Distributed optical fiber sensor for investigation of normal zone propagation and hot spot location in REBCO cables

The use of high temperature superconducting (HTS) materials offers the potential to increase the operating temperature or field strength of coils used in a fusion reactor such as the Chinese Fusion Engineering Test Reactor (CFETR). REBCO cable is one of considered solution to be CFETR central solenoid (CS) coils. However, quench detection is one of the key challenges in the use of HTS magnets. HTS magnets exhibit much lower quenching velocity, and their thermal properties play an important role in HTS quench detection. In addition to voltage-based quench detection schemes, novel methods should be considered for quench detection in the HTS coils. In this paper, the distributed optical fiber sensing equipment was used to measure the normal zone propagation and hot spot location of a REBCO sample cable bathing in liquid nitrogen. A heater was mounted on the surface of the cable to induce a quench. Meanwhile, voltage taps and thermocouples were used to monitor the quench. The results showed that the cable was provided with high stability, and recoverable quench events occurred under different heater energies, with a propagation velocity in the normal zone concentrated at 3-8 cm/s. The distributed fiber can accurately measure the normal zone propagation length of the cable, and estimate the quench boundary of the REBCO tape. The spectral offset of the optical fiber responds before of voltage signals, showing more sensitive than the conventional thermometers. In this paper, distributed optical fibers were first used in REBCO cables. Optical fibers can be implanted into cables and can achieve a high spatial resolution of 5 mm. The fibers can accurately express the quench propagation characteristics of the REBCO cable, and capture the location of the weak point, which provides a new way for quench detection of HTS magnets.