Magnetostrictive Heat Switch for Cryogenic Use

NASA's Jet Propulsion Laboratory, Pasadena, California

A magnetostrictively actuated heat switch has been proposed for use in a variety of cryogenic equipment, including adiabatic-demagnetization refrigerators, calorimeters, and coolers for high-performance infrared cameras. Heretofore, the heat switches in such equipment have generally been of two types: gas-gap and externally mechanically actuated. The gas-gap switches are limited to long cycle times and tend to exhibit both poor isolation in their "open" states and low "closed"/"open" heat-transfer ratios. In the case of externally mechanically actuated switches, the mechanical connections between the switches and the outside environment are heat-conduction paths, along which heat leaks into the affected cryogenic chambers. In contrast, the proposed magnetostrictively actuated switch would feature short cycle times, low heat leakage, high isolation in the "open" state, and a high "closed"/"open" ratio.

The Heat Switch Would Be Closed or Opened by exploiting the magnetostrictive effect to make or break a mechanical contact along the main heat-conduction path.

As shown in the figure, the main thermal contact in the switch would be made or broken by making or breaking, respectively, the mechanical contact between (1) the moving end of a rod of magnetostrictive material and (2) a fixed contact pad. The magnetostrictive material would be a terbium/dysprosium alloy, which exhibits a large magnetostrictive effect at low temperature. The use of a polycrystalline form of this alloy would eliminate the need for a return spring that must be used with the single-crystal form of the alloy, enabling a reduction in the weight and complexity of the switch. The magnetic field needed for actuation would be generated by use of a superconducting solenoid made of Nb/Ti alloy.

In operation, the superconducting solenoid would generate no waste heat. The entire switch, including the magnetostrictive actuator and superconducting solenoid, would be mounted at the cold stage used for temperature control. By using superconducting leads to the cold stage, the heat leak to the cold stage would be minimized. The switch would be mounted on a stainless-steel tube with a doubly re-entrant design that provides good thermal isolation in a small space; the estimated open-state thermal conductance of the assembly is 15 µW/K.

This work was done by Robert Chave, Christian Lindensmith, Brent Fultz, and Marius Birsan of Caltech for NASA's Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to