C. D. Thomson

Evolution of the Electron Yield Curves of Insulators as a Function of Impinging Electron Fluence and Energy

Electron emission and concomitant charge accumulation near the surface of insulators is central to understanding spacecraft charging. A study of changes in electron emission yields as a result of internal charge buildup due to electron dose is presented. Evolution of total, backscattered, and secondary yield results over a broad range of incident energies are presented for two representative insulators, Kapton and Al2O3. Reliable yield curves for uncharged insulators are measured, and quantifiable changes in yields are observed due to <100-fC/mm2 fluences. Excellent agreement with a phenomenological argument based on insulator charging predicted by the yield curve is found; this includes a decrease in the rate of change of the yield as incident energies approach the crossover energies and as accumulated internal charge reduces the landing energy to asymptotically approach a steady state surface charge and unity yield. It is also found that the exponential decay of yield curves with fluence exhibit an energy-dependent decay constant alpha(E). Finally, physics-based models for this energy dependence are discussed. Understanding fluence and energy dependence of these charging processes requires knowledge of how charge is deposited within the insulator, the mechanisms for charge trapping and transport within the insulator, and how the profile of trapped charge affects the transport and emission of charges from insulators

Low-Fluence Electron Yields of Highly Insulating Materials

Electron-induced electron yields of high-resistivity high-yield materials - ceramic polycrystalline aluminum oxide and polymer polyimide (Kapton HN) - were made by using a low-fluence pulsed incident electron beam and charge neutralization electron source to minimize charge accumulation. Large changes in the energy-dependent total yield curves and yield decay curves were observed, even for incident electron fluences of < 3 fC/mm2. The evolution of the electron yield as charge accumulates in the material is modeled in terms of electron recapture based on an extended Chung-Everhart model of the electron emission spectrum. This model is used to explain the anomalies measured in highly insulating high-yield materials and to provide a method for determining the limiting yield spectra of uncharged dielectrics. The relevance of these results to spacecraft charging is also discussed.

The effect of low energy electron and UV/VIS radiation aging on the electron emission properties and breakdown of thin-film dielectrics

Studies of secondary and backscattered electron yield curves of thin-film dielectrics have recently been made using pulsed, low current electron beam methods to minimize insulator charging. These capabilities have allowed us to investigate the evolution of surface and internal charge profiles as a function of low energy electron (<1 keV to 20 keV) pulsed-electron fluence to determine how quickly insulators charge, and how this can affect subsequent electron emission properties. We have also studied critical incident electron energies that result in electrical breakdown of insulator materials and the effect of breakdown on subsequent emission, charging and conduction. The qualitative physics of such processes in solid dielectrics has long been known; this work begins to place such studies on a quantitative basis.

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