The 14-T magnetic resonance imaging (MRI) superconducting magnet utilizes both Nb3Sn and NbTi wires to generate the required magnetic field. Due to the brittleness of heat-treated Nb3Sn wires, the Nb3Sn coil is usually reacted after winding. There are two main challenges in the manufacture of Nb3Sn coils. Firstly, the Nb3Sn coil usually uses the glass fiber as insulation. The heat treatment process will decrease the insulation strength of the glass fiber, which may lead to insulation breakdown of the Nb3Sn coil during quench. Secondly, the dimensions and positions of the Nb3Sn coil could also change during heat treatment, which would affect the stress and strain distributions in the magnet and have a negative effect on the magnetic field homogeneity.
This paper presents the manufacture process and cryogenic test of reinforced bronze-processed Nb3Sn coils for a 14-T animal MRI magnet. The magnet has a warm bore of 175 mm and consists of two Nb3Sn solenoid coils and six NbTi solenoid coils. The Nb3Sn wires are manufactured by the bronze process and reinforced with CuNi/NbTi. Carbon-free fiberglass cloth is wrapped between the layers when winding the Nb3Sn coils to enhance interlayer insulation. After winding, the Nb3Sn coils are subjected to heat treatment at 670℃ in an argon environment. To control the changes in Nb3Sn coil dimensions and positions after heat treatment, a heat treatment tooling for Nb3Sn solenoid coils is designed and utilized, which takes into account the rate of expansion of Nb3Sn wires during heat treatment. After heat treatment, the Nb3Sn coils undergo vacuum pressure impregnation to enhance the mechanical and insulation strength. The Nb3Sn-Nb3Sn/Nb3Sn-NbTi superconducting joints are fabricated and an external aluminum cylinder reinforcement is installed. The stainless steel bobbin is removed to further increase the ground insulation of the Nb3Sn coils. In the cryogenic test, the current of Nb3Sn coils reaches 380 A, corresponding to a central magnetic field of 11 T.
This work was supported by the Strategic Priority Research Program of Chinese Academy of Sciences, China (Grant No.XDB25000000).
Keywords: Bronze-processed Nb3Sn, Heat treatment, Superconducting magnets, Magnetic resonance imaging