The high speed camera was utilised for plasma diagnostics of the DC pin-hole discharge in electrolyte solutions. Two discharge modes were determined. Plasma channels were observed either in the bubble or outside the bubble in the bulk solution, which confirms both thermal and electron theory of the discharge ignition in liquid. In the diaphragm discharge, plasma streamers were better visible on the cathode side of the dielectric barrier because they formed significantly longer channels

Drimalkova, L.

Hlochova, L.

Kozakova, Z.

Krcma, F.

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

doi:10.14311/ppt.2017.1.20Plasma Physics and Technology 4(1):20–23, 2017 © Department of Physics, FEE CTU in Prague, 2017UTILISATION OF THE HIGH SPEED CAMERA FOR THE PIN-HOLEDISCHARGE DIAGNOSTICSZ. Kozáková∗, F. Krčma, L. Dřímalková, L. HlochováFaculty of Chemistry, Brno University of Technology, Purkyňnova 118, 612 00 Brno, Czech Republic∗ kozakova@fch.vut.czAbstract. The high speed camera was utilised for plasma diagnostics of the DC pin-hole discharge inelectrolyte solutions. Two discharge modes were determined. Plasma channels were observed either inthe bubble or outside the bubble in the bulk solution, which confirms both thermal and electron theoryof the discharge ignition in liquid. In the diaphragm discharge, plasma streamers were better visible onthe cathode side of the dielectric barrier because they formed significantly longer channels.Keywords: discharge in liquid, pin-hole discharge, high speed camera, plasma diagnostics.1. IntroductionThe problem of discharges in liquids has been inan intensive focus of researches for the last threedecades, only. Although several applications such aswater treatment [1], surface treatment [2], or plasmasterilization[3] have been developed during the un-derwater research, exact mechanisms of the dischargecreation and related processes still desire a detail studyand description.Non-thermal plasma of underwater discharges canbe generated by various electrode configurations andvoltage regimes. The most studied configuration is apoint-to-plate electrode geometry[4] where DC pulsedhigh voltage up to tens of kV is applied. Next con-figurations suitable for the discharge creation un-der the water surface are coaxial[5] and pin-holesystems[6][7][8]. Besides the DC pulsed high volt-age, also high frequency, microwave or DC non-pulsedvoltage regimes can be used for the discharge genera-tion.The pin-hole discharge can be distinguished as adiaphragm or capillary configuration. In the case ofthe diaphragm system, high voltage electrodes areseparated by a thin dielectric barrier which thicknessis approximately the same size as the diameter ofan orifice connecting both reactor parts[9]. On theother hand, the capillary system uses a thick dielectricbarrier which thickness is remarkably bigger than theorifice diameter[6]. In both cases, the discharge canbe generated in the orifice using either DC or AC highvoltage. The liquid medium is an electrolyte solution.Plasma diagnostics by the ICCD camera has an im-portant force among other diagnostic methods utilisedfor the characterisation of the discharge in liquids.Due to the still indefinite mechanism of the dischargeignition and propagation in the liquid phase, the usageof the high speed camera enables a proper captureof different processes taking place before, within andafter the discharge breakdown moment in the system.Together with an employment of electric measure-ments, the ICCD imaging can reveal processes suchas the bubble formation, the discharge breakdown inthe bubble, the propagation of plasma streamers inand outside the bubble, the bubble cavitation, etc.Following pages demonstrate a complex image studyof the pin-hole discharge in the electrolyte solution. Itis focused on the determination of particular processestaking place in the plasma reactor during the dischargeignition, and on the confirmation of the dischargebreakdown theory (electron or thermal theory[10]).Further, a comparison of the diaphragm and capillarydischarge is enabled due to the utilisation of the highspeed camera records.2. Experimental setupPlasma diagnostics of the pin-hole discharge