Temperature rise measurement method for HTS magnets based on Michelson interfering principle by optical fibers

Abstract High temperature superconducting (HTS) magnet is an option for applications of high magnetic field above 20 T due to high upper critical magnetic field at low temperature. However, the temperature rise measurement is a challenge for HTS magnet since its low quench propagation velocity coming from low thermal conductivity and wide operation range of temperature. In this paper, we propose a novel temperature rise measurement method based on Michelson interfering principle. The laser emits light which is transmitted through a fiber splitter to two arms, one arm optical fiber was co-wound with measurement object and the other path, so called reference arm, has same length with co-wound optical fiber and locates outside of object in bath cooling with constant temperature environment. The two beams of light interfere afterwards and form interfering intensity which is detected by photoelectric conversion device. The interfering effect associates closely with optical path difference coming from two arms since the refractive index of optical fiber depends on temperature. Since the measuring arm has same temperature with measurement object. While the temperature of object rises, although the refractive index of optical fiber in reference arm keeps constant since it in constant temperature so that its optical path also keeps constant, the refractive index of co-wound optical fiber changes so that the optical path difference between two arms changes and the intensity of interfering occurs. Then, the temperature rise is reflected by the phase difference change of the interference light from two arms. The experiment is performed in liquid nitrogen temperature. The results show that the proposed method has the significant advantages of fast response, high sensitivity and immunity from electromagnetic interference compared to conventional techniques, which is potential for not only bottleneck of temperature rise measurement but also the quench detection for HTS magnet.