Strain Responses of Superconducting Magnets Based on Embedded Polymer-FBG and Cryogenic Resistance Strain Gauge Measurements

Large Lorentz forces commonly arise from the intense field and high current in superconducting coils, which can induce maximum stress up to several hundred megapascal (MPa) in magnets. It is a big challenge in compact high-field magnets with small mechanical margin during energization. In this paper, the mechanical characteristics and strain responses of two superconducting magnets with different configurations and low-temperature superconducting (LTS) materials are investigated experimentally during their excitation process and quench events. To explore the strain behavior in the magnets, several cryogenic resistance strain gauges (CRSGs) and homemade polymer-fiber Bragg grating (FBG) sensors are used for measurements. The strain responses during excitation and quench training tests of the two LTS magnets are recorded to show that both measurement techniques have good precision and repeatability. Compared to the CRSG with complex compensation bridges, the polymer-FBG sensors exhibit more advantages like smaller size, noncircuit compensation, and insensitivity to electromagnetic interferences, and most important of all is that the polymer-FBG can be embedded in coils with good survival to measure the internal strain in the magnets. Based on FEM, we further calculated the electromagnetic field and deformation of the two magnets, and the predicted results show good agreement with the experimental data.