Atomic-oxygen irradiation of metal-coated carbon-nanotube-based filters developed for the NASA’s MUSE solar mission

The Multi-slit Solar Explorer (MUSE) will use a multi-slit spectrograph and a context imager to observe the solar transition region and corona in the extreme ultraviolet. Metal-coated carbon-nanotube (CNT) pellicles have been developed as optical blocking filters for the two instruments. Because MUSE will operate in low Earth orbit, the filters must tolerate atomic oxygen (AtOx) preserving in-band throughput and out-of-band rejection. We report results of a conservative AtOx irradiation test campaign performed on four flight-model witness filters: 150-nm and 600-nm Al coatings and 200-nm and 600-nm Zr coatings on dual-layer single-wall CNT pellicles. SPENVIS/NRLMSISE-00 calculations gave mission-equivalent fluences of 5.1 × 1018 and 1.2 × 1019 atoms cm−2 for two and four years, respectively. The samples were exposed at 60° angle of incidence to a four-year-equivalent fluence, while approximately one quarter of each aperture was shielded. UV/Vis/NIR transmission measurements show no irradiation-induced degradation of the optical rejection, and 30–1800-eV X-ray synchrotron transmission measurements show a moderate change in the MUSE band, more pronounced in Al-coated samples. A best fit elemental analysis performed on the wide-band X-ray transmission measurements shows that exposed and protected regions have metal and metal-oxide thicknesses mutually compatible within the fit uncertainties and manufacturing-uniformity tolerances. The exposed regions consistently show 2–4 nm less aluminum oxide than the protected regions, although the differences are consistent within the errors. Both regions contain significantly more oxide than measured on pristine reference filters measured nearly five months earlier, pointing to an AtOx exposure effect.