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/mm(2) 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.