Design and Fabrication of a Conduction-Cooled Superconducting Magnet for Gyrotron

A conduction-cooled superconducting magnet for gyrotron that produced the central field in 3.57 T, with less than 0.1% of field nonhomogeneity and less than 1.5% of the field error rate, was developed. The magnet consists of NbTi split-type solenoids, a cryostat with a thermal shield, a Gifford-McMahon cryocooler, and a pair of binary current leads. The magnet was installed in a vacuum cryostat with 90-mm warm bore, 645-mm depth, 370-mm width, and 220-mm height. In order to minimize the heat load to the superconducting magnet, a thermal shield was thermally attached to the first stage. In addition, thermal contact resistance was carefully considered to minimize the temperature difference between the coils and the second stage of the cryocooler. It took 20 h for the magnet to cool down from 300- to 4-K level. The temperature of the superconducting magnet was stable below 3.5 K, which corresponded to about 0.6 W of total heat loads. After charging the magnet to 88 A of current with 0.1-A/s ramp rate, it produced 3.57 T of the central field with less than 0.07% of field nonhomogeneity and 1.5% of field error rate. The detailed development of the magnet was presented in this paper.