The advent of cryocooler technology ushers in an era where a cryogenic environment is created and maintained locally. It is no longer necessary to transport cryogen from a factory where it is produced to the location where it is needed. One problem remains for cooling large cryogenic systems with cryocoolers that extract more than 50 W of power. Although high-capacity cryocoolers are available from several commercial sources, the thermal link for connecting the cryocooler to a large system is not yet available. For small systems requiring less heat extraction, flexible heat straps made of braided copper wires are available for the thermal link. For heat extraction of more than 50 W at −100 K, this type of heat strap cannot be made short enough and flexible enough.

The flexibility is determined by the mechanical load that the cryocooler can withstand and the differential movement of the cryocooler relative to the system it cools, due to thermal contraction. Typically, the rated load that a cryocooler can handle is 10 kg at any direction. If the heat strap is not flexible enough, thermal contraction would break the cryocooler. One can make the strap longer to increase its flexibility, but that also increases the thermal resistance. For heat extraction of more than 50 W, the thermal resistance would become unacceptably large if it is made flexible enough.

Using a cryocooler with the flexible, gravity-fed heat pipe for cooling reduces the required space to about 2 square meters.

A flexible heat pipe was designed, fabricated, and used to cool a radiation shield of a large cryogenic/vacuum chamber to −100 K. The chamber has a diameter of 1.8 m and a height of 3.5 m. A tank (not shown) is prefilled with a known amount of nitrogen gas. The gas is piped into a conical-shaped condenser through a flexible bellows, where it condenses into liquid nitrogen. The condensed liquid nitrogen is channeled into another flexible bellows by a conical-shaped funnel. The bellows feed the liquid nitrogen into an evaporator using gravity. The evaporator is attached to the object being cooled by a good thermal joint. Liquid nitrogen turns into gaseous nitrogen at the evaporator, and absorbs heat from the object being cooled. The evaporated gas returns to the condenser through another bellows, and the cycle repeats. A heater and a thermometer are mounted on the condenser. A control loop is used to keep the condenser at a temperature slightly above the freezing temperature of nitrogen. Instead of using nitrogen, other gases can be used to make the heat pipe work at other temperatures close to the boiling points of these gases.

NASA is seeking partners to further develop this technology through joint cooperative research and development. Please contact Mark Homer at This email address is being protected from spambots. You need JavaScript enabled to view it. to initiate licensing discussions. Follow this link here  for more information.

Tech Briefs Magazine

This article first appeared in the March, 2018 issue of Tech Briefs Magazine.

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