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SOLAR ARRAY EXPERIMENTS ON THE

SPHINX SATELLITE

by N. John StevensLewis Research CenterCleveland, Ohio

TECHNICAL PAPER proposed for presentation at TenthPhotovoltaic Specialists Conference sponsored by theInstitute of Electrical and Electronics EngineersPalo Alto, California, November 13-15, 1973

https://ntrs.nasa.gov/search.jsp?R=19740001965 2020-06-04T05:18:03+00:00Z

SOLAR ARRAY EXPERIMENTS OH THE SPHIHX SATELLITE

N. John Stevens, National Aeronautics and Space Administration

Lewis Research CenterCleveland, Ohio 44135

Abstract

The Space Hasina, High Voltage Interaction Ex-periment (SPHINX) is the name given to an auxiliarypayload satellite scheduled to be launched in January1974. The principal experiments carried on this sat- •ellite are specifically designed to obtain the engi-neering data on the interaction of high voltage sys-tems with the space plasma. The classes of experi-ments are solar.array segments, insulators, insula-tors with pin holes and conductors. The satellite isalso carrying experiments to obtain flight data onthree new solar array configurations; the edgellluminated-multijunction cells, the teflon encasedcells and the violet cells.

Introduction

Each subsequent generation of satellites tendstowards higher power missions. Current electric pro-pulsion missions under study will require solar ar-rays capable of generating up to 15 kilowatts of pow-er. As the power levals increase, the satellite over-all size and weight will also increase. It becomescritical to improve the efficiency of the satellitesystems to keep the weight, size and power require-ments within practical bounds.

For electrical systems, higher efficiencies areusually associated with higher operating voltages.Studies have been conducted on the high voltage solararray (HVSA) with integral power conditioning thatcan operate at voltages up to 16 kilovolts.1-5 Thesestudies conclude that such systems are feasible.

These high voltage arrays, however, will requireexposure of high voltage surfaces to the space plasma.Possible interactions between the surfaces and theplasma must be considered in the system design. Theinteractions that must be evaluated are the currentdrains between the high voltage system and the plasma,the charge stored on insulator surfaces, and plasmainitiated discharges. These interactions must beknown as a function of voltage and time in space.Unfortunately, engineering data to design systems thatoperate over 100 volts in the space environment is notavailable. As a result of the need for better engi-neering data, a flight experiment to obtain informa-tion on these interactions will be flown. The satel-lite designed to carry this experimental package hasbeen named SPHINX, an acroynm standing for Space Plas-ma, High Voltage Interaction Experiment. The experi-ments will include interaction studies of conductors,insulators, and small solar array segments which canbe biased, independently or collectively, in Binallsteps to positive or negative 16 kilovolts. TheSPHINX satellite is also carrying experiments to ob-tain flight data on three new types of solar arrayconfigurations; the multifunction, edge illuminatedarray (MINX), the teflon covered array. (FEPSA), andthe violet cell array.

Discussion Of The Plasma Interaction

In the standard solar array construction thecover glass does not completely cover the metallicinterconnects between cells (see fig. 1). These cellinterconnects are at various voltages depending on '. .

Aerospace Engineer, Systems Engineering BranchPrincipal Investigator, SPHINX Project.

their location on the array, and when the array is ex-posed to the space plasma, the interconnects will actas a biased plasma probe attracting or repellingcharged particles. At some location on the array thegenerated voltage will be equal to the space plasmapotential (see fig. Z). The cell interconnects thatare at voltages above the space potential will attractelectrons proportional to the voltage difference.These interconnects that are at voltages below thespace plasma potential will repel electrons and at-tract ions proportional to the voltage difforence.This particle flow is a plasma current loop which isin parallel with the electrical load. This current iscalled the plasma coupling or plasma drain current.Since this coupling current is in parallel with theload current, it will act as a loss, reducing the pow-er available from the array. For solar arrays thatgenerate less than 100 volts, this plasma current issmall and the effect can be neglected. As the arrayvoltage is increased, the plasma coupling current rep-resents a progressively larger loss and -the effectmust be evaluated.

