A liquid helium-free superconducting magnet was realized using high-temperature superconducting current leads as the practical application after the discovery of high-temperature superconductors. A tiny cryocooler with a small refrigeration capacity was successfully linked with a superconducting magnet employing conventional superconducting wires. This results in getting rid of liquid helium from the inevitable condition for a magnet operation. Although the high-field properties at low temperatures below 2.2K for NbTi alloy and Nb3Sn compound superconducting wires have been concentrated on so far, the superconducting properties at high temperatures above 4.2K become rather important now, and the operating temperature from 5 to 6K is adopted as typical coil temperature. Since the liquid helium-free superconducting magnet works in a vacuum, cryogenic stability and quench protection based on a superconducting magnet immersed in liquid helium have to be reconsidered. In the near future, a liquid helium-free superconducting magnet wound with conventional low-temperature superconductors as well as high-temperature superconductors will make rapid progress, and high-field applications will be outstandingly enlarged. This article describes several key technologies for liquid helium-free superconducting magnets.
A half-wave rectifier-type flux pump cooled by a refrigerator has been built and its performance tested. The flux pump is composed of four elements as follows: a transformer, two thermal superconducting switches and a load coil (NbTi superconducting magnet). The transformer consists of an iron core, a primary winding with copper wire and a secondary winding with a NbTi/Cu-30wt% Ni superconductor. The primary winding and iron core of the transformer are cooled at the 1st stage of a 4K two-stage Gifford-McMahon (G-M) refrigerator. The load coil, superconducting switch and secondary winding are cooled by the 2nd stage of the G-M refrigerator. The load coil was cooled from room temperature to 3.8K in about 4h, indicating that the structure of the flux pump has no effect on G-M refrigerator cooling performance. The flux pump was operated at a speed of 1/7 [cycle/s]. The load coil current attained 57.7A in about 90min. Persistent mode operation has also been achieved.
The behavior of a 4K-GM (Gifford-McMahon) cryocooler in a magnetic field has been investigated. Characteristics of the magnetic regenerator materials were researched. The specific heat of ErNi0.9Co0.1, ferromagnetic regenerator material decreased under a magnetic field of 1T, while that of HoCu2, an antiferromagnetic regenerator material, had little effect of the field. The SRDK-408BW 4K-GM cryocooler, made by Sumitomo Heavy Industries, Ltd., was installed in a cryocooler-cooled superconducting magnet. The cryocooler was operated in a magnetic field under 1T. The cryocooler with ErNi0.9Co0.1 rose to its reachable temperature after increasing the magnetic field. In contrast to ErNi0.9Co0.1, the cryocooler with HoCu2 has a constant cooling capacity under 1T.
We have designed and constructed a 15T cryocooled superconducting magnet with a vertical room temperature bore of 52mm. The main components of the magnet are four Nb3Sn superconducting coils, a NbTi superconducting coil, two pairs of bismuth-based oxide superconducting current leads (Bi2223 current lead) and two Gifford-McMahon (GM) cryocoolers. The coils were directly cooled down from room temperature to 3.6K in 114h by compact GM cryocoolers without liquid helium. The magnet generated a magnetic field of 15.1T at the center of the bore and was excited from 0 to 15.1T in 38min. The magnet succeeded in holding the field at 15.1T for 24h at the constant coil temperature of 3.8K.