU (卢新培)

1 Anhui Province Key Laboratory of Power Electronics and Electrical Control,Anhui University of Technology,Maanshan 243032,People’s Republic of China

2 State Key Laboratory of Advanced Electromagnetic Engineering and Technology,Huazhong University of Science and Technology,Wuhan 430074,People’s Republic of China

Abstract In the present study,a coaxial transmission line resonator is constructed,which is always capable of generating cold microwave plasma jet plumes in ambient air in spite of using argon,nitrogen,or even air,respectively.Although the different kinds of working gas induce the different discharge performance,their ionization processes all indicate that the ionization enhancement has taken place twice in each pulsed periods,and the electron densities measured by the method of microwave Rayleigh scattering are higher than the amplitude order of 1018 m−3.The tail region of plasma jets all contain a large number of active particles,like NO,O,emitted photons,etc,but without O3.The formation mechanism and the distinctive characteristics are attributed to the resonance excitation of the locally enhanced electric fields,the ionization wave propulsion,and the temporal and spatial distribution of different particles in the pulsed microwave plasma jets.The parameters of plasma jet could be modulated by adjusting microwave power,modulation pulse parameters (modulation frequency and duty ratio),gas type and its flow rate,according to the requirements of application scenarios.

Keywords:pulsed microwave discharge,cold plasma jet,transient ionization process,ionization enhancement

1.Introduction

Atmospheric pressure plasma jets (APPJs) have attracted much more applications such as plasma ignition and combustion,material surface modification,and plasma medical treatment[1-4].Obviously,different application scenarios require different parameters of APPJs [5,6].The application of APPJs in ambient air,like the organisms which are difficult to resist the high temperature and plasma treatment directly,needs low temperature plasma jet and rich reactive particles[7-12].As far as the cold APPJs are concerned,researchers have studied the plasma jet devices generated by discharge under different voltage forms (working frequencies from DC to laser).Due to the difficulty of the plasma generation in ambient air,most of APPJs apply the inert gas (helium or argon) as working gas [13-16].For better and rapid using,the portable air plasma devices are preferred.Thence,various types of APPJs with air have been constructed to meet the requirements of different plasma applications [17-23].However,the APPJs driven by lower frequency voltages suffer from the problem of excessive ozone production and the low plasma density.The APPJs generated by the high frequency voltages (radio frequency (RF) signal or microwave) can effectively avoid the problem of too much ozone,but the temperature of the plasmas produced is relatively high (ozone is easy to decompose under high temperature conditions).Besides,the generation of RF plasma jet often requires RF matching network [24,25],which makes the RF plasma jet device often cumbersome.Meanwhile,microwave plasmas are difficult in generating in ambient air,especially in lager space scale,but are applied widely primarily because they often contain various high concentrations of active particles[26-33].

To pay more concern on the decreases the temperature of APPJs driven by microwave,there are two technological methods.One is to use higher air flow to cool the microwave APPJs.Kang et al[34,35]have proposed a useful microwave air plasma jet for cold plasma wound healing.Although the plasma in generation region has high temperature about 1800 K,the room temperature of plasma in tail region is obtained by the main air gas flow rate of 2-4 slm plus auxiliary cooling air flow rate of 1-3 slm.Besides the drawback of complex in microwave generating and plasma control,the very thin radius of microwave APPJs (the diameter of the nozzle only 0.5 mm) also limits their application performance.Another method is to use the pulsed modulation technology in view of decreasing the plasma temperature,that is so called the pulsed microwave discharge.Unlike the plasma jet excited by continuous microwave,there exists the effect of enhanced ionization in the pulsed microwave discharge.Kwon et al[36]have numerically indicated that an enhanced ionization exists in the pulsed microwave discharge.The ionization enhancement is originated from the heating of number of secondary electrons located in the sheath region,at the rising edge of the pulse.Our previous works have observed the enhanced ionization of three types of pulsed microwave APPJs: coaxial transmission line resonator (CTLR) [29,32,37],hairpin shaped launcher[27,33],and surface wave resonator [28,38].In our experiments with argon discharges,twice ionization enhancements have been acquired in each pulsed period.One enhanced ionization happens at the rising edge of each pulse and the second one takes place at the falling edge of each pulse.That is to say,using pulsed modulation microwave instead of continuous microwave,not only the temperature of APPJs could be decreased,but also the plasma density would be increased due to the effect of ionization enhancement.

