The current–voltage characteristics of hollow cathode discharges and their predischarges in argon under dc and pulsed conditions were found to have a positive slope at pressures up to approximately 50 Torr, and currents up to 20 mA, at a hole diameter of 0.7 mm. In this range of pressure and current, parallel operation of hollow cathode discharges, without ballast, was demonstrated. Scaling to higher pressure is possible by reducing the hole diameter. Pulsed experiments with an array of cathode rings of 75 μm diameter allowed us to obtain parallel operation of more than 50 discharges at a pressure of 350 Torr in air

Byszewski, W. W.

Peterkin, F. E.

Schoenbach, K. H.

Tessnow, R.

Verhappen, R.

English

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Old Dominion University

Old Dominion UniversityODU Digital CommonsBioelectrics Publications Frank Reidy Research Center for Bioelectrics1996Microhollow Cathode DischargesK. H. SchoenbachOld Dominion UniversityR. VerhappenOld Dominion UniversityR. TessnowOld Dominion UniversityF. E. PeterkinOld Dominion UniversityW. W. ByszewskiFollow this and additional works at: https://digitalcommons.odu.edu/bioelectrics_pubsPart of the Electrical and Computer Engineering Commons, and the Inorganic ChemistryCommonsThis Article is brought to you for free and open access by the Frank Reidy Research Center for Bioelectrics at ODU Digital Commons. It has beenaccepted for inclusion in Bioelectrics Publications by an authorized administrator of ODU Digital Commons. For more information, please contactdigitalcommons@odu.edu.Repository CitationSchoenbach, K. H.; Verhappen, R.; Tessnow, R.; Peterkin, F. E.; and Byszewski, W. W., "Microhollow Cathode Discharges" (1996).Bioelectrics Publications. 248.https://digitalcommons.odu.edu/bioelectrics_pubs/248Original Publication CitationSchoenbach, K. H., Verhappen, R., Tessnow, T., Peterkin, F. E., & Byszewski, W. W. (1996). Microhollow cathode discharges. AppliedPhysics Letters, 68(1), 13-15. doi:10.1063/1.116739Microhollow cathode dischargesK. H. Schoenbach, R. Verhappen, T. Tessnow, and F. E. PeterkinPhysical Electronics Research Institute, Old Dominion University, Norfolk, Virginia 23529W. W. ByszewskiOSRAM SYLVANIA Inc., 71 Cherry Hill Drive, Beverly, Massachusetts 01915~Received 25 September 1995; accepted for publication 6 November 1995!The current–voltage characteristics of hollow cathode discharges and their predischarges in argonunder dc and pulsed conditions were found to have a positive slope at pressures up to approximately50 Torr, and currents up to 20 mA, at a hole diameter of 0.7 mm. In this range of pressure andcurrent, parallel operation of hollow cathode discharges, without ballast, was demonstrated. Scalingto higher pressure is possible by reducing the hole diameter. Pulsed experiments with an array ofcathode rings of 75 mm diameter allowed us to obtain parallel operation of more than 50 dischargesat a pressure of 350 Torr in air. © 1996 American Institute of Physics. @S0003-6951~96!04001-8#The current density in hollow cathode discharges ex-ceeds that of linear discharges by orders of magnitude forcomparable discharge voltages. The main mechanism re-sponsible for this current enhancement is assumed to be the‘‘Pendel’’ effect,1 the oscillatory motion of electrons betweenthe opposite cathode fall regions of the hollow cathode, aneffect which causes a drastic increase in the number of ion-ization processes.The lower limit in pressure for this effect to occur isaccording to Helm2 determined by the loss of ‘‘Pendel’’ elec-trons, which reach the opposite cathode wall and are re-moved from the discharge. This condition leads to an expres-sion for the minimum value of gas pressure p times cathodehole diameter D for which the hollow cathode discharge canbe sustained:2pD5ln f21/~^s&n0!, ~1!with n0 being the gas density at a pressure of 1 Torr, and f aloss factor. The average collision cross section ^s& is definedas the integral of the energy dependent elastic and inelasticcross sections over the electron accelerating cathode fall, thenegative glow, and the electron decelerating cathode fall atthe opposite cathode wall, divided by the cathode hole diam-eter. For argon the average cross section was measured as5.5310216 cm2, about twice the value of the maximum crosssection for ionization ~3.2310216 cm2).Comparison of Eq. ~1! with experimental results3 for thecritical pressure and hole diameter at which the hollow cath-ode discharge can be sustained, showed that the critical lossfactor is approximately 0.6 for helium, neon, and argon as fillgases, for cathode holes with dimensions on the order of 2cm. Accordingly, for argon the minimum or critical value ofpD is 0.026 Torr cm. The upper value of hollow cathodedischarge operation is at a pD on the order of 10 Torr cm.4Typically hollow cathode discharges are operated at sub-Torr pressures and cathode diameters in the centimeter range.Experiments at higher pressures ~p.25 Torr! in noble gaseshave been performed