Cryogenic Fiber Optic Sensors for Superconducting Magnets and Power Transmission Lines in High Energy Physics Applications

In the framework of the Luminosity upgrade of the Large Hadron Collider (HL - LHC), a remarkable R&D effort is now ongoing at the European Organization for Nuclear Research (CERN) in order to develop a new generation of accelerator magnets and superconducting power transmission lines. The magnet technology will be based on Nb$_{3}$Sn enabling to operate in the 11 - 13 T range. In parallel, in order to preserve the power converters from the increasing radiation level, high power transmission lines are foreseen to feed the magnets from free - radiation zones. These will be based on high temperature superconductors cooled down with helium gas in the range 5 - 30 K. The new technologies will require advanced design and fabrication approaches as well as adapted instrumentation for monitoring both the R&D phase and operation. Resistive sensors have been used so far for voltage, temperature and strain monitoring but their integration still suffers from the number of electrical wires and the complex compensation of magnetic and thermal effects. These issues might be overcome by developing a new technology based on fiber optic sensors for their well-known advantages like the small size, the intrinsic electrical insulation, immunity to electromagnetic interferences and multiplexing capability although the environmental complexity makes the technology not well assessed yet in the field of superconductivity. This thesis presents the progress done in the material selection and temperature characterization (in the range 300 - 4.2 K) of coated FBG sensors. Results of their implementation in the 20-m-long power transmission line for the helium gas temperature monitoring are also reported. FBG sensors in bonded and embedded configuration have been also integrated in sub-scale Nb$_{3}$Sn dipole magnets for monitoring the main stages of the magnet service life. Experimental results are presented during magnet assembly and thermal cycle down to 1.9 K, when applied compressive forces reach up to 200 MPa, during energization up to 20 kA and quench monitoring under high magnetic fields (up to 13 T).