Electron heating mode transitions in radio-frequency driven micro atmospheric pressure plasma jets in He/O_{2}: A fluid dynamics approach
Yue Liu, Ihor Korolov, Torben Hemke, Lena Bischoff, Gerrit Hübner, Julian Schulze, Thomas Mussenbrock
EElectron heating mode transitions inradio-frequency driven micro atmospheric pressureplasma jets in He/O : A fluid dynamics approach Yue Liu , , Ihor Korolov , Torben Hemke , Lena Bischoff ,Gerrit H¨ubner , Julian Schulze , , and Thomas Mussenbrock Department of Electrical Engineering and Information Science, Ruhr-UniversityBochum, D-44780, Bochum, Germany Key Laboratory of Materials Modification by Laser, Ion and Electron Beams,School of Physics, Dalian University of Technology, Dalian, 116024, China Author to whom correspondence should be addressedE-mail: [email protected]
Abstract.
A two-dimensional fluid model is used to investigate the electron heatingdynamics and the production of neutral species in a capacitively coupled radio-frequency micro atmospheric pressure helium plasma jet – specifically the COST jet– with a small oxygen admixture. Electron heating mode transitions are found to beinduced by varying the driving voltage amplitude and the O concentration numericallyand experimentally. The helium metastable density, and the charged species densitiesare highly relevant to the electron heating dynamics. By analyzing the creation anddestruction mechanisms of the negative ions, we find that the generation of negativeions strongly depends on the O concentration. The increase of the electronegativitywith the increasing O concentration leads to an enhancement of the bulk drift electricfield. The distributions of the different neutral species densities along the direction ofthe gas flow inside the jet, as well as in the effluent differ a lot due to the relevantchemical reaction rates and the effect of the gas flow. The simulated results show thata fluid model can be an effective tool for qualitative investigations of micro atmosphericpressure plasma jets. Keywords : micro-atmospheric pressure plasma jet, COST jet, electron heating,production of neutral species, fluid simulations
1. Introduction
Radio-frequency (RF) micro atmospheric pressure plasma jets ( µ -APPJs) have becomean attractive plasma source for surface treatment [1–6] and, in particular, for biomedicalapplications [7–11]. Such jets are usually operated in helium or argon with a smalladmixture of molecular gases, such as nitrogen, oxygen or combinations of both. Thecontrol of the production of reactive oxygen and nitrogen species (RONS) within the a r X i v : . [ phy s i c s . p l a s m - ph ] F e b ischarge volume is crucial for these applications. One method is the use of voltagewaveform tailoring [12, 13], which has been shown the capability of tuning the dynamicsof electron energy distributions (EED) [14–18]. Typically, the EED deviates from aMaxwellian distribution particularly for high energies even at atmospheric pressure sincethe energy relaxation length can be larger than length scale of the reduced electric fieldgradients. Furthermore, the energy relaxation frequency can be faster than the temporalchange of the reduced electric field.Initial investigations on electron power absorption dynamics in µ -APPJs wereperformed in nominally pure argon or helium. The electron heating modes were assumedto be similar to those of low pressure capacitive discharges, i.e., the α -mode and the γ -mode [19–22]. The α to γ transition was later discussed in the context of addingimpurities [23, 24]. Hemke et al. [25] firstly pointed out based on results of Particle-in-Cell/Monte Carlo Collision (PIC/MCC) simulations for a pure helium discharge thatthe ionization dynamics is mainly produced by Ohmic heating in the so-called Ω-mode.It was demonstrated that even a small impurity in the noble gases can change thedominant ionization path to Penning ionization, leading to a decrease of the breakdownvoltage [26, 27]. Experimentally, phase resolved optical emission spectroscopy (PROES)was used to study the dynamics of energetic electrons based on the wavelength integratedoptical emission [28–31] and the use of selected emission lines (Ar: 750 nm [22], O: 844nm [32, 33]). Bischoff et al. [15] proposed that the 706.5 nm helium line can be usedto probe the ionization dynamics in helium when using a small nitrogen admixture.Electron power absorption mode transitions were observed as well in that work by bothexperiments and PIC/MCC simulations for µ -APPJs operated in He/N mixtures.Low pressure electropositive radio frequency capacitively coupled plasmas (RF-CCPs) can operate in two modes, the α -mode and the γ -mode [34]. Similarly, RFdriven µ -APPJs can also operate in two modes, the aforementioned Ω-mode and thePenning-mode. In the Ω-mode, electrons are accelerated by a bulk electric field whilehaving a high neutral collision frequency leading to a decreased electron conductivityin spite of the electron conduction current being high. Although the spatio-temporalionization dynamics of the Ω-mode is similar to those of the α -mode, the physicalmechanism is different. In the α -mode in low pressure RF-CCPs, energetic electronsare mainly generated by sheath expansion. The electric field adjacent to the sheath canbe either the ambipolar field or a drift field, or even a combination of both due to lowelectron density [35–37].There is also a fundamental difference between the Penning-mode in µ -APPJs andthe γ -mode in low pressure RF-CCPs. The largest electron impact ionization rateoccurs inside the sheath in both modes at maximum sheath extension. In the γ -modethis ionization is produced by secondary electron emission from the electrodes. Whereasin the µ -APPJs, the maximum ionization in the Penning-mode is caused by energeticelectrons producing highly excited states of the rare gas, followed by Penning ionization,for example, in He/O mixtures, He* + O → e + He + O +2 .Strongly electronegative capacitive RF discharges at low pressure, such as O or2F , operate in a drift-ambipolar mode [38], dominated by a drift electric field in thebulk and an ambipolar electric field near the sheath edge. The drift field here resultsfrom a low electron density in the bulk due to depletion by attachment rather than thehigh collision frequency in the Ω-mode. The ambipolar field is a consequence of a localmaximum in the electron density at the sheath edge.Another important aspect of µ -APPJs for any kind of applications is the productionof reactive species [39], which are highly relevant for chemical reactions. Complexplasma chemistries, such as He/O mixtures, consist of hundreds of reactions. Asensitivity analysis [40], as well as a discussion of uncertainties and errors [41] inducedby individual reactions were performed in the frame of a global model. Simulated speciesdensities (atomic oxygen densities [33], ozone densities [42]) were benchmarked againstexperiments. The major reactions that lead to the generation and loss of these twospecies were discussed. Reactive species generations were also found to be affected bydischarge parameters. The pressure dependence of O (a ∆ g ) densities [43], the effect ofpower pulse control on O (a ∆ g ) production [44], as well as the influence of the electrodeconfiguration on RONS production [45] were investigated based on two-dimensionalfluid simulations. Additional reactive neutral species are generated through interactionsbetween the effluent and its surroundings. Parametric studies on RONS production in aAPPJ flowing into humid air [46], or interacting with water [47] were performed basedon a two-dimensional plasma hydrodynamics model.In this work, we investigate a radio frequency capacitively coupled microatmospheric pressure He/O plasma jet based on a two-dimensional fluid dynamicsapproach. The purpose of this study is to address the effect of voltage amplitude andmolecular reactive admixtures on the electron heating mode transition, the chargedspecies dynamics and the neutral species densities. Based on the simulation results andcomparisons to experimental results, it is shown that a fluid model can qualitativelydescribe the variation tendency induced by control parameters and capture the mainphysical mechanism in such micro atmospheric pressure discharges. The remainingcontent of this paper is structured as follows: a brief introduction of the simulationmodel is provided in section 2, followed by discussions of results in section 3. Finally,conclusions are drawn and prospects are made.
