Thermal analysis of the cryocooled superconducting magnet for the liquid helium-free hybrid magnet

Abstract

The liquid helium-free hybrid magnet, which consists of an outer large bore cryocooled superconducting magnet and an inner water-cooled resistive magnet, was developed for magneto-science in high fields. The characteristic features of the cryogen-free outsert superconducting magnet are described in detail in this paper. The superconducting magnet cooled by Gifford–McMahon cryocoolers, which has a 360mm room temperature bore in diameter, was designed to generate high magnetic fields up to 10T. The hybrid magnet has generated the magnetic field of 27.5T by combining 8.5T generation of the cryogen-free superconducting magnet with 19T generation of the water-cooled resistive magnet. The superconducting magnet was composed of inner Nb3Sn coils and outer NbTi coils. In particular, inner Nb3Sn coils were wound using high-strength CuNi–NbTi/Nb3Sn wires in consideration of large hoop stress. Although the cryocooled outsert superconducting magnet achieved 9.5T, we found that the outsert magnet has a thermal problem to generate the designed maximum field of 10T in the hybrid magnet operation. This problem is associated with unexpected AC losses in Nb3Sn wires.