Thermal Coupling Device For Cryogenic Refrigeration

Abstract

Coupling between a refrigerant source and a device to be refrigerated is mechanically accomplished at room temperature by a liquid metal-filled bellows between the refrigeration source and the device. When the refrigeration source provides cooling, the liquid metal freezes to provide inter-engaging clamping force and a conductive thermal coupling.

Description

BACKGROUND OF THE INVENTION

This invention is directed to a thermal coupling interconnecting a device to be cooled and a cryogenic refrigerator.

When a cold finger of a refrigerator is mechanically coupled to a device to be cooled and the device is mounted on a glass dewar, misalignment can cause breakage of the glass dewar. The usual prior art includes no provision for accommodating for such misalignment. Thus, the prior art structures were either adequately aligned or the dewar was broken.

An example of prior thermal coupling is disclosed in K. W. Cowans' U.S. Pat. No. 3,306,075, the entire disclosure of which is incorporated herein by this reference.

SUMMARY OF THE INVENTION

In order to aid in the understanding of this invention, it can be stated in essentially summary form that it is directed to a thermal coupling for coupling a device to be refrigerated with a cryogenic refrigerator. The thermal coupling includes an annular member filled with metal which is liquid at room temperature connected so that mechanical coupling is accomplished within the annulus and, upon refrigeration, the liquid metal freezes and contracts to make a rigid mechanical coupling within the annulus for good thermal conductivity.

It is thus an object of the invention to provide a thermal coupling for coupling a device with a cryogenic refrigerator. It is another object to provide a thermal coupling which accommodates for mechanical misalignment at room temperature resulting from assembly or manufacture, and yet provide a rigid, firm thermal coupling while thermal transfer at cryogenic temperatures is incurred. It is a further object to provide a bellows which has liquid mercury therein and a thermal transfer neck which extends into the bellows so that, when refrigeration occurs, the mercury solidifies and clamps the bellows onto the thermal transfer neck.

Other objects and advantages of this invention will become apparent from a study of the following portion of the specification, the claims and the attached drawings.

FIG. 1 is a perspective view of a dewar installed upon a refrigerator, the refrigerator being broken away.

FIG. 2 is a vertical section taken generally along the line 2--2 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The drawings illustrate a dewar 10 mounted upon a refrigerator 12. Dwwar 10 has an interior wall 14 and an exterior wall 16 which enclose a space 18. Conventionally, space 18 is evacuated and the surfaces of the walls 14 and 16 facing into the space are silvered to minimize heat transfer by radiation and by conduction. The major portion of dewar 10 is made of glass, and thus has the breakability of glass structures. The front of dewar 10 includes a window 20 of suitable material for transmission of radiation of interest. When the radiation of interest is infra-red, a germanium window can be employed to transmit radiation in the 10-micron band.

Interior wall 14 is in the form of a cylindrical tube, and mounted upon the front end of this tube behind window 20 is device 22 to be cooled. Device 22 may be an infra-red radiation detector. On the other hand, some devices, such as parametric amplifiers, need to be cooled for electronic reasons and do not need to face a window. Device 22 could be such a device. Interior wall 14 engages baseplate 24 upon which device 22 is mounted. Extending inwardly, down the interior of the tube defined by interior wall 14 is thermal transfer neck 26. Neck 26 is in good thermal communication with baseplate 24, and, in order to maintain this good thermal communication, it is preferably formed of the same piece of material. A thermally conductive metal such as copper is preferred. Thus, device 22 is in good thermal communication with thermal transfer neck 26. Thermal transfer neck 26 is of circular cross-section and is preferably cylindrical or slightly tapered. When the device is tapered, the smaller end is to the right, as shown in FIG. 2.

Cold finger 28 is the refrigerated portion of a cryogenic refrigerator. Cold finger 28 can be a Joule Thomson type cold finger, such as is shown in E. W. Peterson et al. U.S. Pat. No. 3,269,140 or in J. S. Buller et al. U.S. Pat. No. 3,640,091. On the other hand, the cold finger can be the expansion cylinder of a cryogenic refrigerator, such as is shown in K. W. Cowans' U.S. Pat. No. 3,379,026 or A. G. Dehne U.S. Pat. No. 3,530,681. The entire disclosures of each of these patents is incorporated herein by this reference. Thus, cold finger 28 is the source of refrigeration.

