Development of two-colour pyroreflectometry technique for temperature monitoring of tungsten plasma facing components

For reflective metals like tungsten used in certain plasma facing components of fusion devices, the black body or grey assumption is clearly impossible. For optical measurements of their temperature under heat fluxes coming from the plasma, knowledge of their thermo-optical properties is required. The two-colour pyroreflectometry technique presented in this paper is a potential solution to measure ‘in situ’ the temperature on tungsten above 600 °C. The main assumption of the technique is that the indicatrix of reflection does not vary within the narrow wavelength range used. With this technique, the diffusivity factor ηd is introduced to determine the convergence temperature T* that is assumed to be equal to the true temperature T. Aim of the study presented in this paper was to validate the two-colour pyroreflectometry technique for tungsten plasma components and to develop and test dedicated systems for plasma facing components application. The validation was performed in two steps: comparison of the temperature measured by the proposed technique with thermocouple, comparison of the diffusion factor ηd measurements by the proposed technique with direct measurements using multi-directional reflectometer. Finally, two specific optical probes allowing remote measurements (in the range 0.5–2 m) were developed and successfully tested at a high heat flux facility dedicated to the thermal fatigue of plasma facing components using electron beam scanning, this facility is considered thermally and geometrically relevant for fusion environment. The results obtained demonstrated the suitability of the two-colour pyroreflectometry technique to determine the true temperature on tungsten plasma facing components and to detect possible modifications of its surface properties through the measurements of the diffusivity factor. However, an ultimate validation in real fusion environment including parasitic signal from reflections from other locations or bremsstrahlung from plasma is still necessary before routinely using such a diagnostic in a fusion machine.