Fiber Bragg Grating sensors to measure the coefficient of thermal expansion of polymers at cryogenic temperatures

One of the fundamental advantages in the employment of the Fiber Bragg Grating (FBG) sensors lies in their capability to allow measurements in extreme environmental conditions with also very high immunity toward external electromagnetic interference factors. The behavior of a polymer-coated FBG sensor, tested in cryogenic conditions at the laboratories of the European Organization for the Nuclear Research (CERN) in Geneva, has been analyzed and will be discussed in this paper. Magnets used in the Large Hadron Collider (LHC) for the High Energy Physics Researches at CERN need in fact extreme cooling conditions to preserve the internal superconductivity highly crucial for their performances. The magnets, built with NbTi based superconductors, are cooled with liquid helium and they operate at 1.9 K. The aim of the present work is to estimate the thermal expansion coefficient of two polymers based on epoxy and methacrylate (PMMA) used as coating of FBGs, in the temperature range from 4 K to 300 K and to check their suitability for the use in temperature monitoring of the superconducting magnets. A standard numerical derivative method has been employed to estimate thermal expansion coefficients; moreover the correlated fluctuation analysis (CFA) based procedure is proposed, as a reliable alternative, to overcome numerical derivative drawbacks at very low temperatures within the range of 4-20 K. The calculated values of thermal expansion coefficient for both systems are in agreement with literature data on similar material