Development of Barrier Discharges Operation Modes and Structure Formation

Contrib. Plasma Phys. 52, No. 10, 847 855 (2012) / DOI 10.1002/ctpp.201200041

Development of Barrier Discharges: Operation Modes andStructure Formation

M. Bogaczyk1, S. Nemschokmichal1, R. Wild1, L. Stollenwerk1, R. Brandenburg2,J. Meichsner1, and H.-E. Wagner11 University of Greifswald, Institute of Physics, F.-Hausdorff-Str. 6, 17489 Greifswald, Germany2 Leibniz Institute for Plasma Science and Technology (INP), F.-Hausdorff-Str. 2, 17489 Greifswald, Germany

Received 15 May 2012, accepted 14 June 2012Published online 08 November 2012

Key words Non-thermal plasma, atmospheric pressure, barrier discharge, operation modes, pattern formation.

A discharge cell conguration combining three high-end diagnostic techniques (cross-correlation spectroscopy,electro-optic Pockels effect and laser induced uorescence spectroscopy) has been developed. The joint ap-plication of these diagnostics enables the investigation of surface and bulk processes in barrier discharges indifferent arrangements. Surface charge density, N2(A) metastable state density, and optical radiation of plasmaas three key parameters can be measured and correlated under identical and well-dened experimental condi-tions. Systematic measurements of absolute surface charge densities and nitrogen metastable concentrationsare presented for different operation modes. The operation mode is mainly controlled by the gas composi-tion, the discharge cell geometry, and the properties of the applied voltage. In particular, in pure helium theunusual Townsend-like discharge was investigated in detail. The described arrangements and the knowledgeabout reproducible control of discharge modes are an excellent base for the systematic investigation of the in-teraction between plasma and dielectric surface and its role on the formation, development, and structure ofbarrier discharges.

1 Introduction

Non-thermal plasmas at atmospheric pressure are effective sources of radicals and excited species. Therefore,they have found many technical applications in the plasma chemistry, lighting, and recently also in life-science[1,2]. In this group the barrier discharges (BDs) have a key position. It is well-known that BDs can be operated invarious discharge modes, namely the lamentary and diffuse mode. In the latter it has to be distinguished betweena Townsend-like and glow-like mode [37]. The appearance of different operation modes depends on the feedinggas mixture and ow rate, gas pressure, operation frequency and shape of applied voltage, dielectric barriermaterial and the reactor geometry (e.g. gap spacing). Especially, for large aspect ratios (ratio of lateral extensionto gap distance) an increased tendency of pattern formation is observed [79]. Finally, the discharge operationmode (lamentary or diffuse) is related to the ratio of the secondary processes at the electrodes (e.g. exoemissionof electrons) to the ionization in the discharge volume (Townsend mechanism and Penning ionization). Here,surface charges play an important role, because of the low energy necessary to release electrons from such asurface. This energy can be provided by metastable species. Many investigation in the past have contributed tothe better understanding of the complex processes. Despite the great progress in this eld of complex plasmas inthe last decade [1013], the diagnostics of these phenomena is still a challenge.

To study the discharge modes and the lateral structuring, a discharge cell conguration has been developed tocombine three high-end diagnostic techniques: The spatio-temporally and spectrally resolved discharge develop-ment in the volume was investigated by the well-known cross-correlation spectroscopy (CCS) [3, 1416]. Thetemporally resolved detection of surface charges on the dielectric succeeded by the application of the electroopticPockels effect in combination with a CCD camera [1719]. Metastable N2(A) molecules were determined by the

Corresponding author. E-mail: [email protected]

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848 M. Bogaczyk et al.: Development of Barrier Discharges: Operation Modes and Structure Formation

laser induced uorescence spectroscopy [11]. This way, it was possible for the rst time to correlate key param-eters of the discharge operation modes under identical and well-dened experimental conditions. Additionally,a separate surface BD conguration was used to investigate the propagation of microdischarges in air along theinterface of the gas and the dielectric surface.

The presented results are an important and extensive source for the comparison with kinetic models to geta deeper (nally quantitative) understanding of the complex mechanisms in the BD evolution. The modelingactivities - under progress - consider the dominant processes in the discharge volume as well as the interactionsof relevant plasma species with dielectric surfaces [2023].

2 Experimental set-up and diagnostic techniques

Fig. 1 Sketch of the principal outline of experimental investigations: Different diagnostic techniques are applied to the samedischarge cell conguration with dielectric BSO crystal as one of the dielectrics.