2. Model
The simulations in this investigation were performed using the computational modelingplatform nonPDPSIM developed by Mark Kushner [48, 49]. It is a two-dimensionalmulti-species fluid dynamics code based on an unstructured grid, used for medium tohigh pressure weakly ionized plasmas. The model and prior applications have beenpreviously discussed in detail [48, 49]. nonPDPSIM has been successfully applied tothe study of micro atmospheric pressure plasma jets [21, 39, 43–47]. Here, only a brief3 igure 1.
Schematic of the simulation geometry based on the COST reference micro-plasma jet introduced in [50]. The black dotted line perpendicular to the electrodesrepresents positions where results shown in figures 2-8 are taken. The blue dotted lineparallel to the electrodes represents positions where results shown in figure 9 and 10are taken. description of nonPDPSIM is provided.For each charged species, the particle conservation equation is solved. The particleflux is expressed in terms of the drift-diffusion approximation. In order to allow for thenon-Maxwellian characteristics of the electron energy distribution function (EEDF),the electron transport coefficients, as well as the rate constants in the source and lossterms, are obtained by solving a 0-dimensional Boltzmann equation based on the 2-termapproximation. These generated coefficients are firstly tabulated as a function of thereduced electric field, subsequently altered in dependence of the mean electron energy,or the effective mean electron temperature. The effective mean electron temperature isobtained by solving the electron energy conservation equation. The fluid equations ofcharged species are coupled to incompressible (or if needed compressible) Navier-Stokesequations, which are used to describe the neutral species transport. Poisson’s equationis solved for the electric potential.A schematic of our simulation domain is shown in figure 1. It is based on theCOST reference micro-plasma jet introduced in [50]. In our simulations, a finite volumeof the model is considered, whose edges are grounded. The powered electrode is at thebottom and the grounded electrode is at the top. Two dielectrics are placed betweenthe respective electrode and the adjacent grounded wall, with a relative permittivityequal to 4. Gases flow in on the left side and are mixed in the gas mixing volume. Itis 5 mm in height. Therefore, the discharge cannot be ignited in this region at voltagestypically used to drive the jet. After the mixing region there is the discharge channelof 30 mm in length and 1 mm in height. A side chamber is located after the dischargedomain for the effluent before gases flow out on the right. The unstructured mesh ofdesign includes approximately 12000 nodes for the plasma region. 4he discharge is operated in He/O mixtures at atmospheric pressure. The gasflow rate can significantly affect plasma discharges and species generations. A fast gasflow will carry most of the generated neutral species outside the jet, meanwhile, it willcause turbulence in the jet. Those together lead to unsteady discharges. While a slowgas flow will lead to reactive species fully built up in the jet so that the treatmentdistance is limited. As a result, the gas flow is influenced by the length of the jet,and it is required to generate high fluxes of reactive species at the nozzle in an energyefficient way. Considering the feature size of a COST jet, it usually works in a gasflow rate approximate 1 slm [15–17, 33, 39, 50, 51]. The gas flow is fixed at 1 slm insimulations. The oxygen concentration ratio is set to 0.05% , 0.25%, and 0.5% fordifferent simulation cases. The voltage source is fixed at a frequency of 13.56 MHz andthe voltage amplitude is varied from 400 V to 600 V. The included species are groundstate neutral species He, O , O, O , excited state neutral species O (v=1-4) (first fourvibrational levels of O ), O (v), O (a ∆ g ), O (b Σ + g ), O( D), He* (ensemble of He(2 S)and He(2 S)), positive ions O +2 , O + , He + , negative ions O − , O − , O − , and electrons.For electron impact collisions with helium atoms, cross sections of the elastic [52], theexcitations [52, 53] and the ionization process [52] are considered. For electron impactcollisions with oxygen molecules, the reactions and the corresponding cross sectionsproposed by Gudmundsson et al. [54] are considered. These cross sections are used togenerate transport coefficients by solving the Boltzmann equation. Reactions betweenelectrons and other neutral species are neglected, since those neutral densities are at leastone order of magnitude lower than the molecular oxygen density. Reactions betweenheavy species (ions and neutrals) are based on Turner [41]. The number of total reactionsis reduced according to the sensitivity analyses [40]. The chemical reactions consideredin this work are the same as those listed in [18]. Surface coefficients for neutral speciesare treated as a loss probability [33, 55]. The coefficients are identical to those listedin [18]. The ion induced secondary electron emission coefficients are chosen to be 0.2,0.06, and 0.1 for He + , O +2 , and O + respectively based on the formula given in [56]. The experimental set-up has been introduced in detail by Bischoff et al. [15] and Korolov et al. [16]. Here only a brief description is demonstrated. Experiments are performedusing a RF driven COST-jet [50] operated in He combined with different O admixtures.The jet consists of two parallel stainless-steel electrodes of 30 mm in length. The gapbetween the electrodes is 1 mm. 5.0 purity helium and oxygen gases are used. The gasflow of He is fixed at 1 slm and the O flow is varied from 0.5 sccm to 5 sccm. TheRF voltage is applied to the powered electrode by a power generator via a matchingnetwork. The voltage waveform at the powered electrode is measured by a voltage probe(Tektronix P6015A with a bandwidth of 75 MHz). Phased resolved optical emissionspectroscopy (PROES) is used to observe the helium emission line at 706.5 nm via aninterference filter at 700 nm wavelength and 15 nm of full width at half maximum. The5hreshold of the corresponding electron impact helium excitation reaction is 22.7 eV. Inthis case only energetic electrons are detected as discussed by Bischoff et al. [15]. Thespatial-temporally resolved emission is recorded by an ICCD camera with a gate widthof 1 ns. The measurements are taken at the position of -10 mm from the nozzle. (Thecoordinate system is shown in figure 9.) The image resolution between the electrodegap corresponds 149 pixels. To monitor the impurity level, time integrated opticalemission spectroscopy (OES) is also conducted by a Universal Serial Bus (USB) gratingspectrometer.