Experimental studies have been conducted inground facilities to study the interaction of solararray segments biased to high voltages with a plasmaenvironment. A solar array segment of 1058 cm2 area(476 two cm by two cm cells) was tested in plasma den-sities of 104 to 26 electrons per cubic centimeter.6In these tests the segment was not illuminated and thebias voltages were applied from power supplies outsideof the vacuum chamber. The results for an array biasedto positive voltages are shown in Fig. 3. In general,these results tend to support the prior analyticalstudies7)8 that for altitudes approaching synchronousthe effect is snia.11. However, these results point outthat higher voltage arrays may have to be used only athigher altitudes to avoid serious plasma coupling cur-rent losses.

The results for an array biased to negative volt-6;are shown in Fig. 4. In this case the results

are considerably different. The current collectionrises rapidly and terminates in an arc which shuts offthe laboratory power supplies. The voltage at whichthe array arced to the. plasma is a function of theplasma density and varied from a few hundred volts at adensity corresponding to about 15,000 km to severalthousand volts at synchronous altitude plasmaidensi-ties.

A similar study has been conducted at the LewisResearch Center. The array segment was 100 cm2 area(24 two cm by two cm cells in series) and the biasvoltages were provided by an isolated power supplywithin a fiberglass enclosure mounted inside the vacu-um chamber (see fig. 5). The results of this studyessentially agree with the results obtained with thelarger segment (see fig. 6). The electron currentscollected with the segment biased positively increasedwith voltage for the tests conducted.

The results obtained for negatively biased volt-ages were the same as those obtained with the largersegment. The array segment arced to the plasma at var-ious voltages depending upon the plasma density. Thepower supply was designed to shut down, restart andramp back to the voltage setting when an arc occurred.This arragement allowed observations of several con-secutive arcs during a test. These observations of thearcing revealed that glowing spots appeared randomly atthe cell interconnects; there was no one specific areathat initiated the discharges. This segment was tested

several times, each test giving the same results.Aside from some discoloration of the metal, there wasno apparent damage to the segment (see fig. 7).

This plasma coupling current with the cell inter-connects is only one aspect of the high voltage inter-action phenomenon. It has been found that insulatorimperfections such as small pin holes can result indisproportionally large electron current collection.6,8,9,10 Furthermore, in the vacuum tank tests, thesecollected currents have been found to be proportionalto the total area of the insulator (see fig. 8). Thiscurrent collection effect will be evaluated in spaceby the SPHIHX to determine if it must be considered inthe design of high voltage systems.

It is recognized that there are serious limita-tions in the ground test facilities. The uncertainsimulation of the plasma environment, the distortionof the electric fields in the plasma by metallic vacu-um tankj walls, and the high background pressure re-quired to operate the plasma sources probably influ-ence the test results. Hence, it is not possible tostate conclusively that the test results obtained todate represent those that will be found in space. Aspace flight experiment is required to verify the plas-ma interaction and to obtain the engineering dataneeded to design high voltage systems for space appli-cations. Such an experiment will be conducted usingthe SPHINX satellite.

Discussion Of The SPHINX Experiments

SPHIHX is a self contained satellite that will beplaced into a highly elliptical orbit, 880 km by 34,300 km, in January 1974. Most of the experiment sur-face will always face towards the sun. With this or-bit the experiments can be conducted in plasma densi-ties that will vary from 1 to 1000 electrons per cubiccentimeter over the nominal mission life of one year.

The experimental system on this satellite is de-signed to study the space plasma interactions over afull range of voltages. Measurements can be made todetermine the interaction for voltages up to .±1000volts. This information can be used in the design ofintermediate voltage solar arrays and systems. Thesatellite experimental system is also capable of bias-ing the test surface to ±16 kV to obtain the engineer-ing information required to design high voltage sys-tems.

