Enhanced strain and temperature sensing in copper-coated fiber Bragg Grating sensors across a wide temperature range from cryogenic to elevated levels

Preliminary demonstrations show Fiber Bragg Grating (FBG) sensors effectively measure low temperatures, such as in Helium cryostats. These sensors, resistant to electromagnetic interference, offer significant potential for monitoring strain and temperature within superconducting coils and cryogenic structures due to their compact size. Standard FBG sensors are constrained by the thermal expansion coefficient of the polyimide coating, restricting their use as embedded sensors during the heating process of Nb₃Sn superconducting coils. These sensors are currently unable to endure temperatures exceeding 400 °C, essential for fabricating Nb₃Sn coils through the wind and react technique. This study explores enhancing the endurance of FBG sensors through magnetron sputtering coating process with copper (thickness of 20 μm), which has a thermal expansion coefficient similar to Nb₃Sn coils. The results showed improved repeatability and survival for the copper-coated FBG in comparison to the standard polyimide-coated FBG from room temperature up to 939 K. Subsequently, the strain and temperature sensing capabilities of the FBG sensors with copper coating are evaluated using a separately developed controllable conduction cooling system, ranging from room temperature to 4.2 K. The findings in this paper demonstrate that the copper coating significantly enhances the durability of the FBG sensors under high temperatures. Additionally, the strain and temperature sensing characteristics of the sensors remain effective across a broad range from cryogenic to elevated temperatures. The homemade FBGs were independent of temperature below 30 K and responsible for the larger internal thermal strain of Nb₃Sn magnets during its pre-heat treatment and operation.