Atmospheric Pressure Non-thermal Plasma in Flexible Tubings

Low-temperature non-thermal plasma configurations for medical applicationRuixue Wang 1,3,4 , WeiDong Zhu 2,3 , Jose L. Lopez 2,31 Academy for Advanced Interdisciplinary Studies, Peking University, Beijing, China;2Center for Microplasma Science and Technology, Saint Peter’s College, Jersey City, NJ, USA3 Department of Applied Science and Technology, Saint Peter’s College, Jersey City, NJ, USA4 Department of Physics, Polytechnic Institute of NYU, Brooklyn, NY, USAAbstractAtmospheric pressure non-thermal helium plasmas are generated in flexible tubings (Teflon, Peek) aswell as in glass tubings. Plasma is ignited when a low frequency (a few tens of kilohertz) AC voltage (15-25 kV) was supplied to two annular electrodes on the tubing. “Plasma brush” (or plasma jet array) isrealized by opening circular holes on the side walls of the tubings. Individual ballasting on each hole isnot necessary, although better stability can be achieved with it. Jets in the jet array can be delivered tovarious distances from the tubing wall by connecting to the openings dielectric tubings with smaller innerdiameters. Electrical and optical characteristics of the plasmas are studied. The flexibility and lowtemperature operation of these devices make them more attractive in applications such as disinfection ofcatheters and cleaning of irregular surfaces.Keywords: Non-thermal plasma flexible tubing1. IntroductionNon-thermal plasma produced at atmosphericpressure has received considerable attention due totheir potential in various industrial and scientificapplications such as material treatment andbiomedical applications[ 1 ]. A large number ofdesigns has been developed and tested already, suchas DBD, Plasma Jet, Plasma Torch [ 2 ]. Among these,plasma jet has attracted extensive attention due toexcellent plasma stability and also rich reactionchemistry with downstream [ 3 ]. The low temperaturefeature of these plasma jets makes it possible forthem to be placed in direct contact with plant, animaland human tissues, in particular for localizedtreatment. However, to achieve widespread use inmedicine, the typically small plasma jet must bescaled up and be capable of treating uneven surfaces,such as complex surgical instruments. Twodimensionalarrays of spatially confined plasma jetshave been reported particularly by Tachibana group[ 4 ] and MG Kong group [ 3,5,6 ]. Here we developedatmospheric pressure non-thermal helium plasmas inflexible tubings (Teflon, Peek) as well as in glasstubings driven by a low frequency (a few tens ofkilohertz) AC voltage (15-25KV) power supply. Theplasma jet can be expanded to 3-D when openingsare created on the side walls of the tubings.2. Plasma Configurations2.1 Plasma BrushPlasma Brush in this study is shown schematicallyin figure 1(a). The dielectric tube was made ofTeflon with an inner diameter of 1/4” and outerdiameter of 1/2”, through which He gas flows. Apair of tubular electrodes (copper) is attached tothe dielectric tubing, and connected to the HVpower supply and ground, respectively. While theend of tubing is sealed, several holes can be drilledthrough the sidewall (1/8”thick) in differentpositions to let out working gas. The plasmagenerated in the Teflon tubing can expel fromthese holes. Dielectric (such as alumina) tubingswith the same outer diameter as these holes can beinserted to the holes to further extend the emergingplasma and the length of the tubing can vary from0.5” to 2”. Holes can be drilled in both upstreamand downstream of the dielectrics, the maximumplasma branches can be supported depending onthe applied voltage, the arrangement of the holesand also the distance between adjacent holes. Inone example, six holes were drilled in a row –three before the powered electrode and three after(Figure 1(b)). In another example, eight holeswere drilled in 2 sets of 4 every 90° around thetube in a circular fashion (Figure1(c)). In the six-

