A cryocooler cooled 6 T NbTi superconducting magnet with room temperature bore of 220 mm

We are developing a cryocooler cooled 6 T superconducting magnet without liquid helium and/or liquid nitrogen. This magnet, which is directly cooled by two 4 K Gifford-McMahon (GM) cryocoolers in vacuum, consists of an graded NbTi coil, Bi-based oxide superconducting current leads and a persistent current switch. The coil is made using two types of NbTi wires and a copper bobbin. The coil with an inner diameter of 260 mm, an outer diameter of 329 mm, and a height of 334 mm, is impregnated with grease to ensure a good heat conduction. The operating current for central field of 6 T in the 220 mm room temperature bore is 152 A. A mechanical persistent current switch is used to limit the load from the switch into the 4 K GM cryocooler. Designed temperature of the magnet is 4.9 K at the operating current of 152 A