J.T. GalofaroGlenn Research Center, Cleveland, Ohio

B.V. VaynerOhio Aerospace Institute, Brook Park, Ohio

W.A. DegrootDynacs Engineering Company, Inc., Brook Park, Ohio

D.C. FergusonGlenn Research Center, Cleveland, Ohio

C.D. Thomson, J.R. Dennison, and R.E. DaviesUtah State University, Logan, Utah

Inception of Snapover and GasInduced Glow Discharges

NASA/TM—2000-209645

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NASA/TM—2000-209645

January 2000

National Aeronautics andSpace Administration

Glenn Research Center

Inception of Snapover and GasInduced Glow Discharges

J.T. GalofaroGlenn Research Center, Cleveland, Ohio

B.V. VaynerOhio Aerospace Institute, Brook Park, Ohio

W.A. DegrootDynacs Engineering Company, Inc., Brook Park, Ohio

D.C. FergusonGlenn Research Center, Cleveland, Ohio

C.D. Thomson, J.R. Dennison, and R.E. DaviesUtah State University, Logan, Utah

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NASA Center for Aerospace Information7121 Standard DriveHanover, MD 21076Price Code: A03

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NASA/TM—2000-209645 1

INCEPTION OF SNAPOVER AND GAS INDUCED GLOW DISCHARGES

J.T. GalofaroNational Aeronautics and Space Administration

Glenn Research CenterCleveland, Ohio

B.V. VaynerOhio Aerospace Institute

Brook Park, Ohio

W.A. DegrootDynacs Engineering Company, Inc.

Brook Park, Ohio

D.C. FergusonNational Aeronautics and Space Administration

Glenn Research CenterCleveland, Ohio

C.D. Thomson, J.R. Dennison, and R.E. DaviesUtah State University

Logan, Utah

AbstractGround based experiments of the snapover

phenomenon were conducted in the large verticalsimulation chamber at the Glenn Research Center(GRC) Plasma Interaction Facility (PIF). TwoPenning sources provided both argon and xenonplasmas for the experiments. The sources were usedto simulate a variety of ionospheric densitiespertaining to a spacecraft in a Low Earth Orbital(LEO) environment1–4. Secondary electron emissionis believed responsible for dielectric surfacecharging, and all subsequent snapover phenomenaobserved2,5. Voltage sweeps of conductor potentialsversus collected current were recorded in order toexamine the specific charging history of each sample.The average time constant for sample charging wasestimated between 25 and 50 seconds for all samples.It appears that current drops off by approximately afactor of 3 over the charging time of the sample. Allsamples charged in the forward and reverse biasdirections, demonstrated hysteresis. Current jumpswere only observed in the forward or positive sweptvoltage direction. There is large dispersion in thecritical snapover potential when repeating sweeps onany one sample. The current ratio for the firstsnapover region jumps between 2 and 4.6 times, witha standard deviation less than 1.6. Two of the

samples showed even larger current ratios. It isbelieved the second large snapover region is due tosample outgassing. Under certain preset conditions,namely at the higher neutral gas backgroundpressures, a perceptible blue-green glow wasobserved around the conductor. The glow is believedto be a result of secondary electrons undergoingcollisions with an expelled tenuous cloud of gas, thatis outgassed from the sample. Spectroscopicmeasurements of the glow discharge were made in anattempt to identify specific lines contributing to theobserved glow.

I. IntroductionSnapover describes a sudden and rather dramatic

change in the current collection regime in and aroundpositively biased conductors that are surrounded by adielectric6–9. Specifically there is a dramatictransition from the normal current collection regimeto a regime exhibiting high current collection. Suchcurrent jumps can only be brought about bysuccessively biasing a conductor to increasingpositive bias potentials, in a time interval, that iscomparable to the charging time of the dielectric.Dielectric surface charging is brought about bysecondary electron emission5,6,9. A small percentageof primary electrons, having the proper energy and

NASA/TM—2000-209645 2

trajectory, will be focused onto the surface of thedielectric by the E-field. The focusing effects aretypically limited to a small area of the dielectric,extending over a small radial distance ≈ 1–2 cm fromthe outward edge of the conductor/ dielectricinterface. As primary electrons strike the dielectricone or more secondaries can be liberated. As thesheath area grows, the surface of the dielectricquickly charges to a positive potential. Secondaryelectrons, suffering collisions with the outgassedspecies, are responsible for excitation and theresulting glow. Ferguson et al. reported snapover-induced glow in ground tests using argon, xenon andneon plasmas10.