The developed discharge cell conguration enabled the detailed investigation of the discharge evolution andprocesses in the volume as well as the interaction with the dielectric surfaces. As shown in gure 1, it consists oftwo parallel electrodes covered with dielectrics. The driven one is a transparent and conductive ITO layer whichcovers the topside of a glass plate (glass = 7.6). The grounded electrode is a polished aluminum mirror. On itstop an electro-optic BSO crystal (Bi12SiO20, BSO = 56) is placed to permit the measurement of surface charges.The gap distance between the dielectrics can be varied from 0.5mm to 1 mm. Sidewise orices enable a gas owand the application of the optical diagnostics (CCS and LIF). The working gases are pure helium, pure nitrogen,and mixtures of both. The pressure varies from 200 hPa to 1000 hPa and the frequency is in the range from 2 kHzto 200 kHz. The applied sinusoidal voltage amounts up to 6 kV depending on the pressure and gas.

The surface charge measurement is based on the electro-optic Pockels effect [24]. The BSO crystal is homo-geneously illuminated ( = 634 nm) and the initially linearly polarized light is changed by the optical set-up andthe BSO crystal to elliptic polarization. The ellipticity depends on the voltage drop across the BSO crystal andthus on the deposited charges on its surface. Via a linear polarization lter, the surface charge distribution (signand value) is made visible by means of a high-speed camera (exposure time 10 s).

To investigate the lateral patterns in the luminescence distribution in dependence of the applied voltage anICCD camera is directly placed in front of the discharge cell, recording the discharge emission. The exposuretime is typically 200 s. For a driving frequency of 100 kHz, each image is accumulated over several tens ofsubsequent periods.

The optical emission evolution can be investigated with the highly sensitive CCS diagnostic technique. It isbased on time-correlated single photon counting from about 106 discharge events. In case of diffuse BDs thetime-information is derived from phase resolved measurements as described in [12]. The spatial resolution isin the range of 0.05 mm to 0.01 mm. A monochromator provides a spectral resolution of 0.1 nm of the opticalemission signal. The most intensive lines of N2, namely the N2 second positive system (SPS, 0-0 transition at = 337 nm) and the rst negative system (FNS, 0-0 transition = 391 nm), as well as the He line at = 706 nmhave been investigated.

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Contrib. Plasma Phys. 52, No. 10 (2012) / www.cpp-journal.org 849

For the detection of the metastable N2(A3+

u ) state a LIF set-up is used. The laser excites the transition fromthe A3

+u , v = 0 state to the B

3g, v = 3 state at the wavelength of = 687.44 nm. The uorescence band ofthe following transition to the A3

+u , v = 1 state at about 762 nm is measured perpendicularly to the laser beam

via a monochromator and a photomultiplier tube. For the absolute density calibration the Rayleigh scattering atcarbon dioxide has been measured for equal conditions [25, 26].

All diagnostics are completed by the measurement of applied voltage, discharge current, and transferredcharge.

3 Results and discussion

3.1 Discharge operation modes

Different BD modes can be generated in the described discharge cell. Working with pure gases of helium andnitrogen generally results in diffuse BD. The admixture of a few percent of nitrogen to helium leads to thelamentary discharge mode. The BD in pure He (purity 99.999%) is dominated by several bands of the secondpositive system (SPS) and rst negative system (FNS) on nitrogen which is an inevitable impurity. The FNSemission of the exited N+2 (B

2+

u ) ions results from the Penning ionization between helium metastables andnitrogen in the ground state. The emission of the SPS is caused by inelastic collisions of electrons with nitrogenin the ground state to the excited state N2(C3u) followed by the de-excitation to N2(B3g). The admixture ofN2 gas reduces an effective Penning ionization due to a reduction of He metastables. But the emission intensityof the SPS becomes more intensive because the N2 concentration is increased.

The SPS and FNS emission developments have been investigated in the pure helium ( 10 ppm N2 impurities)and pure nitrogen diffuse BDs, as illustrated in gure 2. Both discharges have the emission maximum near theanode. The emission duration corresponds to the current pulse duration, namely 30 s in case of helium andseveral tens of s (100 s) in case of nitrogen. These results conrm the generation of a Townsend-like BD inboth gases, nitrogen as well as helium. For nitrogen this operation mode is well-known from literature. However,for pure helium the discharges usually operate in the glow-like mode with an emission maximum near the cathode[4, 6]. In these investigations mainly gap distances of 2 mm to 5 mm were used, and the formation of a cathodesheath (glow) is possible. Under our conditions, the gap distance at sinusoidal excitation is too small for thedevelopment of a cathode fall region. This suggestion is supported by the fact that also in our set-up the operationof the glow-like mode in helium was possible by the application of applied voltages with much steeper edge asin case of sinusoidal voltage, namely rectangle or sawtooth. The higher dV /dt induces much stronger electricaleld strengths in a short time resulting in faster ionization and space charge build-up [27].

