Ralph C. Longsworth

4 K Refrigerator and Interface for MRI Cryostats

A 4 K refrigerator and sleeve have been designed to eliminate cryogen boil-off from superconducting Magnetic Resonance Imaging magnets. The refrigerator and sleeve have heat stations that cool radiation shields at nominal temperatures of 77 K and 20 K. A heat exchanger at 4.2 K recondenses helium boil-off. The unit is designed for use in a cryostat which has no liquid nitrogen in the 77 K shield. It has been demonstrated that the refrigerator can be removed and replaced in less than one hour with a loss of only 0.6 L of helium.

Maglev Cooling Options for Nb3Sn Cables in Conduit Superconducting Magnets

This paper reports on studies of cooling Nb3Sn superconducting magnets in magnetically-levitated trains. Each car is designed to have six bogies with two cryostats per bogie and multiple coils in each cryostat which have gas-cooled cables in conduit windings that can have exit temperatures as high as 8 K. Refrigeration systems that are considered include the use of liquid helium (LHe) and liquid nitrogen (LN2), supplied from central stations on a daily basis, on-board refrigerators that receive power from the guide way, and hybrid systems that use LN2 and LHe from central stations to reduce the power input and weight of an on-board refrigeration system. It is assumed that LN2 which is supplied from a central station, can be vented but that He would be recovered. The system for recovering and storing He on board the train for 24 hours of operation adds appreciable weight to the system and requires a significant amount of power for the recovery compressor. An on-board refrigerator that uses LN2 for 77 K refrigeration is lighter and uses less power than a refrigeration system that uses no cryogens The system that is lightest and requires very little power input is one that uses LHe in a sealed system for refrigeration between 4.2 K and 8 K and LN2 for 77 K refrigeration.