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Abstract-Fibre Bragg Grating (FBG) sensors are expected to provide valuable data in extreme radiation environments associated with nuclear research reactors. However, when the fast neutron fluence reaches 10 18 to 10 19 n/cm², the radiation induced changes in the material density and refractive index may drastically bias the measurements. The present study evaluates the radiation effect on the FBG performances by comparing their properties before and after their exposure to fast neutron fluences exceeding 10 19 n/cm² (E > 1 MeV). We studied responses of FBGs manufactured by three different laboratories in the same single-mode optical fibre but using different inscription conditions. The Bragg wavelength and the reflectivity were measured before and after irradiation thanks to a dedicated mounting. For nearly all FBGs, the Bragg peak remains visible after the irradiation while the radiation-induced Bragg wavelength shifts (RI-BWS) vary from a few pm (equivalent temperature error < 1°C) to nearly 1 nm (~100°C error) depending of the FBG inscription conditions. Such high RI-BWSs can be explained by the huge refractive-index variation and compaction observed for bare fibre samples through other experimental techniques. Our results show that by using specific hardening techniques the FBG-based temperature measurements in a nuclear research reactor experiment may become feasible.

Fiber Bragg gratings (FBGs) are point optical fiber sensors that allow the monitoring of a diversity of environmental parameters, e.g., temperature or strain. Several research groups have studied radiation effects on the grating response, as they are implemented in harsh environments: high energy physics, space, and nuclear facilities. We report here the advances made to date in studies regarding the vulnerability and hardening of this sensor under radiation. First, we introduce its principle of operation. Second, the different grating inscription techniques are briefly illustrated as well as the differences among the various types. Then, we focus on the radiation effects induced on different FBGs. Radiation induces a shift in their Bragg wavelengths, which is a property serving to measure environmental parameters. This radiation-induced Bragg wavelength shift (RI-BWS) leads to a measurement error, whose amplitude and kinetics depend on many parameters: inscription conditions, fiber type, pre- or post-treatments, and irradiation conditions (nature, dose, dose rate, and temperature). Indeed, the radiation hardness of an FBG is not directly related to that of the fiber where it has been photo-inscribed by a laser. We review the influence of all these parameters and discuss how it is possible to manufacture FBGs with limited RI-BWS, opening the way to their implementation in radiation-rich environments.

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The results on the first successful application of Laser Induced Breakdown Spectroscopy (LIBS) for remote in situ diagnostics of plasma facing components (a deposited layer on a divertor tile) in Joint European Torus (JET) are presented. The studies were performed with an available JET EDGE LIDAR laser system. For in-depth analysis of deposited layers on JET divertor tiles, a number of laser shots were applied onto the same divertor place without laser beam displacement. The spectral lines of D, CII and impurity elements (CrI, BeII, …) were identified in a wide spectral range (400-670 nm). With the increase in a number of laser shots applied onto the same divertor place, we observed consecutive changes in spectral line intensities of deuterium, carbon, and impurities with the appearance of spectral lines of tungsten substrate (WI). In-depth analysis of deposited layers on JET divertor tiles was made on the basis of the spectral line behaviour in reference to the applied laser shots. The possibility of surface cartography with laser beam displacement on the tile surface was demonstrated as well. Based on the results obtained, we may conclude that LIBS method is applicable for in situ remote analysis of deposited layers of JET plasma facing components.

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In the final objective of elaborating an optical