Persistent Current Operation of a Cryogen-Free NbTi Superconducting Magnet

A cryocooler-cooled and thermally controlled type persistent current switch (PCS) using CuNi/NbTi superconductor for a cryogen-free superconducting magnet has been investigated. This PCS was combined with the cryogen-free NbTi superconducting magnet, and we succeeded in generating a magnetic field of 5.0T under persistent current operation. The PCS was covered with a thermal insulator and embedded in a copper cup with a cryogenic grease, and the copper cup was fixed to the second stage of the cryocooler. Heat transfer from the PCS to the cryocooler was controlled by a thermal insulator between the PCS and the copper cup, enabling the PCS to turn off with minimum power input. A practical PCS exhibited stable operation with 40mW of power input, and 18s and 100s of switching time from on-to-off and from off-to-on, respectively. When the magnet is energized, current sharing is induced to the PCS due to self inductance of the magnet. In order to reduce the self-heating of the PCS due to current sharing, enlargement of the off-resistivity of the PCS was important. The cryogen-free NbTi superconducting magnet with 52.6A/5T of constant magnetism and 59.6H of self-inductance was able to energize within 18min from 0T to 5T. The decreasing rate of generating 5T on persistent current operation was 39.5ppm/h (on average) in the initial 100h. This value corresponds well with the results of calculations based on an LR-circuit discharge due to the resistivity of soldered joints in the closed circuit.