IEEE Transactions on Nuclear Science, Vol. NS-29, No. 6, December 1982ENVIRONMENTALLY INDUCED DISCHARGES IN A SOLAR ARRAY*

D. B. SnyderDepartment of Physics

Case Western Reserve UniversityCleveland, Ohio 44106

Abstract

In the continuing effort to simulate dischargesseen during geomagnetic substorms, the charging anddischarging characteristics of an electrically iso-lated solar array segment are being studied. Asolar array segment is floated while bombarded withmonoenergetic electrons at various angles of incidence.The potentials of the array surface and of the inter-connects are monitored using Trek voltage probes, tomaintain electrical isolation. A back plate is capa-citively coupled to the array to provide informationon the transients accompanying the discharges.

Several modes of discharging of the array wereobserved at relatively low differential and absolutepotentials (a few kilovolts). A relatively slow dis-charge response in the array was observed dischargingover one second, with currents of nanoamps. Singlefaster discharges were also seen which lasted a fewtenths of a millisecond and with currents on the orderof microamps.

Introduction

Analytical predictions of solar array potentialsin geomagnetic substorm environments have indicatedthat solar cell cover slides are at a positive poten-tial with respect to the interconnects. 1,2 It is be-lieved that such voltage distributions can give riseto breakdowns, which could produce the spacecraftcharging anomalies observed in satellites.

In the past, the discharges studied on solararrays have been generated by irradiating the arrayswith electron beams. The interconnect circuits havebeen either grounded3, biased5, or floated on alarge resistor.6 Each of these techniques has yieldeduseful information, but these test results may havebeen influenced by the test arrangement by influencingthe amount of charge on the interconnects.

This work represents another step in attemptingto simulate environmentally induced discharges. Asmall solar array segment is electrically floated andirradiated by a monoenergetic electron beam. The angleof incidence of the beam can be adjusted to promotedifferential charging in the array. A plate on theback of the array mounting is used as a capacitivelycoupled probe, to monitor the charge leaving the arrayduring transients in the interconnect potential.

In this paper, the details of the test apparatusare described, the surface voltage profiles as a func-tion of the beam angle of incidence are discussed andthe discharge transient characteristics are presented.The results from the biased array are presented to pro-vide a comparison with the floating array results.

Experimental Apparatus

This work was conducted in one of the large vacuumchambers (2.1 m x 1.05 m diameter) at NASA Lewis Re-search Center. The chamber is an ion pumped system.During these tests the pressure was typically 1.5 x 10Pa. The electron gun used a hot filament to produceelectron densities of up to 15 nA/cm2 over an area of300 cm2, at energies of up to 10 KeV.

The solar array segment used for these experimentswas from the SPHINX satellite, and has been used insimilar testing before (Fig. 1).5 It is constructedfrom 24, 2 cm square solar cells connected in series toform a 6 x 4 matrix. The interconnects are a silvermesh, and the cover slides are 0.15 mm thick, fusedsilica. The gap between the cells for the intercon-nects is 0.5 to 1 mm wide. This assembly is attachedto a sheet of kapton, which, in turn, is attached to afiberglass printed circuit board. A 2.5 cm radius cop-per disk has been etched on the back of the board nearthe center of the array, and covered with kapton. Thisback plate provides a cpacitively coupled probe (60 pF)which is used to monitor the discharging of the array.

The array is mounted on a rotatable platform (Fig.2(a)) so that the angle of incidence of the electronbeam can be varied. The potential of the array can bemeasured by using a noncontacting Trek voltage probe.One probe is located above the array and is capable ofmoving along that column. In addition, a second probemonitors a connection to the array interconnects.Tests were run with this connection outside the vacuumsystem and behind the array (shielded from the beam).A power supply was used initially to evaluate behaviorwhere the interconnects were biased.

The electrical characteristics of the back plate/array capacitor were investigated by applying a squarepulse to the interconnects of the array. The backplate was connected to an oscilloscope with with a 1megaohm input impedance. The decay observed in Fig.2(b) is consistent with an RC discharge with a timeconstant of 0.7 msec. The voltage of the back platerises with the input pulse to within a tenth of amicrosecond. The loss in signal is due to additionalcapacitances between the cable and shield (700 pF).For fast discharges the currents can be calculatedfrom the peak voltage (charge) and the rise time. Forslow discharges the current is voltage/l M ohm.