In short,although the cold air APPJs generated by microwave pulses are required for room temperature plasma applications,the mechanism of dominant ionization and their characteristics are still unclear.In this work,a cold microwave APPJ device has been generated by microwave pulses in ambient air,using the working gas of argon,nitrogen and air,respectively.The mechanism of ionization enhancements and their distinctive characteristics should be focused on.

2.Discharge experiments and operations

Figure 1 shows the schematic structure of CTLR and the discharge images of three types of cold microwave APPJs.The CTLR device shown in figure 1(a) is developed as a quarter wavelength coaxial resonator with one open end and one short end.The total length of resonate cavity approximates 31 mm (a quarter of wavelength of 2.45 GHz microwave).The resonator is made of copper.The inner radius of the resonant cavity is 5 mm,and the structure at the open end adopts a conical cavity (the port diameter is 6 mm),so as to make the strongest electric field intensity located at the open end.The coaxial powered electrode is sharpened to a stainless-steel needle in order to overcome large breakdown and sustain power of atmospheric gas discharge.The microwave pulses are applied on the central electrode by a Sub-Miniature version A (SMA) connector at the coupled point 3 mm apart from the short end.The sharp tip of the central electrode is retracted to 2 mm at the open end.The reason is in view of exciting easily the discharge in the strongest electric field area.Besides,the temperature of the excitation area is very high,leaving a distance of 2 mm to the open end,so temperature of the plasma jet outside the resonant cavity decreases slightly.

The discharge experiments operate as the followed schedule.(1)The first stage starts at opening the valve of the working gas tanks (99.999% argon,99.999% nitrogen,or air compressor of 5 MPa and 8 l).The gas flow rate(0-20 slm)is adjusted by another valve controller.Each gas flow rate is monitored by each flow meter.The gas enters the resonant cavity from five 1 mm diameter holes at the bottom of the resonator,as shown in figure 1(a).This central symmetrical gas inlet mode will produce rotating gas flow,which is helpful to reduce the temperature of plasma jet.(2) The second stage is to set the signal generator (frequency range of 2.4-2.5 GHz,output power of 1 mW) and set the pulsed modulator (square wave of 5 V,duty ratio of 0.01-0.99,and modulated pulse frequency of 10 Hz-200 kHz).The forward microwave power is amplified by a 53 dB amplifier (maximum power of 200 W)to a prefixed value.At normal cases,the gas discharge would not start until an incident power reaches a higher value (normally,20 W for argon,50 W for air and 60 W for nitrogen).After the gas discharge starts,the applied power could be decreased and the plasma jet could be maintained until the peak value of microwave power being smaller than the minimum power (depending on the type of gas and pulse duty ratio,generally speaking,2 W for argon,10 W for air and 6 W for nitrogen).(3) The third stage is to take photos on the plasma jet by using two cameras.The discharge photos with the longer exposure time are captured by a common digital camera (Canon 60D) and the discharge photos with the nanosecond exposure time are taken by a fastgated ICCD camera (Andor iStar DH340T).The ICCD camera and the 53 dB amplifier are triggered by the same pulse signal so that the collected data are synchronized.(4)The fourth stage is to measure the temporal and spatial evolution of the electron density by using microwave Rayleigh scattering setup.The microwave Rayleigh scattering setup is homemade in our laboratory and its operations could be referred in our previous works[26,39].(5)The fifth stage is to measure the temperature of plasma jet with infrared thermal imager (Testo AG0515,the infrared thermal sensitivity is＜0.05 °C,and the temperature measurement range is from −30 °C to+350 °C) and the composition of plasma jet with optical fiber spectrometer (Avantes AvaSpec.2048).Because the light radiation intensity of nitrogen or air plasma jet is low,it should be to use the acquisition time of 2 s to measure the spectrum.(6) The last stage is to end the discharge experiment.Decrease the microwave power to zero firstly,after then turn off the electrical power switch,and thereafter close all gas valves terminally.