3. Results
The first row of figure 2 shows the computational spatio-temporally resolved He(3 S)excitation rates as a function of the driving voltage amplitude at 400 V, 500 V, and 600V (different columns), with the gas flow fixed at 1 slm and the oxygen concentration keptconstant at 0.05%. The results are taken at the center of the discharge channel (markedby the black dotted line in figure 1). The maximal excitation rates are different for eachcase. The results are normalized by the respective maximum. As the driving voltageamplitude increases, the spatio-temporal dynamics of the excitation rate changes, i.e., anelectron heating mode transition is induced. At lower voltage, the majority of energeticelectrons are generated in the plasma bulk when the sheaths are expanding; while athigher voltages, strong electron impact excitation rates appear inside the sheaths at thetime of maximal sheath voltage. It should be noted that even though the electric fieldin the sheath is much stronger than in the bulk at 400 V, the electron density in thebulk is much higher compared to the sheath. As a consequence, most energetic electrons(above 22.7 eV) are generated by the acceleration due to the bulk drift field and thedischarge is operated in the Ω-mode [15, 25].When the voltage is increased to 600 V, the electric field in the sheath becomesstrong enough to dominate the production of energetic electrons, causing the dischargeto be operated in the Penning-mode [15, 25]. As can be seen in figure 2, the strongelectron impact excitation rates occur when the sheaths expand to the maximum, i.e.when the electric field in the sheath is strongest. Those electrons mainly originatefrom Penning ionization inside the sheath. The same electron heating mode transitionsare found via PROES measurements in a lower voltage amplitude range from 270 Vto 355 V, as shown in the second row of figure 2. Each case is normalized by therespective maximum. A larger grounded electrode compared to the powered electrodeleads to the weak asymmetry of the patterns in the experimental results. The verticalstripes in experimental results are due to synchronization issues of the camera. Asthe camera is directly triggered by the applied waveform, at some moments the internaldelay generator of the camera is not very stable, thus, leading to an inaccurate set of thedelay time. The difference of the working voltage amplitude between simulations andexperiments is due to the ignorance of the electron kinetic effects in fluid simulations,particularly for the electrons at high energy, such as the electrons generated from6enning ionization and surface emissions, which are then accelerated by the electricfield. On the other hand, the total number of non-uniform unstructured cells is limitedto a reasonable number to save the computational cost. Due to the far larger effectiveelectrode length than the electrode gap, a relatively coarse meshing along the electrodegap is used, which can lead to a reduced precision of the spatial resolution between theelectrodes. However, the simulation results still show a qualitative agreement with theexperiments. It is very important to understand such a mode transition, since it resultsin a lot of plasma parameter variations. For example, as shown in the third row of figure2, the time-averaged helium metastable density (simulated) is increased, with two peaksnear the sheath edges at 600 V. Helium metastables are generated via electron impactexcitations, while mainly destructed by Penning ionizations with O . Both reactionsproceed fast. Since O is uniformly distributed, the helium metastable density profiledepends on the generation rate distribution, i.e., it is the highest at the center of thedischarge gap in the Ω-mode, while two peaks near the sheath edges are formed in thePenning-mode.The first row of figure 3 shows the computational spatio-temporally resolvedHe(3 S) excitation rates as a function of the O concentration. The voltage amplitudeis fixed at 500 V. The results are taken again at the center of the gas flow channel(marked by the black dotted line in figure 1). The maximal excitation rates aredifferent for each case. The results are normalized by the respective maximum. Forthe 0.05% O concentration case, the excitation rates are stronger in the sheaths whenthe sheaths expand to the maximum, indicating that the discharge is predominantlyoperated in the Penning-mode. Increasing the O concentration induces the discharge tobe predominantly operated in the Ω-mode, since the excitation rates are stronger in theplasma bulk when the sheaths are expanding. Such a transition is also shown by PROESmeasurements in the second row of figure 3 at 355 V. The slight asymmetry is causedby the larger area of the grounded electrode. The reason for the use of higher voltageamplitudes in the simulations has been discussed above. This transition is caused by thecombination of two effects: electronegativity and collisions. More electronegative gascan lead to more negative ions and a lower electron density in the bulk (shown below),leading to a stronger bulk drift electric field, since the electron conductivity is inverselyproportional to the electron density. This is similar to the drift pattern of the drift-ambipolar mode [38] in low pressure RF strongly electronegative capacitive discharges.Besides, the destruction rate of He* is enhanced by the increased O density. However,the majority of electrons generated via the Penning ionization cannot be acceleratedto high energy due to the more frequent inelastic collisions in the presence of moremolecular gas, leading to a decreased population of He*. Correspondingly, the heliummetastable density decreases significantly by adding more O as shown in the third rowof figure 3.As shown above, the helium metastable density is highly relevant to the electronheating dynamics, and so are the charged species densities. Figure 4 shows the simulatedtime-averaged charged species density profiles between the electrodes as a function of7 igure 2. Spatio-temporally resolved He(3 S) excitation rates from simulations (firstrow) and from experiments (second row), and the computationally obtained time-averaged helium metastable (ensemble of the triplet and the singlet) density profiles(third row) between the two electrodes as a function of the driving voltage amplitude.The gas flow is fixed at 1 slm and the oxygen concentration is kept constant at 0.05%. the voltage amplitude. The O concentration is kept constant at 0.05%. All the resultsare taken at the center