The increased complexity of modern spacecraftis fueling the requirement for higher voltage powersystem designs. Others have previously demonstratedthat the high voltage spacecraft systems are capableof undergoing significant physical interactions withthe plasma environment11,12. The 160 V solar array,designed to provide power, for the InternationalSpace Station represents a prime example9,11.A solar array is most vulnerable to snapover at theinstant it comes out of eclipse into full sunlight. Thesolar array voltage will quickly ramp up to fulloperating potential. With current spacecraft solararray voltages, and the large number of conductorand dielectric surfaces in close proximity, there isincreased danger of snapover induced electricaldischarges disrupting spacecraft power systems13–15.

II. Experimental Test ApparatusAll snapover electrical and optical experiments

were run in a 1.8m diameter by 3m high verticalvacuum chamber. Two penning type plasma sourcesprovided plasma for the experiments. The penningsources use a hot filament to ionize either argon orxenon neutral gas which is slowly bleed into thechamber through a controllable leak valve. Anionization gauge was used to measure chamberpressure as gas was added to the chamber via the leakvalve. Electron plasma number densities for theexperiments ranged from 2×105 to 4.0×106

electrons/cm3 and electron temperatures were on theorder of 1–3.5 eV.

A sample table was constructed of fiberglass andwas mounted in a vertical position in the vacuumchamber. Twenty sample coupons of various types(see figure 1) were then affixed to the sample table sothat they could be clearly observed through an opticalviewing port on the chamber wall. The samplecoupons consisted of a central conductor of a specificmetal type (copper or aluminum) and geometry(cylinder or hemispherical), imbedded in a 0.635cmthick by 10.16cm long by 10.16cm wide square ofdielectric material. Cylinder diameters for both

copper and aluminum conductors are given on thesample key at the bottom of figure 1. All conductors,with the exception of the hemisphere, were mountedflush with the top surface of the dielectric. Samplecoupons were then prepared with a dielectriccomposed of Kapton, Teflon, or silicon dioxide(Sio2).

Figure 1.—Sample coupon testbed used insnapover experiments.

Electrical connections were made to the backsideof the conductor and each sample coupon wasconnected to a separate electrical feed through. Theexposed back of each conductor was then insulatedwith a silicone type adhesive. All sample couponswere electrically floated in the vacuum chamber. Aprogrammable source and measure unit allowed asingle conductor sample to be swept with respect to thechamber ground, while simultaneously measuring thecurrent at each bias voltage step. In this way it waspossible to obtain a volt/ampere curve for each sample.

For the snapover-induced glow experiments asingle 1.27cm aluminum-Teflon coupon test samplewas mounted on a separate insulated stand. Thecoupon was positioned so that it could be clearly seenthrough a 15.24cm diameter quartz window mountedon the vacuum chamber wall. Insulated electricalconnections were then made between the sample andthe electrical feedthrough port.

A 10cm diameter double convex lens with afocal distance of 24cm was mounted inside thechamber, between the sample and quartz opticalviewing port. The position between the lens and thesample was adjusted at 85cm, so that the nearlyparallel light rays from the sample would be focussedjust outside the optical view port. The position of thespectrometer was then adjusted so that the focal pointof the lens would fall exactly on the 50µ wideentrance slit. A black cylindrical light shield tube wasconstructed around the lens and the optical view portto shield stray light generated by the plasma sources.

NASA/TM—2000-209645 3

This spectrometer employs a Czerny-Turneroptical design (see figure 2(a)). Light passing throughthe entrance slit undergoes 3 specular reflectionsbefore passing through the exit assembly. The focallength of the instrument is 156mm. The grating has1200 groves per mm, and was cut with a blaze anglecentered on 300nm peak intensity. The usablewavelength range for this particular grating is 190nmto 600nm. The manufacturer claims a resolution of1nm for the instrument.

