Effect of the second curing cycle on performance of superconducting magnet insulating system

In large-scale, high-field superconducting magnets used for magnetic confinement fusion, high energy accelerators, and magnetic resonance imaging, the insulating system made from glass fiber reinforced resin-based composites is the key component, which mainly plays the role of mechanical support, fixing and protecting superconducting conductors, as well as electrical insulation. Vacuum Pressure Impregnation (VPI) approach is widely used in the manufacturing of the insulation system. The second curing cycle is generally required after the first VPI and curing process. For example, after the superconducting coil is cured in the mold, the de-molding process requires the superconducting coil to be reheated according the curing temperature. Moreover, for large-scale superconducting magnets, the superconducting coil needs to undergo a second VPI process after the first VPI process to fix the coil in the coil case. In this work, the tensile and shear properties of pure epoxy resin and the glass fiber reinforced resin-based composite, were investigated at both room and cryogenic temperatures and the effect of the second curing cycle on the mechanical properties was analyzed. Additionally, the strain evolution of the Nb-Ti superconducting coil during the second curing cycle was measured using the Fiber Bragg Grating (FBG) sensors embedded in the composite. The results indicate that the second curing cycle will not introduce additional strain to the previously cured resin matrix, but the defective or weak parts of the resin matrix may be affected by the new added epoxy resin and a little extra strain has been observed.