Closed-Cycle Cooling System for a Superconducting Magnet and 2K – 400K Variable Temperature Insert

Abstract

A closed-cycle cooling system based on a combined Gifford-McMahon and Joule-Thomson cycle refrigerator has been successfully coupled to an 8 Tesla laboratory magnet resulting in completely closed-cycle operation. The G-M/J-T cooler is used both as a reliquefier and as a liquefier. The J-T loop cools a heat exchanger in the liquid bath surrounding the superconducting magnet eliminating the liquid helium boiloff. In addition, room temperature helium gas is injected, cooled to 80K by the first G-M stage, then to 15K by the second G-M stage and finally liquefied at 4.0K by the J-T stage. Some liquid is used by the vapour cooled magnet current leads and the remaining liquid is used by a fourth stage of cooling for the sample. J-T expansion of liquid helium into the variable temperature sample insert allows continuous operation at any sample temperature between 2K and 400K, while simultaneously allowing the superconducting magnet to operate at 4.3K. This modular cooling technology has also been applied to cooling Magnetic Resonance Imaging (MRI) machines, Dilution Refrigerators, SQUID Magnetometers and small Helium Liquefiers.