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NASA Technical Memorandum 827231T

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Vesting of a Spacecraft Model in a CombinedEnvironment Simulator

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Prepared for theEighteenth Annual Conference on Nuclear and Space Radiation Effects 41sponsored by the Institute of Electrical and Electronics EngineersSeattle, "Vashington, July 20-24, 1981

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TESTING OF A SPACECRAFT MODEL IN A COMBINED ENVIRONMENT SINVI ATUR

J. V. Staskus and J. C. Roche

National Aeronautics and Space AdministrationLewis Research CenterCleveland. Ohio 44135

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Abstract

A scale model of a satellite was tested in alarge vacuum facility under electron bombardment andvacuum ultraviolet radiation to investigate thecharging of dielectric materials on curved surfaces.The model was tested both stationary and rotatingrelative to the electron sources as well as groundedthrough one megohm and floating relative to the chawber. Surface potential measurements are presentedand compared with the predictions of computermo.lelling of the stationary tests. Uischarge activ-ity observed during the stationary tests is discussedand signals from sensing devices located inside andoutside of the model are presented.

Introduction

As part of the continuing effort in the jointNASAIUSAF spacecraft charging investigation, electronspraying tests were performed on a model of theSCATMA (P78-k) satellite in one of the large vacuumfacilities at NASA-LeRC. Considerable testing atatmospheril pressure had previously been performed onthe model. The testing was concerned with mea-surement and analysis of the model's response tosimulated electron induced oiscnarges occurring atvarious po 4 nts on tole model. The LeRC electronspraying tests were part of a comoined experimentaland analytical investigation of the charging ofdielectric materials in geometric patterns on acurved surtace. Previous charging tests had beenperformed on 3U cm square or smaller samples of sin-gle dielectric materia l s un groutided substrates withnormally incident electron beams.

Electron spraying tests of a satellite were con-ducted by the European space Agency on the METEUSATP1 model.,'9 3 The tests were an attempt to re-produce anomalies observed on the orbiting space-craft. During those tests. it was repo rted that dis-charges were unexpectedly observed to ot.cur with thesatellite floating. In the course of We LeRC test-ing. it was also observed that discharges occurredwith the SCATSAT floating. Furthermore, dischargesoccurred for electron beam energies well below thoserequired to produce discharges in the tests of smallsamples on grounded substrates.

The testing, originally planned to investigatecharging, was expanded to include investigation ofthe low level discharges. Sensors were placed in andaround the model to determine the nature and locationof the discharging. That invest i gation has only re-cently been initiated and is still being pursued.

Experiment Uescri lion

Facility

The tests were conducted in the 4.6-meter diame-

ter by 18.3-meter long vacuum chamber in the NASALewis ReSearCh Electric Propulsion Laboratory(Fig. 1). The test object, called SCATSAT. was a

tl3 scale model of the SCATMA satellite launched inJanuary of i979. The model was Suspended on threeU.76-meter long Uelrin rods providing UC isolationfrom the chamber. The support assemo:y was mountedon a hollow-shaft, rotary vacuum foedthrough with amotorized drive to permit simulation of satellite

rotation. The hollow shaft permitted data cables tobe brouiht out of the vacuum chamber before twisting

during rotation. The model was centered on the cham-ber axis. Four electron guns were located about two

meters from the model approximately every 90 aroundthe model at the elevation of the chamber axis. Sur-face voltage probes mounted on vertical arms scannedthe cylindrical surface of the model as it rotated.A surface voltage probe mounted beneath the modelviewed the stainless steel forward skin to continu-rusly monitor the structure potential. Two vacuumultraviolet (VUV) sources were mounted near one ofthe electron guns at elevations above and below thechamber axis (Fig. 2). Electron flux sensors werelocated between each electron gun and the model atabout 15 cm from the model. The chamber was pumpedby giftusion pumps agd was operated in the midlU- torr to low 10 torr pressure range.

SCATSAT Model

Figure 3 is a detailed view of the SCATSAT.

The model is a 1.1-meter long by 1.1-meter diameter.right circular cylinder. The fiberglass cylindricalsurface is covered with 2 cm by 4 cm tused silicasolar cell cover slides over copper foil to simulatethe solar cells ct the SCATMA satellite. A smallpatch of indium-tin-oxide (ITU) coated cover slidesis located near the forward end of the model. Theconductive 1TO surfaces are electrically connected tothe SCATSAT structure. The aluminum central band hastwo area: of silvered Teflon tape about 180 degreesapart around the model. The silver layer on the tapewas elect r ically connected to toe model structurethrough the tape's conductive adnesive. Two squarealuminum patches about 180 degrees apart and onopposite sides of the central band represent thespacecraft surtace potential monitor (SSPM) simula-tions for these tests. Tney are uoth electricallyconnected to the model's structure. The forward endof the model is a skin of stainless steel foil. Theaft end of the model is an aluminum skin with a non-cunductive white paint. Thus the model duplicated

the general features of the dielectric surfaces onSCATMA.

