Monitoring of a high temperature superconducting magnet by means of distributed optical fiber sensing

Distributed optical fiber sensor is a unique technology that offers unprecedented advantages and performance especially in those experimental fields where the environmental harshness limits the applicability of standard sensors. By measuring the faint light backscattered by the fiber in response to a well-tailored probing signal, distributed sensors allow mapping the variation of physical parameters along the fiber path with high spatial resolution. In this work we report on the application of this technology to the monitoring of a complete mockup prototype of high-temperature superconducting magnet, developed for the future High Luminosity Large Hadron Collider at CERN. Four optical fibers have been embedded in different areas of the magnet and have been measured by optical frequency-domain reflectometry. The magnet was first monitored during the cooling phase from room temperature down to 4.5 K; results show that, despite the huge temperature variation, the structure reacts uniformly, without suffering from localized thermal stress, confirming the design targets. In a second phase, the magnet was monitored while it was powered with electric currents up to 2.5 kA, at the operational temperature of 50 K. In this case results show non-negligible localized strain accumulations due to the Lorentz forces, which are marginally higher than what was expected by design. The experiment confirms the unique advantages that distributed optical fiber sensors offer to both the design and operation control of structures as critical and complex as superconducting magnets. • A full-scale superconducting quadrupole magnet has been realized with eight optical fibers embedded for operational and structural monitoring. • Distributed sensing has been performed during the cooldown and the powering of the magnet in operative conditions at cryogenic temperatures. • Full details of the embedding process and the difficulties encountered are reported. • Successful accurate monitoring of the magnet is reported, confirming the viability of embedded distributed optical fiber sensors for the monitoring of superconducting magnet.