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.

High-temperature superconductor (HTS) magnet systems, especially those designed for fusion reactors, require effective and reliable monitoring to avoid damaging anomalies. In tokamaks, some of the magnetic coils are time-dependent, which causes strain and large inductive voltages within the magnet, rendering detection of incipient quench challenging. Ionizing radiation can also create material defects and lead to non-uniform degradation of conductors. The resulting decrease in critical current uniformity across the magnet, along with manufacturing defects, such as failure of structural materials or cooling systems, can all potentially initiate a quench. HTS magnets have a lower normal zone propagation velocity than low-temperature superconductors, and this causes normal zones to be localized, increasing the risk of permanent damage. Fiber optic sensors have several qualities that are essential in fusion systems. Unlike traditional voltage-based sensors, fiber optic cables are immune to the large electromagnetic fields present. This study presents and validates a fiber optic interrogation technique for monitoring magnetic confinement fusion and other high-temperature superconducting magnet systems. Coherent-phase optical time domain reflectometry (OTDR) allows for the high sampling rates (tens of kHz) necessary to quickly detect and mitigate quench events over the long distances required to monitor fusion magnet systems. This technique was demonstrated to successfully detect localized thermal transients at cryogenic temperatures as low as 6 K. These outcomes were also demonstrated using fibers embedded in HTS magnet coils at 77 K, verifying the potential for this interrogation technique’s use for failure detection in HTS coils.