Influence of oxygen addition on the discharge characteristics of an argon plasma jet at atmospheric pressure

Junyu Chen(陈俊宇) Na Zhao(赵娜) Jiacun Wu(武珈存) Kaiyue Wu(吴凯玥) Furong Zhang(张芙蓉)Junxia Ran(冉俊霞) Pengying Jia(贾鹏英) Xuexia Pang(庞学霞) and Xuechen Li(李雪辰)

1College of Physics Science&Technology,Hebei University,Baoding 071002,China

2School of Mathematics and Physics,Handan University,Handan 056005,China

3Institute of Life Science&Green Development,Hebei University,Baoding 071002,China

Keywords: plasma jet,oxygen addition,fast photography,optical emission spectra

1. Introduction

Without the need for any vacuum device, atmospheric pressure plasma jet can produce a kind of remote plasma in open atmosphere, which is also referred to as a plasma plume. Being abundant with active species in plasma plume,[1,2]plasma jet has extensive applications, such as material synthesis,[3–5]surface modification,[6–8]ozone generation,[9]water purification,[10]methane conversion,[11]sterilization,[12–14]catalysis,[15]and medicine.[16,17]

For an inert-gas plasma jet in a barrier discharge configuration,three distinct operating modes are witnessed with varying input power, which include a chaotic mode, a bullet one,and a continuous one.[18]For the plasma jet operated in the bullet mode, a fast-moving bullet-like plasma is observed in plasma plume when imaged by fast photography.[19]The formation of plasma bullet is attributed to a streamer regime.[20]In addition to discharge regime, various discharge characteristics, such as plume length,[21–24]plume morphology,[25–30]and plasma parameters,[27–29,31–33]are investigated for inertgas plasma jet.

In fact, oxygen is often added to promote the production of active species,[34]thus increasing the plasma treatment efficiency.[35]However, oxygen addition is harmful for plume length.[36,37]Besides,vibrational and gas temperatures increase with increasing oxygen content of a helium jet.[37]Due to the electronegativity of oxygen molecules, electron density decreases with increasing oxygen content of a helium jet.[38]For low-cost argon plasma jet,[39,40]a constant oxygen dose has been investigated.[41–43]In fact, discharge aspects are influenced by oxygen concentration(CO). Preliminary results have revealed thatCOaffects plume length,[36,37]electron density and gas temperature.[37,38]Detailed investigations are needed for the influence ofCOon the discharge characteristic of an argon plasma jet.

In this paper, a single-electrode argon plasma jet is employed to investigate in detail the influence of oxygen addition on the discharge characteristics including plume morphology, discharge intensity, atomic oxygen concentration, electron density and electron temperature.

2. Experimental setup

The plasma jet is in a single-electrode geometry, whose schematic diagram is drawn in Fig. 1. A 12.0 cm long tungsten needle (both radius and tip radius are 0.5 mm) is poised at the axis of a quartz tube with inner and outer diameters of 5.0 mm and 8.0 mm, respectively. The needle tip is aligned with the quartz-tube nozzle. Argon and oxygen, both of which have a purity of 99.999%, are regulated by two independent gas flow meters (Sevenstar CS200A). Hence, oxygen content (CO, volume ratio) in argon is variable with a total flow rate (Q). A homemade power source that produces a sinusoidal voltage with an amplitude(Vp)of 10.0 kV and a frequency(f)of 3.0 kHz is electrically connected with the plasma jet. The voltage is detected by a probe (Tektronix P6015A).A lens is used to focus integrated light emitted from the jet, which is then collected by a photomultiplier tube(PMT)(ET 9130/100B).Utilizing a 4-channel digital oscilloscope(Tektronix DPO4104),waveforms of applied voltage and integrated light signal can be simultaneously obtained.In addition,a digital camera(Canon EOS 5D Mark IV)and an electron-multiplying intensified charge-coupled device(emICCD,PI MAX4)are utilized to capture the plume images.A spectrometer(ACTON SP2750)installed with an emICCD(PI MAX4) at the outlet slit is used to collect optical emission spectrum. A maximal spectral resolution is realized with a grating of 2400 grooves/mm. Temporally resolved spectrum is obtained with a method similar to that reported previously by us.[30]A TTL signal to trigger the emICCD is displayed along with the light emission signal by the digital oscilloscope. Hence, the optical gate of the emICCD is presented on the oscilloscope with reference to the discharge. Through varying the gate time,temporally resolved spectra can be obtained. The spatially resolved spectrum is realized by varying the detection position of an optical fiber connecting with the entrance slit of the spectrometer.

