Development of the In Situ Calibration Method for ITER Divertor IR Thermography

An important issue for ITER divertor infrared (IR) thermography (IRTh) is that changes in the emissivity of tungsten divertor targets resulting from depositions; erosions; and dependences on temperature, wavelength, and surface roughness affect the temperature measurement, which requires an accuracy of 10%. Therefore, we investigated the emissivity dependences of tungsten samples in ITER-grade tungsten and validated the proposed in situ calibration method for emissivity evaluation by using an IR laser in laboratory experiments. The emissivity of the tungsten samples had a strong dependence on surface roughness of 1.0 to 5.9 μm. In the two-color method, by measuring the radiances of the tungsten sample in two wavelengths of 3.35 and 4.67 μm, the change of the ratio of the emissivities did not satisfy the measurement requirement. Thus, an in situ calibration method of emissivity is needed. The emissivity evaluated using the in situ calibration method was in good agreement with the emissivity evaluated from the radiance for tungsten samples at temperatures of 22°C, 100°C, and 400°C. Consequently, the in situ calibration method for emissivity evaluation using an IR laser was successfully validated. More work is needed for the application in IRTh.