Figures 1(b)-(d)show the photograph of the images with the working gas of nitrogen,air and argon,respectively.The left column shows the temperature of the plasma jet measured by the infrared thermal imager,the middle column shows the discharge photos taken by the Canon 60D camera,and the right column shows the finger touching the tail of the plasma jet.The pulse modulated frequency(10 kHz),duty ratio(0.5)and peak microwave power (30 W) are used as the input conditions.Considering that the temperature of diatomic molecular gas (nitrogen or air) microwave plasma jet is higher,compared with argon microwave plasma jet,it generally needs higher gas flow to acquire a low-temperature plasma jet.The gas flow rate for nitrogen or air is 10 slm,but the argon gas flow rate for is 1.5 slm in the present experiments.As shown in figure 1(b),when the nitrogen gas flow rate of 10 slm is applied on the proposed CTLR,the length of APPJ approximates 22 mm.The temperature of nitrogen plasma jet is lower than 15 °C,and the finger can touch directly to the tail region of the nitrogen plasma jet without electric shock or hot feeling.As shown in figure 1(c),fixing the gas flow at 10 slm unchangeable,using air instead of nitrogen,the air plasma jet with the length of about 10 mm can be generated.The temperature of air plasma jet is around 18°C,and the finger can also touch safely to the tail region of the air plasma jet.Compared with nitrogen plasma jet,air plasma jet appears to shrink a lot.Interestingly,the air plasma jet has a core region generated at the tip of the powered electrode.The temperature distribution measured by the infrared thermal imager also shows that the hot spot is located in the plasma excitation region.Figure 1(d) shows the argon plasma jet.The argon gas flow rate is fixed at 1.5 slm and the length of plasma jet reaches 13 mm.At this time,the light radiation intensity of argon plasma jet is relatively strongest,and the temperature of plasma jet is also relatively highest.Moreover,during the experiment,we also have found that if the conductor (or human body) contacts with the plasma jet with a residence time greater than 3 s,it will rise rapidly.The physical mechanism might be that the microwave is transmitted to the contacted conductor (or human body) along the highly conductive plasma jet,and the skin effect of microwave on the conductor surface heats the conductor(or human body).Therefore,when using microwave plasma jet to treat the conductor,it is necessary to keep the plasma jet moving relative to the treated conductor,otherwise there will be the risk of burns,although the temperature of the plasma jet is cold as room temperature.Of course,this occurs under the discharge power of 30 W.If the peak power is less than 20 W,this problem will not exist.

Figure 1.(a) Schematic structure of CTLR,and (b)-(d) discharge images with the different working gases,respectively.(b) Nitrogen plasma jet (the pulse modulation frequency is 10 kHz,the peak microwave power is 30 W,the duty cycle is 0.5 and the argon flow is 10 slm).(c) Air plasma jet (the pulse modulation frequency is 10 kHz,the peak microwave power is 30 W,the duty cycle is 0.5 and the argon flow is 10 slm),and (d) argon plasma jet (the pulse modulation frequency is 10 kHz,the peak microwave power is 30 W,the duty cycle is 0.5 and the argon flow is 1.5 slm).

3.Experimental results and discussions

3.1.Transient discharge images

Figure 2 shows one group of fast-gated photos with argon plasma jet.The pulse modulated frequency is 10 kHz (pulse period of 100 μs),the duty ratio is 0.5,the peak incident power is 30 W,the argon gas flow rate is 1.5 slm.The ICCD camera is preset by program control software.The triggered mode uses external trigger,the acquiring mode chooses dynamic,the exposure time is 50 ns,the photoelectric gain inputs 3000,the image number types 100,and the dynamic delay time controls to 1 μs.As shown in figure 2,there illustrates twice dominant ionizations in one pulse period.One enhanced ionization occurs at the rising edge of each pulse,and the second enhanced ionization occurs at the falling edge of each pulse.At the time of 1 μs,one brighten plasma bullet produces at the outlet of CTLR.From 2 to 8 μs,the length of plasma jet increases quickly,but its diameter becomes thinner,and the brightness of light radiation becomes weaker.At this time,one ionization front moves forwards and a relatively weak discharge ionization channel follows.The microwave pulse disappears at the moment of 50 μs and then the argon plasma jet reaches its longest length.The strongest intensity of emitted light arrives at the time in the range of 51-52 μs.After then,the intensity of emitted light shrinks slowly to disppear.It is worth noting that the light radiation in the middle region of the plasma jet disappears finally,in the attenuation stage.

Figure 2.Discharge images with argon plasma jet obtained using the ICCD camera with exposure times of 50 ns.The discharge condition is the same with that of figure 1(d).

