A regenerative magnetic refrigerator operating between liquid helium and liquid hydrogen reservoirs

A regenerative refrigerator with a GGG (Gd₃Ga₅O₁₂) salt core built and tested in the Cryogenics Engineering Laboratory at MIT is discussed. The core remains stationary as supercritical helium is forced through it via the action of a displacer system. At the same time, the magnetic field is varied by an AC superconducting magnet. Preliminary data obtained from this system suggested the need for modification of (1) the helium flow system, (2) the reservoir temperature control system, (3) the cycle control system, and (4) the instrumentation. A heat exchanger has been constructed which enables the heat of rejection to be deposited in a liquid-hydrogen bath. The refrigeration load is provided by an electric heater. Real-time feedback is used to synchronize the helium flow with the magnetic-field variation. The modifications are described and various schemes for producing the required helium flow without the use of moving mechanical parts are noted.