Fig. 2 Spatio-temporally resolved discharge developments of the SPS and FNS in pure helium and nitrogen. 500 hPa, 2 kHz,0.45 kV in pure helium, and 3.7 kV in pure nitrogen.

In gure 3 a) the discharge net current (solid line) and surface charge development (circle) over one period areshown for pure nitrogen. The low current and its long pulse duration corresponds to the Townsend-like mode.During the appearance of a current pulse, the surface charge changes its sign. After that, it remains constant onthe BSO crystal until the next current pulse in the next half period occurs. The temporally integrated net current(dashed line) shows an excellent agreement with the measured surface charges. One reason for the generation of adiffuse discharge mode in pure nitrogen is the effective secondary emission of electrons by plasma species. Here,

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metastable nitrogen molecules (in competition with energetic ions) play an important role [12, 13]. Therefore,the time dependence of the N2(A3

+u , v = 0) density is measured and plotted with the applied voltage and the

discharge current in gure 3 b). The averaged density is about 21013 cm3 in the center of the gap which is inthe same order of magnitude as in [11]. The N2(A) density increases during the discharge pulses and is clearlylarger in the positive half period than in the negative one. This is reasonable because of the asymmetric dischargecell set-up due to the different dielectrics (glass, BSO).

Fig. 3 a): Phase resolved surface charge from the electro-optic measurement (circles) with the corresponding net current(solid line) and the temporally integration of the net current (dashed line). b): Time dependence of metastable N2(A3

+u ,

v = 0) density in the center of the discharge gap. 500 hPa, 2 kHz, 4 kV.

For small admixtures of nitrogen to helium in percent range a lamentary BD is observed [28]. The spatialresolved surface charge density for a 9:1 He/N2 mixture is shown in gure 4 a) for both half periods after thedischarge breakdown. Both images show deposited surface charge spots which are caused by previous microdis-charges [29]. Negative surface charges are electrons deposited on the dielectric. Positive surface charges resultmost probably from the recombination of ions with electrons from the bulk material. Notably, the negative sur-face charges are more extended than the positive ones due to the higher electron mobility in the volume in frontof the electrodes compared to the ions [30, 31]. Their radial proles can be tted well by a Gaussian functionas shown in gure 4 b). Also, the discharge re-ignition is preferred on deposited surface charges, the so-calledmemory effect. In gure 4 c) the N2(A3

+u , v = 0) metastable density is shown after one microdischarge in

pure nitrogen. It has its maximum several microseconds after the few nanoseconds lasting discharge pulse.

Fig. 4 a) Charge distribution in the lamentary mode (90%He and 10%N2 mixture) of positive and negative surface chargesin the positive and negative half-period, respectively. b) Radial prole of positive and negative surface charges, FWHM: + =0.9 mm and = 1.6 mm, respectively. c) Density of N2(A3

+u , v = 0) metastables in the afterglow of one microdischarge

in pure nitrogen. 500 hPa, 2 kHz, a)-b) 0.91 kV, and c) 5.5 kV.



Hence, the production of N2(A3+

u , v = 0) is dominated by depletion of energetically higher states insteadof electron impact excitation. This corresponds to the delay of the metastable production with respect to thedischarge current in the diffuse mode as shown in gure 3 and in literature [11, 32].

3.2 Lateral pattern formation

A laterally extended helium BD (i. e. electrode diameters much larger than the electrode distance) has beenfound to produce irregularities in the lateral homogeneity of the luminescence density under certain conditions.The discharge breaks up into a number of strongly localized discharge spots. Due to their formation mechanism,i.e. by an effective electron focus into the spots and pattern conservation by surface charges [17, 18, 31], the spotshave to be distinguished from the streamer-based laments in the lamentary discharge mode. Instead, all thedischarge spots behave glow-like and ignite synchronously. As the discharge spots interact among one anotheron short distances, they may show a collective behavior yielding higher order structures, e.g. hexagonal patterns[31].

Starting at approximately U = 400 V, the applied voltage is constantly reduced until the discharge disappears.While the lateral discharge pattern at the applied voltage down to about U = 363V is clearly hexagonal, see gure5 a), the arrangement of the discharge spots becomes random for values of U < 363 V, see gures 5 b)-d). Thespot positions are analyzed with the triple correlation function (TCF), as shown on the right-hand side in gure 5.

Fig. 5 Left column: Light density distribution images atvoltages close to the bifurcation point. Right column: Triplecorrelation functions of the corresponding light density dis-tribution. f = 80kHz sinusoidal, helium, without gas ow,exposure time 200 s. a) U = 363 V, b) U = 358V, c) U =350V, d) U = 342V.

