Abstract-Electron-induced electron yields of high-resistivity, high-yield materials -ceramic polycrystalline aluminum oxide and the 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 energy-dependent total yield curves and yield decay curves were observed, even for incident electron fluences of <3 fC/mm 2 . 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 anomalies measured in highly insulating, high-yield materials, and to provide a method for determining the limiting yield spectra of uncharged dielectrics. Relevance of these results to spacecraft charging is also discussed.
Electron emission and concomitant charge accumulation near the surface of insulators is central to understanding spacecraft charging. We present a study of changes in electron emission yields as a result of internal charge build up due to electron dose. Evolution of total, backscattered and secondary yield results over a broad range of incident energies are presented for two representative insulators, Kapton TM and Al 2 O 3. Reliable yield curves for un-charged insulators are measured and quantifiable changes in yields are observed due to <100 fC/mm 2 fluences. We find excellent agreement with a phenomenological argument based on insulator charging predicted by the yield curve; 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. We also find that the exponential decay of yield curves with fluence exhibit an energy dependant decay constant, α(E). Finally, we discuss physics based models for this energy dependence. To understand 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.
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 ( 4 keV to 20 keV) pulsed-electron fluence to determine how quickly insulators charge, and how this can affect subsequent eleclron 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.
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