Figure 3 shows two groups of discharge images of pulsed microwave APPJs:(a)the air discharge shown in left column and(b)the nitrogen discharge displayed in right column.The discharge conditions and the parameters of ICCD camera are unchanged,except for (1) using air or nitrogen instead of argon,(2) the working gas flow rate of 10 slm and (3) the exposure time of 500 ns.Because the plasma jet temperature of air or nitrogen discharge is relatively hot,a higher gas flow rate is required to reduce the temperature.While the light emission intensity is very low.If the exposure time of ICCD camera is too short,the discharge image of plasma jet cannot be captured.As shown in figure 3,the discharge images of air and nitrogen have no periodicity,and the discharge photos are approximately the similar whether there is pulsed microwave power or not.Comparing the discharge images of air and nitrogen in ambient air,it is shown that the air plasma jet is brighter and the volume of nitrogen plasma jet is larger.

Figure 3.Two groups of discharge images of pulsed microwave APPJs obtained using the ICCD camera with exposure times of 500 ns.(a)Air discharge located in the left column and (b) nitrogen discharge arranged in the right column,respectively.During the experiment,the pulse modulation frequency is 10 kHz,the peak microwave power is 30 W,the duty cycle is 0.5,and the gas flow rate is 10 slm.

3.2.Measurement on electron density

Next,we will measure the spatiotemporal distribution of the electron density of pulsed microwave APPJs.The originated signal is captured by using a homemade microwave Rayleigh scattering device [26,39].The calculation of the originated signal (for obtaining the transient electron density) is determined by the calibration parameter A and the transient shape of the plasma jet (its radius and length).The calibration parameter A is 3.72 × 105· Ω·−

V m2in the measurement.The transient discharge images taken by the ICCD camera could be adopted to confirm its radius and length.As can be seen in figure 2,the shape of argon plasma jet is varied along with the microwave pulse.As calculating the data,the equivalent radius of the plasma jet is counted by the average radius of the jet-shaped lighten regions.The signal data captured from microwave Rayleigh scattering device is about 2500 numbers in one pulsed period.For completing the data calculation,the data of the equivalent radius and length should be fitted versus time to both 2500 numbers.As shown in figure 4(a),the fitted data of the equivalent radius and length are calculated.In the time of 0-70 μs,the equivalent radius and length are determined by gray value method base on the light intensity of the jet-shaped highlighted regions as shown in figure 2.In other times,the equivalent radius and length are counted as 2 mm and 10 mm,respectively.This is to prevent numerical divergence when calculating the electron density,because the light radiations at these times are too weak to be detected by the ICCD camera.

Figure 4(b) shows the varied distribution of electron density of the argon plasma jet.In general trends,the electron density increases at first and then decreases,that is along with the pulsed microwave power.As shown in figure 4(b),the curve trend of the electron density is consistent with the pulsed trigger signal or the incident microwave.At the first time of 8 μs,the electron density increases quickly,which indicates that there happens the first ionization enhancement.From 8 to 50 μs,the electron density increases gradually from 7.5 × 1019to 1.5 × 1020m−3.However,after the pulsed microwave power disappears,the electron density suddenly decreases and rises immediately,the peak electron density takes place to 1.8 × 1020m−3at 61 μs.This indicates that the second dominant ionization arrives at,although the pulsed microwave power has been removed for 11 μs.

Figure 4.(a)Equivalent radius and length and(b)transient evolution of electron density of the argon plasma jet.The discharge condition is the same with that of figure 1(d).

As for air and nitrogen pulsed microwave APPJs,the time-varying equivalent length and radius of plasma jet cannot be obtained from the discharge photos taken by ICCD camera,due to the discharge images of air and nitrogen without periodicity,as shown in figure 3.In other words,the equivalent length and radius of air or nitrogen pulsed microwave plasma jet are not varied along with the pulsed microwave power.Thence,for air plasma jet,the equivalent radius is 0.9 mm and the equivalent length equals 1.8 mm,and for nitrogen plasma jet,the equivalent radius is 1.2 mm and the equivalent length equals 1.1 mm.These data all are determined by gray value method base on the light intensity of discharge images(figure 3).Thence,the Rayleigh scattered signals of air and nitrogen plasma jets have been measured by Rayleigh scattering device and their transient electron densities could be acquired,as indicated in figure 5.No matter there is air plasma jet or nitrogen plasma jet,their electron density has twice dominant ionizations in each pulse period,and occurs at the rising and falling edges of the pulse.Compared with argon plasma jet,the peak electron densities of air and nitrogen plasma jets are lower about one order of magnitude,and the peak electron density of nitrogen plasma jet is the lowest(about 9.1 × 1018m−3).In addition,there are many small pulse waveforms for the electron density of air and nitrogen plasma jets,which might be caused by the characteristic of atmospheric air streamer discharge (selfpulsed discharge) in ambient air.