Fig. 6 Principle of the triple correlation function [33].

Fig. 7 Analysis of the transition from the hexagonal pat-tern to the stochastic arrangement of the laments. Evalu-ated are the ratios of the 60-peak at nearest neighbor dis-tance to the mean background in angular and radial direc-tion, Hang = y60/ < y(d0, ) > and Hrad = y60/ r , respectively.



It evaluates the positions of any triple of discharge spots and returns a probability distribution y(r, ) of ndinga second spot in a distance r and an angle with respect to an arbitrarily selected nearest neighbor, see gure 6[33]. Qualitatively, the degradation of the hexagonal order starts with a blurring of order in angular direction.A decay of the radial order follows. However, whereas the angular order is entirely removed at the end of thetransition, a small modulation in the radial order remains because two discharge spots can not move arbitrarilyclose as they inhibit each other [18, 34].

In order to characterize the transition quantitatively, convenient measures for the hexagonal order are chosen.Hrad and Hang are the ratios of y60 = y(d0, 60) to the TCF mean background along a line in radial and angulardirection, respectively. d0 is the nearest neighbour distance. Figure 7 shows the previously observed transitionusing Hrad and Hang . Both plots show principally the same behavior: at higher voltages, the measures of orderremain nearly constant. At decreasing voltages occurs a sudden change of the measures slope, and the measureof order decreases. This abrupt transition is a supercritical bifurcation. At the smallest used voltages, the peaky60 vanishes in the background noise due to the low order and low lament number. Hence, Hang and Hrad arenot meaningful anymore. In all measurements, the bifurcation point for the radial order Ub,rad occurs at lowervoltages than the bifurcation point for the angular order Ub,ang , which is consistent with the previous qualitativeobservations.

3.3 Surface discharges

The propagation of microdischarges along the interface between the gas and dielectric surface can be investi-gated by the discharge cell shown in gure 8. The proposed electrode arrangement was carried out after testingdifferent surface BD arrangements for the investigation of single repetitive microdischarges in air [35, 36]. Thedischarge cell consists of two needle electrodes (syringe hollow needles made of chrome-nickel-steel alloy, 0.4mm diameter) placed on the opposite sites of a 0.6 mm thick Al2O3 ceramic plate (96% purity, p = 10). Thetips of the electrodes faced each other with a gap of g = 1.15 mm. To prevent parasitic discharges, the drivenelectrode was covered by additional isolating material. The discharge was driven in dry air (gas ow 300 sccm)by means of sinusoidal voltage with several kV amplitude at a frequency of about 60 kHz.

In gure 8 results of ICCD-photography are shown. There is a higher discharge activity if the exposed elec-trode is the anode. Only these results are presented in the following. The side-on ICCD photos (upper rightpictures) show that in the chosen electrode conguration the discharge channel develops directly on the dielectricsurface. The discharge channel is about 100 m thick and its length increases with the amplitude of the appliedvoltage. At 3.2 kV the transferred charge of a single channel was about 1.5 nC. The increase of the voltage am-plitude leads to the generation of further discharge pulses in the same half period. Four to six individual currentpulses could be detected [37]. The top-view ICCD camera photo (left lower part of gure 8) at 3.2 kV shows thatthe rst microdischarge in the half period develops on the direct path between the electrode positions. At 3.75 kVtwo microdischarges per half period are observed. The corresponding ICCD-photo was taken with gate settingonly recording the second microdischarge event (20 accumulations). The second discharge channel evades theregion of the preceding breakdown taking a curved and thus longer path resulting in larger current amplitude andtransferred charge by trend. This result demonstrates that charge carriers are deposited on the dielectric surfaceleading to specic discharge pattern.

The gure 9 presents the result of CCS measurement for the 0-0 transition of SPS, the most intensive band inthe emission spectrum. The microdischarge starts with a short Townsend prephase at the tip of the anode, similaras observed in volume BD and coplanar discharges [16]. A positive space charge accumulates in front of theanode up to a level incepting the propagation of a cathode directed ionization front or streamer. The maximumvelocity of the ionization front is about 2.8105 m/s [36] at the distance of 0.2mm in front of the the anode. Withincreasing distance from the anode, the velocity decreases to 1.5105 m/s at the cathode region. This behavioris quite different from volume BDs where the velocity increases towards the cathode. The presented surface BDconguration is a good object for the detailed and quantitative investigation of surface effects on the breakdownand discharge development since the channel propagates along the interface between gas and dielectric. Similarlyas the discharge re-ignition or uniformity described in the previous subsections, the propagation of dischargechannels is determined by the elementary interaction with the charged dielectric surface. Surface ionization dueto detrapping, ion impact and photo effect or surface attachment of charge carriers may have a strong inuence[38, 39] which should be investigated in more detail.



