A 7.7 T NbTi Superconducting Magnet System Cooled by a 4 K Gm Refrigerator

Abstract

This paper describes the experimental result of a superconducting magnet system, which is cooled by only a 4 K Gifford-McMahon (GM) refrigerator. This magnetic system doesn’t use any cryogens, such as liquid helium or liquid nitrogen. The central magnetic field of 7.7 T and the maximum magnetic field of 9.1 T were obtained at the electrical current of 53.7 A. Points of technical interest in this study were use of the 4 K GM refrigerator, which used the magnetic regenerator material instead of usual material of lead (Pb), and use of high Tc superconductor current leads. The Niobium Titanium (NbTi) superconducting magnet, 36 mm in inner diameter, 120 mm in outer diameter, and 28 mm in length, was cooled at 4 K level by the 4 K GM refrigerator. Current leads between the first (45 K) and second (4.2 K) stages of the 4 K GM refrigerator were made of high Tc superconductor (Bi-2212) to remove the ohmic heating and to reduce the heat load to the 4 K level for the 4 K GM refrigerator. The cool down time from room temperature to the 4 K level was about G hours.