The buildup of static charge on satellite surfaces is an important issue in the utilization of satellite systems. The analysis of this phenomenon has required important advances in basic charging theory and the development of complex codes to evaluate the plasma sheaths that surround satellites. The results of these theories and calculations have wide application in space physics in the design of systems and in the interpretation of low-energy plasma measurements. In this review, those aspects of charge buildup on satellite surfaces relevant to the space physics community are summarized. The types of charging processes, models of charge buildup, satellite sheath theories, and charging observations are described with emphasis on basic concepts.

The charging of spacecraft surfaces

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

It has been known for many years that the grounding resistance of a concentrated electrode drops when it is subjected to a high current charge. This helps reduce the ground potential rise. The degree of the resistance depends on the magnitude of the ionization gradient of the soil E/sub o/. Based on both a theoretical analysis and a critical review of the large number of available measurements, this paper recommends that E/sub o/ be taken equal to 300 kV/m for typical soils. This is significantly less than the 1000 kV/m value used by some authors. Graphs are also given describing the behaviour of the rod electrodes which are used in many field installations. >

The soil ionization gradient associated with discharge of high currents into concentrated electrodes

When the ANIK El and £2 communications spacecraft suffered serious failures of their momentum wheel control systems, it was postulated that the satellites were subjected to bulk (internal dielectric) charging followed by discharge that disabled key circuitry. This paper effectively confirms the hypothesis by linking the events to a well-established pattern of operational anomalies on another spacecraft. Since March 1991, a commercial geosynchronous satellite has experienced over 50 specific, but relatively trivial, mode switches. These are analyzed in relation to energetic electron fluences measured at GOES-7 and METEOSAT-3; without exception they coincide with periods of relatively high flux. Combining the measurements, the critical directed energy fluence is calculated to be 1 x 10 MeV cm". This figure is consistent with the theoretical breakdown threshold and CRRES data characterizing the discharge process; it fits with an effective shielding thickness of less than 0.2 mm of aluminum. Three switches in the days preceding the ANIK failures strongly argue for a common explanation, although it may be impossible to even identify actual discharge sites. Modern communications spacecraft employ tried and tested techniques for electrostatic discharge protection, but it is clear that the recognized hazard of internal dielectric charging has often been underestimated and shielding guidelines overlooked.

Conclusive evidence for internal dielectric charging anomalies on geosynchronous communications spacecraft

Twenty years after the landmark SCATHA program, spacecraft charging and its associated effects continue to be major issues for Earth-orbiting spacecraft. Since the time of SCATHA, spacecraft charging investigations were focused primarily on surface effects and spacecraft external surface design issues. Today, however, a significant proportion of spacecraft anomalies are believed to be caused by internal charging effects (charging and ESD events internal to the spacecraft Faraday cage envelope). This review will, following a brief summary of the state of the art in surface charging, concentrate on the problems introduced by penetrating electrons ("internal charging") and related processes (buried charge and deep dielectric charging). With the advent of tethered spacecraft and the deployment of the International Space Station, low altitude charging has taken on a new significance as well. These and issues tied to the dense, low altitude plasma environment and the auroral zone will also be briefly reviewed.

Spacecraft charging, an update

To assess the effect of dielectric charging on the performance of operational spacecraft, consideration must be given to extending laboratory simulations and measurements to operational conditions. Some of the key issues that must be addressed are the scaling of discharge characteristics from laboratory simulation exposure fluxes to the expected fluxes in space and the scaling from laboratory-sized objects to satellite-sized objects. In addition, consideration must be given to the effects of grounding laboratory test objects through relatively low impedances as opposed to high impedance, which is more representative of the actual space conditions. Some results have been reported previously (Refs. 1-4) on area and pulse width scaling, but little systematic investigation has been reported of the effects on the discharge characteristics of other simulation parameters. This paper presents results of a recent experimental investigation, part of which addresses these issues. Twelveand 36-inch-diameter 2-mil Kapton samples, a 24-inch-square Kapton thermal blanket, a 36-inch 4-mil fiberglass sample, and a second-surface mirror array were exposed to 25-keV electrons. Measurements were made of the discharge current and time history, blowoff current and time history, and the surface potential before and after discharge. RE-02092 EDGE SIGNAL

Effect of Laboratory Simulation Parameters on Spacecraft Dielectric Discharges

The pulsed electrical breakdown characteristics of soil between a hemispherical and a planar electrode have been measured in meter-size geometries for three soil types with water contents from 1 to 7% by volume. The threshold electric field for breakdown is a function of the soil type and the water content, varying from about 1 to 2 MV/m locally near the hemispherical electrode. The delay time (tD) between application of the voltage and the occurrence of the breakdown is primarily a function of the applied voltage, which varies approximately as tD-3/8 for these samples. After breakdown, the impedance of the soil decreases with time until it reaches a minimum impedance of 0.2 to 5 ?/m for these meter-size samples, depending on the current that the pulser can drive through the arc. The discharge arc prefers to travel underground rather than on the surface of the soil unless there is some low-impedance object on the soil surface.

Electrical Breakdown Characteristics of Soil

Experimental investigations of spacecraft charging effects produced by energetic electrons capable of penetrating the spacecraft exterior have been performed. Electron exposures of printed circuit boards have revealed relatively large discharges resulting in correspondingly large transients induced onto the PC board strips to which electronic components are directly attached. Voltages in excess of 50 volts have been measured across 50-ohm impedances. Coupled signals had rise times of 10 nsec or less and pulse widths of from 10 nsec to hundreds of nanoseconds. Spacecraft charging and discharging within spacecraft electronics appear to be a new and important aspect of the spacecraft charging problem.

High-Energy Electron-Induced Discharges in Printed Circuit Boards

The electrical breakdown characteristics of small-scale soil samples subjected to voltage pulses have been measured. The threshold breakdown field for these samples with a water content of 4.5 percent by volume is about 2.7 to 3.0 MV/m. The delay time from application of the voltage to breakdown varied from about 30 ns at the highest available voltages to milliseconds close to the threshold field. The postbreakdown I-V characteristics of the samples are strong functions of the amount of current that can be supplied by the pulser/circuit combination. The breakdown process in the test soils appears to be due to air ionization in the voids between the soil particles. A semiempirical mathematical model for the postbreakdown I-V characteristics, based on a competition between air ionization and a recombination (quenching) process, gives good correlation with the experimental results.

Electrical Breakdown Properties of Soil

The electrical breakdown characteristics of soil were measured with the test sample in air and then with the air in the soil voids replaced with SF6. All of the measured breakdown properties that are associated with arc initiation, such as threshold field and energy to breakdown, were consistently different for air and SF6. The ratios of these quantities are comparable to the ratio of the breakdown electric-field strengths of free air and SF6 at one atmosphere. This agreement between the breakdown strengths of soil and free gases is strong evidence that the initiation of arc breakdowns in soil involves ionization of the air in the voids between the soil particles.

T. M. Flanagan

Papers

Effect of Laboratory Simulation Parameters on Spacecraft Dielectric Discharges

Electrical Breakdown Characteristics of Soil

High-Energy Electron-Induced Discharges in Printed Circuit Boards

Electrical Breakdown Properties of Soil

Effect of Ambient Gas on ARC Initiation Characteristics in Soil