Surface preparation of SS316L(N)-IG second mirrors for ITER optical diagnostics

The challenges of developing large-scale SS316L(N)-IG mirrors for the collection system of the ITER divertor Thomson scattering (DTS) diagnostic are under consideration. These are second mirrors (M2), located out of direct plasma sight, require an optically smooth surface to ensure efficient light collection. Two approaches to surface finishing-abrasive and magnetorheological polishing-were investigated. On small-scale samples (22 mm and 6 $\times$ 6 mm), the target roughness ($R_q/\lambda<$ 0.012) was achieved, while scaling to larger substrates revealed degradation of surface quality, indicating the need for further process optimization. The results confirm the applicability of the developed polishing techniques and provide a basis for a scalable preparation route for ITER optical components.