Predicting Environmental Charging with a "Patch" Model Analysis of Electron Yield

Understanding electron yield (EY), the ratio of emitted electrons to incident electrons, is crucial for predicting charging effects of materials under charged particle bombardment. We describe the development and validation of new methods to analyze EY of complex, multi-component materials in terms of the EY of their constituent materials. This “patch” model has been applied to measurements of a set of highly angular, insulating granular lunar simulant samples and their constituent materials, which serves to illustrate application of the “patch” model. Measurements were performed in an ultrahigh vacuum chamber across a wide energy range (15 eV to 5 keV) using low-fluence, pulsed methods tailored to EY measurements of highly insulating materials. Agreement between estimates of the fractional contribution of the constituents to EY curves determined with the “patch” model and independently with energy dispersive X-ray (EDX) and bulk assay measurements are discussed. The “patch” model can be further enhanced by incorporating additional constituent EY data, exploring effects of particle size and preparation methods for granular materials, attention to surface coverage and cleanliness, and repeated experiments. Results presented here, when compared with very limited Apollo regolith EY data, show clear differences between simulants and actual lunar regolith. Charging of lunar dust and regolith poses significant challenges for future spacecraft explorations and operations. Additional studies proposed above and their analysis with the “patch” model, along with access to actual lunar regolith, will allow more accurate predictions of charging behavior of lunar dust and will better support engineers in their simulations of space environment effects and their efforts to mitigate lunar regolith charging effects.