Fiber optic quench detection for large-scale HTS magnets demonstrated on VIPER cable during high-fidelity testing at the SULTAN facility

Fiber-optic thermometry has the potential to provide rapid and reliable quench detection for emerging large-scale, high-field superconducting magnets fabricated with high-temperature-superconductor (HTS) cables. Developing non-voltage-based quench detection schemes, such as fiber Bragg grating technology, are particularly important for applications such as magnetic fusion devices where a high degree of induced electromagnetic noise impose significant challenges on traditional voltage-based quench detection methods. To this end, two fiber optic quench detection techniques – fiber Bragg grating (FBG) and ultra-long fiber Bragg grating (ULFBG) – were incorporated into two VIPER high-current HTS cables and tested in the SULTAN facility, which provides high-fidelity operating conditions to large-scale superconducting magnets. During surface heater induced quench-like events under a variety of operating conditions, FBG and ULFBG demonstrated strong signal-to-noise ratios (SNR) ranging from 4 to 32 and measured single-digit temperature excursions; both the SNR and temperature sensitivity increase with temperature. Fiber thermal response times ranged between effectively instantaneous to a few seconds depending on the operating temperature. Strain sensitivity dominates the thermal sensitivity in the conditions achievable at SULTAN; however, measurements at higher quench evolution temperatures, coupled to future work to increase the thermal-to-strain signal, show promise for quench detection capability in full-scale magnets where temperature and strain may occur simultaneously. Overall, FBG and ULFBG were proven capable to quickly and reliably detect small temperature disturbances which induced quench initiation events for high current VIPER HTS conductors in realistic operating conditions, motivating further work to develop FBG and ULFGB quench detection systems for full-scale HTS magnets.

光纤测温技术（fiber-optic thermometry）有望为采用高温超导体（High-Temperature Superconductor, HTS）线缆制备的新型大型高场超导磁体提供快速可靠的失超检测（quench detection）方案。开发基于非电压的失超检测方案——例如光纤布拉格光栅（fiber Bragg grating, FBG）技术——对于磁聚变装置（magnetic fusion devices）等应用尤为重要，这类场景中高强度的感应电磁噪声会给传统基于电压的失超检测方法带来极大挑战。为此，研究人员将两种光纤失超检测技术——光纤布拉格光栅（FBG）与超长光纤布拉格光栅（ultra-long fiber Bragg grating, ULFBG）——集成至两根VIPER大电流高温超导体线缆中，并在SULTAN装置内开展测试；该装置可为大型超导磁体提供高保真的运行工况。在多种运行条件下由表面加热器诱发的类失超事件（quench-like events）中，FBG与ULFBG展现出4至32的信噪比（signal-to-noise ratio, SNR），并测得个位数量级的温度偏移；二者的信噪比与温度灵敏度均随温度升高而提升。光纤的热响应时间介于近乎瞬时至数秒之间，具体取决于运行温度。在SULTAN装置可实现的工况下，应变灵敏度主导了热灵敏度；但针对更高失超演化温度下的测量结果，结合后续用于提升热-应变信号比的研究工作，该技术在温度与应变同步产生的全尺寸磁体中展现出失超检测的应用潜力。总体而言，FBG与ULFBG已被证实能够在真实运行工况中快速可靠地检测到诱发VIPER大电流高温超导体导体失超起始事件的微小温度扰动，这为后续进一步研发面向全尺寸高温超导体磁体的FBG与ULFBG失超检测系统提供了动力。