Investigation of the influence of surface composition on the charge state distribution of ∼keV hydrogen exiting thin carbon foils for space plasma instrumentation

Abstract

Energetic neutral atom (ENA) imaging techniques have become a powerful tool for remotely probing plasma environments in space. ENA imagers cover energies from 0.01keV up to a few MeV, and they use different techniques to cover such a broad energy range. Most of them convert the ENA into a charged particle to remove the converted ENA from the initial neutral direction. In the >∼0.2keV/nuc to 10’s of keV/nuc range, the conversion subsystem is usually an ultra-thin carbon foil. The sensitivity of ENA imagers based on charge conversion by carbon foils is driven by the ability of these foils to convert a neutral atom into an ion. The charge state distribution after the carbon foils is a strong function of the chemical and physical properties of the exit surface.In this study, we analyze the composition and structure of the surface using X-ray photoelectron spectroscopy. The surface is roughly 88% carbon and 12% oxygen, forming strong CO bonds. Annealing the foil lowers the oxygen content to about 9%. We coat the surface of the foils with Au, Al2O3, or MgO. We compare the exit charge state distributions of hydrogen prior to and post coatings. While no significant difference is observed in the exit charge state for the Au and Al2O3 coatings, there is a slight decrease of the positive fraction after MgO. The annealing of the foil has the benefit of reducing the angular scattering of hydrogen by a factor of ∼1.2. This is a significant improvement that has the potential to increase sensitivity of ENA imagers.