hole case, all of the alumina tubings can be ignited(have a visible plasma jet) except the one in themiddle of upstream. The plasma started betweenthe two tubular electrodes and then spread to thebranches when voltage was increased.(a)((b)(c)emissions from NO, N 2 , N 2(Figure 3 (b) inset),He and O.Current(mA)0.030.020.010.00-0.01-0.02ABCDEF151050-5-10Voltage(KV)-0.03-15Figur .1. (a) Schematic diagrams of the plasma brush; (b)ignited plasma brush with six holes and (c) 2 sets of 4 every90° holes around the tube2.2 Characterization of plasma brushThe electrical characteristics of the plasma brushwere evaluated with a high voltage probe(Tektronix 6015A) connected to the poweredelectrode and a current probe (Pearson 6585)placed at six different locations (A-F, as noted infigure 1(a)). The results at V p =10 kV without anyballasting capacitor are shown in figure 2. Thedischarge currents at all six locations are more orless the same. One current pulse is observed forevery half cycle of the applied voltage, whichresembles what was typically observed indielectric-barrier discharges [ 7 ].Optical emission was monitored via amonochromater (Princeton Instrument Acton 2750)connected with an Intensified Charge CoupledDevice camera (Princeton Instrument IMAX-1024)at alumina port A and B (as noted in figure 1 (a))to determine the composition of the plasma. Thelight was introduced via a quartz fiber opticsbundle placed at ~1 cm away from the exit of thealumina tubings. Figure 3 shows the emissionspectra taken at these two locations. The overallemission intensity from A (downstream) is higherthan that from B (upstream) although sameemission lines are found in both. Identifiable are-20 0 20 40 60 80 100Time(ns)Figure 2. Voltage (blue) and currents for plasma brush indifferent position (as noted in figure 1 (a))Emission Intensity(a.u)1000000800000600000400000200000010000008000006000004000000A_dowstream(a)200000 (b)B_upstream200 250 300 350 400 450200 300 400 500 600 700 800 900Wavelength(nm)Figure 3. Optical Emission Spectra taken at alumina port(a) A (downstream) and (b) B (upstream) (A and B are asnoted in figure 1 (a)). Inset: light emission from 200-450nmshowing emission from NO, N 2 and N 2+2.3 Plasma in various forms in TeflonIntensity(A.U)Plasma can be generated in flexible tubings madeof Teflon or Peek (OD: ¼” and ID: ½”). Figure 4(a) and (b) show helium plasma generated in asingle loop and a spiral tubing arrangement. Forthe single loop arrangement (Figure 4 (a)), heliumwas introduced through a three way insulatingconnector and evenly spread to both branches.Light emission from one branch appears to bestronger than the other due to the electrode120000100000800006000040000200000N 2N + 22 +1 -NOv=0(0,0)v=-1v=-2Wavelength(nm)(2,3)(1,2)(0,1)(3,5)(2,4)(1,3)(0,2)(3,6)(2,5)(1,4)(0,3)(3,7)(2,6)(1,5)(0,4)(2,2)(1,1)(0,0)v=-3v=0v=-4(4,5)(2,3)(1,2)(0,1)

arrangement. At the far end of the loop, anopening was made and connected to an aluminatube, where a plasma jet was created as expected.For the spiral tubing case, helium plasma couldnot bend over one single loop if the copperelectrode was attached only to a section on oneloop. A strip of copper electrode (1” wide) wasthen attached across all loops. A seeminglycontinuous plasma through the spiral tubing wasthen generated (Figure 4 (b)).alumina ranking the highest in the materials used inthis study). Charge accumulation eventually reacheda critical value at which the electric field was strongenough to break down air around it at the presenceof a third electrode.(a)(a)He gas(b)PoweredelectrodeFigure 4. Plasma generated in (a) a single loop and (b)spiral tubing2.4 Plasma with a third electrodeIt has been well documented that these heliumplasmas can be generated in glass/quartz tubings atvarious diameters [ 8 , 9 ]. We repeated thoseexperiments in glass tubings of different sizes: 1) ID:0.218” and OD: 3/8”; 2) ID: 0.312”, OD: 0.5” and 3)ID: 0.376” and OD: 0.5” with our AC power supply.The appearance of the plasma in the glass tubing ishighly dependent on the size of the tubing and theinput voltage. At a low voltage, the luminescence ofthe plasma is confined to an axial filament ofdiameter below 1 mm at the center of the tubing (asshown in figure 5 (a), dashed line represents theglass boundary). The diameter as well as thebrightness of the filament increase with the appliedvoltage. At a voltage of V p = 25 kV, the plasma fillsthe whole glass tube with the axial filamentremaining at the center (as shown in figure 5 (b).Also worth noting is that at approximately the samepeak voltage, when a metal object (such as a razorblade or a round head hex Allen