Cryogenic cooler thermal coupler

A thermal coupler assembly mounted to the coldfinger of a cryogenic cooler which provides improved thermal transfer between the coldfinger and the detector assembly mounted on the dewar endwell. The thermal coupler design comprises a stud and spring-loaded cap mounted on the coldfinger assembly. Thermal transfer is made primarily through the air space between the cap and coldwell walls along the radial surfaces. The cap is spring loaded to provide thermal contact between the cap and endwell end surfaces.

BACKGROUND OF THE INVENTION 
The present invention relates to infrared energy receivers which use a 
cryogenic refrigerator to cool an infrared detector assembly, and more 
specifically to the design of the thermal interface between the 
refrigerator and the vessel which holds the detector assembly. 
In infrared receivers, a detector array comprising, for example, 
semiconductor materials, is mounted in a vacuum vessel (or "dewar"). The 
outer wall of the dewar forms a well ("coldwell"), typically cylindrical 
in shape, which intrudes into the body of the vessel, forming a sleeve 
which holds the cooling member (or "coldfinger") of the refrigerator. The 
detector assembly is mounted inside the dewar at the end of the 
cylindrical well ("endwell"), in thermal contact with the coldfinger. 
Infrared energy passes through a window in the outer wall of the dewar, 
striking the detector assembly mounted on the endwell. 
The design of the thermal interface between detector and refrigerator 
assemblies is difficult because of the modular design of the assemblies 
(which is the result of other system constraints) and because of the range 
of temperatures the device is exposed to (typically 77K to 300K). First, 
the coupler must provide good thermal conductance between the coldwell and 
coldfinger. Second, in order to do this, the thermal coupler must include 
some means of adjusting to the variable distance between the coldfinger 
and coldwell ends caused by the accumulation of mechanical tolerances in 
the cooler/coldfinger and dewar and for differences in material 
contractions when cooled to operating temperature. Third, the thermal 
coupler must minimize vibration transmitted to the detector assembly from 
the refrigerator to the coldwell, because the glass endwell and detector 
assemblies are fragile, and because transmitted vibrations may result in 
microphonic image degradation. 
The thermal coupler design problem has been solved in a number of ways. One 
method has been to insert a "fuzz button" comprising gold coated copper 
wool which has been impregnated with a thermally conductive grease into 
the gap between coldfinger and endwell, thus providing physical as well as 
thermal contact between subassemblies. This approach has had several 
problems. First, because of the inelasticity of the copper wool, the 
button has to be replaced each time the unit is disassembled, making field 
repairs difficult. Second, it is difficult to guage the amount of wool 
required to fill the gap which varies from unit to unit because of the 
different tolerance build-ups, resulting in unpredictable thermal 
conductivity. As a result, a highly skilled technician is required to 
install the fuzz button. Third, if too large a button is inserted in the 
coldwell, the glass endwell may fracture when the unit is assembled. 
In a second thermal coupler design (U.S. Pat. No. 3,999,403, entitled 
"Thermal Interface for Cryogen Coolers"), heat transfer is provided by a 
thermally conductive bellows placed between the coldfinger and endwell. 
The bellows expands or contracts as the coldfinger length changes in 
response to temperature changes, or to accommodate differences in 
tolerance build-up between units. This design has the disadvantages that 
the bellows travel is generally limited, and that the thermal conductivity 
is generally low because of the bellows structure. 
In another design, (U.S. Pat. No. 3,851,173, entitled "Thermal Energy 
Receiver"), the thermal coupler comprises a spring loaded cap mounted over 
the end of the coldfinger, such that the cap and endwell are in physical 
and thermal contact. A flexible conductive material connects cap and 
coldfinger to provide increased thermal transfer between the two. An 
adapter having an "H"-shaped cross-section fits over, and is brazed onto 
the coldfinger. The upper portion of the adapter is open and seats a coil 
spring together with the flanged cap which engages the detector endwell by 
pressure of the spring. The heat transfer mechanism is primarily through 
the spring and cap, or if necessary through an additional flexible 
conductive cable placed between the cap and "H"-shaped adapter. This 
configuration has several disadvantages. First, because of the high spring 
pressure required to affect good thermal transfer, the coupler also tends 
to transmit vibration from the refrigerator motor to the detector 
assembly, which may stress or fracture the endwell, detector assembly or 
both. A second disadvantage is that it requires a larger coldwell diameter 
which may not be practical given other system design constraints. Another 
disadvantage of this design is that thermal grease placed between the cap 
and endwell to improve thermal transfer may enter the inner gap between 
the adapter and cap. As a result, when the device cools, thermal grease in 
the inner gap may freeze, locking the cap to the coldfinger so that as the 
coldfinger shrinks, the cap pulls away from the endwell, thereby 
decreasing the coupler's thermal conductivity, and thereby increasing the 
possibility of mechanical vibration of the detector. Another disadvantage 
is that the maximum length of the assembly is not mechanically constrained 
so that repair of the unit in the field is more complicated. 
