Distributed fiber optic sensing (DFOS) for strain and post-quench diagnosis in the CSMC magnet: validation and cryogenic challenges

Reliable electromechanical and thermal monitoring is essential for the safe operation of high-field superconducting magnets such as the central solenoid (CS) of the China Fusion Engineering Test Reactor. To validate key CS technologies, a CS model coil (CSMC) was developed as a pre-research platform. Conventional diagnostics, including voltage taps and thermocouples, provide only discrete measurements and cannot offer distributed, high-resolution, multiphysics characterization under cryogenic and high-field conditions. In this work, a distributed fiber optic sensing (DFOS) system based on Rayleigh optical frequency-domain reflectometry was implemented and tested on the CSMC. Stainless-steel-encapsulated single-mode fibers were installed on the inner Nb₃Sn coil and helium outlet tubes to monitor distributed strain and temperature during 20/30 kA electromagnetic cycling, quench events, and warm-up tests. The DFOS accurately captured the linear dependence of spectral offset on the square of current (Δλ ∝ I2) with a coefficient of variation below 5%, revealing nonuniform Lorentz-force-induced stress distribution. Due to the system’s limitations in Hz-level temporal resolution and external fiber placement, it was unable to capture local anomalies characteristic of the pre-quench phase. However, the system clearly recorded abrupt strain drops synchronized with the activation of the quench protection system during current discharge. During the warm-up phase, fiber signals reproduced temperature evolution from 4.2 to 200 K and visualized local thermal inhomogeneities in modules P14 and P15, demonstrating potential for hotspot detection. These results confirm the feasibility of DFOS for distributed electromechanical and thermal monitoring in the CSMC, validating its robust capability for post-quench diagnosis and health monitoring, while highlighting the necessary upgrades for future early warning functionality.