• First study on EAST: Plasma exposure reduces first-wall BRDF by ∼two orders of magnitude at 656 nm. • Spherical deposits cause shadowing/masking; non-metallic components lower BRDF. • Erosion-deposition dynamically affects BRDF, with complex impacts on optical diagnostics. Optical diagnostics is an important method for obtaining basic parameters and operational conditions of the plasma in the tokamak. The bidirectional reflectance distribution function (BRDF) of the wall will have a degradation during the plasma experiment, which will then affect the signal-to-noise ratio of optical diagnostics. Studying the BRDF change of the metal first wall could help to understand the change of wall reflection level and its possible influence on optical diagnostics. Two molybdenum-titanium-zirconium (TZM) first wall tiles with different levels of impurity deposition after exposure in EAST for one experimental campaign operation were analyzed. The surface morphology and elemental composition were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray Photoelectron Spectroscopy (XPS). The BRDF value of the samples was also measured. The results show that after about 7426 plasma pulses, the peak BRDF value of the material decreased by about two orders of magnitude, and the BRDF curve exhibits asymmetry about the specular direction. The SEM observations showed that the microscopic spherical-like structures of the deposited layer are responsible for the shadowing and masking effects, thus accounting for the asymmetry observed in the BRDF curve. The EDS results indicated that the introduction of non-metallic components within the deposition layer will introduce the degradation of the BRDF value. These findings provide valuable references for analyzing wall reflectance behavior in EAST and other tokamaks.
Zhu et al. (Fri,) studied this question.