of the discharge channel (marked by the black dotted line infigure 1). In such cases, merely electrons, O +2 and O − are dominant, while the othercharged species are negligible. Their densities are increased by increasing the voltageamplitude, but the variation for O − is weak. This is due to the competition between theenhanced major generation (electron impact dissociation attachment) and destruction(recombination with O +2 and reactions with oxygen neutrals). It can be seen that thetime-averaged quasi-neutrality is broken, since the O +2 density is higher than the sum of8 igure 3. Spatio-temporally resolved He(3 S) excitation rates from simulations (firstrow) and from experiments (second row), and the computationally obtained time-averaged helium metastable (ensemble of the triplet and the singlet) density profiles(third row) between the two electrodes as a function of the O concentration. Thegas flow is 1 slm. The applied voltage amplitude is 500 V in simulations and 355 Vin experiments. The black dotted lines in the first row correspond to the moment atwhich results are taken and shown in figure 8 for each case. electron and O − densities, for example, at the center of the discharge gap. We believethat it is an averaging effect. The boundary loss for electrons is pronounced due to asmall discharge gap (1 mm) and the strong oscillation driven by the RF electric field,which results in a narrow profile of the electron density at each moment. Ion densitiesare almost time independent in steady state, thus, the spatio-temporal profiles of iondensities are not shown. The instantaneous quasi-neutrality is fulfilled locally (shown in9 igure 4. Simulated time-averaged charged species density profiles between theelectrodes as a function of the voltage amplitude at the longitudinal position indicatedin figure 1. The O concentration is kept constant at 0.05%. Figure 5.
Computational electron, positive ion and negative ion densities at differentmoments within one RF period at the longitudinal position indicated in figure 1. Thevoltage amplitude is 500 V and the O concentration is kept constant at 0.05%. figure 5), while the mean electron density is decreased by averaging over one RF period.Figure 6 shows the simulated time-averaged charged species density profiles betweenthe electrodes as a function of the O concentration. The voltage amplitude is keptconstant at 500 V. All the density profiles are shown at the center of the dischargechannel (marked by the black dotted line in figure 1). At low O concentration (0.05%),the electron density is higher than the negative ion densities. The major negativeion is O − , while O − and O − are negligible. As the O flow is increased, the electrondensity decreases, and the hump in the density profile becomes significant due to theenhancement of the negative ion population at the center of the electrode gap. At 0.5%O concentration, the O − and O − densities predominate over the electron density. The10 igure 6. Simulated time-averaged charged species density profiles between theelectrodes as a function of the O concentration at the longitudinal position indicatedin figure 1. The voltage amplitude is kept at 500 V. O − density increases slightly compared to the significant enhancement of the O − andO − densities. To understand such a variation of the negative ion density as a function ofthe O concentration, it is necessary to investigate the major chemical reactions leadingto the construction and destruction of those negative ions. We estimate the productionrate and loss rate induced by each relevant reaction based on the time-averaged densityof each species and the corresponding rate constant. By comparing those estimatedproduction and loss rates, we find that O − , O − and O − are mainly produced by thereactions e + O → O + O − , (1)e + O + He → O − + He , (2)O − + O + He → O − + He , (3)while the destruction rates, as mentioned above for negative ions, differ merely slightlybetween those different species. It can be seen that the generation of negative ionsis proportional to the O concentration. However, reaction (3) also corresponds toa destruction mechanism of O − , causing the O − density to increase only slightly byincreasing the O concentration. Such variations of the negative ion and electrondensities as a function of the voltage amplitude and the O concentration can affectthe electronegativity. Here the electronegativity is defined as the ratio between the sumof the time-averaged negative ion densities and the time-averaged electron density atthe center of the discharge gap. As shown in figure 7, this ratio decreases as the voltageamplitude increases (red square). However, a significant increase of the electronegativityresults from the increasing O concentration (blue dot).As a consequence of the variation of the electronegativity, the electric fieldperpendicular to the electrode changes with the O concentration. In figure 8, theleft figure shows the electric field component perpendicular to the electrodes at t = 8.3ns corresponding to the moment of the maximal excitation rate in the bulk (marked in11 igure 7. Ratios between the sum of the time-averaged negative ion densities andthe time-averaged electron densities at the center of the discharge gap as a function ofthe voltage amplitude (red square, 0.05% O concentration) and the O concentration(blue dot, the voltage amplitude of 500 V). The densities are taken at the longitudinalposition indicated in figure 1. the first row of figure 3). We restrict the region from 0.2 mm to 0.6 mm to magnify thedifferences among the bulk electric fields. The electric field in this region mainly causesthe Ω-pattern of the electron heating dynamics. This drift electric field becomes largeras the O concentration increases, since the electron density gets lower, inducing anattenuation of the electron conductivity. It is known that the strong bulk electric fieldin atmospheric pressure discharges is caused by the very high collisionality of electrons,mainly due to electron elastic collisions, since cross sections of electron elastic collisionsare much larger than those of electron inelastic collisions. In our study cases, thepredominant contribution is electron helium elastic collisions due to the high flow ofthe helium gas. The increasing O concentration from 0.05% to 0.5% cannot lead toa significant increase of the total collision frequency because O is still a very smalladmixture and the cross section of electron oxygen elastic collisions is comparable withthe one of electron helium