Fiber Bragg Grating Sensors for Strain Measurement at Multiple Points in an NbTi Superconducting Sample Coil

Multipoint stress/strain measurement in a simple single layered Niobium Titanium wire wounded superconducting sample coil (SSC) was carried out using fiber Bragg grating sensors (FBGs) at low temperature (4.2 K) and at high background magnetic field (8 T) excited by power supply. Ten gratings with different spatial period were fabricated at various positions along a single mode fibre (SMF). The grating location in the SMF sensing array was then mounted at the measuring points on the SSC using stycast 2850 FT. The change in the strain in the SSC. This induced strain, changes the Bragg wavelength of the mounted FBG sensor, which was then recorded using a FS5100 Bragg meter. Experimental results showed that the FBG sensor array was highly reproducible with error of ±0.8 μm/m for high external magnetic field and high current. Also the Young's modulus of the SSC at 900 A, 8 T, and 4.2 K was calculated to be ~ 117 GPa, which is below to the average Young's modulus of the NbTi/Cu superconducting composite wire, which is in the range of 130 GPa at 4.2 K. The loading and unloading characteristic curves of the SSC exhibit reversible behavior without any hysteresis in the operated range. This ensures that the SSC operates linearly within the elastic limit for the applied current and magnetic field. In this paper, we report the behavior of each single turn of the single layered SSC and its corresponding strain due to Lorentz force. The FBG sensing principle is validated by executing various modes of SSC excitation.