Optical fiber integration into Bi2Sr2CaCu2Ox/Ag/AgX and (RE)Ba2Cu3Ox superconducting coils

High-temperature superconductors (HTS) are a vital technology for future particle accelerators, motors, generators and other electric power systems, fusion reactors, and many other medical and defense applications requiring high magnetic fields. One remaining limiting factor limiting to the implementation of HTS systems is the lack of adequate sensors to monitor the temperature and strain states of the superconducting magnets (SCMs), and in particular for rapid and early quench detection, as the slow normal zone propagation velocity of HTS conductors results in a particularly difficult quench protection challenge. Without effective quench protection, HTS magnets are likely to fail catastrophically, so addressing this challenge is critical to technological success. Previously, it has been shown that the quench detection challenge may be addressed by integrating optical fibers interrogated by Rayleigh scattering into HTS SCMs, providing a novel, fast quench detection system which is particularly impactful on but not limited to HTS SCMs. Optical fibers can be integrated with conductor or cables providing a distributed measurement of temperature and strain with very high spatial and temporal resolutions. Many key questions remain, however, for Rayleigh-interrogated optical fiber (RIOF) quench detection to become an accepted sensor within SCMs. One of these questions was whether RIOF would provide sufficient sensitivity at very low temperature (4.2 K); this was successfully addressed in Phase I. In Phase I, we integrated optical fibers into HTS coils and demonstrated effective quench detection at 4.2
K. Small coils were wound with integrated optical fibers, voltage taps (VTs) and embedded heaters to generate thermal instabilities, cooled in liquid helium and tested at 4.2 K. Through these experiments RIOF performance at 4.2 K was shown quantitatively to be effective. In Phase II, we propose to address remaining scale-up and reliability issues, improve overall performance, and integrate fibers into significantly larger magnets. The aim is to demonstrate that RIOF can be implemented along every step of the way to operating a successful HTS magnet, including integration into the winding, monitoring of
cool-down, ramp-up, steady-state operation, and quench detection. Furthermore, we recognize that a key non-technical issue for implementation of RIOF in magnets is going to be customer confidence, which only comes with time and experience. Through the Phase II objectives, the SCM community will obtain more evidence that RIOF can be effective and reliable. Superconducting magnets based on high temperature superconductors are limited by the lack of an effective distributed sensor to prevent magnet failure. Optical fiber sensors have shown the ability to meet this need, but many challenges related to scale-up and reliability remain; these will be addressed, opening the door for superconducting applications.