3.3.Measurement on reactive particles

Most of APPJs contain a large number of active particles[40-44].The composition and proportion of active particles in different plasma jets and their different regions are different.In the practical application of plasma jet,the tail region of plasma jet is often used to deal with the target object,so the active particle composition in the tail region of plasma jet is worthy of attention.Figure 6 shows the radiation spectrum that is measured by an optical fiber spectrometer.The optic fiber detector is fixed to a distance of 5 mm away from the axis of plasma jet.For argon discharge,the acquired time is applied at 200 ms due to its stronger light radiation.But for air or nitrogen discharge,the integration time is prefixed at 2 s and averaged 5 times in view of the emitted spectrum being captured.

Figure 6(a) shows the spectrum acquired from the tail region of argon plasma jet.Reactive particles,like NO,N2,N+2,Ar∗,Ar+,O,have been detected.As for air plasma jet,the emission spectrum indicates that it contains particles of NO,N2,N+2,O,as shown in figure 6(b).For nitrogen plasma jet,the optical spectrum captured from the nozzle is without NO and O,as shown in figure 6(c).But the optical spectrum captured from the tail region of nitrogen plasma jet is similar with the air discharge.Compared with air plasma jet,the light emission intensity of nitrogen plasma jet is relatively weak,as shown in figure 6(d).

Figure 5.Transient electron densities of the pulsed microwave air and nitrogen APPJs,respectively.The discharge condition is the same with that of figure 3.

Figure 6.Optical spectra captured by optical fiber spectrometer.(a)Argon plasma jet,the tail region of plasma jet,(b)air plasma jet,the tail region of plasma jet,(c) nitrogen plasma jet,the nozzle region of plasma jet,and (d) nitrogen plasma jet,the tail region of plasma jet.

3.4.Discussion on ionization process

The mechanism of ionization process and characteristics of the proposed plasma jets can be acquired by comprehensively analyzing the discharge images,electron density evolution and electromagnetic interaction.The incident microwave changes its mode into the surface wave of surface plasmon polaritons(SPPs)in the CTLR device[45].The surface wave field of SPPs is locally enhanced at the open end of CTLR.When the microwave pulse with the pulse modulated frequency of 10 kHz is coupled to the CTLR device,the local enhanced electric field of SPPs will be excited resonantly along the powered electrode.Then the ionized gas discharge should be taken place near the tip of the powered electrode.At atmospheric pressure,the ionization develops in the form of streamer,and the front of ionization wave develops as the propulsion of plasma bullet [28].

In one discharge period,when the pulsed microwave excitation exists,the development of discharge consumes microwave energy partly,meanwhile,the microwave transmits its energy forward in the form of surface waves,maintaining the ionization development of plasma jet.At this time (pulsed microwave power on),the skin effect occurs,because the electron density of the microwave plasma jet(the amplitude order higher than 1018m−3as shown in figures 5 and 6)is higher than the cutoff value for incident microwave(about 1017m−3for 2.45 GHz).Due to the skin effect,the plasma jet has a donut cross section (that is,high-energy electrons only exist around the shell of plasma jet,and the number of high-energy electrons and excited particles in the shell is small) [29].When the microwave pulse excitation passes,the attenuation of the microwave plasma jet starts.In this stage (pulsed microwave power off),the luminous light emission undergoes a process of rapid enhancement and attenuation,as shown in figure 2.Along with the pulsed microwave excitation disappears,the number of electrons decreases synchronously and the skin effect disappears.Then,the high-energy electrons outside the shell move rapidly into the inner region of the plasma jet.Meanwhile,the microwave stored in the CTLR penetrates into the plasma jet and heats the low-energy electrons to higher energy.The superposition of both factors increases the density of high-energy electrons,and the light emission is enhanced by frequent collisions between high-energy electrons and excited particles.Therefore,the first ionization enhancement is caused by the interaction between the atmospheric pressure streamer discharge and the local enhanced electric field of SPPs at the tip of the powered electrode.The second ionization enhancement is induced by combined action both the skin effect of dense plasma jet generated by microwave and the microwave energy storied in the CTLR device.