Light passing from the exit assembly is detectedby a charge-coupled device (CCD). The detector/controller is plumed with 2 lines, a chilled water tocool the detector head and a dry gaseous nitrogen lineto prevent against condensation. Figure 2(b) shows ablock diagram of the apparatus used in obtaining theoptical spectra. Information that is gathered by thecontroller/detector is passed to a computer via aparallel interface and card. Appropriate software,provided by the manufacturer, was used to capture andstore information displayed on the computer screen.

Figure 2(a).—Spectrometer setup used in analyzingsnapover-induced glow.

Figure 2(b).—Applied voltage bias between coupon andtank ground causes gaseous discharge. Current pulsedetected by current probe triggers channel 1 of thescope. Output trigger from scope causes HV Gate

Pulser to open Gate and obtain spectra.

III. Experimental Snapover ResultsThe first series of tests were run to determine the

time constant of the sample. The sample was biasedfrom V=0 volts potential and then immediately tosome large positive potential. Figures 3(a) and 3(b)show the current to the sample recorded at varioustime intervals. These hand plots show the chargingconstant, τ to be roughly between 25s and 50s. Notethat the current drops by a factor of 3 over the timeconstant in figures 3(a) and 3(b). The floatingpotential, Vf was measured at 5.5 volts in these tests.

Figure 3(a).—Sample curve for sample biasedfrom 0 to +600 volts.

Figure 3(b). —Sample curve for sample biasedfrom 0 to +700 volts.

The next series of tests involved snapover. The1.27cm copper-Teflon sample was biased up in anargon plasma. The neutral gas background pressure,Po = 7.3×10-5 Torr, the plasma number density,Ne = 2×105 cm-3 and the electron temperature,Te = 3.5eV. A sweep was made by biasing theconductor from –100V to +400V in 10V steps with astep time 0.5s at each bias voltage. There are severalirregularities in the forward biased voltage-currenttrace (figure 3(a)). The observed hook at the firstlarge current jump at 200V is believed to be due tooutgassing of the sample (more will be said aboutoutgassing later). A sweep in the reverse biasdirection (figure 3(b)) does not show anypeculiarities. Note the hysteresis between the forwardand reverse bias traces in figures 4(a) and 4(b).

NASA/TM—2000-209645 4

Figure 4(a).—Forward bias sweep showing currentjumps at 200V, 300V, and 380V.

Figure 4(b).—Reverse bias sweep shows a smooth curvesweep typical of a conductor.

In order to verify the effect of current jumpingtwo more sweeps have been obtained from the same1.27cm copper-Teflon coupon (sample 19). Figure 5(a)demonstrates the single large current jump at 250V isthought to be a result of outgassing. The “hook” in thecurve at 250V is characteristic of a gas discharge athigh voltage. Figure 5(b) appears to demonstrate ashifting of the critical voltage to a higher snapoverpotential. If this effect is do to outgassing we shouldobserve the disappearance of the large current jump at250V in the second sweep of sample 19. Also note theabsence of the hook in figure 5(b) appears todemonstrate that the sample is well outgassed.

Figure 5(a).—Current-voltage sweep at 10:30 a.m.,10V steps and 0.5s per step.

Figure 5(b).—Current-voltage sweep at 2:30 p.m.,10V steps and 0.5s per step. The critical voltage

shifted at 300V.

Figure 5(c).—Critical voltage decreases from300V to 275V but the effect is repeatable.

NASA/TM—2000-209645 5

In order to verify the effect of current jumpingtwo more sweeps have been obtained from the same1.27cm copper-Teflon coupon (sample 19). Figure5(a) demonstrates the single large current jump at250V is thought to be a result of outgassing. The“hook” in the curve at 250V is characteristic of a gasdischarge at high voltage. Figure 5(b) appears todemonstrate a shifting of the critical voltage to ahigher snapover potential. If this effect is do tooutgassing we should observe the disappearance ofthe large current jump at 250V in the second sweepof sample 19. Also note the absence of the hook infigure 5(b) appears to demonstrate that the sample iswell outgassed.