The experimental surfaces of the satellite areshown in Fig. 9. The solar array segment experimentsare mounted on tabs at three of the corners of theexperiment enclosure. The MDIX array is located atthe fourth corner. The insulator experiments are lo-cated on the experiment enclosure deck, outboard ofthe solar array segments and on booms. The FEPSA andViolet Cell experiments are mounted adjacent to themain housekeeping solar array on small, tabs.

Solar Array Segment Experiments

The solar array segment experiments will be usedto obtain engineering data relating to the performanceof a high voltage solar array in a space plasma. Eachof the 3 segments consists of 24 cells in series. Thecells are 10 mil thick, 10 ohm centimeter cells of H-on-P polarity. The covers are 0.012 inch fused silicaglass. The nominal output of the array segment isabout 1 watt at the expected operating temperature of65° C.

The potential of two of the segments can be variedover the low and high voltage ranges. The plasma cou-pling currents will be measured by an electrometerfloating in the high voltage circuit. This electro-meter is capable of reading currents between 10"10 and10"̂ amps. An 8 point voltage-current characteristicof the array segment can be measured at any bias volt-

age.One of the two array segments has been constructed

in the conventional manner with the cell interconnectsexposed to allow coupling with the space plasma. Thisarray will be used to determine if the interactionsfound in the ground facility tests are real or are vac-uum facility phenomena. This array will be operatedover long periods of time to determine if the plasmainteractions change with time.

The second of the array segments that will bebiased has the interconnects insulated by a layer ofETV. If the segment performance is similar to thatexperienced in the ground facility testing (see figs.3,4, & 6) then the insulation of the interconnectswill prevent, or at least reduce, .the plasma inter-actions. The operation of this array over long periodsof time in space will tell how well the insulation per-forms in the space enviroment.

The third segment is identical to the first andwill be used as a control surface to separate environ-mental degradation effects from the high voltage ef-fects.

Insi.il ator Experiments

The insulators being tested are 5 mil polyimidefilm (kapton) and 5 mil FEP teflon. The bulk resis-tance of these materials will be determined for bothsunlight and shade conditions. Ten experiments arededicated to the study of the current collectionthrough pin holes in insulators. The insulator exper-iment construction is shown in Fig. 10. The insula-tion is about 10 cm in dia and covers the 5 cm diaelectrode that is connected to the power supply. Theexperiment holder is made out of epoxy-fiberglass lam-inate. The insulation is bonded to both the fiberglassand electrode with a silicone adhesive (Dow Corning A-4000). The pin holes are located in the center of theinsulator. The pin hole diameter are 0.041 cm and0.38 cm. These are the common pin hole diameters forground test specimens and therefore it will be possi-ble to correlate the data obtained from the SPHIHXsatellite with existing data.

MINX •

The Miniature Interaction Experiment (MINX) is asolar array experiment that uses mult i- junction, edgeilluminated photovoltaic devices to generate highvoltages. ̂ In this experimental array, nine segmentsare used. Shorting switches are used to vary the arrayoutput voltages from about 135 to 1125 volts. Theoverall size of the array is about 380 cm2. Hence, theexperiment will be a small area simulation of a highvoltage array. The data obtained from this flightwill be used to predict the performance of a full scalehigh voltage array of this construction.

FEFSA

The purpose of the Lewis Eesearch Center designedFEP teflon encapsulated solar array segment experiment13, 13 is ^0 verify its performance in a space envi-ronment. This type of array which incorporates lowcost, light weight construction consists of a laminatedpackage made up .of a kapton substrate FEP teflon filmsolar cell array and an FEP teflon film cover. Thepackage is bonded together in vacuum under heat andpressure to form an encapsulated array.

The flight experiment consists of three FEP cov-ered arrays and one glass covered array as a control.A 16 point voltage current curve will be obtained.This experiment will not be biased to high voltages.

Violet Cell Array

The violet cell array experiment is similar insize to one of the FEPSA experiments. The array uti-lizes the improved silicon solar cell developed byCOMSAT14 and will determine the effect of the spaceenvironment on this type of cell. Glass covers areused. This experiment is not connected to the highvoltage circuits. •

Concluding Remarks

The NASA is considering a number of missions thatwill require high power solar arrays. Such missions .could achieve significant benefits if the solar arrayvoltages could be increased to 300 volts or tiigher.

