A proposed technique would make it possible to maintain two reservoirs of superfluid helium at the same pressure but at different temperatures. Heretofore, a fountain effect (described below) has made this impossible. The proposed technique could be useful for low-temperature experimentation and for the general processing of liquid helium.
Liquid helium becomes a superfluid when cooled below a temperature of 2.177 K. One of the characteristics of a superfluid is nearly infinite thermal conductivity. Thus, if two reservoirs containing superfluid helium are put in fluid communication to equalize their pressures, thermal conduction through the connecting fluid equalizes their temperatures. Even a connection as narrow as a capillary tube acts as a thermal short circuit.
One way to obtain a temperature difference between the two reservoirs is to make the fluid connection through a porous material (e.g., a ceramic). However, such a temperature difference is accompanied by a pressure difference; this is the phenomenon known as the fountain effect.
According to the proposal, the two reservoirs would be connected via an electrically conductive capillary tube, part of the interior of which would be occupied by an electrically insulated wire. A high voltage would be applied between the wire and the capillary tube. The resulting high electric field inside the tube would cause electrostriction in the fluid, so that the pressure in the capillary tube would increase. The increase in pressure would depress the superfluid-transition temperature, causing the fluid in the capillary tube to revert to a normal fluid. Because the fountain effect would not occur in the normal fluid, it would be possible to maintain a temperature difference between the opposite ends of the capillary tube.
The increase in pressure with electrostriction is given by
P = (e0/6)( d - 1)(d + 2)E2,
where e0 is the vacuum permittivity, d (which equals 1.06) is the relative permittivity of liquid helium, and E is the electric field. A suitable device to implement the technique could be made from a wire of 0.003 in. (0.076 mm) outside diameter and a tube of 0.004 in. (0.102 mm) inside diameter. At the breakdown electric field of 108 V/m, it should be possible to depress the superfluid-transition temperature by 0.23 mK.
This work was done by Talso Chui and Yury Mukharsky of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the category.
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Electrostrictive Thermal Break Between Superfluid Reservoirs
(reference NPO20406) is currently available for download from the TSP library.
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