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 that are under construction for ITER. 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, 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 considered include neutrons and gamma rays. A detailed study of the effects of radiation will include both the radiation induced optical attenuation and the more subtle impact that radiation can have on the RIOF technique via the potential creation of new scattering centers. This study 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. The Phase I plan of work includes down-selection of optical fibers to be studied, development of experimental procedures and irradiation plans based on the target doses, and characterization of un- irradiated fibers. In Phase II, we plan to conduct extensive irradiation experiments based on the results of phase I, using the Breazeale Nuclear Reactor within the Pennsylvania State University, as well as analyze the data to draw conclusions on the radiation tolerance of different commercial-off-the-shelf fibers.