elastic collisions. Attentively, here we only point out that theenhancement of the bulk electric field is not mainly caused by the elastic collisions, butby the increased electronegativity induced by adding more O . The right plot of figure 8shows the electric field perpendicular to the electrodes at t = 19.2 ns corresponding tothe moment of the maximal excitation rate in the top sheath (marked in the first row offigure 3). Again, only the region between 0.6 mm and 0.9 mm is shown to magnify thedifferences. The electric field in the top sheath becomes smaller as the O concentrationincreases, which weakens the Penning-pattern of the electron heating dynamics there.Interestingly, the low electric field in the sheath leads to a self-amplification mechanism12 igure 8. Simulated electric field perpendicular to the electrode along the electrodegap at t = 8.3 ns when the excitation rate is maximal in the bulk (left) and t = 19.2ns when the excitation rate is maximal in the sheath (right) as a function of the O concentration at the fixed voltage amplitude of 500 V. The corresponding moments aremarked by black dotted lines in the first row of figure 3. The results are taken alongthe black dotted line indicated in figure 1. of this electron heating mode transition. Since the voltage is fixed across the dischargegap, the enhanced electric field in the bulk corresponds to a larger potential drop there.As a result, the voltage drop across the sheath is reduced, leading to the decreasedelectric field there.So far, we have discussed the electron heating mode transition dependence onthe voltage amplitude and the O concentration. We have also analyzed the chargedspecies density variations induced by the control parameters and the effect of theelectronegativity on the electron heating dynamics. Another important aspect forpractical applications is the production of the desired reactive neutral species, such asatomic oxygen and ozone. We firstly focus on the computational distributions of neutralspecies densities along the direction of the gas flow at the fixed voltage amplitude of500 V and the O concentration of 0.05%, as shown in figure 9. The densities aretaken at the center of the electrode gap (highlighted by the blue dotted line in figure1). Regarding the coordinate system, the gas nozzle is located at 0, indicating thatthe coordinate is negative inside the jet (discharge domain), while the coordinate of theeffluent is positive. The He*, O( D) and O (v=1-4) densities are flat distributed insidethe jet, and rapidly decrease to be negligible after the nozzle. The O, O (a ∆ g ) andO (b Σ + g ) densities increase in the jet and reach their highest values in a short distancefrom the nozzle outside the jet, and decrease slightly in the effluent. The O density iscontinuously increasing until the end of the effluent propagation. Similar results havebeen reported by Hemke et al. [39]. Such neutral species distributions along the gasflow result from the major generation and destruction reaction rates, together with theeffect of the gas flow. According to the sensitivity analyses based on chemical reactions13n He/O plasma jets in [33, 40], He*, O( D) and O (v=1-4) are generated via electronimpact excitation and quenched by He or O . Both processes are fast so that theirdensity distributions are uniform in the jet. No source is provided outside the jet, sothat they vanish rapidly. O, O (a ∆ g ) and O (b Σ + g ) are also generated by electronimpact molecular oxygen reactions at fast rates, but vanish slowly by reactions betweenneutrals. In such cases, the gas flow affects the transport of those species to shownon-uniform distributions in the jet. Due to their relatively slow destruction rates, thespecies generated in the jet can be carried by the gas flow to the effluent. The decreasein the effluent is because of the lack of sources, but is not significant. O is generatedand destructed by neutrals slowly. The main generation reaction isO + O + He → O + He , (4)which provides a source in the effluent.Figure 10 shows three reactive oxygen species (O (a ∆ g ), O and O ) densitydistributions along the gas flow as a function of the voltage amplitude and the O concentration. Densities are taken at the center of the electrode gap (highlighted bythe blue dotted line in figure 1). Those species densities can be enhanced by eitherincreasing the voltage amplitude or the O concentration within the range discussed inthis work. But increasing the O admixtures is more effective.
4. Conclusions
In this work, the spatio-temporal electron heating dynamics in a He/O RF capacitiveatmospheric pressure micro plasma jet is investigated by 2-dimensional fluid dynamicssimulations and PROES measurements. Two electron heating modes (the Ω-mode andthe Penning-mode) are found and the electron heating dynamics can be changed byvarying the voltage amplitude as well as the O concentration. Based on the analysesof the construction and destruction mechanisms of the negative ions, it is pointed outthat the generation of the negative ions is largely dependent on the O concentration.The increased negative ion density induced by adding more O enhances the drift bulkelectric field, which contributes to the electron heating mode transition. The densitiesof different neutral species are found to show different distributions along the directionof the gas flow inside the jet and in the effluent due to the species relevant chemicalreaction rates as well as the effect of the gas flow.We notice that the electron heating mode transition can be induced within alower range of the voltage amplitude in experiments (the second rows of figure 2 and3). However, our simulated results clearly show that the fluid model is capable ofinvestigating RF micro atmospheric pressure discharges qualitatively and providingguidance for practical applications. To achieve quantitative agreement of the resultsbetween the electrodes with experimental measurements, a kinetic treatment of electronsis needed. Complex chemistry between heavy species can hardly be treated by Particle-in-Cell/Monte Carlo Collision (PIC/MCC) simulations to the best of our knowledge.14 igure 9. Computationally obtained distributions of neutral species densities at thecenter of the electrode gap along the direction of the gas flow (highlighted by the bluedotted line in figure 1). The gas nozzle is located at 0. The discharge domain is from-30 mm to 0. The effluent is at the position larger than 0. The voltage amplitude is500 V and the O concentration is 0.05%. Figure 10.