le;t Results

Material Charging Tests

Testing of the SCATSAT started with all investi-

gation of the charging of the dielectric materials onthe curved surface. The first tests were conductedwith a one megohm resistive connection between themodel and the chamber. A resistive voltage dividerwas included for structure potential measurement.The stationary model was irradiated by tour electronguns for approximately 30 minutes. The guns werethen turned off and the model was rotated at 1 rpm toobtain the surface voltage profiles. It is knownfrom repetitive measurements that the surface volt-ages do not change in the one minute required toobtain the data. Figure 4(a) shows the solar arraysurface voltage profile resulting from ir • adiation byfour 8-keV. approximately 1 nAll , electronbeams. The four broad peaks were produced by thefour electron beaws. The narrow peaks correspond tothe individual cover slides. The area at zero

corresponds to the simulation of the SSPN. The maxi-mum potentials of -6 kV occurred where the electronbeam angle of incidence was minimum. The angle aasnot exactly zero since the path viewed by the probewas not in the plane of the electron guns. The elec-tron guns were turned back on while the model wasrotating at I rpm and after several minutes equilib-rium was reached. The surface voltage profile shownin Fig. 4(o) was the result. The potential peakswere reduced by 700 to 800 volts and the valleys werefilled in. Finally, the two VUV sources were turnedon and the equilibrium surface voltage profile shownin Fig. 4(c) resulted. During the stationary irra-diation tests surface voltage probe and Current sen-sor transients indicated that some low-level, non-visible discharges were occurring randomly at quitelow electron beam energies. The discnarge activitywas halted or diminished when the model was rotated.

Comparsion to Modelling Predictions

The NASA Charging Analyzer Prog •am (NASCAP)4-6was used to generate computer predictions of the sur-face voltage profiles on a co.-.Vuter simulation of aground, stationary SCATSAT being irradiated by fourelectron beams. The SCATSAT was modelled as an octa-gon (Fig. 5) suspended in a representation of thevacuum chamber. Figure 6 shows the solar array andcenter band surface voltage profiles recorded op themodel following irradiation by the four 1 nA/cWelectron beams at 6 keV while the model was station-ary. The segmented line connects the potentialspredicted for the surface cells of the NASCAP modelof the SCATSAT. ;he values predicted are in goodagreement (withii 25 percent) with the recordedda-a. The discrepancy is probably due to the use ofan octagonal rather than circular analytical model.It is encouraging to note that the structure seen inthe recorded data is also evident in the computerpredictions within the constraints of the computermodel's spatial resolution. Simulation of therotating SCATSAT was not attempted at this .e.7

Discharge Tests

The potential of the floating SCATSAT irradiatedby four appromxiately 1 nA/cm4 electron beams at2.5 keV is shown in Figure. 7(a). Ignoring theeffects of electron gun warm-up, the model's poten-tial rapidly achieved a nearly stable negative valuewith periodic discharges taking place. The dis-charges were more frequent than observed during testswith one megohm between the model and ground. Thediscnarge rate increased with :ncreasing electronflux and/or beam energy. Since the model wasstationary, the voltage on the model's structure wasthe only voltage data obtainable. For reasons as yetunknown, the structure potential data underwentperiodic step changes in levPi as indicated at ap-proximately 400, 1000, and 1200 sec. The surfacevoltage probe transients i.idicative of dischargingare shown between 2UO an(. 400 sec and between 1000and 1200 sec.

The NASCAP prediCLIOns of the material poten-ials shown in Fig. 7(b) indicate the structure to behe most negative with the surfaces of the solar cellover slides and Teflon several hundred volts posi-ive relative to the structure. Uepsite the largeiscrepancy between the data and prediction of theime required for the structure to reach equilibrium,he agreement in the equilibrium value is quite goodwithin 100 volts). The time discrepancy is relatedo the size of the timesteps used in the computeriodelling.

A final observation that requires furthernvestigation is the change in structure potential at.urn-off of the electron guns. In the case shown,

the structure potential rapidly jumped from -750 to•350 volts. On other occasions, at Similar testconditions, the change was much less rapid and onstill others there was no observable change. At pre-sent the structure potential change at electron gunturn-off does not appear to be chamber pressuredependent.

