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Infrared (IR) diagnostics are used to measure plasma-facing components (PFC) surface temperature in fusion devices. However, the interpretation of such images is complex in all-reflective environments because of unknown emissivity and multiple reflections issues. In order to assess these challenges an iterative inversion method based on a fast photonic model, the radiosity method, has been developed. The radiosity method is based on strong hypotheses including all diffuse surfaces. The inversion method allows retrieving the true surface temperature of PFC in two steps: a step of the target emissivity estimation in a baking scene and the use of the emissivity map to retrieve the temperature of metallic components with errors up to 3% during a plasma scenario.

This paper presents the measurement of the Bidirectional Reflectance Distribution Function (BRDF) of tungsten (W) samples and the resulting reflection models in the nuclear fusion device WEST (tokamak). For this, an experimental gonio-spectrophotometer was developed to fully characterize the material optical and thermal-radiative properties of metallic samples with different roughness. A ray-tracing photonic simulation was then used to predict the photon behavior in a fully metallic environment as a function of reflectance measurement. Low emissivity ( 0.1 at 4 µm) and highly specular reflectance (fitting with a Gaussian distribution around the specular direction with small width lower than 10 • ) are found for W samples. These measurements have been used as input for the photonic simulation and the resulting synthetic image reproduced well the reflection features on the upper divertor detected in WEST infrared experimental images.