Computationally obtained distributions of reactive oxygen speciesdensities at the center of the electrode gap along the direction of the gas flow(highlighted by the blue dotted line in figure 1). The gas nozzle is located at 0.The discharge domain is from -30 mm to 0. The effluent is located at the positionlarger than 0.
A real hybrid model would be a more effective solution for simulating sophisticateddischarges with multiple species more accurately. This has been already shown by theauthors elsewhere based on a coupling between a 1d3v (one dimension in displacement,three dimensions in velocity) PIC/MCC model and a 2D simplified fluid model [18].15owever, in order to consider the lateral gas flow, a sophisticated hybrid model including2d3v PIC/MCC algorithm and 2D fluid model would be ultimately needed. Moreover,a large number of cells are required to resolve the plasma dynamics precisely in bothdirections. The implementation and application of such model is a future goal. In themeantime, one has to rely either on a hybrid 1d3v PIC/MCC model that allows for acorrect kinetic description of electrons but omits the actual gas transport, or on thefluid model which considers the transport of all species, but treats electrons not fullykinetically.
Acknowledgments
This work is supported by the German Research Foundation in the frame of SFB 1316(project A4 and project A5) and MU2332/11-1. We gratefully thank Prof. MarkKushner for his support and guidance of using nonPDPSIM . We also thank Dr. Zolt´anDonk´o for fruitful discussions.
ORCID IDs
T. Mussenbrock: http://orcid.org/0000-0001-6445-4990I. Korolov: https://orcid.org/0000-0003-2384-1243J. Schulze: https://orcid.org/0000-0001-7929-5734Y. Liu: https://orcid.org/0000-0002-2680-1338
References [1] M. Laroussi, “Low Temperature Plasma-Based Sterilization: Overview and State-of-the-Art,”
Plasma Processes and Polymers , vol. 2, no. 5, pp. 391–400, 2005.[2] K. H. Becker, K. H. Schoenbach, and J. G. Eden, “Microplasmas and applications,”
Journal ofPhysics D: Applied Physics , vol. 39, pp. R55–R70, Jan. 2006.[3] I. Adamovich, S. D. Baalrud, A. Bogaerts, P. J. Bruggeman, M. Cappelli, V. Colombo,U. Czarnetzki, U. Ebert, J. G. Eden, P. Favia, D. B. Graves, S. Hamaguchi, G. Hieftje, M. Hori,I. D. Kaganovich, U. Kortshagen, M. J. Kushner, N. J. Mason, S. Mazouffre, S. M. Thagard,H.-R. Metelmann, A. Mizuno, E. Moreau, A. B. Murphy, B. A. Niemira, G. S. Oehrlein,Z. L. Petrovic, L. C. Pitchford, Y.-K. Pu, S. Rauf, O. Sakai, S. Samukawa, S. Starikovskaia,J. Tennyson, K. Terashima, M. M. Turner, M. C. M. van de Sanden, and A. Vardelle, “The2017 Plasma Roadmap: Low temperature plasma science and technology,”
Journal of PhysicsD: Applied Physics , vol. 50, p. 323001, July 2017.[4] O. V. Penkov, M. Khadem, W.-S. Lim, and D.-E. Kim, “A review of recent applications ofatmospheric pressure plasma jets for materials processing,”
Journal of Coatings Technologyand Research , vol. 12, pp. 225–235, Mar. 2015.[5] S. E. Babayan, J. Y. Jeong, V. J. Tu, J. Park, G. S. Selwyn, and R. F. Hicks, “Deposition of silicondioxide films with an atmospheric-pressure plasma jet,”
Plasma Sources Science and Technology ,vol. 7, pp. 286–288, Aug. 1998.[6] T. Ichiki, R. Taura, and Y. Horiike, “Localized and ultrahigh-rate etching of silicon wafers usingatmospheric-pressure microplasma jets,”
Journal of Applied Physics , vol. 95, pp. 35–39, Dec.2003.
7] S. J. Kim, T. H. Chung, S. H. Bae, and S. H. Leem, “Characterization of Atmospheric PressureMicroplasma Jet Source and its Application to Bacterial Inactivation,”
Plasma Processes andPolymers , vol. 6, no. 10, pp. 676–685, 2009.[8] D. B. Graves, “Low temperature plasma biomedicine: A tutorial review,”
Physics of Plasmas ,vol. 21, p. 080901, Aug. 2014.[9] K.-D. Weltmann and T. von Woedtke, “Plasma medicine—current state of research and medicalapplication,”
Plasma Physics and Controlled Fusion , vol. 59, p. 014031, Nov. 2016.[10] M. G. Kong, G. Kroesen, G. Morfill, T. Nosenko, T. Shimizu, J. van Dijk, and J. L. Zimmermann,“Plasma medicine: An introductory review,”
New Journal of Physics , vol. 11, p. 115012, Nov.2009.[11] S. Bekeschus, A. Schmidt, K.-D. Weltmann, and T. von Woedtke, “The plasma jet kINPen – Apowerful tool for wound healing,”
Clinical Plasma Medicine , vol. 4, pp. 19–28, July 2016.[12] B. G. Heil, U. Czarnetzki, R. P. Brinkmann, and T. Mussenbrock, “On the possibility of makinga geometrically symmetric RF-CCP discharge electrically asymmetric,”
Journal of Physics D:Applied Physics , vol. 41, p. 165202, July 2008.[13] J. Schulze, E. Sch¨ungel, Z. Donk´o, and U. Czarnetzki, “The electrical asymmetry effect inmulti-frequency capacitively coupled radio frequency discharges,”
