Plasma tests of platinum and rhodium optical coatings in 40.68MHz gas discharge in hydrogen and helium to assess first mirror performance in fusion diagnostics

First mirrors (FM) are essential optical components in fusion diagnostics, directly impacting the accuracy of plasma monitoring and machine control in ITER. These mirrors are exposed to high-flux, low-energy ion bombardment and periodic plasma cleaning, both of which contribute to surface degradation. This study evaluates the erosion and degradation of micrometer-thick platinum and rhodium coatings deposited on stainless steel substrates. The coatings were subjected to hydrogen and helium plasmas with ion fluxes of (0.5–7)·10¹⁹ ions·m⁻²·s⁻¹ and energies of up to 200 eV, reaching cumulative fluences of (0.4–2)·10²⁴ ions·m⁻². The resistance of these coatings to radiofrequency plasma as a potential in-situ maintenance approach was also investigated. Characterization techniques including X-ray photoelectron spectroscopy, scanning electron microscopy, optical interferometry, and reflectometry were employed to assess sputtering resistance, structural integrity, and optical reflectivity. Rhodium coatings exhibited somewhat better resistance to sputtering and blistering compared to amorphous platinum, maintaining higher performance over time. Given the high cost of rhodium, investigation focused on optimizing amorphous platinum coatings. Although erosion rates remain a limiting factor for both rhodium and platinum deployment, newly developed platinum coatings demonstrated promising improvements with increased thickness. The results highlight the critical role of material selection and surface engineering in enhancing the longevity and reliability of FM under fusion environments.