was car-ried out in plasma reactor shown in Figure 1. Its totalvolume was 100ml. Two high voltage electrodes madeof platinum were separated by a wall made of POM-C(Polyoxymethylene (Copolymer), thickness of 1mm).There was an orifice (10mm in diameter) in the centreof the wall into which a dielectric barrier made ofceramics Shapal-MTM (thickness variable from 0.3 to2mm) was installed. One central pin-hole with thediameter of 0.2–0.6mm was connecting both electrodeparts.High voltage was supplied from the DC self-pulsingsource (high voltage amplitude up to 4 kV, power up to300W). Water solutions containing NaCl electrolytewith initial conductivity of 275µS cm−1 were used inpresented experiments.An intensified CCD camera (ICCD) Andor iStarwith an objective Cosmicar/Pentax TV lens 50 mm(1:2.8) was used for visual records of bubbles andplasma streamers. The microsecond time resolutionwas used for these measurements. The ICCD cam-era was synchronized with the discharge operationby current evaluation determined by a four-channeloscilloscope LeCroy LT374L with a high voltage probeELDITEST GE 3830 30 kV (attenuation 1:1000).20vol. 4 no. 1/2017 Pin-hole discharge diagnosticsFigure 1. Plasma reactor used for the image diagnos-tics of the pin-hole discharge.3. ResultsThe utilisation of the ICCD high speed camera had en-abled the detail characterization of the discharge igni-tion, its regular operation as well as the identificationof related processes such as the bubble formation. Themoment representing the discharge breakdown in thepin-hole was confirmed both by time resolved current-voltage characteristics and light emission recordsby optical emission spectroscopy[9][11][12]. Camerarecords represented by photographs of a particulardischarge moment had been completed by time re-solved electric characteristics of discharge current andcamera triggering. The ICCD camera was switchedon automatically when the current signal rapidly in-creased which indicated the discharge ignition (seeFigure 2).The camera images were taken vertically to thedielectric barrier separating the anode and cathodeparts of the plasma reactor. In the photograph, thedielectric barrier is the white central stripe from theleft to the right (its thickness is approximately 1mm).There is an orifice in the barrier onto which a particu-lar diaphragm or capillary is installed. The diaphragmor capillary is not visible in the photograph becauseit is hidden within the barrier. The pin-hole con-necting both electrode parts is approximately in themiddle of the picture. The anode is set in the upperpart while the cathode lies in the bottom part of thephotographs.Increasing the applied voltage on the electrodes,tiny microbubbles appear in the pin-hole due to theliquid heating. Microbubbles are dragged from thepin-hole to the bulk solution. When the applied volt-age exceeds the breakdown value (over 1200V), thedischarge is ignited in the pin-hole. According to theimage records, the discharge is created primary in thebubble of the evaporated liquid. This fact confirmsthe thermal theory of the discharge ignition in liquids.Formation of the bubble is associated with a sub-stantial increase of system resistance. Subsequently,different potentials formed on the inner and outersurface of the bubble lead to the breakdown of the liq-uid vapour inside the bubble. Simultaneous decreaseof system resistance induces an intensive current in-crease which is represented by a significant peak onthe current characteristics. The camera triggering wasadjusted to this current enhancement, as it seen inthe Figure 2.Further increase of the applied voltage ensures thestable discharge operation. However, two dischargemodes have been detected using the high speed cam-era images. The first one is represented by the furtherdischarge propagation in the bubble (Figure 3), ac-cording to the thermal theory. The photograph clearlyshows plasma streamers in the bubble on the cathodeside of the dielectric barrier.The second mode of the pin-hole discharge opera-tion is represented by further propagation of plasmastreamers outside the bubble (Figure 4). We supposethat this discharge mode corresponds to the electrontheory. When plasma