The intention of this work, was to produce aninverted potential gradient (the interconnects morenegative than the surround glass) in the vicinity ofthe interconnect by increasing the secondary emissionyields. This would produce an intense electric fieldat the coverslide/interconnect boundary and may allowcharge to escape from the interconnects via a fieldemission mechanism. It was assumed that this could bedone by increasing the angle of incidence between thesample and the electron beam. Increasing the yieldshould make the equilibrium potential of the glass morepositive. Metals, typically, have lower yields thaninsulators and increasing the angle of incidenceshould have enhanced the difference between the metaland glass potentials.

Fig. 3 demonstrates the angular dependence of thesurface potentials on the angle of incidence of theelectron beam. At normal incidence, the cover slidesreach a potential of about -3 KeV. The interconnectis at a potential of -1 KeV, substantially more posi-tive than was expected. At normal incidence there isa substantial potential gradient in the gap, but it isin the wrong direction to trigger discharges.

Increasing the angle of incidence forces the coverslides more positive as expected. The interconnect

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potential does not approach the beam energy as anti-cipated. This may be due to either of two reasons.Either the beam may be deflected by the electricfields at the edges and not reach the interconnects,or the interconnects may have secondary yields differ-ent from silver. In recent work, Hoffman and co-workers7 have noted that the secondary yield for alumi-num on kapton tends to look more like aluminum oxidethan aluminum. The silver in this case, may have con-taminants, changing the secondary yields.

Another interesting feature is the negative peakat the edge of the cover slide. This is consistentwith an effect seen by Reeves and Balmain8 in a two-dimensional charging model. To check whether or notthis was a feature of the interconnect geometry, theprobe was moved to an adjacent column where the inter-connect geometry was reversed. The peak stays on theedge of the glass facing the beam, rather than follow-ing the interconnect geometry. This tends to increasethe electric field in the interconnect region. Thisedge effect provides an inverted gradient, but thereis insufficient charge stored there to account for theobserved discharges. The inverted gradient is clearlynot the sole criteria for the occurrence of discharges.

The attempt to create an inverted potential gradi-ent at the interconnect was unsuccessful. Based onthe assumed breakdown mechanism then, discharges shouldnot have occurred. However, discharges were observed.

Discharges

Several forms of discharges have been seen. Figure4 shows a type of relatively slow, repetitive dis-charge. During a discharge, the potential of theinterconnect would drop over a time scale of millisecto seconds. It would then rise, recharging to nearlythe peak potential over something on the order of 10sec. before decharging again. At low current densitiesdischarges were not seen. In the beam current range of2-5 nA/cm2, these repetitive discharges occurred. Athigher current densities, the array discharged as des-cribed above, but did not recharge, and maintained anequilibrium potential closer to ground. Once thisoccurs, the interconnect potential is noisy, as is thesignal from the back plate. This mode of dischargingappears to be related to the "zenering" (dropping to a

less negative potential) observed by Inouye and Sel-len.5

The changes in potentialof the charge is lost in theto half of the charge may bezenering.

indicate that about 10%repetitive mode, while uplost at the initiation of

Due to difficulties in reproducing these dischar-ges reliably, the conditions for their existence havenot been established. So far, the following have beenobserved:

1) There is a dependence on beam current density.

2) These discharges have not been seen at beamenergies of 3 KeV or less, breakdowns are more

frequent at higher beam energies.

3) The incident beam angle also appears to influ-ence this discharge mode, since these dischar-ges have not been seen at a normal angle ofincidence.

A second, faster discharge has also been seen on thefloating array (Fig. 5). These discharges are lessfrequent and since the interconnects maintain a con-

stant potential between discharges are essentiallysingle events. During these single discharges, the

interconnect potential drops 100 to 2000 volts (bothminor and major events occur). From a minor transientobserved on the back plate, the discharge lasts a fewtenths of a millisecond. The current from the arrayis on the order of a microamp. A calculation of thechange in voltage, from the peak of the back plate sig-nal indicates that the voltage change is greater than80 V, consistent with the change in interconnect poten-tial actually seen. Such a minor discharge accountsfor 4% of the total charge on the array. A major dis-charge can result in a 90% loss of charge. The currentduring these discharges was too high for the instru-ments to measure. The discharges described above arenot visible except the major discharges produce a dimflash of light over all of the solar cells. The in-tensity is comparable to the glow when the cover slidesare bombarded with an intense electron beam. Thisindicates that the entire array is involved in thedischarge. However, the conditions which initiate thedischarge, may be an as yet, unknown local effect.

