R. E. Davies

Effects of Evolving Surface Contamination on Spacecraft Charging

The effects of evolving surface contamination on spacecraft charging have been investigated through (i) ground-based measurements of the change in electron emission properties of a conducting surface undergoing contamination and (ii) modeling of the charging of such surfaces using the NASCAP code. Specifically, we studied a Au surface as adsorbed species were removed and a very thin disordered carbon film was deposited as a result of exposure to an intense, normal incidence electron beam. As a result of this contamination, we found an ~50% decrease in secondary electron yield and an ~20% reduction in backscattered yield. The type and rates of contamination observed are similar to those encountered by operational spacecraft. Charging potentials of an isolated panel of the material were determined under both sunlit and eclipse conditions in geosynchronous orbits for typical and extreme environments. In all environments studied, just monolayers of contamination lead to predictions of an abrupt threshold effect for spacecraft charging; panels that charged to small positive values when uncontaminated developed kilovolt negative potentials. The relative effect of NASCAP parameters for modeling secondary and backscattered electron emission and plasma electron distributions were also investigated. We conclude that surface contamination must be considered to avoid the serious detrimental effects associated with severe spacecraft charging.

Inception of Snapover and Gas Induced Glow Discharges

AbstractGround based experiments of the snapoverphenomenon were conducted in the large verticalsimulation chamber at the Glenn Research Center(GRC) Plasma Interaction Facility (PIF). TwoPenning sources provided both argon and xenonplasmas for the experiments. The sources were usedto simulate a variety of ionospheric densitiespertaining to a spacecraft in a Low Earth Orbital(LEO) environment 1–4 . Secondary electron emissionis believed responsible for dielectric surfacecharging, and all subsequent snapover phenomenaobserved 2,5 . Voltage sweeps of conductor potentialsversus collected current were recorded in order toexamine the specific charging history of each sample.The average time constant for sample charging wasestimated between 25 and 50 seconds for all samples.It appears that current drops off by approximately afactor of 3 over the charging time of the sample. Allsamples charged in the forward and reverse biasdirections, demonstrated hysteresis. Current jumpswere only observed in the forward or positive sweptvoltage direction. There is large dispersion in thecritical snapover potential when repeating sweeps onany one sample. The current ratio for the firstsnapover region jumps between 2 and 4.6 times, witha standard deviation less than 1.6. Two of thesamples showed even larger current ratios. It isbelieved the second large snapover region is due tosample outgassing. Under certain preset conditions,namely at the higher neutral gas backgroundpressures, a perceptible blue-green glow wasobserved around the conductor. The glow is believedto be a result of secondary electrons undergoingcollisions with an expelled tenuous cloud of gas, thatis outgassed from the sample. Spectroscopicmeasurements of the glow discharge were made in anattempt to identify specific lines contributing to theobserved glow.I. IntroductionSnapover describes a sudden and rather dramaticchange in the current collection regime in and aroundpositively biased conductors that are surrounded by adielectric

Evolution of secondary electron emission characteristics of spacecraft surfaces

Secondary electron emission (SEE) plays a key role in spacecraft charging [Garrett, 1981; Frooninckx and Sojka, 1992] . As a result, spacecraft charging codes require knowledge of the SEE characteristics of various materials in order to predict vehicle potentials in various orbital environments [Katz, et. al., 1986]. Because SEE is a surface phenomenon, occurring in the first few atomic layers of a material, the SEE characteristics of a given surface are extremely sensitive to changes in surface condition—e.g., the addition or removal of surface contaminants, or changes in surface morphology. That spacecraft surfaces can and generally do undergo significant evolution during their operational lifetimes is a fact well established by NASAs Long Duration Exposure Facility (LDEF) [Crutcher, et al., 1991a]. Deposition and removal of contaminants can occur as a result of preferential adsorption of gases on cooler surfaces, the collection of ionized gases on negatively charged surfaces, atomicoxygen-induced oxidation, photodissociation under vacuum uv bombardment, and ion-induced desorption. Since SEE is materialdependent phenomenon, it is reasonable to assume that as a spacecrafts surfaces evolve, so too do its SEE characteristics. In order to determine whether or not charging models need incorporate the effects of changing surface conditions aboard operating spacecraft, data assessing the impact of these changes on the SEE characteristics of various surfaces are required. Measurements have therefore been made investigating the dynamic evolution of secondary electron (SE) yields resulting from energetic electron bombardment of typical spacecraft materials in a rarefied atmosphere representative of the microenvironment surrounding space vehicles. A detailed report of the experiment and results has been given elsewhere [Davies, 1996; Davies and Dennison, 1997]; what follows here is a brief summary.