Implications of the measured BRDF for the ITER divertor impurity monitor

Abstract

This study employed a goniophotometer to investigate the three-dimensional bidirectional reflectance distribution function (BRDF) of tungsten, a material utilized in the ITER divertor. The measured BRDFs were incorporated into ray-tracing simulations to assess the impact of reflected light on the divertor impurity monitor (DIM), a spectroscopic measurement for ITER. The results demonstrate that surface conditions, which are expected to be modified by the plasma-wall interaction, exert a considerable influence on the characteristics of reflected light. In comparison to mirror-polished surfaces, the sputtered and recrystallized surfaces exhibited broader reflection patterns. In addition, the presence of a fuzzy surface resulted in a notable reduction in reflected light, including multiple reflections. Furthermore, this study investigated the potential for errors to arise from the use of optical reflection models with a small number of free parameters. It was found that discrepancies in the BRDF at shallow angles result in errors in the reflected light observed in the DIM. However, these discrepancies became relatively small in DIM measurements when the surface exhibited low specular reflection, such as sputtered or fuzzy surfaces. These results indicate that while some discrepancy exists between the predicted and actual BRDFs, the impact on DIM diagnostics may not be as significant as the error in the BRDF itself.