Platinum as a first mirror material for fusion applications: a comparison with rhodium

Abstract The metallic first mirrors (FMs) of ITER optical diagnostics will face erosion caused by particles from the fusion plasma, as well as redeposition of materials originating from the first wall. Both processes can degrade their reflectivity over time. In vacuo plasma cleaning is planned to restore their optical properties. Several mirror materials were tested for the FM application. Currently, mirrors based on nanocrystalline rhodium (Rh) coatings or single crystal molybdenum (Mo) are considered as the mainstream approach. However, previous research indicates that repetitive cleaning of nanocrystalline mirrors can cause surface patterning, which negatively impacts their optical performance. Additionally, FMs are vulnerable to in-vessel coolant leaks, which could severely degrade their optical properties. Literature shows that exposing Rh mirrors to steam results in the formation of a thin layer of Rh oxide and the development of voids in the top 1 µ m of the coating. Despite this, plasma cleaning has been shown to restore the pristine reflectivity of the mirrors. This paper discusses the potential of utilizing platinum (Pt) as an FM material. Pt and Rh were subjected to cyclic plasma cleaning and steam ingress tests, after which their morphology and optical properties were measured and analyzed using XPS, SEM, FIB and spectrophotometry. Additionally, the neutron-induced transmutation of Pt and Rh was assessed with respect to ITER and DEMO irradiation scenarios.