The evid-_•nce of discharging consisted of tran-sients on the surface voltage probe signals and theelectron flux sensor electrometer signals. Dis-charges were not visible to the naked eye and3U minute time exposure photographs did not show anyvisible evidence of discharges. When tests were con-ducted with the model electrically floating, the dis-charges occurred more frequently for a given electronbeam energy and flux. The surface voltage probemechanism was roved away from the model to eliminateit as a possible discharge site and the dischargescontinued. The discharge rate was found to increasewith increasing electron beam energy and/or flux.The rate was also found to be dependent on the por-tion of the model being irradiated. When an area ofthe model with a Teflon-aluminum interface on thecentral band was being irradiated, the greatest dis-charge activity was observed. Discharge activitycould be diminished by rotating the model to irra-diate a different portion of the central band.Each of the electron guns, when operated separately,produced similar variations in discharge activitywith variations in the portion of the model beingirradiated.

Additional sensing devices were then installedto further investigate the discharging. A devicethat responds to the time derivative of a magneticfield, a B-dot sensor, was placed in the interior ofthe model near a Teflon-aluminum interface on thecentral band. Insulation capable of withstandingover 20 kv was used between the SCATSAT and the sen-sor and its cables. A 15 cm diameter loop antennawas placed outside of the model and opposite theelectron gun being used. Figure 8 shows transientsignals from these devices during a 4 keV, 1 to2 nAlsquare cm, test of the floating stationarySCATSAT. Both signals were of low amplitude. Thoughthe signals shown were of about the same duration,they were not from the same event. A small fractionof the discharges that were detected by the antennaoutside the model produced sufficient signal from theB-dot sensor to trigger its digital waveform re-corder. This was probably due to the shielding ofthe model's interior by the Faraday cage constructionrepresentative of the SCATKA satellite. The oscilla-tion frequency of the antenna signal was typically20 mHz while the calculated vacuum chamber resonantfrequency was 50 mhz.

Sumriary

Electron beam charging tests were conducted on a2/3 scale spacecraft model in a vacuum chamber usingup to four electron guns to irradiate the model'scylindrical surface. The tests were conducted withthe model stationary as well as rotating and with themodel connected to ground through one megohm as wellas floating. Computer modelling of tests of thestationary model, both grounded and floating, pro-duced results in good agreement with the recordeddata. The computer model predicted a positive volt-age gradient in the insulators on the floating model.

Discharging was noted which occurred for elec-tron beam energies as low as 2.5 keV. The frequencyof discharging increased with increasing electronflux and/or beam energy. The discharges were morefrequent on the floating model than on the model withone megohm to ground. The discharges were not visi-ble and produced low amplitude signals from sensingdevices placed in the vacuum chamber. The rf fre-

OMGINAL PAGE ISOf POOR OUALM

1. Kati, J. J. Cassidy, M. J. Mandell, G. W.Schnuelle, P. b. Steen, and J. C. Roche, "TheCapabilities of the NASA Charging AnalyzerProgram,* in "Spacecraft Charging Technology" -1978, NASA CP-2071 and ANL TR-79-0062, 1979,pp. 101-122.1. Katz, U. E. Parks, M. J. Mandell, J. N. Harvey,U. M. Brownell, Jr., S. S. Wang, and M. Rotenberg,"A Three-Dimensional Dynamic Study ofElectrostatic Charging in Materials," SystemsScience and Software, LaJolla, CA, rep.SSS-R-17-3361. 1971. (NASA CR-135256).1. Katz, J. J. Cassidy, III, M. J. Mandell, U. W.Schnuelle, P. a. Steen, U. E. Parks, M. Rotenberg,and J. H. Alexand_r, "Extension, Validation, andApplication of the NASCAP Code," Systems Scienceand Software, LaJolla, CA, rep. SSS-R-79-3904,1979. (NASA LR-159595).M. Mandell, 1. Katz, and U. E. Parks, "NASCAPSimulation of Laboratory Spacecraft Charging TestsUsing Mulitple Electron buns" late news paperpresented at session 1 of the 18th annual IEEEconference on Nuclear and Space Radiation effects.July 21-24, 1981, Seattle, WA.

quency of the discharges was 2r MHz while the vacuum 4.chamber resonant frequency wCa calculated to be about50 MHz.

All of the phenomena that have been observedduring the testing are not fully understood and theinvestigation is continuing.

5.Reference£

1. R. C. Keyser, R. E. Leadon, A. Weiman, and J. M.Wilkenfeld, "Electron Induced Uischarge Modeling,Testing, and Analysis for SCATHA." vol. 1, IRTCorporation, San Diego. CA, rep. no. IRT81b1-015, 6.1918.

2. J. Reddy. "Electron Irradiation Tests on theEuropean Meteorological Satellite," in "SpacecraftCharging Technology" - 19..80, NASA CP-2182, 1981,pp. 835-866.

3. U. 6. Hoge, METEOSAT Spacecraft Charging 1.Investigation" in "Spacecraft ChargingTechnol0gy0 - 1980. NASA CP-2182. 1981,pp. 814-834.

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