The sample coupons have been sitting in thevacuum chamber for some 48 hours. Assuming theeffect of current jumping is caused by outgassing oneshould observe a rise in the critical voltage forsnapover on coupon sample 19. In reality the criticalvoltage was measured at 200V for the virgin sample.After sitting in vacuum for a few hours the criticalvoltage increased to about 300V (figure 5(b)). Thecritical voltage then decreases to 275V after sitting inthe vacuum chamber for another 20 hours (figure 5(c)).

It was decided to keep the sample coupons in thevacuum for a much greater length of time (3 moredays) before continuing the tests on the other samples.For the proceeding tests Po = 7.3×10-5 Torr (consistingof argon gas neutrals), Ne = 3.3×10+5 cm-3, andTe = 2.2 eV for the experiments. In order to obtaingood statistical results a minimum of 10 sweeps wereperformed on each sample coupon. The results of theseresults are summarized in Table 1.

Table 1.—Summary of snapover resultsfor various sample coupons.

The snapover results reported in table 1 needfurther explanation. Three standard measurements areshown in table 1: Snapover Voltage, Current Ratio,and Increase in area. These tabular values represent

the mean value for 10 independent measurements ofthe particular parameter in of interest for a givensample. To the right of each standard measurement isa field containing the standard deviation or spread ofvalues about the mean determination.

The snapover voltage refers to the inceptionvoltage at the first current jump (first snapoverregion). The current ratio is a scalar factor referringto the magnitude of the collected current. Thismagnitude is computed by dividing the currentmaximum by the current minimum in the firstsnapover region. Finally the increase in area is afictitious term referring to the effective increase insheath collecting area. The increase in collected areais governed by sheath size; orbit limited currentcollection, secondary yield, E-field, and surfaceconductivity. The increase in area is computed fromthe ratio of the slopes of the current-voltage curvedirectly before and after the first jump in current atthe inception voltage.

Statistical data for snapover inception voltageappears to be widely scattered for all samples. It isinteresting to note that sample 16, the largestconductor of all samples tested, showed a secondlarge current jump in all ten sweeps. Samples 4 and 8showed large dispersion in their standard deviationsfor all three standard measurements. The reason forthe large dispersions is believed to be due tocontamination of dielectric surfaces. Aftercompleting the various snapover tests on the samplesand after completing the initial glow observations, itwas discovered that yellow stains were found aroundall electrodes of the biased samples. An inspection ofthese stains, performed by three pairs of experiencedeyes, have confirmed that the contamination isdiffusion pump oil possibly due to a valve failureexperienced earlier. None of the unbiased virginsamples inspected showed stains of any kind. Thesamples were cleaned with a methyl alcohol washand returned to the vacuum chamber after drying.

IV. Optical ResultsIn order to verify the hypothesis of a gas

discharge at high voltage, our eyes and a color videocamcorder has been employed. Previous attempts atGRC to see snapover induced glow with the humaneye proved allusive, even at voltages as high as600V. Such gaseous discharges were strongly felt toresult from Paschen breakdown. If vacuum arcignition is indeed the cause of such gaseousdischarges then the glow would be stronglydependent on two parameters: pressure and voltage16.Since changes in voltage alone failed to produce theglow, it was decided that the other parameter pressureneeded to be adjusted.

NASA/TM—2000-209645 6

An argon plasma was established in the vacuumchamber with a much greater neutral gas backgroundpressure (Po = 3×10-4 Torr) then previously used.Next a previously unbiased virgin sample(sample 11) was slowly swept from +100V to +600Vin 5V steps. The voltage was applied over a time of500ms at each step. As the sample was swept,observations of the sample were made with theunaided eye through the vacuum chamber’s opticalview port. An 8cm diameter blue-green glowing ringof gas was detected around the electrode atapproximately 515V. The glow lasted about 10sbefore extinguishing. A second attempt at biasingsample 11 failed to produce the glow. The absence ofthe glow appears to confirm the initial suspicion ofsample outgassing. It is suggested here that sampleoutgassing, due to gas and/or water vapor trappedbetween the conductor-dielectric interface, isresponsible. In fact a blue color is often seen inplasmas where water vapor is present.

