Electron-beam Generated Plasma to Enhance Performance of Protected Aluminum Mirrors for Large Space Telescopes

This proposal addresses the need to develop and advance the state-of-the-art in optical coating technology that will be key to providing improvements that are needed to coat the large astronomical mirrors planned for the advanced NASA mission concepts such as the Large UV/Optical/IR (LUVOIR) Surveyor observatory. The proposed LUVOIR mission is a 8-16 m aperture telescope covering 90 - 2500 nm in spectral range, that will unveil fundamental information related to primary NASA goals, such as the history of galaxies, both the Milky Way and its neighbors, the origins of stars and planets, the demographics of planetary systems, and the search for life. Importantly, the short-wavelength coverage and performance of the LUVOIR mission will be greatly influenced by the performance of optical coatings. Accordingly, improving optical coating technology has been identified as an "Essential Goal" in the technology needs for the LUVOIR Surveyor. Obtaining broadband coatings with a performance of at least 50-70% reflectance below 120 nm is an immediate, high-priority technology investment area, fundamental to mission feasibility that needs further development. Pure, un-oxidized, aluminum presents high reflectance over the whole LUVOIR target spectral range, however it has to be protected from oxidation with a thin film of a transparent material. While no coatings offer transparency between 91-102 nm, above 102 nm there a few candidate transparent materials (LiF, MgF2, AlF3) that provide efficient coatings. Currently, Al protected with the fluorides LiF or MgF2 is a frequently used solution for far ultra-violet optical systems. Importantly, the reflectance of the fluoride protected Al is limited by the residual absorption of the fluoride layer. This proposal aims to enhance the performance of Al-based reflectors by employing a highly scalable, large area, plasma processing system developed at the Naval Research Laboratory to passivate the surface of aluminum coatings with a fluoride layer. The Large Area Plasma Processing System (LAPPS) at NRL is well suited for this task due to its inherently low ion energy (< 5 eV) which allows essentially damage free plasma processing, a critical attribute for maintaining high quality optical coatings. Previous work at the Naval Research Laboratory has already demonstrated the ability of this system to generate 1 m2, highly uniform, sheet-like, plasma ideally suited for the processing of large optical components. In addition, preliminary studies on the passivation of thin Al films have already demonstrated the feasibility of a plasma process, based on benign pre-cursors, for effectively fluorinating the surface of small Al mirror samples. Further advancement of this initial work will develop a scalable treatment of an aluminum surface on a scale as large as 1+ meter diameter. A successful completion of this task would lead to a Lyman Ultra- Violet (LUV) technology which would ultimately enable new scientific imaging capabilities.

The Strategic Astrophysics Technology program (SAT) supports focused development efforts for key technologies to the point at which they are ready to feed into major missions in the three science themes of the Astrophysics Division: Exoplanet Exploration, Cosmic Origins, and the Physics of the Cosmos. This program is specifically designed to address middle technology readiness level (TRL) "gaps" between levels 3 and 6: the maturation of technologies that have been established as feasible, but which are not yet sufficiently mature to incorporate into flight missions without introducing an unacceptable level of risk. NASA does not require a data management plan for proposals to SAT.

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