Superconducting cables based on high temperature superconductors (HTS) are necessary for applications requiring large currents and low inductance, such as compact fusion reactors. In this paper, we report the proof-of-concept of a SMART Conductor on Round Core (CORC ^® ) wire realized via integration of optical fibers into the copper core. A SMART CORC ^® wire with integrated optical fibers was manufactured and its capabilities have been experimentally demonstrated. Results show that by interrogating the optical fibers via Rayleigh backscattering, a Spectral Shift signal as a function of time and position along the cable can be used to detect and locate hot-spots that are developed within the wire or its terminations. It has been found that highly localized current injection into the terminations could initiate hot-spots within the cable at locations where current redistribution between tapes occur. This effect is virtually eliminated when adequate current connections are used that inject current evenly along the cable terminations. Normal zone propagation velocities have been calculated as a function of time using Spectral Shift data for a heater-induced quench as well as a quench induced by overcurrent. In both cases the normal zone propagation velocity was about 6 cm s ⁻¹ , but in the heater-induced experiment it was preceded by 500 ms of slower propagation at 2.5 cm s ⁻¹ .

(RE)Ba₂Cu₃O_7-x (REBCO) conductors have overcome technical challenges related to manufacturing quality, length homogeneity, scale-up of piece-length, and joints. There is one remaining technical challenge, however, which is common to all high temperature superconductors and lies in effective detection of failure to prevent material degradation. An innovative technique based on optical fibers interrogated by Rayleigh backscattering has been shown to have advantages over voltage taps at detecting incipient faults. Prior work has experimentally demonstrated the technique in several implementation scenarios, including direct integration of optical fibers into superconducting conductors and cables to create a class of ‘SMART’ conductors and cables that are able to monitor their own health. In this paper, the magnet monitoring technique based on Rayleigh backscattering interrogated optical fibers has been experimentally studied in a model coil subject to external magnetic field, where different fiber integration methods are used to increase selectivity of the fiber sensor to temperature. Results show that the spectral shift displays different features during strain and thermal transients. The implications of the results in terms of potential and limitations of each sensor as well as strain-temperature decoupling are discussed.

Presented in this article are experimental results of an investigation on the performance of distributed fiber optic temperature sensors at temperatures up to $800~^{\circ }$ C. The experimental results produced in this work assess the performance of fiber optic temperature sensors for use in instrumenting liquid metal heat pipes. Distributed fiber optic temperature sensors are capable of providing high spatial and temporal resolution temperature measurements across a wide range of operating temperatures and conditions, making them intriguing candidates for many advanced nuclear reactor technologies. Tests were conducted at high temperature on the prolonged survivability, short-term performance, and high-temperature cycling effects of distributed optical fiber temperature sensors. A quartic fit of the spectral shift produced by the fiber sensors was developed to fit with thermocouple (TC) measurements of the experiment and was compared with fits available in literature. An upper limit of $700~^{\circ }$ C was established for the prolonged use of distributed fiber optic sensors. No significant hysteresis effects were observed when the fiber sensors were cycled at high temperatures. Distributed fiber optic temperature sensors were determined to be viable for instrumenting liquid metal heat pipes under limited operational conditions.

High-temperature superconductors (HTS) are prone to the formation of localized hot spots, which can lead to irreversible material degradation and system-wide failure. The challenge of detecting these hot spots is compounded in HTS coils operated under alternating current (AC) due to the significant baseline voltage overshadowing quench signals. Rayleigh-backscattering Interrogated Optical Fibers (RIOF) offer a promising solution by providing distributed sensing with immunity to electromagnetic noise and high spatial resolution for detecting temperature and strain variations. This study presents the first comprehensive analysis of RIOF quench detection in AC-operated HTS coils, including both straight REBCO samples and pancake coils under various low-frequency AC conditions, in both self- and external magnetic field. The effects of Lorentz forces on the optical fiber signal and the ability to detect thermal transients are studied. The results indicate that AC operation does not hinder the detection of thermal anomalies, though it does introduce a periodic spectral shift in the fiber. This periodic signal corresponds to the AC frequency in the presence of an external magnetic field, and to twice the AC frequency in self-field conditions, consistent with the expected time-dependence of the Lorentz force under these respective conditions. These findings are applicable to AC-operated HTS defense systems and time-dependent magnets, such as those used in fusion reactors, enabling more reliable quench detection in demanding environments.

Coils wound with High Temperature Superconductors have found increasing use in recent years in the development of high field magnets for several applications, including nuclear fusion. This is primarily due to their unparalleled ability to carry large currents in the presence of high magnetic fields, as well as their mechanical robustness. This work presents a novel approach to hot-spot detection and coil diagnostics that is applicable to both insulated and non-insulated REBCO pancake coils. The essence of the idea consists of embedding one or more optical fibers within the coil-to-coil insulation, or spacer material, that is typically used in magnets made of single or double pancake coils. We show a proof of concept of this approach via fabrication of SMART insulation samples and experimental demonstration of its capabilities on a REBCO NI Coil.

This study investigates the radiation-induced attenuation (RIA) of an Anti-Resonant Hollow-Core Fiber (AR-HCF) exposed to neutron and gamma radiation in a nuclear reactor environment. The AR-HCF—with a revolver-style structure—was characterized for RIA and compared to a solid, pure-silica-core fiber. Experimental results demonstrate that the AR-HCF exhibits substantially higher radiation tolerance compared to pure-silica, solid-core fibers, by about a factor of six in terms of dB/m. Numerical modeling in COMSOL Multiphysics (Version 6.3) was performed to simulate potential contributors of RIA, including silica compaction due to neutron fluence and changes in light confinement. These simulations ruled out these effects as primary causes of the measured attenuation. We also show that our results are consistent with the radiolytic generation of nitric acid within the hollow core, an interpretation that aligns with findings from a prior study. The results included in this manuscript provide insight into the behavior of AR-HCFs in the radiation field of a nuclear reactor, a topic with very limited prior literature, and underscore their potential for use in high-radiation environments such as fission and fusion reactors, particle accelerators, and space applications. The findings also point to promising future directions, including spectral characterization and dosimetry applications that leverage the unique properties of these fibers.