streamers primary created inthe vapour phase reach the bubble inner surface, ioni-sation of liquid molecules starts outside the bubble,and plasma channels can propagate further into thebulk solution. If the applied voltage is high enough,the discharge operation is possible even though thebubble explodes.Based on obtained images we have assumed thatboth breakdown theories fit for the discharge igni-tion in liquids. Primary, the discharge ignition startsin the bubble of the liquid vapour due to the lowerionisation potential needed for the breakdown. Thisphase corresponds to the thermal (bubble) theory.On the other hand, further propagation of plasmachannels into the liquid is possible outside the bubbleas well. This fact reflects on the electron theory ofelectron avalanches. Thus mechanisms of the pin-holedischarge operation are a combination of both theo-ries. This fact is valid in systems where a slow voltageincrease is used for the discharge ignition. Moreover,this phenomenon was also observed when gas bubbleswere additionally introduced into the system throughthe HV electrode[13]. In the case of pulsed nanosec-ond discharges, a proceeding of the thermal theoryis impossible due to a lack of time necessary for theliquid evaporation. In such systems, the dischargeoperates directly by streamers in the liquid phase.3.1. Comparison of the diaphragm andcapillary dischargeThis result section compares the shape of plasmastreamers created in the diaphragm and capillary con-figuration. The diaphragm configuration means thatthe dielectric barrier parameters are approximatelyequal, i.e. the barrier thickness of 0.3mm and the pin-hole diameter of 0.3mm. In the capillary configura-tion, the barrier thickness is substantially higher thanthe pin-hole diameter, i.e. the thickness of 1.5mmand the pin-hole diameter of 0.3mm.The camera imaging of the discharge created in thepin-hole was adjusted according the same conditions21Z. Kozáková, F. Krčma, L. Dřímalková, L. Hlochová Plasma Physics and TechnologyFigure 2. Time resolved record of discharge currentand camera signal during the discharge breakdown inthe diaphragm.Figure 3. Discharge creation in the bubble on the cath-ode side of the dielectric barrier; mean applied voltageof 1540V (diaphragm configuration: ceramic barrierthickness of 0.3mm, central pin-hole with diameter of0.3mm).as in the previously mentioned experiments. Thecamera triggering was set at the current increasingphase. Both discharge modes (i.e. plasma streamerspropagation in the bubble or outside the bubble) hasbeen recorded in all tested configurations. Imagesobtained in the capillary configuration are presented inFigures 5 and 6. Figure 5 represents plasma streamersformed in the bubble while Figure 6 shows plasmastreamers propagating outside the bubble into thebulk solution.In the diaphragm configuration (Figures 3 and 4),plasma channels are well visible only on one side ofthe dielectric barrier where the cathode is installed.This phenomenon indicates a different shape of plasmastreamers in the DC voltage regime. According to thetheoretical assumption[14], longer plasma channelsare created on the cathode side while shorter plasmachannels on a spherical shape are formed on the anodeside. Presented images confirm this assumption forFigure 4. Plasma streamers propagating outside thebubble from the pin-hole towards the cathode; meanapplied voltage of 1540V (diaphragm configuration:ceramic barrier thickness of 0.3mm, central pin-holewith diameter of 0.3mm).the diaphragm discharge because longer streamers areobserved just on the cathode side.In the capillary configuration (Figures 5 and 6),plasma streamers are visible on both sides of the di-electric barrier. However, the shape of the channelsseems different similarly to the diaphragm configura-tion. Recorded plasma streamers are more intensiveon the cathode side of the barrier. Moreover, thereis an interesting difference of the discharge creationin the bubble or in the liquid, only. When a bubbleis formed in the pin-hole and the discharge is ignitedin it, plasma channels fill almost the whole space ofthe bubble. This effect is more obvious in the case ofthe thicker barrier (1.5mm). Therefore, the dischargeseems more intensive in the bubble mode because en-ergy is mostly dissipated inside the bubble, i.e. inthe gas phase. Another effect of the capillary con-figuration is that both bubbles and plasma channelsare dragged from the pin-hole (capillary) towards thebulk solution. Especially in the case of the thickerbarrier (1.5mm), the bubble on the cathode side issignificantly enhanced, and it forms a conical shape.It corresponds to the effect of pumping which wasalready observed in the capillary configuration[6].4. ConclusionsThe high speed camera was utilised for plasma di-agnostics of the DC pin-hole discharge in electrolytesolutions. This device has been assumed as an impor-tant tool for a detailed imaging of plasma streamersand bubble formation. Based on the camera records to-gether with the time resolved characteristics of electricparameters, particular processes before, within andafter the discharge breakdown have been detected. Ac-cording to detailed photographs, two discharge modeshave been determined. Plasma channels were observedeither in the bubble or outside the bubble in the bulk22vol. 4 no. 1/2017 Pin-hole discharge diagnosticsFigure 5. Discharge creation in the bubble; mean ap-plied voltage of 2020V (capillary configuration: ce-ramic barrier thickness of 1.5mm, central pin-holewith diameter of 0.3mm).Figure 6. Discharge creation outside the bubble; meanapplied voltage of 1920V (capillary configuration: ce-ramic barrier thickness of 1.5mm, central pin-holewith diameter of 0.3mm).solution. These results confirm both theories of thedischarge ignition in liquid, thermal and electron the-ory.The ICCD imaging also helped to compare thedischarge creation in the diaphragm and capillary con-figuration. In the case of the diaphragm discharge,plasma streamers were better visible on the cathodeside of the dielectric barrier because they formed sig-nificantly longer channels. In the case of the capillary,plasma channels propagate further from the pin-holeinto the solution and thus they were well visible onboth sides of the dielectric barrier.AcknowledgementsThis work was supported by the Czech Ministry of Educa-tion, Youth and Sports, project No. LD14014.References[1] P. Lukeš et al. Degradation of phenol by underwaterpulsed corona discharge in combination with tio2photocatalysis. Res. Chem. Intermed., 31(4-6):285–294,2005. doi:10.1163/1568567053956734.[2] M. Klíma et al. Hf plasma pencil and dc diaphragmdischarge in liquids - diagnostics and applications.Czech J. Phys., 56:B1051–B1056, 2006.doi:10.1007/s10582-006-0325-x.[3] K.R. Stalder et al. Electrosurgical plasmas. J. Phys.D: Appl. Phys., 38:1728–1738, 2005.doi:10.1088/0022-3727/38/11/014.[4] M.J. Kirkpatrick and B.R. Locke. Hydrogen, oxygen,and hydrogen peroxide formation in aqueous phasepulsed corona electrical discharge. Ind. Eng. Chem.Res., 44(12):4243–4248, 2005. doi:10.1021/ie048807d.[5] P. Šunka et al. Potential applications of pulseelectrical discharges in water. Acta Physica Slovaca,54(2):135–145, 2004.[6] F. De Baerdemaeker et al. Characteristics of accapillary discharge produced in electrically conductivewater solution. Plasma Source Sci. Technol.,16:341–354, 2007. doi:10.1088/0963-0252/16/2/018.[7] M. Sato et al. Decoloration of dyes in aqueoussolution by pulsed discharge plasma in water throughthe pinhole. Transaction of the institute of fluid-flowmachinery, (107):95–100, 2000.[8] Z. Stará et al. Organic dye decomposition by dcdiaphragm discharge in water: Effect of solutionproperties on dye removal. Desalination, 239(1-3):283–294, 2009. doi:10.1016/j.desal.2008.03.025.[9] F. Krčma et al. Diaphragm discharge in liquids:Fundamentals and applications. J. Phys.- Conf. Series,207:012010(6 pages), 2010.doi:10.1088/1742-6596/207/1/012010.[10] R.P. Joshi et al. Electrical network-basedtime-dependent model of electrical breakdown in water.J. Appl. 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Utilisation of the High Speed Camera for the Pin-hole Discharge Diagnostics

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Utilisation of the High Speed Camera for the Pin-hole Discharge Diagnostics

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