This exposure of a high voltage surface to thespace environment can result in an interaction betweenthat surface and the space plasma. These interactionsmust be taken into consideration in designing a highpower space system. Unfortunately, no prior -flightengineering data is available on the interactions be-tween insulated surfaces at various potentials and thespace plasma. The available experimental data obtainedin ground facilities indicates that the interaction canbe severe, but it is not certain whether or not theseresults have been influenced by the facility. A com-parison with data obtained in the space environment isrequired to establish the validity of the ground testdata.

A flight experiment will be conducted using sur-faces designed to give the engineering data needed.These experiments will be operated in plasma densitiesthat range from 1 to 1000 electrons per cubic centi-meter for a period of about 1 year. The interactionsstudied will be between the plasma and conductors,insulators, insulators with pin holes and solar arraysegments for a range of bias voltages from 0 to ±16 kV.

References

1. P. Wiener| and H. Rasmussen, "Concept for a HighVoltage Solar Array with Integral Power Condition-ing" Proceedings of the Fifth Intersociety EnergyConversion Engineering Conference, Vol. 1, AmericanNuclear Society, 1972. i

2. T. Ebersole, j. H. Hayden, R. Rasmussen, and P.Wiener, "Study of High Voltage Solar Array Config-

. rations with Integrated Power Control Electronics"General Electric Co. Rep. 705D4256, MSA CR-72725,June 1970.

3. B. G. Herron, D. E. Creed, R. W. Opjorden, and G.T. Todd, "High Voltage Solar Array ConfigurationStudy", Hughes Aircraft Co., NASA CR-72724,July 1970.

4. W. F. Springgate, "High Voltage Solar Array Elec-trical Configuration Study", Boeing Co. Rep. D180-10037-1, NASA CR-72723, Apr. 1970.

5. J. E. Triner, "A Digital Regulated Solar ArrayPower Module", HASA TM X-2314, July 1971.

6. K. Kennerud, "High Voltage Solar Array Experi-ments, " Boeing Co., NASA CR-121280, July 1973.

7. W. Knauer, J. R. Bayless, G. T. Todd, and J. W.Ward, "High Voltage Solar Array Study" HughesResearch Labs., NASA CR-72675 May 1970.

8. W. F. Springgate, and H. Oman, "High Voltage SolarArray Study", Boeing Co. Rep. D2-121734-1, HASACR-72674, 1969.

9. . R. K. Cole, H. S. Ogawa, and J. M. Sellen.JJr.,"Operation of Solar Cell Arrays in Dilute StreamingPlasmas," AIAA Paper 69-262, Mar. 1969.

ID. N. T. Grier and D. J. McKlnzie, "Current Deainageto a High Voltage Probe in a Dilute Plasma," AIAAPaper 72-105, Jan. 1972.

11. T. J. Riley, J. E. Triner, B. L. Sater, D. Cohen,and H. Somberg, "A Miniature High Voltage PlasmaInteraction Fligh Experiment, Project MIHX" Paperto be given at IEEE Photovoltiaic SpecialistConference, Nov. 13-15, 1973.

12. S. A. Greenbery, M. McCargo, and W. L. Palmer,"Investigation of FPF'Teflon as a Cover for SiliconSolar Cells," Lockheed Missile and Space Co. Rep.LMSC-D 243070, NASA CE-72970, Aug. 1971.

13. A. Forrestierri, S. A. Greenbery, M.'McCargo, andW. L. Palmer, "FEP Covers for Silicon Solar Cells,"NASA TM X-67824, Aug. 1971.

14. J. Lindmayer, and J. Allison, "An Improved SiliconSolar Cell - The Violet Cell," 9th TTreE photovol-taic Specialists Conference Record, IEEE, 1972,pp. 83-84.

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