Speaker
Description
Visible optical diagnostics in tokamaks can be significantly affected by stray light, which is mainly generated by multiple reflections of plasma radiation from plasma-facing components. Because plasma-wall interactions continuously modify wall morphology and optical properties, models based on unexposed materials may fail during long-term operation. This study investigates the evolution of visible stray-light effects in EAST by combining optical characterization of molybdenum-titanium-zirconium first-wall tiles with LightTools ray tracing. The samples included one unexposed reference tile and two tiles exposed in the 2024 EAST campaign, comprising 7426 discharges with a total effective discharge duration of 94,064.7 s. The bidirectional reflectance distribution function (BRDF) provides an angular description of wall scattering and is useful for stray-light suppression analysis. To quantify directional scattering, BRDF was measured at 656 nm, while spectral reflectance was measured from 400 to 800 nm; surface morphology and composition were characterized by SEM, EDS, and XPS. After exposure, impurity deposition layers mainly containing C, O, and Li formed on the surfaces, with average thicknesses of about 42 and 54 μm. The peak BRDF decreased by up to nearly two orders of magnitude, and the angular distribution became asymmetric, indicating redistribution from specular reflection to broader diffuse scattering. Ray-tracing results show that these wall changes alter both the intensity and spatial distribution of stray light entering diagnostic lines of sight. Thus, the evolution of stray light cannot be described by simply scaling the wall reflectance, but is governed by the wall condition, scattering properties, and diagnostic geometry. Future work will combine BRDF data after plasma exposure with diagnostic measurements from individual discharges to develop correction models for specific diagnostics during long pulse operation.