3. Results and discussion

Images of the argon plume are presented in Fig. 2 with varyingCO. When the working gas is pure argon(CO=0),the plasma jet emanates a solid plume, which is composed of a white part and a purple one from side-view images(left row).The white part shortens with increasingCO(0.2% to 0.4%).However,the length of the whole plume keeps almost constant with varyingCOin the small range. The purple part is diffuse,which transits to a hollow void whenCOreaches 0.6%. The morphology transition is observed more clearly from the frontview images(right row).Here,the central spot comes from the white part. Obviously, the cross section of the focus plane is diffuse with a lowCO. A purple ring resulting from the hollow structure is observed withCOof 0.6%. In brief, there is a transition from a diffuse morphology to a hollow structure with increasingCO.

Figure 3 illustrates waveforms of applied voltage and light emission signal from the plasma plume with varyingCO.For the sake of convenience,we define positive discharge and negative one,which correspond to the discharges initiating at positive negative voltages, respectively. When the working gas is pure argon, some positive and negative discharges appear underVpof 10.0 kV,which is similar to that reported by Ouyanget al.[44]With increasingCO(0.2% and 0.4%), the number of positive and negative discharges decreases per voltage cycle. At the same time,the maximal intensity of positive discharges increases, while that of negative ones decreases.For the hollow plume(CO=0.6%),one can see that only one positive discharge initiates per voltage cycle and negative discharge is almost ignorable.

Fast photography implemented by an ICCD was often used to reveal the propagation of streamers in a plasma jet.[1]Through using the emICCD that has a higher amplification than ICCD, negative discharge and positive one with varyingCOare imaged, as illustrated in Fig. 4. Discharge duration is around 300 ns, andtexpof 1.0 μs is used to capture single discharge image in Fig. 4. Apparently, diffuse negative discharge extends along the argon channel, which looks like a cone. With increasingCO,the length of the cone decreases. In contrast to the cone-like discharge,positive discharge is slimmer. The left side seems like a thin column, which appears mainly at the axis of the argon stream. The right side tends to be stochastically branched. Moreover, the thin column shortens and the branched part lengthens with increasingCO.WhenCOreaches 0.6%,the branches appear at the boundary layer of the argon channel (or the interface between the argon stream and the surrounding air),[27]which results in the hollow structure of the plume. From Fig. 4, it can be found that negative and positive discharges contribute to the white and purple parts of the argon plume,respectively.

Figure 5 illustrates 300 nm to 900 nm scanned optical emission spectra of the argon plasma plume. The spectra mainly include the lines from OH (A2Σ+→X2Π) at 308.9 nm,[45]and those from the second positive system of N2(C3Πu→B3Πg).[46]Both of them come from the diffusion of H2O and N2in ambient air.[47,48]Besides, there are lots of spectral lines of Ar I(2p3→1s4,738.4 nm;2p1→1s2,750.4 nm; 2p6→1s5, 763.7 nm; 2p2→1s3, 772.7 nm) from the 4p→4s transitions, which are clearly presented in the spectra.[49]Moreover, the dissociation of oxygen molecule contributes to atomic oxygen emission at 844.6 nm.[50]

Optical actinometry is used to investigate atomicCO,which is positively related with intensity ratio of the spectral lines (844.6 nm to 750.4 nm) in a small range ofCO.[21,51]From temporally and spatially resolved spectra,the spatial distribution of atomicCOcan be obtained for negative discharge and positive one,respectively,as indicated in Fig.6. Figure 6 reveals that with increasingCO,averaged atomic oxygen concentration (reflected by the intensity ratio) increases for both positive and negative discharges. Compared with that of negative discharge, atomic oxygen concentration of positive discharge is higher.AtomicCOincreases with increasing distance away from the needle tip. The above mentioned phenomenon can be explained as follows.

With increasingCO, more oxygen molecules participate in the discharge process, leading to the production of more oxygen atoms by electron impact dissociation.[21]As a result,atomicCOincreases asCOincreases. On account of the same reason, more oxygen molecules that diffuse into the working gas contribute to the growing atomicCOwith increasing distance from the needle tip. Moreover,the difference of atomicCObetween negative discharge and positive one may come from their different plasma parameters (density and temperature of electrons), which will be shown later. With lower plasma parameters,less oxygen molecules will be dissociated by electron impact,leading to lower atomicCOin negative discharge.

Intensity ratios of spectral lines can reflect plasma parameters,[52]such as density and temperature for electrons.[49,53]The line intensity ratios (738.4 nm to 763.7 nm, positively related with electron density) and(763.7 nm to 772.7 nm, positively related with electron temperature)as functions ofCOare shown in Fig.7. Here,intensity ratios are calculated from integrated spectra,which reflect space-averaged density and temperature of electrons. Apparently, with increasingCO, average electron density (reflected by the ratio of 738.4 nm to 763.7 nm) presents a decreasing tendency,while average electron temperature(the ratio of 763.7 nm to 772.7 nm) increases. Compared with those of negative discharge, average density and temperature of electrons are higher for positive discharge.

