Superconducting coils that generate a high magnetic field are subjected to a large electromagnetic force. In general, compound superconductors such as Nb3Sn are sensitive to stress and strain; the superconducting properties for instance, critical current density decrease, as force increases. To prevent a reduction in superconducting properties, several kinds of reinforced Nb3Sn wires have been developed. Reinforced materials such as Cu-Nb, Ta, Alumina-Cu and Nb-Ti-Cu, have been developed. To fabricate a cryocooled magnet using those reinforced wires, we experimentally measured the minimum quench energy (MQE) under the cryocooled conditions of some reinforced Nb3Sn wires. As a result, it became clear that thermal stability expressed as MQE was controlled by the temperature margin between the temperature of the operating condition and the superconductivity to normal transition temperature. Using the FEM analysis, it is realized that cause of the decline in thermal stability at the time of reinforcement was the low thermal conductivity of the reinforce materials.