Fig. 8 Surface BD electrode arrangement (left upper part,g = 1.15 mm), gated ICCD-photos in top view and side view of mi-crodischarges in air.

Fig. 9 Development of the intensity of 0-0 transitionof SPS at = 337.1 nm with 3.2 kV. The solid linerepresents the exposed anode tip and the dashed linethe tip position of the hidden cathode. The number ofcounted photons is color-coded in logarithmic scale.

4 Summary and outlook

For the rst time a discharge cell conguration was developed allowing the joint observation and correlationof key parameters of the BD discharge operation modes under identical and well-dened conditions. Namely,the diffuse and lamentary discharge operation as well as the formation of patterns were investigated. The spa-tiotemporally and spectrally resolved discharge development in the volume was studied by the cross-correlationspectroscopy. The temporally resolved quantitative measurement of surface charges on the dielectric surfaces suc-ceeded by the application electro-optic Pockels effect in combination with the CCD camera technique. MetastableN2(A) molecules were detected by laser induced uorescence.

The various modes have been investigated systematically in pure nitrogen, helium and small admixtures ofnitrogen (in percent range) to helium. Additionally, the propagation of a microdischarge along the interface ofthe gas and the dielectric surface was studied in air.

In pure nitrogen the expected diffuse Townsend-like discharge mode was generated which is nally character-ized by a radiation maximum near the anode. Surprisingly, under our experimental conditions (small gap distanceof 1 mm, sinusoidal applied feeding voltage) this mode appeared also in pure helium. Here, the gap distance istoo small for the development of a cathode fall region. Usually in helium the glow-like operation with radiationmaxima near the cathode is described in literature, but at gap distances of 2 mm to 5 mm.

The measured surface charge densities at the diffuse discharge operation in nitrogen and helium are in excellentagreement with the calculated charge transfer by discharge current integration over time. After the dischargebreakdown the surface charge density stays constantly until the breakdown in the next half-period.

One reason for the generation of a diffuse discharge mode in pure nitrogen is the effective secondary emissionof electrons by plasma species. Here, metastable nitrogen molecules play an important role. Its production isdominated by the depletion of higher excited states of N2. The measured nitrogen metastable concentrations arein the range of about 1013 cm3 which is in good agreement with the literature.

Small admixtures of nitrogen to helium in percent range result in the lamentary discharge mode. At theseconditions the nitrogen molecules are efciently ionized by the Penning effect with metastable helium atoms. Thedeposited surface charges are strongly determined by the distribution of microdischarges or discharge spots. Theproles of negative and positive charge spots can be tted by a Gaussian plot. They have signicantly differenthalf widths. One reason is the higher mobility of the electrons in the discharge volume close to the dielectricsurface. It has been shown that the re-ignition of microdischarges in the following half period of the appliedvoltage is preferred on locations with high surface charge densities. In case of multiple breakdowns in the samehalf period the microdischarges are generated where no surface charges are present. Beside this so-called memory



effect, an inuence of the propagation of repetitive microdischarge channels along the interface of the gas volumeand the dielectric surface has been investigated.

In the case of patterned discharges the voltage amplitude controls the number and the order of the dischargespots. The breakdown of the hexagonal order is a supercritical bifurcation appearing with decreasing voltageamplitude.

The presented experimental results are an extensive source for the comparison with kinetic models to geta deeper (nally quantitative) understanding of the complex mechanisms in the BD evolution. The modellingactivities - under progress - consider the dominant processes in the discharge volume as well as the interactionsof relevant plasma species with dielectric surfaces [2023, 40].

In order to gain more information about the elementary processes of plasma-dielectric interaction furtherexperiments in discharge arrangements with the breakdown appearing directly on and along the surface are pro-posed. The application of the surface BD geometry described in this contribution to a BSO as the dielectric wouldenable the measurement of surface charges in correlation with the plasma propagation along the surface. In orderto control the microdischarge the implementation of a third electrode is foreseen, a so-called sliding discharge[41]. In particular, it is planned to investigate the role of negative ions by the laser photodetachment technique[42].

Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft (DFG) through the transregionalcollaborative research center SFB/TRR 24. The authors are grateful to Tomas Hoder (INP Greifswald) and Helge Grosch(DTU Risoe, formerly INP) for supporting surface BD investigations.

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Development of Barrier Discharges Operation Modes and Structure Formation

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