Plasma Sources Science andTechnology , vol. 20, p. 015017, Jan. 2011.[14] A. R. Gibson, Z. Donk´o, L. Alelyani, L. Bischoff, G. H¨ubner, J. Bredin, S. Doyle, I. Korolov,K. Niemi, T. Mussenbrock, P. Hartmann, J. P. Dedrick, J. Schulze, T. Gans, and D. O’Connell,“Disrupting the spatio-temporal symmetry of the electron dynamics in atmospheric pressureplasmas by voltage waveform tailoring,”
Plasma Sources Science and Technology , vol. 28,p. 01LT01, Jan. 2019.[15] L. Bischoff, G. H¨ubner, I. Korolov, Z. Donk´o, P. Hartmann, T. Gans, J. Held, V. S.-v. der Gathen,Y. Liu, T. Mussenbrock, and J. Schulze, “Experimental and computational investigations ofelectron dynamics in micro atmospheric pressure radio-frequency plasma jets operated in He/N mixtures,” Plasma Sources Science and Technology , vol. 27, p. 125009, Dec. 2018.[16] I. Korolov, Z. Donk´o, G. H¨ubner, L. Bischoff, P. Hartmann, T. Gans, Y. Liu, T. Mussenbrock,and J. Schulze, “Control of electron dynamics, radical and metastable species generation inatmospheric pressure RF plasma jets by Voltage Waveform Tailoring,”
Plasma Sources Scienceand Technology , vol. 28, p. 094001, Sept. 2019.[17] I. Korolov, M. Leimk¨uhler, M. B¨oke, Z. Donk´o, V. S.-v. der Gathen, L. Bischoff, G. H¨ubner,P. Hartmann, T. Gans, Y. Liu, T. Mussenbrock, and J. Schulze, “Helium metastable speciesgeneration in atmospheric pressure RF plasma jets driven by tailored voltage waveforms inmixtures of He and N ,” Journal of Physics D: Applied Physics , vol. 53, p. 185201, Feb. 2020.[18] Y. Liu, I. Korolov, J. Trieschmann, D. Steuer, V. S.-v. der Gathen, M. Boeke, L. Bischoff,G. H¨ubner, J. Schulze, and T. Mussenbrock, “Micro atmospheric pressure plasma jets excitedin He/O by voltage waveform tailoring: A study based on a numerical hybrid model andexperiments,” Plasma Sources Science and Technology , 2020.[19] F. Iza, J. K. Lee, and M. G. Kong, “Electron Kinetics in Radio-Frequency Atmospheric-PressureMicroplasmas,”
Physical Review Letters , vol. 99, p. 075004, Aug. 2007.[20] K. Niemi, J. Waskoenig, N. Sadeghi, T. Gans, and D. O’Connell, “The role of helium metastablestates in radio-frequency driven helium–oxygen atmospheric pressure plasma jets: Measurementand numerical simulation,”
Plasma Sources Science and Technology , vol. 20, p. 055005, Aug.2011.[21] B. Niermann, T. Hemke, N. Y. Babaeva, M. B¨oke, M. J. Kushner, T. Mussenbrock, and J. Winter,“Spatial dynamics of helium metastables in sheath or bulk dominated rf micro-plasma jets,”
Journal of Physics D: Applied Physics , vol. 44, p. 485204, Nov. 2011.[22] M. D¨unnbier, M. M. Becker, S. Iseni, R. Bansemer, D. Loffhagen, S. Reuter, and K.-D. Weltmann,“Stability and excitation dynamics of an argon micro-scaled atmospheric pressure plasma jet,”
Plasma Sources Science and Technology , vol. 24, p. 065018, Nov. 2015.
23] A. Chirokov, S. N. Khot, S. P. Gangoli, A. Fridman, P. Henderson, A. F. Gutsol, andA. Dolgopolsky, “Numerical and experimental investigation of the stability of radio-frequency(RF) discharges at atmospheric pressure,”
Plasma Sources Science and Technology , vol. 18,p. 025025, Mar. 2009.[24] E. Kawamura, M. A. Lieberman, A. J. Lichtenberg, P. Chabert, and C. Lazzaroni, “Particle-in-cell and global simulations of α to γ transition in atmospheric pressure Penning-dominatedcapacitive discharges,” Plasma Sources Science and Technology , vol. 23, p. 035014, May 2014.[25] T. Hemke, D. Eremin, T. Mussenbrock, A. Derzsi, Z. Donk´o, K. Dittmann, J. Meichsner, andJ. Schulze, “Ionization by bulk heating of electrons in capacitive radio frequency atmosphericpressure microplasmas,”
Plasma Sources Science and Technology , vol. 22, p. 015012, Dec. 2012.[26] T. Martens, A. Bogaerts, W. J. M. Brok, and J. V. Dijk, “The dominant role of impurities in thecomposition of high pressure noble gas plasmas,”
Applied Physics Letters , vol. 92, p. 041504,Jan. 2008.[27] I. Radu, R. Bartnikas, and M. R. Wertheimer, “Frequency and voltage dependence of glow andpseudoglow discharges in helium under atmospheric pressure,”
IEEE Transactions on PlasmaScience , vol. 31, pp. 1363–1378, Dec. 2003.[28] V. S.-v. der Gathen, L. Schaper, N. Knake, S. Reuter, K. Niemi, T. Gans, and J. Winter, “Spatiallyresolved diagnostics on a microscale atmospheric pressure plasma jet,”
Journal of Physics D:Applied Physics , vol. 41, p. 194004, Sept. 2008.[29] J. Benedikt, S. Hofmann, N. Knake, H. B¨ottner, R. Reuter, A. von Keudell, and V. Schulz-vonder Gathen, “Phase resolved optical emission spectroscopy of coaxial microplasma jet operatedwith He and Ar,”
The European Physical Journal D , vol. 60, pp. 539–546, Dec. 2010.[30] S. Reuter, J. Winter, S. Iseni, S. Peters, A. Schmidt-Bleker, M. D¨unnbier, J. Sch¨afer, R. Foest,and K.-D. Weltmann, “Detection of ozone in a MHz argon plasma bullet jet,”
Plasma SourcesScience and Technology , vol. 21, p. 034015, May 2012.[31] L. Schaper, J. Waskoenig, M. G. Kong, V. S.-v. der Gathen, and T. Gans, “Electron Dynamicsin a Radio-Frequency-Driven Microatmospheric Pressure Plasma Jet,”