Moving to a new virgin sample (sample 2) acamcorder was set up to record the glow. The samplewas biased positive and the glow (see figure 6) wasrecorded. Figure 7 shows the voltage-current sweepfor the glow recorded shown in figure 6.

Figure 6.—Example of snapover inducedglow discharge.

Figure 7.—Gaseous discharge beginsat about 430V potential.

The finial series experiments were performed inan attempt to obtain a glow discharge spectrum.Sample 19 was cleaned and mounted in the tankfacing the quartz optical view port. An argon lampwas mounted in mounted in front of the sample. Thedistance of the culminating lens was adjusted to asharp focus on the slit. For calibration purposes, anargon spectrum was obtained by rotating the gratingand obtaining a calibration spectrum for argon atthe following central wavelengths, (λ): λ = 385nm,λ = 400nm, λ = 420nm, λ = 435nm, λ = 450nm, andλ = 470nm. A sample argon spectrum with thespectrometer adjusted on the 420nm line is shown infigure 8. The overall estimate of error for thisconversion process is 1nm (10 angstroms) 17.

Figure 8.—Sample argon calibration spectrum withtwo of the most intense lines labeled.

In reality the output obtained from theinstrument software is given in terms of pixels(scalar) and intensity (relative units). Conversionfrom pixels to wavelength, λ, is obtained from thefollowing equation:

( ) 420.1 ( ( ))cp k p bλ λ≅ − −

where p = pixel number, k = 0.125 nm per pixel,λc = 385nm, 400nm, 420nm, 435nm, 450nm, and470nm (central wavelength adjustment set on thespectrometer), b(385) = 246.5, b(400) = 370.5,b(420) = 534.5, b(435) = 654.5 and b(450) = 774.5.Exact details of the conversion process as well asestimate of errors can be found in Vayner et. al.(reference 17).

Having completed the calibration of thespectrometer the argon lamp was removed and thevacuum chamber was pumped down. The tankpressure with the argon plasma sources operating wasPo = 3.2×10-4 Torr.

NASA/TM—2000-209645 7

In practice the spectrometer is adjusted to aspecific central wavelength, λc and a backgroundtrace was acquired. The plasma source hot filamentsproduce a modest wide band background signalcentered at about 550nm. This background needs tobe subtracted from every glow spectrum acquired.The background also needs to be newly acquired evertime the spectrometer central wavelength is re-adjusted. A minimum of two glow spectra wasacquired at each λc specified in the argon calibrationspectrum. Figure 9 shows a typical glow spectrumobtained in an argon plasma.

Figure 9.—Glow spectrum in argon plasma.Spectrometer set for λc = 420nm.

It should be noted that a shift to higherfrequencies in λc translates to a shift to the right (shiftto higher λ’s) in the acquired spectrum.

The finial series of measurements were aimed atobtaining a glow discharge spectrum in the presenceof a xenon plasma. The base operating pressure withthe plasma sources on was Po = 2×10-4 Torr. Onceagain, a minimum of two glow discharge spectra wasacquired at each λc specified in the argon calibrationspectrum. Background traces obtained at each shift inλc were subtracted from the corresponding glowdischarge spectrum. Figure 10 demonstrates a typicalglow spectrum obtained in a xenon plasma.

Figure 10.—Glow spectrum in a xenon plasma.Spectrometer set λc = 385nm.

A direct comparison of spectrum measurementsperformed for different λc in both argon and xenonplasmas are made. These comparisons were used todetermine the most reliable lines and common linesin the glow discharge spectrum.

For argon plasmas the following lines wereidentified: 403, 410, 414, 418, 420.6, 425, 428.6,438, 444, 456, and 461/63. In xenon plasmas thefollowing lines were also identified: 371, 398/401,412/415, 419, 422/24, and 454. The MIT Tables18

show the most intensive lines for xenon occur at419nm, 414nm and 398nm. Lines at 414nm in argonand the 412/415 lines for xenon may belong to someother species in the glow spectrum. No lines forxenon were found at 401nm. Lines for oxygen at412nm, for copper at 412nm and for OII at 414nmwere found. There is a strong line for xenon at454nm. No suitable species was found at 456nm.There are however lines at 457.7nm for OI and a lineat 457.9nm for ArII.

