Rayleigh-backscattering interrogated optical fibers in HTS coils: effects of AC currents on optical fiber signals

High-temperature superconductors (HTSs) 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 (RIOFs) offer a promising solution by providing distributed sensing with minimal interference from electromagnetic noise and high spatial resolution for detecting temperature and strain variations. This study presents the first comprehensive analysis of RIOF hot-spot detection in AC-operated HTS conductors, 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 between 1–10 Hz 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.