Biased Array

A solar Array can be biased to induce discharges,presumably through the inverted gradiant mechanism. Inorder to compare the above results with the dischargefrom a biased array, discharges were observed on anarray with the interconnects biased to -2 KV (Fig. 6).An electron beam energy of 2 KeV incident on the arrayat an angle of 450, pushed the cover slides -800 V.The power supply keeps the potential of the intercon-nects constant, until it becomes overloaded. The backplate sees a current from the array of 20 mA. Theinstrumentation did not have the range to see the topof the curve. The decay of the transient is the RCtime constant for the back plate since the power supplytakes several milliseconds to recover from the overload.

After the discharge the cover slide potential isnearly equal to the interconnect potential, so at leastsome of the charge is being redeposited on the surfaceof the cover slides.

Though this experiment does supply information onthe conditions necessary for discharge, the character-istics of the array breakdown are swamped out by theadditional charge supplied by the power supply.

Conclusions

This experiment allows the study of dischargesfrom an electrically floating array. Discharges can bestimulated by irradiating the array with a monoener-getic electron beam at various angles of incidence.

Various modes of discharge were seen. A relativelyslow, repetitive discharge is seen at low electron den-sities. These discharges release about 10% of thecharge on the array. Single faster discharges are alsoseen which release currents on the order of microamps,for a few tenths of a millisecond. Minor dischargesemit about 4% of the charge, while major dischargesemit about 90% of the charge stored on the array.

The slow and fast minor discharges appear to besmaller than the discharges induced by biasing theinterconnects negative with respect to the cover slides.The power supply provides additional charge whichallows much more intense discharges.

The potential gradient at the interconnect is not

the sole criteria for discharges to occur. In thefloating array the interconnect potentials are slightlypositive with respect to the cover slides. However,there is a region at the edge of the cover slide whichis more negative than the center of the cover slide.

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Further study is still needed to determine moreprecisely the threshold conditions for these dis-charges. The angle of incidence effects, currentdensity, and electron beam energy effects need to bedetermined. The difficulty in reproducing dischargeconditions indicate that the history of the array maybe important, and that contamination of the surfacesmay influence the conditions for initiating these dis-charges.

References

1. Sanders, N. L. and Inouye, G. T., "NASCAP Charg-ing Calculations for a Synchronous Orbit Satel-lite," Spacecraft Charging Technology - 1980,NASA CP-2182, 1981, pp. 684-708.

2. Stevens, N. J., "Analytical Modeling of Satellitein Geosynchronous Environment," Spacecraft Charg-ing Technology - 1980, NASA CP-2182, 1981, pp.717-729.

3. Stevens, N. J., Berkopec, F. D., Staskus, J. V.,Blech, R. A., and Narisco, S. J. "Testing ofTypical Spacecraft Materials in a Simulated Sub-Storm Environment," Proc. Spacecraft ChargingTechnology Conference - 1977, AFGL-TR-770051,1977, pp. 431-457.

4. Bogus, K. P., "Investigation of a CTS Solar CellTest Patch Under Simulated Geomagnetic SubstormCharging Conditions", Proc. Spacecraft ChargingTechnology Conference - 1977, AFGL-TR-770051,1977, pp. 487-501.

5. Stevens, N. J., Mills, H. E., and Orange, L.,"Voltage Gradients in Solar Array Cavities asPossible Breakdown Sites in Spacecraft-Charging-Induced Discharges," NASA TM 82710, 1981.

6. Inouye, G. T. and Sellen, J. M., "TDRSS SolarArray Arc Discharge Tests," Spacecraft ChargingTechnology - 1978, NASA CP-2071, 1979, pp. 834-852.

7. Gordon, W. L., Yang, K. Y., and Hoffman, R. W.,private communication.

8. Reeves, R. D. and Balmain, K. G., "Two-DimensionalElectron Beam Charging Model for Polymer Films",IEEE Transactions on Nuclear Science, NS-28, 1981,pp. 4547-4552.

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