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.
NPO-20502
This Brief includes a Technical Support Package (TSP).
Magnetostrictive Heat Switches Actuated by Flux Tubes
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Overview
The document discusses a novel approach to magnetostrictive heat switches for cryogenic applications, specifically focusing on the use of superconducting flux tubes (SFTs) instead of traditional superconducting solenoids. The primary innovation is that SFTs allow the heat switch to maintain its state—either "open" or "closed"—without the need for continuous power, making it bistable. This is particularly advantageous for long-duration operations, such as devices in dormant states awaiting activation.
In traditional designs, the magnetic field required for actuation is generated by a solenoid, which necessitates a constant current supply to keep the switch in its desired state. If the current is turned off, the switch reverts to the opposite state. In contrast, SFTs, made from bismuth strontium calcium copper oxide (BSCCO), can maintain a constant magnetic flux without continuous power, only requiring a pulsed current to toggle the switch's state. This feature significantly reduces heat leakage compared to externally actuated mechanical and gas-gap heat switches.
The document highlights the technical specifications of SFTs, noting that they can produce flux densities ranging from 0 to 10 Tesla at temperatures between 77 K (liquid nitrogen) and 4 K (liquid helium). These flux densities are more than sufficient for the magnetostrictive materials used in heat switches, which typically require much lower flux levels (0 to 0.2 T).
The advantages of SFT-actuated magnetostrictive heat switches include minimal thermal disturbance in steady-state conditions, rapid switching times (approximately one thousandth of that of gas-gap heat switches), and the ability to function effectively at the operational temperatures of the devices they serve. The work was conducted by Robert Chave at Caltech for NASA's Jet Propulsion Laboratory, emphasizing the potential for increased utility and application range of these heat switches.
Overall, the document presents a significant advancement in cryogenic thermal management technology, showcasing the benefits of SFTs in enhancing the efficiency and reliability of heat switches in space exploration and other applications requiring precise thermal control.
