Localized hotspot detection for quench prevention in HTS magnets using distributed fiber Bragg gratings

High-temperature superconductors (HTS) can carry and withstand enormously high current densities and magnetic field respectively, which provide great promise for future energy generation, e.g. high-power generators and compact fusion systems. However, an open issue with the use of HTS is the challenge of rapidly detecting a potentially catastrophic quench, which tends to be localized. To overcome the challenge, we propose to proactively monitor the event of a hot spot (temperature rise) using fiber Bragg grating (FBG) sensors. A 10 m long ultra-long fiber Bragg grating (ULFBG) which consists of quasi-continuous FBGs with identical Bragg wavelengths is wound and bonded around a G10 former and six resistive heaters distributed along the length of the sensors are used to induce localized hot spots. Using a novel Spectral Intensity Change (SIC) algorithm, the hotspot induced spectral changes can be extracted based on the unique change patterns and processed to trigger the occurrence of a hot spot. The 10 m ULFBG is demonstrated to respond to temperature rises within 5 K from 30-40 mm hot spots through different cryogenic bonding materials (Apiezon N, epoxy and silicone) at 80 K via heat and strain transfer. The results show that although only 0.03% of the total length of the sensors are affected by the hot spots, the ULFBG exhibits sufficiently large signal to noise ratio (SNR) for a 10 K temperature rise in the challenging cryogenic environment. ULFBG is also shown to behave almost like 'distributed' sensors in detecting the event of a hot spot, which significantly improves the limitation of discrete FBGs.