wrench) wasbrought close to the glass tubing wall, far away fromthe powered electrode (either upstream ordownstream), air breakdown was observed. Thisphenomenon was more clearly observed whenceramic tubings were used (as shown in figure 5 (c)and (d)). It is very likely that charges in the plasmaaccumulated on the tubing walls, which wasessentially proportional to the applied voltage andthe dielectric constant of the tubing material (withFigure 5.Plasma generated in glass tubing (ID: 0.218,OD:03/8”) at (a) V p = 7 kV: a clear axial filament is seen and (b)V p = 25 kV: plasma fills the whole glass with an axial filamentpresent; Air break down outside of the alumina tubings wereobserved at V p = 25 kV when (c) a razor blade and (d) a roundhead hex Allen wrench was brought closeFigure 6. At V p =7 kV, a spiral shape plasma developswhen a round head hex Allen wrench is brought close tothe glass tubingIn the case of lower voltage (V p =7 kV) while theplasma was still sustained, no air break wasobserved. However, when moving close a roundhead hex Allen wrench, a spiral shape plasmaform along the inner surface of the tubing (Figure6). This could be that the electric field due to thecharges accumulated on the tubing walls beinglocally perturbed by the metal item. Furtherinvestigation of this phenomenon is underway. Ithas to be noted that this spiral shape plasmaappears only toward the grounded electrode or in

the case of the gas was expelled into air, it appearstoward the tubing opening (where we consider as avirtual ground). Meanwhile, the filament in themiddle of the tubing became weak but still visible.ConclusionIn conclusion, we demonstrated the generationof helium plasma in flexible tubings as well as inglass tubings. The plasma can be split into manybranches by opening holes on flexible tubings.OES show emission mostly from NO, N 2 , N 2 + ,He and O. The appearance of the plasma in glasstubing depended on the size of the tubing andthe applied voltage. Clear axial filament wereobserved at both high voltage (V p =25 kV) andlow voltage (V p =7 kV). At V p =25 kV, airbreakdown between the tubing and thirdelectrode was observed due to the strong electricfield from the accumulated charged on thetubing walls. Different plasma geometry and lowtemperature property make it suitable formedical applications where instruments withirregular surfaces are involved.Acknowledgement(b) (d)This research was funded in part by the Electro-Energetic Physics Program of the U.S. Air ForceOffice of Scientific Research (AFOSR) undergrant number FA9550-08-1-0332.Acknowledgment is also made to the Donors ofthe American Chemical Society PetroleumResearch Fund and Council Scholarship ofChina for support of this research. RW wouldlike to thank Drs. Jue Zhang and Jing Fang fortheir constructive advices.and gas flow fields for three-dimensional surfacetreatment. APL. 94, 021501(2009)4 O Sakai, T Sakaguchi and K Tachibana.Photonic bands in two-dimensional microplasmaarrays.I. Theoretical derivation of bandstructures of electromagnetic waves. JAP 101,073304(2007).5 QY Nie, Z Cao, C S Ren, DZ Wang and MGKong. A two-dimensional cold atmosphericplasma jet array for uniform treatment of largeareasurfaces for plasma medicine. NJP. 11(2009)115015.6 Z Cao, Q Nie, DL Bayliss, JL Walsh, CS Ren,DZ Wang and MG Kong. Spatially extendedatmospheric plasma arrays. PPST. 19(2010)0250030.7 U Kogelschatz, Dielectric-barrier Discharges:Their History, Discharge Physics, and IndustrialApplications. Plasma Chemistry and PlasmaProcessing. Vol.23, No1, March 2003.8KD Weltmann, R Brandenburg, T vonWoedtke, J Ehlheck, R Foest, M Stieber and EKindel. Antimicrobial treatment of heat sensitiveproducts by miniaturized atmospheric pressureplasma jets(APPPs). J.Phys.D: Appl.Phys.41(2008)1940089 N Jiang, A Ji and Z Cao, Atmospheric pressureplasma jet: Effect of electrode configuration,discharge behavior, and its formationmechanism. JAP, 2009.References1 K.H.Becker, U.Kogelschatz, K.H.Schoenbach,and R.J.Barker, Non-Equilibrium Air Plasma atAtmospheric Pressure(IOP, Bristol, 2004)2 M G Kong, G Kroesen, G Morfill, T Nosenko,T Shimizu, J van Dijk and J L Zimmermann.Plasma medicine: an introductory review. NewJ.Phys. 11(2009) 115012.3 Z. Cao, J.L. Walsh, and M.G. Kong.Atmospheric plasma jet array in parallel electric