## 2 Degradation of Materials in Space--Stability Issues on Mirrors
### Charged Particles
The reflectance degradation in the full 93-250 nm range presented no spectral structure, which may be due to the lack of interference, contrary to what was observed for Al/MgF\({}_{2}\) and Al/LiF mirrors. A similar behavior was observed with He ions. Low-energy He ion irradiations on metallic thin films of Au and Cu demonstrate that, with fluences in the order of 10\({}^{15}\)-10\({}^{16}\) ion/cm\({}^{2}\), a faint dislocation band starts forming, with the preservation of the optical performance in the visible spectral range and a  
Figure 3: Effect of protons on mirror reflectivity. **(a)** Evolution of the UV-VIS reflectance of a SiO\({}_{2}\)-protected Al mirror irradiated with protons at 60 keV with different fluences (data retrieved from [110]). **(b)** Evolution of the UV-VIS reflectance of SiO\({}_{2}\)-protected Al mirror irradiated with protons at different energies and keeping a fluence of 1:10\({}^{17}\) p/cm\({}^{2}\) (data retrieved from [110]). fluence-proportional degradation in the ultraviolet range [115]. Fluences in the order of 10\({}^{17}\) ions/cm\({}^{2}\) were found to induce a large formation of bubbles inside the films and a deep transformation of the surface morphology [116; 117], with a consequent degradation of the visible and UV reflectance. The diameter and the density of such bubbles increase with the fluence due to the tendency of helium ions to migrate and form agglomerates. This behavior has been confirmed not only in metals, but also in semiconductors [118]. A particular case is instead that of multilayer (ML) stacks for the EUV. Several studies have demonstrated that protons and alpha particles with energy of a few keV can already lead to dramatic degradations of performance with fluences in the order of 10\({}^{16}\) ions/cm\({}^{2}\)[119; 120; 121]. For example, Mo/Si structures with different capping layers were irradiated by protons at 1 keV with fluences of 9\(\cdot\)10\({}^{15}\) and 36\(\cdot\)10\({}^{15}\) p/cm\({}^{2}\), showing a change in the peak position and a degradation of the reflectance (Figure 4a). Such an effect was attributed to the expansion and delamination occurring in the topmost layers of the ML stack (see the TEM image reported in Figure 4b) [122; 123; 124]. After the He\({}^{+}\) ion irradiation with fluences of 2.5\(\cdot\)10\({}^{15}\), 5\(\cdot\)10\({}^{15}\), and 10\({}^{16}\) ions/cm\({}^{2}\), the exposed MLs showed a drop of reflectance but no appreciable reflectance peak shifts [120]. In this case, the degradation was attributed to an increase in the intermixing at the interfaces in the topmost layers. Recently, Al/Mo/B\({}_{4}\)C and Al/Mo/SiC were also irradiated with 1 and 100 keV protons with doses of 7.4\(\cdot\)10\({}^{12}\) and 9\(\cdot\)10\({}^{15}\) p/cm\({}^{2}\). The lowest dose was chosen in order to simulate the situation expected inside the High Resolution Imager (HRI) and Full Sun Imager (FSI) telescopes on board of the ESA Solar Orbiter mission, where the mechanical structure and the front filters drastically reduce the proton flux impinging on the multilayers. None of the irradiated structures showed appreciable changes in performance, suggesting that at these values of fluence MLs can be considered stable.