Irradiation Sensitivity of Rayleigh-backscattering Interrogated Optical Fibers for Real-Time Magnet Monitoring

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 REBa2Cu3O7x 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, voltagebased 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 Rayleighbackscattering 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 mmrange. Although the proposing team has shown that RIOF is a transformational method to solve the failure detection challenge in HTS magnets, for RIOF to be used in fusion applications, the radiation sensitivity of optical fibers needs to be studied. In response to this FOA, we propose to study the effects of ionizing radiation on a range of optical fibers that can be used as the sensing element in RIOF. Radiation fields that will be used include neutrons and gamma rays. After successfully developing experimental methods and data analysis protocols during phase I, the phase II focuses on carrying out the irradiation experiments, analyzing the results, and studying potential solutions to mitigate undesired effects. The effects of radiation will be determined via the measurement of intensity of Rayleigh backscattering, radiation induced optical attenuation, and direct sensor interrogation. These measurements will be carried out with each fiber sample at progressively higher neutron fluences and doses from gamma rays. The successful completion of the proposed work will advance RIOF quench detection technology closer to maturity for those applications that expose the magnets to ionizing radiation, while also expanding the knowledge on the use of optical fibers in radiation environments, like nuclear fission reactors.