In the shrinking stage,the middle region of the discharge channel finally disappears,as shown in figure 2.That is because there is a stronger electric field at the tip of the powered electrode.After the pulse microwave excitation disappears,the density of high-energy electrons near the tip of the powered electrode is more than that in the middle of the plasma jet,and the probability of collision with excited particles is frequent.As a result,the density of excited particles in the middle of the plasma jet decays slower than that around the tip of the powered electrode,and then the light emission exists longer in the middle of the plasma jet.In addition,the argon excited particles could be alive with the lifetime longer than 200 μs and the nitrogen excited particles could live to 2 ms [2],which indicates that there exists a considerable number of excited particles in the time without the microwave power [27].Even during the time without microwave power(between pulses),there also exists the weak light radiation in the discharge channel,and the light emission intensity gradually decreases to disappear.It is precisely because the lifetime of nitrogen excited particles is long enough,which is much longer than the duration of a pulsed microwave power(50 μs),then there is always a bright shape of plasma jet in the transient photo images of air and nitrogen plasma jets.However,the ionization development only exists when the pulsed microwave power exists,and the electron density only increases when the pulsed microwave power exists,as shown in figure 5.Therefore,although the peak microwave power required to trigger the discharge of air and nitrogen is high relatively,once the discharge occurs,the maintenance power of air and nitrogen plasma jet is low.This advantage is very favorable for the biomedical application of cold plasma jet,especially for nitrogen plasma jet driven by microwave pulses(considering that the nitrogen plasma jet is thick and long).In summary,the formation mechanism of periodic discharge morphology can be attributed to the combined interaction of resonant excitation of the local enhanced surface waves,the ionization wave propulsion,and the temporal and spatial distributions of different excited particles in the pulse modulated microwave plasma jets.

We have selected the most typical discharge phenomena of three gases (argon,air,nitrogen) to study.The ionization processes of these three gases all have the phenomenon of twice ionization enhancements,that is,ionization enhancement occurs at the rising edge and the falling edge in each modulated pulse.Different gas discharges have different plasma properties.From the experimental results,it can be seen that the argon plasma jet has the highest electron density,followed by air discharge,and the nitrogen plasma jet is the lowest electron density.Different from the nitrogen plasma jet or the air plasma jet,the electron density of argon plasma jet has a hysteresis phenomenon (the peak electron density lagging to about 10 μs).As air or nitrogen is used as the working gas,the electron density peak appears just at the moment as the pulse disappears.The length of air plasma jet is relatively short,and its radius is relatively concentrated.Under the same discharge parameters,the length of nitrogen plasma jet is three times that of air plasma jet,and the plume region of nitrogen plasma jet can occupy the entire nozzle of the resonator.

4.Conclusions

In conclusion,three types of cold pulsed microwave APPJs have been generated in ambient air,using the working gas of argon,nitrogen and air,respectively.The physical mechanism of twice ionization enhancements in each pulse periods and the distinctive characteristics of pulsed microwave APPJs have been indicated by comprehensively analyzing the transient discharge images,electron density evolution and electromagnetic interaction.The electron density measured via the method of microwave Rayleigh scattering has suggested that the proposed plasma jet has the highest plasma densities.The tail regions of plasma jets all include a large number of active particles,like NO,O,emitted photons,etc,but without O3.The parameters of plasma jet could be modulated by adjusting microwave power,modulation pulse parameters (modulation frequency and duty cycle),gas type and its flow rate,according to the requirements of application scenarios.According to the characteristics of three plasma jets,we recommend the use of nitrogen plasma jet generated by microwave pulses,because its comprehensive performance is best suitable for the biomedical application of cold plasma jet.For example,the proposed nitrogen plasma jet can be touched by finger directly,which suggests that the room temperature plasma jet rich in reactive particles could be valuable in cold plasma biomedical applications in ambient air(wound treatment,coagulation,skin surface sterilization,etc).

Acknowledgments

This work is supported by National Natural Science Foundation of China (Nos.52177126 and 11575003) and Anhui Province University Excellent Youth Foundation (No.gxyqzd2021104).