The need for uniform criteria, or guidelines, to be used in all phases of spacecraft design is discussed. Guidelines were developed for the control of absolute and differential charging of spacecraft surfaces by the lower energy space charged particle environment. Interior charging due to higher energy particles is not considered. A guide to good design practices for assessing and controlling charging effects is presented. Uniform design practices for all space vehicles are outlined.

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Design guidelines for assessing and controlling spacecraft charging effects

A two-dimensional model is developed to describe the charging of strips of thin polymer films above a grounded substrate exposed to a uniform mono-energetic electron beam. The study is motivated by the observed anomalous behaviour of geosynchronous satellites, which has been attributed to differential charging of the satellite surfaces exposed to magnetospheric electrons. Surface and bulk electric fields are calculated at steady state in order to identify regions of high electrical stress, with emphasis on behaviour near the material's edge. The model is used to study the effects of some of the experimental parameters, notably beam energy, beam angle of incidence, beam current density, material thickness and material width. Also examined are the consequences of a central gap in the material and a discontinuity in the material thickness.

Two-Dimensional Electron Beam Charging Model for Polymer Films

A possible explanation for environmentally-induced discharges on geosynchronous satellites exists in the electric fields formed in the cavities between solar cells - the small gaps formed by the cover slides, solar cells, metallic interconnects and insulating substrate. When exposed to a substorm environment, the cover slides become less negatively charged than the spacecraft ground. Hence, it is possible for metallic surfaces (usually silver mesh) to be at a negative potential in a cavity that has a "positive" surface above it. If the resultant electric field becomes large enough, then the interconnect could emit electrons (probably by field emission) which could be accelerated to space by the positive voltage on the covers. An experimental study was conducted using a small solar array segment in which the interconnect potential was controlled by a power supply while the cover slides were irradiated by monoenergetic electrons. It was found that discharges could be triggered when the interconnect potential became at least 500 volts negative with respect to the cover slides. Analytical modeling of satellites exposed to substorm environments indicates that such gradients are possible. Therefore, it appears that this trigger mechanism for discharges is possible. Details of the experiment and modeling study are presented.

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Voltage Gradients in Solar Array Cavities as Possible Beeakdown Sites in Spacecraft-Charging-Induced Discharges

The primary purpose of this paper is to suggest that low differential voltage, small energy discharges associated with internal buried charge may be a dominant mechanism by which stored electrostatic energy is released from dielectrics on board orbiting spacecraft. The evidence from space as given by Stevens is noted. Laboratory experimental evidence of Frederickson is invoked to demonstrate that discharges occur under circumstances with no external potential drop. Previous calculations are then reviewed which indicate that significant internal electric fields can exist in dielectrics charged with multiple-kilovolt electron beams under conditions involving little or no external potential drop. The internal discharge mechanism of Meulenberg is recalled, and new calculations suggesting that the space environment is conducive to the formation of the conditions required by this mechanism are presented. Experimental procedures for checking the suggestions of the paper are developed.

Internal Breakdown of Charged Spacecraft Dielectrics

In the continuing effort to simulate discharges seen during geomagnetic substorms, the charging and discharging characteristics of an electrically isolated solar array segment are being studied. A solar array segment is floated while bombarded with monoenergetic electrons at various angles of incidence. The potentials of the array surface and of the interconnects are monitored using Trek voltage probes, to maintain electrical isolation. A back plate is capacitively coupled to the array to provide information on the transients accompanying the discharges. Several modes of discharging of the array were observed at relatively low differential and absolute potentials (a few kilovolts). A relatively slow discharge response in the array was observed discharging over one second, with currents of nanoamps. Single faster discharges were also seen which lasted a few tenths of a millisecond and with currents on the order of microamps.

Environmentally Induced Discharges in a Solar Array

An investigation to determine spacecraft materials characteristics under charging conditions was conducted at the Lewis Research Center substorm simulation facility. The test specimens were spacecraft paints, silvered Teflon, thermal blankets and solar array segments. The samples, ranging in size from 300 to 1,000 sq cm were exposed to monoenergetic electron energies from 2 to 20 keV at a current density of 1 nA sq cm. The samples generally behaved as capacitors with strong voltage gradients at their edges. The charging characteristics of the silvered Teflon, Kapton, and solar cell covers were controlled by the secondary emission characteristics. Insulators that did not discharge were the spacecraft paints and the quartz fiber cloth thermal blanket sample. All other samples did experience discharges when the surface voltage reached -8 to -16 kV. The discharges were photographed. The breakdown voltage for each sample was determined and the average energy lost in the discharge was computed.

Testing of typical spacecraft materials in a simulated substorm environment

The test specimens were spacecraft paints, silvered Teflon, thermal blankets, and solar array segments. The samples, ranging in size from 300 to 1000 sq cm were exposed to monoenergetic electron energies from 2 to 20 keV at a current density of 1 NA/sq cm. The samples generally behaved as capacitors with strong voltage gradient at their edges. The charging characteristics of the silvered Teflon, Kapton, and solar cell covers were controlled by the secondary emission characteristics. Insulators that did not discharge were the spacecraft paints and the quartz fiber cloth thermal blanket sample. All other samples did experience discharges when the surface voltage reached -8 to -16kV. The discharges were photographed. The breakdown voltage for each sample was determined and the average energy lost in the discharge was computed.

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S. J. Narciso

Papers

Testing of typical spacecraft materials in a simulated substorm environment

Testing of typical spacecraft materials in a simulated substorm environment