Thermal stability of high-strength Nb_3Sn wire and relevant application to the cryocooled superconducting magnet

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A test coil employing CuNb/Nb_3Sn multifilament superconducting wire was prepared, and the cryogenic stability for the CuNb/Nb_3Sn coil was examined in liquid helium cooling and cryocooler cooling conditions. It is found that a minimum quench energy MQE in the cryocooling condition is reduced to 1/3 in comparison with the liquid helium cooling condition, and on the contrary a normal zone propagation velocity is enlarged by a factor of 3 to 4. These behaviors are dependent on the loading current ratio between the operation current and the critical current, and in particular the cryocooling condition reveals the strong dependence of the current ratio.Further, the CuNb/Nb_3Sn test coil fabricated by a react and wind technique was investigated in a large electromagnetic stress state. It is found that the test coil wound by the bending strain of 0.33% stood with the Hoop stress of 330Mpa, and that no degradation of the critical current for the test coil was observed. This surely results in confirmation of the patent for the CuNb/Nb_3Sn wire with reinforcing stabilizer.Finally, we started to construct the world's first cryogen-free hybrid magnet consisting of an outer cryogen-free compact wide bore superconducting magnet and an inner water-cooled resistine magnet. A cryogen-free 23 T hybrid magnet by the combination of a cryogen-free 8 T superconducting magnet with a 360mm warm bore magnet and a 15T water-cooled magnet will realize in near future.