High Temperature Superconductors (HTS) have the potential to enable compact nuclear fusion reactors by allowing the generation of the required magnetic field with magnets that are smaller than those based on low temperature superconductors. The advantage of smaller reactors is in their faster and cheaper development. Although (RE)Ba2Cu3O7-x (REBCO) is the best performing and most mature HTS, it still suffers from two main technical challenges: protection from quench and conductor cost. Because of a much smaller normal zone propagation velocity, a normal zone in an HTS magnet generates a much larger peak electric field and thus a much higher peak temperature than in an LTS magnet. As a result, voltage-based systems are insufficient and put the magnets at risk. Additionally, voltage based detection can be compromised by electromagnetic (EM) noise. Which is an especially big problem in AC-operated fusion magnets and makes voltage based approaches completely ineffective. The current state of the art for quench detection in HTS systems is either no detection systems (unacceptable for large magnets) or voltage taps, which have been shown to be ineffective. The proposing team has been working on Rayleigh-backscattering Interrogated Optical Fibers (RIOF) for several years, and has shown numerous advantages of RIOF compared to voltage taps. Some of the advantages include immunity to electromagnetic noise, higher sensitivity to thermal and mechanical perturbation, smaller response time and higher spatial resolution (mm-range). Although the proposing team has shown that RIOF is a transformational method to solve the failure detection challenge in HTS magnets, fusion applications would greatly benefit from a RIOF technology that is able to distinguish strain from temperature variations. In response to this FOA, we partner with LUNA Innovations, manufacturer of the optical interrogator, to test the viability of a method to decouple strain from temperature variations at cryogenic temperatures. Additionally, the sensing characteristic and thermal sensitivity will be estimated at cryogenic temperatures with and without simultaneous presence of strain onto the optical fiber, thereby quantifying the effectiveness of the decoupling methods. Lastly, the additional computations needed to discriminate temperature and strain will be evaluated in terms of their impact on measurement speed and other sensing parameters.
