Fibre Bragg grating sensors for fusion diagnostics: Temperature monitoring of a tungsten mono-block mock-up under high heat flux

•A femtosecond fiber Bragg grating temperature sensor system was introduced to measure the temperature of a tungsten mono-block mockup under high heat flux loading.•The sensor system showed higher sensitivity to temperature variation and unique advantage in multi-point measurement when compared with thermocouples.•A finite element analysis was carried out and proved the measurement accuracy of the sensors.•A femtosecond fiber Bragg grating temperature sensor system will be integrated in the updated lower divertor of the Experimental Advanced Superconducting Tokamak and the scheme was detailed. The optimum functioning of the divertor is very important for the realisation of the long-pulse H-mode discharge of a tokamak device. This necessitates the development of a component monitoring system for its operation status including temperature and strain. This study introduces a promising technique for monitoring temperature and strain sensor systems based on the fibre Bragg grating (FBG). It was used to monitor the temperature of a W/Cu mono-block mock-up during a high heat flux (HHF) test for the first time, demonstrating its suitability as a temperature monitoring system for the tungsten divertor. Temperatures up to 1000 °C were measured under HHF using five femtosecond FBG sensors inscribed in an optic fibre encapsulated in a stainless-steel capillary tube. The FBG sensors exhibit higher sensitivity to temperature variation while maintaining the same measurement accuracy relative to the five thermocouples (TCs) in symmetrical locations. An ANSYS fluent simulation based on the finite element method yielded an output consistent with the experimental results. The temperature distribution obtained through one channel of optical fibre sensors demonstrates the advantage of FBG sensors as a multipoint measurement method. These sensors are preferred to TCs because the complex installation process and utilisation of a mass of wires are eliminated. In view of the success of this work, the installation of an FBG-based temperature monitoring system as a novel diagnostic system is planned on the upgraded lower divertor of the Experimental Advanced Superconducting Tokamak device.