As is well known, both negative and positive discharges of plasma jet operate in a streamer regime.[1,20,24,27–30,54]Compared with negative discharge(anode-directed streamer),positive discharge (cathode-directed streamer) has a higher electric field strength(E).[29,55]Electrons are mainly produced in the plasma through the impact of argon atoms by electrons,which is dominated by the first Townsend ionization coefficient (α).[56,57]αis a function ofE.[58]A higherEtends to produce a plasma with a higher electron density. Therefore,positive discharge has a higher electron density than negative discharge. Besides, electron temperature depends onEbecause electrons obtain more energy in one mean free path under a strongerE. Hence, a plasma with a higher electron temperature can be generated with a higherE.[59]Due to the discrepancy ofE,negative discharge is lower than positive one in plasma parameters(density and temperature of electrons).

Long-life active species,such as metastable argon atoms(Ar*),play an important role in gas discharge.[60]Since residual Ar*can greatly decrease the field threshold for breakdown(Eth)due to stepwise ionization,[60]which is described by the following reactions:

where e=electron,Ar=ground state argon atom,and Ar+=positive argon ion. Consequently, the forthcoming discharge is inclined to initiate in the argon channel abundant with Ar*.Resultantly,negative discharge always extends along and covers the left side of the argon channel because the previous discharge (positive discharge) distributes the channel axis with Ar*. Based on the same reason, the left side of positive discharge appears in the axis of the argon stream because abundant residual Ar*are concentrated in the axis, which result from the previous positive and negative discharges.

Compared with the left side,more oxygen molecules exist at the right side of the argon stream due to the diffusion of ambient air. Oxygen molecules can quench Ar*through the following reaction:[60,61]

Therefore,the right side of the argon stream has a lower concentration of Ar*. As a result,a positive discharge can only be initiated under a strongerEat the right side. Numerical simulation has revealed that streamer bifurcates under a stronger field.[62]Consequently, the right side of positive discharge is branched. With increasingCOof the working gas,more residual Ar*will be quenched,[26,61]leading to a higherEthof the argon stream. This leads to the shortening column and lengthening branched part of positive discharge. In fact, the argon stream is surrounded by negative oxygen ions,[63]which can provide the forthcoming discharge with seed electrons through detachment. This factor will decreaseEthof the boundary layer. With the increasedEthof the argon stream and the reducedEthin the interfacial layer,discharge tends to appear in the interfacial layer whenCOreaches a certain value(0.6%).

As mentioned above,Ethmonotonously increases with increasingCO. This means that discharge will initiate under a strongerEwith a higherCO. Since electron temperature depends onE, electron temperature is higher under a strongerE. Hence, electron temperature increases for both negative and positive discharges with increasingCO. Coefficientαis not only a function ofE, but also related with gas ingredient.[55]The quenching of Ar*by oxygen molecules decreases the number of electrons produced by stepwise ionization, thus decreasingαcoefficient.[60]Moreover, some electrons are attached by oxygen molecules, which also decrease electron density in the plasma. Consequently, electron density decreases for both positive and negative discharges with increasingCO.

4. Conclusion

In this paper, the influence of oxygen addition on discharge characteristics has been investigated in detail for a single-electrode argon plasma jet. Within a small range ofCO(≤0.6%), the emanated plasma plume keeps almost constant in length, which is composed of the left white and the right purple parts. With increasingCO,the purple part transits from the diffuse morphology to a hollow void. During this process,the number of positive and negative discharges decreases per voltage cycle. At the same time,the maximal intensity shows an increasing trend for positive discharge, while a decreasing trend for negative one. Moreover,emICCD images reveal that negative discharge looks like a cone,while positive discharge is composed of a column and some branches. With increasingCO, both the cone and the column turn shorter, however,the branches become longer. From optical emission spectra,atomicCO, density and temperature of electrons are investigated as functions ofCO. Finally, these variation trends have been analyzed qualitatively.

Acknowledgments

Project supported by the National Natural Science Foundation of China (Grant Nos. 51977057 and 11875121), the Natural Science Foundation of Hebei Province, China(Grant Nos. A2020201025 and A2019201100), the Natural Science Interdisciplinary Research Program of Hebei University(Grant Nos. DXK202011 and DXK201908), Post-graduate’s Innovation Fund Project of Hebei Province, China (Grant Nos. CXZZBS2019023 and CXZZBS2019029), and Postgraduate’s Innovation Fund Project of Hebei University(Grant Nos.HBU2021ss063 and HBU2021bs011).