In a proposed improvement on the basic concept of a magnetostrictive heat switch for cryogenic applications, the magnetic field needed for actuation would be generated by a superconducting flux tube (SFT). A closely related concept for a magnetostrictive heat switch was presented in "Magnetostrictive Heat Switch for Cryogenic Use" (NPO-20274), NASA Tech Briefs, Vol. 23, No. 8, (August, 1999), p.48. To recapitulate: The main thermal contact in the heat 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 magnetic field needed for actuation would be generated by use of a superconducting solenoid.
The use of an SFT instead of a superconducting solenoid would be a significant and advantageous departure from the earlier proposal. In the case of a solenoid, it would be necessary to supply current continuously to the solenoid to maintain the magnetic field needed to keep the switch in either the "open" or "closed" state; turning off the current in the solenoid would cause the switch to revert to the opposite state. In contrast, in the case of an SFT, the magnetic flux would remain constant without power applied (other than the separate power needed in any event to maintain the cryogenic environment). Thus, an SFT-actuated heat switch would be bistable; it would remain in either the "open" or "closed" state until toggled into the opposite state. Toggling would be effected by applying a pulsed current to a coil around the SFT to change the magnetic flux and thereby change the degree of magnetostriction.
In comparison with externally actuated mechanical and gas-gap heat switches, both the superconducting-solenoid SFT-type magnetostrictive heat switches would offer the advantage of less heat leakage. Both magnetostrictive heat switches would function at the temperatures of the devices to be thermally switched, with little or no thermal hysteresis. The switching time of either magnetostrictive heat switch would be only about one thousandth of that of a gas-gap heat switch. The SFT-actuated switch would offer the additional advantage that in the steady state, any thermal disturbance that it would introduce would be almost unmeasurably small and, in most cases, completely negligible.
SFTs made of bismuth strontium calcium copper oxide (BSCCO) are already available. Typical lengths and outer diameters are of the order of centimeters, and bore diameters range upward from 1 cm. Flux densities range from 0 to 10 T in the approximate temperature range from 77 K (liquid nitrogen) to 4 K (liquid helium). These flux densities are more than adequate for a magnetostrictive heat switch; the range of flux densities for magnetostrictive materials that have been investigated for use in heat switches is 0 to 0.2 T in the given temperature range.
This work was done by Robert Chave of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.