V. ConclusionSnapover and related solar array interactions

with the space plasma environment are issues of greatimportance. An understanding of the snapoverphenomenon is therefore valuable to solar arraysystem design and survivability. Modern high powersolar arrays are currently being used in a number ofspacecraft. A number of these spacecraft areexperiencing problems that can be related tosnapover. Large planar arrays, incorporatinginsulators adjacent to exposed conductors in thespace plasma, can effectively collect current as ifthey were conductors. Such enhanced plasma currentcollection was detected in the laboratory on the PASPPlus flight arrays4. The reason for this type of currentcollection behavior is due to enhanced surfaceconductivity resulting from secondary electronemission. The present experiments were conducted tostudy the effects of enhanced current collection in thelaboratory for a number of different dielectrics andconductor types. Snapover induced glow discharges,a related phenomenon, was also studied at pressureranges between 10-4 and 10-5 Torr. Although thesepressures are much higher than what might ordinarilybe found in low Earth orbit, they are more typical ofwhat might be found during brief periods ofspacecraft thruster firings. Spectroscopic analysis ofthe glow discharge has allowed several linescontributing to the glow to be isolated, but not thecontributing species. It is hoped the present study isbeneficial to the scientific community.

NASA/TM—2000-209645 8

VI. References

1. Staskus, J.V., “Electron Beam Charging of SpaceShuttle Protection System Tiles”, SpacecraftEnvironmental Interactions Technology, NASACP–2359, 1983, pp. 91–102.

2. Stevens, N.J. and Domitz, S., “Interaction

Between Spacecraft and the Charged ParticleEnvironment”, Spacecraft Charging Technology,NASA CP–2071, 1978, pp. 268–294.

3. Ferguson, D.C., Snyder, D.B. and Carruth, R,

“Findings of the Joint Workshop on Evaluationof Impacts of Space Station Freedom GroundConfigurations”, NASA TM–103717, 1990.

4. Ferguson, D.C., “Comparison of Electron

Current Collection by Space Station Solararrays as Measured by SAMPIE and PASPlus”,Photovoltaic Array Space Power PlusDiagnostics (PASP Plus) Experiment FinalReport, PL–TR–97–1013, March 1997, pp. 7-1.

5. Katz, I. Et, Mandell, M.J., and Jongeward, G.A.,

“A Three dimensional Dynamic Study ofElectrostatic Charging in Materials. NASACR–135256, 1977.

6. Galofaro, J.T. “Comparison of Currents Predicted

by NASCAP/LEO Model Simulations withElementary Langmuir Type Bare Probe Modelsfor an Insulated Cable Containing a Pinhole”,NASA TM–102486, July 1990, pp. 3–5.

7. Cole, R.K., Ogawa, H.S., and Sellen, J.M.,

“Operation of Solar Cell Arrays in DiluteStreaming Plasmas”, TRW–09357–6006–R000,NASA Contract NAS3–10612, NASA CR–72376,1968.

8. Kennerude, K.L., “High Voltage Solar Array

Experiments”, NASA CR–121280, 1974. 9. Grier, N.T., and McKinzie, D.J., Jr., “Current

Drainage to a High Voltage Probe in a DilutePlasma”, NASA TM–X–67890, 1971.

10. Ferguson, D., Hillard, G., Snyder, D., and Grier,

N., “The Inception of Snapover on Arrays: AVisualization Technique”, 36th AIAA–98–1045,Aerospace Sciences Meeting and Exhibit,Reno, NV, January 12–15, 1998, pp. 1–8.

11. Vaughn, J.A., Carruth, Jr., M.R., Zkatz, I.,Mandell, M.J., and Jongeward, G.A., “ElectricalBreakdown Currents on Large Spacecraft in aLow Earth Orbit,” Journal of Spacecraft andRockets, vol. 31, no. 1, 1994, pp. 54–59.

