Measurements of Quench Propagation Velocity in HTS Cable for Fusion Applications Using Optical Fiber Sensors

Low quench propagation velocity (QPV) in high-temperature superconductors (HTS) undermines conventional voltage-based quench detection, since damaging temperatures can develop locally before appreciable voltages appear. This work investigates an ENEA aluminum slotted-core cable featuring a BRAided STack (BRAST) of REBCO tapes, instrumented with capillary-mounted fiber Bragg gratings (FBGs) in direct thermal contact with the stack, alongside conventional voltage taps. Quench experiments were carried out in liquid nitrogen flow at 77 K, with the current held constant at about 500 A. Electrical and optical signals were processed with a standardized pipeline for the onset quench: statistical thresholds for electric field, and temperature-based arrivals at FBGs. From arrival times at known positions, QPV is estimated via two independent methods based on resistive front and thermal front, respectively. Both methods yield consistent values in the range of 20-60 mm/s. Downstream, where the front is fully developed, the two estimates converge within uncertainty. These results demonstrate that embedded optical fibers provide local temperature tracking, electromagnetically immune, and minimally invasive complement to voltage taps for quench detection and QPV estimation in HTS cables for fusion-magnet applications.