IEEE Transactions onPlasma Science , vol. 39, pp. 2370–2371, Nov. 2011.[32] A. Greb, K. Niemi, D. O’Connell, and T. Gans, “Energy resolved actinometry for simultaneousmeasurement of atomic oxygen densities and local mean electron energies in radio-frequencydriven plasmas,”
Applied Physics Letters , vol. 105, p. 234105, Dec. 2014.[33] J. Waskoenig, K. Niemi, N. Knake, L. M. Graham, S. Reuter, V. S.-v. der Gathen, and T. Gans,“Atomic oxygen formation in a radio-frequency driven micro-atmospheric pressure plasma jet,”
Plasma Sources Science and Technology , vol. 19, p. 045018, June 2010.[34] P. Belenguer and J. P. Boeuf, “Transition between different regimes of rf glow discharges,”
PhysicalReview A , vol. 41, pp. 4447–4459, Apr. 1990.[35] R. P. Brinkmann, “The electric field in capacitively coupled RF discharges: A smooth step modelthat includes thermal and dynamic effects,”
Plasma Sources Science and Technology , vol. 24,p. 064002, Oct. 2015.[36] R. P. Brinkmann, “Electron heating in capacitively coupled RF plasmas: A unified scenario,”
Plasma Sources Science and Technology , vol. 25, p. 014001, Dec. 2015.[37] J. Schulze, Z. Donk´o, T. Lafleur, S. Wilczek, and R. P. Brinkmann, “Spatio-temporal analysis ofthe electron power absorption in electropositive capacitive RF plasmas based on moments of theBoltzmann equation,”
Plasma Sources Science and Technology , vol. 27, p. 055010, May 2018.[38] J. Schulze, A. Derzsi, K. Dittmann, T. Hemke, J. Meichsner, and Z. Donk´o, “Ionization byDrift and Ambipolar Electric Fields in Electronegative Capacitive Radio Frequency Plasmas,”
Physical Review Letters , vol. 107, p. 275001, Dec. 2011.[39] T. Hemke, A. Wollny, M. Gebhardt, R. P. Brinkmann, and T. Mussenbrock, “Spatially resolvedsimulation of a radio-frequency driven micro-atmospheric pressure plasma jet and its effluent,”
Journal of Physics D: Applied Physics , vol. 44, p. 285206, June 2011.[40] M. M. Turner, “Uncertainty and sensitivity analysis in complex plasma chemistry models,”
Plasma ources Science and Technology , vol. 25, p. 015003, Dec. 2015.[41] M. M. Turner, “Uncertainty and error in complex plasma chemistry models,” Plasma SourcesScience and Technology , vol. 24, p. 035027, June 2015.[42] A. Wijaikhum, D. Schr¨oder, S. Schr¨oter, A. R. Gibson, K. Niemi, J. Friderich, A. Greb, V. S.-v.der Gathen, D. O’Connell, and T. Gans, “Absolute ozone densities in a radio-frequency drivenatmospheric pressure plasma using two-beam UV-LED absorption spectroscopy and numericalsimulations,”
Plasma Sources Science and Technology , vol. 26, p. 115004, Oct. 2017.[43] N. Y. Babaeva, R. A. Arakoni, and M. J. Kushner, “Production of O ( ∆) in flowing plasmasusing spiker-sustainer excitation,” Journal of Applied Physics , vol. 99, p. 113306, June 2006.[44] N. Y. Babaeva, R. Arakoni, and M. J. Kushner, “O ( ∆) production in high pressure flowingHe/O plasmas: Scaling and quenching,” Journal of Applied Physics , vol. 101, p. 123306, June2007.[45] A. M. Lietz and M. J. Kushner, “Electrode configurations in atmospheric pressure plasma jets:Production of reactive species,”
Plasma Sources Science and Technology , vol. 27, p. 105020, Oct.2018.[46] S. A. Norberg, E. Johnsen, and M. J. Kushner, “Formation of reactive oxygen and nitrogen speciesby repetitive negatively pulsed helium atmospheric pressure plasma jets propagating into humidair,”
Plasma Sources Science and Technology , vol. 24, p. 035026, June 2015.[47] S. A. Norberg, G. M. Parsey, A. M. Lietz, E. Johnsen, and M. J. Kushner, “Atmospheric pressureplasma jets onto a reactive water layer over tissue: Pulse repetition rate as a control mechanism,”
Journal of Physics D: Applied Physics , vol. 52, p. 015201, Oct. 2018.[48] M. J. Kushner, “Modeling of microdischarge devices: Pyramidal structures,”
Journal of AppliedPhysics , vol. 95, pp. 846–859, Jan. 2004.[49] M. J. Kushner, “Modelling of microdischarge devices: Plasma and gas dynamics,”
Journal ofPhysics D: Applied Physics , vol. 38, pp. 1633–1643, May 2005.[50] J. Golda, J. Held, B. Redeker, M. Konkowski, P. Beijer, A. Sobota, G. Kroesen, N. S. J.Braithwaite, S. Reuter, M. M. Turner, T. Gans, D. O’Connell, and V. S.-v. der Gathen,“Concepts and characteristics of the ‘COST Reference Microplasma Jet’,”
Journal of PhysicsD: Applied Physics , vol. 49, p. 084003, Jan. 2016.[51] N. Knake, K. Niemi, S. Reuter, V. Schulz-von der Gathen, and J. Winter, “Absolute atomic oxygendensity profiles in the discharge core of a microscale atmospheric pressure plasma jet,”
AppliedPhysics Letters
Plasma SourcesScience and Technology , vol. 22, p. 035011, May 2013.[55] D. S. Stafford and M. J. Kushner, “O ( ∆) production in He/O mixtures in flowing low pressureplasmas,” Journal of Applied Physics , vol. 96, pp. 2451–2465, Sept. 2004.[56] Y. P. Raizer,
Gas Discharge Physics . Berlin Heidelberg: Springer-Verlag, 1991.. Berlin Heidelberg: Springer-Verlag, 1991.