Bellows 30 is directly secured to cold finger 28. It is attached so that the open interior of the bellows is directly in engagement with the end of the cold finger. Bellows 30 has a corrugated outer wall and an inner wall 32 which is of such shape as to fit the exterior of thermal transfer neck 26. Thus, inner wall 32 is preferably cylindrical, although it may be optionally frusto-conical, as previously described. Inner wall 32 of bellows 30 is of such dimension that it is in slip-fit relationship with the exterior of thermal transfer neck 26, when the entire structure is at room temperature. Thus, there are several thousandths of an inch clearance between the interior of inner wall 32 and the exterior of thermal transfer neck 26 when they are at room temperature. The corrugated outer wall of bellows 30 is such as to provide a substantial amount of mechanical flexure between the inner wall 32 and cold finger 28. Thus, misalignments between the thermal transfer neck 26 and cold finger 28 are taken up by flexure of the corrugated exterior wall of the bellows.

Mercury 34 fills the bellows. The mercury is liquid at room temperature and permits flexure of the bellows to accommodate misalignment, when the dewar is being installed upon the refrigerator. After installation, with cooldown of the refrigerator, mercury 34 freezes and contracts. The contraction of the mercury as it solidifies contracts inner wall 32 so that inner wall 32 becomes firmly clamped upon the exterior of thermal transfer neck 26 at cryogenic temperatures. Such temperatures are reached below the temperature of evaporating solid carbon dioxide at atmospheric pressure.

Once the mercury is frozen and inner wall 32 is clamped on thermal transfer neck 26, there is a solid metal-to-metal path from the end of the cold finger through the frozen mercury and through the clamp joint to the thermal transfer neck whereby the refrigeration is directly and efficiently transferred to device 22.

By this means, misalignment is accommodated for and good thermal coupling is accomplished.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Accordingly, the scope of this invention is defined by the scope of the following claims.

Claims

What is claimed is:

1. A thermal coupling for a cryogenic refrigerator having a refrigerated surface means for causing cooling which is enclosed by a dewar having wall means for defining a portion of said dewar, a device to be cooled mounted upon said dewar wall means, the improvement comprising:

a metallic thermal transfer neck;

a hollow metallic annulus having an inner wall embracing around said thermal transfer neck, said annulus having metal therein which is liquid at room temperature, said annulus being dimensioned so that it is movable onto and off of said neck when said metal within said hollow annulus is liquid and is clamped in metal-to-metal relationship with said neck when said metal in said annulus is solidified by refrigeration;

said neck being mounted on and being in thermal communication with one of said means and said annulus being mounted on and in communication with the other of said means so that, when said surface means is refrigerated, there is metal-to-metal thermal communication between said surface means and said wall means.

2. The apparatus of claim 1 wherein said refrigerated surface means is on a cold finger which forms a part of said refrigerator, said refrigerated surface means being on said cold finger.

3. The apparatus of claim 2 wherein said annulus is mounted upon said cold finger and said neck is mounted upon said wall means of said dewar.

4. The apparatus of claim 3 wherein said annulus is a bellows having an interior wall which is a surface of revolution and having a corrugated outer wall, said outer wall being mounted upon said cold finger so that liquid metal within said bellows is in direct contact with said refrigerated surface means of said cold finger.

5. The apparatus of claim 4 wherein said thermal transfer neck has an exterior surface which is a surface of revolution corresponding to the surface of revolution of said interior wall of said bellows.

6. The apparatus of claim 3 wherein said dewar has an outer wall spaced from said wall means, said wall means being tubular and receiving said cold finger therein, said outer wall of said dewar substantially protecting said device from thermal conduction with respect to the exterior environment.

7. The apparatus of claim 6 wherein said annulus is a bellows having an interior wall which is a surface revolution and having a corrugated outer wall, said outer wall being mounted upon said cold finger so that liquid metal within said bellows is in direct contact with said refrigerated surface means of said cold finger.

8. The apparatus of claim 7 wherein said thermal transfer neck has an exterior surface which is a surface of revolution corresponding to the surface of revolution of said interior wall of said bellows.