Neon liquefaction system for high Tc experiments

Abstract

Development of high temperature ceramic superconductors is hindered by lack of suitable cooling below nitrogen temperatures. If these new high T c materials are to be useful in power engineering applications they must achieve current densities J of 10 4–10 5 A cm −2 in the superconducting core in magnetic fields H of 6–10 T. The present state-of-the-art indicates that these J-H characteristics will only be achieved using BSCCO materials operating at temperatures below ≈40 K. Bath cooling at these temperatures presents a problem since this is too low for liquid nitrogen, but too high for liquid helium to be a practical or economic choice. However, the boiling point of neon (27.1 K) and its high specific cooling capacity make it an eminently suitable choice of refrigerant at these temperatures. A cryostat is being constructed which employs a two-stage Gifford-McMahon cooler to liquefy neon gas and then maintain a bath of up to 5 dm 3 of liquid for long periods. The system has been designed to allow either in-situ experiments or transfer of liquid to another cryostat. When used in the latter mode a closed loop recovery system must be employed, but unlike a helium recovery system in which high pressure compressors are required, the system is relatively simple since liquefaction is achieved at low pressure.