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The luminosity upgrade of the Large Hadron Collider (HL -LHC) planned at the European Organizat ion for Nuclear Research (CERN) requires the development of a new generation of superconducting magnets based on Nb 3 Sn technology. The instrumentation required fo r the racetrack coils needs the development of reliable sensing systems able to monitor the magnet thermo -mechanical behavior during its service life, from the coil fabrication to the magnet operation. With this purpose , Fiber Bragg Grating (FBG) sensors have been embedded in the coils of the Short Model Coil (SMC) magnet fabricated at CERN . The FBG sensitivity to both temperature and strain required the development of a solution able to separate mechanical and temperature effects. This work presents for th e first time a feasibility study devoted to the implementation of an embedded FBG sensor for the measurement of the "true " temperature in the impregnated Nb 3 Sn coil during t he fabrication process. Keywords: Fiber Optic sensors, Fiber Bragg gratings, superc onducting magnets , Nb

Distributed and multi-point fiber-optic based measurements of cryogenic temperature down to 30 K are presented. Measurements have been performed along the cryostat of a superconducting power transmission line, which is currently being tested at CERN over a length of about 20 m. Multi-point measurements were based on two kinds of FBG with different coatings (epoxy and PMMA). In addition, distributed measurements exploited optical frequency-domain reflectometry to analyze the Rayleigh scattering along two concatenated fibers with different coatings (acrylate and polyimmide). Results confirm the viability of these approaches to monitor cryogenic temperatures along a superconducting transmission line.

The design, fabrication and tests of a new generation of superconducting magnets for the upgrade of the LHC require the support of an adequate, robust and reliable sensing technology. The use of Fiber Optic Sensors is becoming particularly challenging for applications in extreme harsh environments such as ultra-low temperatures, high electromagnetic fields and strong mechanical stresses offering perspectives for the development of technological innovations in several applied disciplines.