Selecting of FBG Coatings for Quench Detection in HTS Coils

Fiber Bragg grating (FBG) temperature sensors are now regarded as a relatively mature sensing technology for temperature measurement in ambient conditions, and may offer potential as a cryogenic temperature sensor for quench detection. In this paper, the temperature response of three different coated FBG sensors from 77 to 293 K was studied; bare (silica) FBG, acrylic coated, and polyimide coated FBG sensors. We examine the effect of fiber coatings on the temperature sensitivity of an embedded FBG sensor down to cryogenic temperatures. Measurements were repeated with sensors from two FBG suppliers. The measured temperature response was then compared and contrasted with the predicted sensor response. In order to assess the sensor sensitivity, repeatability, and any hysteresis, these sensors were then subjected to repeated cool-down / warm-up cycles (77–293 K). The effect of epoxy resin impregnation on the temperature response characteristics was analyzed by comparing the sensor response characteristics before and after embedding into a 10 mm × 2 mm × 4 mm epoxy block. We compare this to the uncoated fiber for embedment in HTS coils. Finally, we recommend the optimum low-cost sensor construction for use in quench detection in coils.