by Sturges and Oskam5 with parallelcathode plates down to millimeter distance. Hollow cathodedischarges at pressures up to 100 Torr in neon in cylindricaland spherical holes, respectively, of 0.75 mm diameter havebeen studied by White,6 at values of pD close to the upperlimit. The measured current–voltage characteristics for a 100Torr neon discharge at pD57.5 Torr cm in a molybdenumhollow cathode showed a shallow, but well-defined fallingcharacteristics extending to 18 mA, except for the currentrange where the discharge transfers from the face of the cav-ity to the interior.Whereas White has concentrated on the upper pD rangefor submillimeter cylindricals hollow cathode discharges, wehave explored the discharge characteristics of these micro-hollow cathode discharges in a range of pD close to thelower limit for the onset of the ‘‘Pendel’’ effect. The crosssection of the hollow electrode geometry is shown in Fig. 1.It consists of a cylindrical hole in molybdenum of 0.7 mmdiameter and 2.1 mm depth. The cathode is separated by a0.25 mm mica spacer from a ring-shaped anode of 2 mm i.d.dc voltages of up to 600 V and pulsed voltages of up to 800V with 1 ms duration were applied. The measurements wereperformed in flowing argon. Discharge current and voltagewere measured using a current viewing resistor and a highvoltage probe, respectively. Also, the discharge was observedend-on, time integrated, by means of a microscope connectedto a CCD camera.Figure 2 shows the dc current–voltage characteristics forpressures of 3.5, 5.0, 7.5, 16.5, and 56 Torr, corresponding topD values of 0.25, 0.35, 0.53, 1.16, and 3.92 Torr cm over acurrent range from 0.01 to 400 mA. In addition, for the pres-FIG. 1. Discharge geometry with hollow cathode and anode ~both molyb-denum!, separated by a mica insulator.13Appl. Phys. Lett. 68 (1), 1 January 1996 0003-6951/96/68(1)/13/3/$6.00 © 1996 American Institute of Physics2.0mm . o.7mmfl I n 2.1mm~ I ~ ' ,,,...,,, ~ t 2.1 mm sure of 56 Torr, the characteristic obtained in the pulsedmode is shown. Except for the lowest pressures the currenttraces show a distinct discontinuity. It becomes more pro-nounced and is shifted to lower voltages with increasingpressure and current, respectively.The visual appearance of the discharges varies depend-ing on gas pressure and current. At lower pressures ~,5Torr!, or low current levels, respectively, the boundaries ofthe plasma column in the cathode hole are rather diffuse. Forhigher pressures, just below the current discontinuity, a well-defined plasma column on the axis of the hollow cathode isobserved with a smaller diameter compared to the hollowcathode diameter; past the transition, the plasma column fillsalmost the entire cathode hole. Photographs of the dischargesin these two modes are shown in Figs. 3~a! and 3~b!. Spectralmeasurements indicate the role of electrode vapor in thistransition from a low to a high current mode. The relativeintensity of the electrode vapor lines ~compared to the gasemission! increases drastically when the discharge transfersinto the higher current mode.The slope of the current–voltage characteristics at pres-sures below 16.5 Torr is positive over the entire range ofobservation. At a pressure of 56 Torr, the slope seems to turnnegative for currents above 30 mA. For dc operation wewere limited to currents of approximately 35 mA due to ex-cessive Joule heating. In the pulsed mode, however, we wereable to extend the current range to almost 500 mA. In thismode the change from positive to negative differential con-ductivity is for the pressure of 56 Torr clearly visible ~Fig. 2!.This negative slope at pD’s considerably above the criticalvalue is consistent with results obtained by White.6The fact that the current–voltage characteristics of lowpD hollow cathode discharges have a positive slope over awide range of currents allows us to place them in parallelwithout using ballast resistors for the single discharges. Thiswas demonstrated by using a set of four holes in molybde-num with the same diameter as before. The discharges in thelow current mode @Fig. 3~a!#, as well as in the high currentmode @Fig. 3~b!# are not affecting each other, as expected.The distance between the holes which in this case was largecompared to the hole diameter, can be reduced to fractions ofthe hole diameter without affecting the discharge array. Asystem with approximately 50 cathode openings of 200 mm,separated by 10 mm, provided a stable array of hollow cath-ode discharges in air at 50 Torr. By reducing the cathodeopenings to values of 75 mm it was possible to increase thepressure ~of air! to 350 Torr. It is expected that further reduc-tion of the hole diameter will allow us to generate arrays ofhollow cathode discharges in atmospheric air.Discharges operated below the current discontinuity areTowsend discharges along a path from the interior of thecathode hole to the backside of the hollow anode. In thismode electrons may obtain energies close to value deter-mined by the applied voltage and form axial electron beams.For discharges operated above the current continuity, theseTownsend discharges or ‘‘predischarges’’ constitute the ini-tial stage in their temporal development.7 The predischargegenerates a virtual anode close to the anode which propa-gates toward the cathode orifice.8 This modifies the initiallydominantly axial electric field at the orifice of the hollowcathode and generates a more radial field structure. Thechange in electric field favors the onset of the Pendel effect,which generates a highly ionized plasma column on the axisFIG. 