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

The optimum functioning of the divertor is very important for the realisation of the long-pulse H-mode dischargeof a tokamak device. This necessitates the development of a component monitoring system for its operationstatus including temperature and strain. This study introduces a promising technique for monitoring temperatureand 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 atemperature monitoring system for the tungsten divertor. Temperatures up to 1000 °C were measured underHHF using five femtosecond FBG sensors inscribed in an optic fibre encapsulated in a stainless-steel capillarytube. The FBG sensors exhibit higher sensitivity to temperature variation while maintaining the same measurementaccuracy relative to the five thermocouples (TCs) in symmetrical locations. An ANSYS fluent simulationbased on the finite element method yielded an output consistent with the experimental results. Thetemperature distribution obtained through one channel of optical fibre sensors demonstrates the advantage ofFBG sensors as a multipoint measurement method. These sensors are preferred to TCs because the complexinstallation process and utilisation of a mass of wires are eliminated. In view of the success of this work, theinstallation of an FBG-based temperature monitoring system as a novel diagnostic system is planned on theupgraded lower divertor of the Experimental Advanced Superconducting Tokamak device.