12. McCoy, J.E., and Konradi, A., “Sheath Effects

Observed on a 100 Meter High Voltage Panel inSimulated Low Earth Orbit,” Proceedings ofSpacecraft Charging Technology 1978,NASA CP–2071, October—November 1978,pp. 201–210.

13. Davis, V. and Gardner, B., “Parasitic Current

Collection by Solar Cells in LEO,” AIAA Paper95–0594, 1995.

14. Ferguson, D.C., “Preliminary Results from the

Flight of the Solar Array Module PlasmaInteractions Experiment (SAMPIE),”Proceedings of the XIII Space Photovoltaic andResearch and Technology Conference, NASACP–3278, 1994.

15. Hillard, G.P., “Plasma Chamber Testing of

Advanced Photovoltaic Solar Array Coupons,”Journal of Spacecraft and Rockets, vol. 31,no. 3, 1993, pp. 530–532.

16. Boxman, R.L., Sanders, D..M., Martin, P. J. andLafferty, J.M., “Handbook of Vacuum ArcScience and Technology,” Fundamentals andApplications, Noyes Publications, New Jersey,USA, pp. 61–62.

17. Vayner, B., Galofaro, J., Ferguson, D., andDegroot, W., “A Comprehensive Study ofDielectric-Conductor Junction in Low DensityPlasmas”, Proc. 38th AIAA Aerospace SciencesMeeting and Exhibit”, 2000–0871, January10–13, Reno, NV.

18. Harrison, G.R., “Massachusetts Institute ofTechnology Wavelength Tables” withIntensities in Arc, Spark, or Discharge Tube ofmore than 100,000 Spectrum Lines Between10,000 A. and 2000 A., M.I.T. Press,Cambridge, Massachusetts, and London,England, 1969, pp. 117–207.

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Inception of Snapover and Gas Induced Glow Discharges

Ground based experiments of the snapover phenomenon were conducted in the large vertical simulation chamber at the GlennResearch Center (GRC) Plasma Interaction Facility (PIF). Two Penning sources provided both argon and xenon plasmas for theexperiments. The sources were used to simulate a variety of ionospheric densities pertaining to a spacecraft in a Low EarthOrbital (LEO) environment. Secondary electron emission is believed responsible for dielectric surface charging, and all subse-quent snapover phenomena observed. Voltage sweeps of conductor potentials versus collected current were recorded in order toexamine the specific charging history of each sample. The average time constant for sample charging was estimated between 25and 50 seconds for all samples. It appears that current drops off by approximately a factor of 3 over the charging time of thesample. All samples charged in the forward and reverse bias directions, demonstrated hysteresis. Current jumps were onlyobserved in the forward or positive swept voltage direction. There is large dispersion in the critical snapover potential whenrepeating sweeps on any one sample. The current ratio for the first snapover region jumps between 2 and 4.6 times, with astandard deviation less than 1.6. Two of the samples showed even larger current ratios. It is believed the second large snapoverregion is due to sample outgassing. Under certain preset conditions, namely at the higher neutral gas background pressures, aperceptible blue-green glow was observed around the conductor. The glow is believed to be a result of secondary electronsundergoing collisions with an expelled tenuous cloud of gas, that is outgassed from the sample. Spectroscopic measurements ofthe glow discharge were made in an attempt to identify specific lines contributing to the observed glow.

J.T. Galofaro and D.C. Ferguson, NASA Glenn Research Center; B.V. Vayner, Ohio Aerospace Institute, 22800 CedarPoint Road, Brook Park, Ohio 44142; W.A. Degroot, Dynacs Engineering Company, Inc., 2001 Aerospace Parkway,Brook Park, Ohio 44142; C.D. Thomson, J.R. Dennison, and R.E. Davies, Utah State University, Logan, Utah84322-0001. Responsible person, J.T. Galofaro, organization code 5410, (216) 433–2294.

J.T. Galofaro, B.V. Vayner, W.A. Degroot, D.C. Ferguson, C.D. Thomson,J.R. Dennison, and R.E. Davies