2. Current–voltage characteristics of the microhollow cathode dis-charge in argon.FIG. 3. Array of hollow cathode discharges in argon at a pressure of 16.5Torr in the ‘‘predischarge’’ mode ~above! and the hollow cathode dischargemode ~below!.14 Appl. Phys. Lett., Vol. 68, No. 1, 1 January 1996 Schoenbach et al.10° pulsed de ~ 0 p = 56.0 Torr • p = 3.5 Torr 0 D p = 5.0Torr 10-1 T p = 7.5 Torr L:. p = 16.5 Torr ♦ ♦ p = 56.0Torr ••• ♦ g 10-2 ; rNAL:. L:. I-z ~ w ~~ T a: a: ::, T () [!Jl 10-3 5'" ~-L:. ~ .. L:. r"' •• 10-4 ~"#~ • 200 250 300 350 400 450 500 550 600 650 VOLTAGE(V) of the hollow cathode. The discharge mode above the currentdiscontinuity therefore constitutes the real hollow cathodedischarge. The lowest pD value for which this hollow cath-ode discharge can be sustained in our discharge configura-tion, for voltages below 600 V, is 0.53 Torr cm ~p57.5 Torr!,20 times the value obtained with Eq. ~1!.Modeling of a hollow cathode discharge in argon hasprovided current–voltage characteristics similar to the ones,which we have obtained experimentally in the lower pDrange, except for the transition region from predischarge tohollow cathode discharge.9 Whereas our measurements showonly a slight, if any reduction in forward voltage when thedischarge enters the hollow cathode mode, in the computedcurrent–voltage characteristics this transition is accompaniedby a considerable drop of the discharge voltage. The ~rela-tively slight! increase in current with voltage above the tran-sition into the hollow cathode mode, which is seen in boththe modeling and experimental curves ~Fig. 2!, is attributedto the fact that the plasma sheath in the hollow cathode modecovers the entire hollow cathode surface, and any increase incurrent requires an increase in cathode fall voltage.9The electron energy distribution in the hollow cathodedischarge mode contains electrons with an energy corre-sponding to the cathode fall.10 Electrons with at least 100 eVwere measured in the negative glow of a parallel plate hol-low cathode discharge.11 The increased ion flux to the cath-ode in the hollow cathode mode causes an increase in theconcentration of sputtered electrode material and explainsthe observed occurrence of metal spectral lines in this dis-charge phase. Eventually heating of the cathode causes ther-mionic electron emission to become the dominant effectrather than electron emission through ion impact, and thevoltage across the discharge drops to values of less than 20V. In recent experiments with dispenser material ~tungstenimpregnated with barium oxide! as cathode material we havedemonstrated this transition to the thermionic mode at cur-rents of approximately 100 mA for a 0.75 mm diameter hol-low cathode.The possibility to generate arrays of microhollow cath-ode discharges without using ballast suggests their use assimple flat panels in light sources at high pressures. Thedischarges can be operated in two stable modes: in the ‘‘pre-discharge’’ or electron-beam mode or as fully developed hol-low cathode discharges. Other applications are large areaelectron and ion sources and, because of the nonthermal elec-tron energy distribution, particularly in the electron-beammode, as gas processing devices.1A. Guentherschulze, Z. Phys. 19, 313 ~1923!.2H. Helm, Z. Naturforsch. Teil A 27, 1812 ~1972!.3C. C. Van Voorhis and A. G. Shenstone, Rev. Sci. Instrum. 12, 257 ~1941!.4 J. W. Gewartowski and H. A. Watson, Principles of Electron Tubes ~VanNostrand, Princeton, NJ, 1965!, p. 561.5D. J. Sturges and H. J. Oskam, J. Appl. Phys. 35, 2887 ~1964!; J. Appl.Phys. 37, 2405 ~1966!.6A. D. White, J. Appl. Phys. 30, 711 ~1959!.7M. T. Ngo, K. H. Schoenbach, G. A. Gerdin, and J. H. Lee, IEEE Trans.Plasma Sci. 18, 669 ~1990!.8P. Choi, R. Aliaga, B. Plottiere, M. Favre, J. Moreno, H. Chuaqui, and E.Wyndham, Appl. Phys. Lett. 63, 2750 ~1993!.9A. Fiala, L. C. Pitchford, and J. P. Boeuf, in Proceedings of the XXIIConference on Phenomena in Ionized Gases, edited by K. H. Becker, W.E. Carr, and E. E. Kunhardt ~Stevens Institute of Technology, Hoboken,NJ, 1995!, Vol. 4, p. 191.10 I. Kuen, F. Howorka, and H. Sto¨ri, Phys. Rev. A 23, 829 ~1981!.11K. Fujii, Jpn. J. Appl. Phys. 16, 1081 ~1977!.15Appl. Phys. Lett., Vol. 68, No. 1, 1 January 1996 Schoenbach et al.

Microhollow Cathode Discharges

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Microhollow Cathode Discharges

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