The incredible current density and high magnetic field generation possible with high-temperature superconductors (HTS) have the potential to revolutionize energy generation, e.g. high-power generators, and compact fusion energy devices. However, an open issue that limits the applications of this class of superconductors is the challenge of rapidly detecting a hot spot which can lead to a quench. Owing to the inherent advantages of fibre optic sensors, they are promising candidates to be integrated in HTS magnets for hot-spot detection. In this paper, closely spaced fibre Bragg gratings (FBG) with the same Bragg wavelength are used to establish an ultra-long FBG (ULFBG) for distributed hot-spot monitoring. We investigate the capability of a 10m ULFBG to detect a small temperature rise at the end of the sensor. The results show that a 10 m long ULFBG can rapidly detect a small hot spot within 1 K temperature rise at 80 K. It is expected that ULFBG can be wavelength-division multiplexed and integrated to superconducting coils to achieve long-distance hot-spot monitoring with extremely high spatial resolution and fast response.

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Optical fiber Bragg gratings (FBGs) are well suited for applications as temperature or/and strain sensors in harsh environments, e.g., detecting thermal hot spots in high-temperature superconductor (HTS) fusion energy magnets at cryogenic temperatures and high radiation environments. To maximize the signal-to-noise ratio (SNR) of the FBGs to a hot spot, we propose to have them mounted in V-shaped grooves of HTS’ copper former. To investigate the differences between different adhesives on transferring strain and heat in this configuration, five arrays of FBGs are mounted in the V-shaped grooves of a copper dog-bone using Scotch-Weld epoxy, Stycast 2850 FT, Apiezon N, and Loctite 5145 silicone. The copper is cycled through tensile forces in a modified universal tensile tester, subjected to a thermal cycle between 293 K and 77 K, and exposed to heat pulse propagations at 293 K and 80 K. The FBGs that are bonded using Stycast show the highest temperature and strain sensitivities at room and cryogenic temperatures. No major differences in the temperature and strain sensitivities have been found between Ormocer and polyimide coated FBGs. Apiezon N is found to transfer strain consistently well below 245 K, which is comparable with other bonding materials in the temperature range between 77 K and 110 K. The FBGs bonded with the four adhesives in the V-groove configurations are shown to have comparable SNRs to a temperature rise of 20 K at 80 K. This paper emphasizes the importance of maximizing the thermal strain transferred from the host material through the bonding agents to achieve high temperature sensitivity of FBGs.

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