An ITER Challenge Absolute surface temperature measurements of low and varying emissivity tungsten plasma facing components

International audience; One of the challenges that ITER will face during plasma operation is the absolute divertor surface temperature determination of actively water cooled (70 °C, 3 MPa) tungsten Plasma Facing Units (PFU) to ensure their integrity. The expected steady state heat flux up to 10 MW/m² is close to the operational limit and so one of the goals of the thermographic system is to make reliable measurements (wavelength band: 3.5µm to 4.5µm) with relatively low error bars to avoid tungsten melting and material damages. The tungsten emissivity is low and dependent on wavelength, temperature, surface state (roughness, cracks, oxidation, and erosion/deposition processes) which can evolve during time along plasma operation. An error on the absolute emissivity can lead to large absolute temperature errors, and consequently to either a reduction of the operational window to fulfil safety limits, or an increase risk regarding the integrity of the components. For the past years, a number of emissivity measurements have been performed with ITER like PFU mock-ups made of different grade of tungsten, with different techniques, from different material makers, and from different manufacturers. The overall emissivity discrepancy is large, indicating that tungsten emissivity is a major issue for temperature measurement and wall protection. The accuracy on the temperature measurement is investigated for both monocolor and bicolor IR thermography techniques. We present laboratory tests performed with IR camera equipped with a rotating filter wheel for bicolor temperature measurement. It shows that the bicolor technique can reach high precision T/T < 10% at high temperature assuming the emissivity ratio is constant regarding the two selected wavelengths.