In a fourth coupler design (U.S. Pat. No. 4,324,104, entitled "Noncontact 
Thermal Interface"), a cap-shaped adapter, matched to the shape of the 
endwell, is fixed to the end of the coldfinger. Thermally conductive shims 
are placed between the cap and the coldfinger to adjust the gap between 
the adapter and endwell. The contours of the adapter/shim and endwell are 
matched such that the gap between the two is approximately one ten 
thousandth (0.0001) of an inch, the smallest gap practicable while taking 
into account the differential expansion rates of the metal coldfinger and 
glass coldwell. A thermally conductive hydrocarbon or inert gas is placed 
in the gap between the endwell and adapter to improve thermal coupling. It 
should be noted that because the cooling function of the refrigerator is 
directed to the end of the coldfinger, the performance of this coupler 
depends primarily on maintenance of the gap between adapter and endwell on 
radial and axial surfaces. Thus, one of the disadvantages of this design 
is that it requires separate measurement of each device, and physical 
accommodation for the differences in length between the coldfinger and 
coldwell. Thus, this design is not generally suited for a production 
environment. A second disadvantage is that because the gap is difficult to 
maintain along the axial surfaces of the adapter and endwell, there is 
generally a large temperature drop across the end surface of the endwell 
and interchangeability of couplers between units is most likely not 
possible. 
It is accordingly an object of the present invention to provide an improved 
heat transfer mechanism between the cooler and detector of an infrared 
receiver mounted in a vacuum dewar vessel. 
Another object of the present invention to provide a device for integration 
of a cryogenic cooler with a detector dewar assembly which will 
automatically accommodate tolerance differences in size between the cooler 
and detector dewar subassemblies. 
It is a further object of the present invention to provide an infrared 
receiver having increased cooling capacity with reduced physical contact 
between the cooler and detector assemblies such that vibration effects are 
minimized. 
Still another object of the present invention is to provide a thermal 
coupler assembly which is both easy to assemble, measure, and test, and is 
easily integrated into an infrared receiver. 
SUMMARY OF THE INVENTION 
The above and other objects of the present invention are achieved by 
providing a thermal coupler which does not rely solely on either direct 
contact designs, which typically have vibration problems, or on noncontact 
thermal transfer design principles, which require accurate measurement of 
the relative lengths of the coldfinger and coldwell to affect optimum 
thermal transfer. The thermal coupler of the present invention uses the 
principle of noncontact thermal transfer between the radial surfaces of a 
cylindrical stud mounted on the coldfinger end, and a surrounding cap, 
which is held in place by a low strength spring between stud and cap and a 
retaining pin. Thermal transfer to the endwell is primarily through gas or 
air in the gap between stud and cap (which is a controlled tolerance), and 
by light but direct contact of the cap and coldwell at the endwell, and by 
gas or air trapped between the cap and coldwell walls on their radial 
surfaces. The stud diameter is less than that of the coldfinger, such that 
the cap may closely fit both the stud and the inner coldwell diameter 
without redesign. The low strength spring placed between the stud and cap 
improves alignment and initial positioning of the cap without creating 
vibration problems. The cap is captured by a pin through the stud so that 
the coupler remains in one piece when the system is disassembled. The pin 
may be bonded, such as by soldering, to the cap after assembly of the 
coupler.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
Referring to FIG. 1, a portion of an infrared receiver is shown, 
specifically the portion comprising the thermal interface between the 
coldfinger 22 of a cryogenic refrigerator (not shown), and a portion of a 
vacuum dewar assembly 10 which houses detector assembly 16. The detector 
assembly 16 is positioned on surface 40 of coldwell 18, facing window 14 
in wall 12 of dewar 10 which allows the transmission of energy of the 
appropriate wavelengths to detector assembly 16. 
Referring to FIG. 2, the thermal coupler of the present invention comprises 
a cylindrical stud 28 with matching up-shaped cap 24, biasing spring 30 
which separates stud 28 and cap 24, and retaining pin 26 which sits in 
slot 42 of stud 28, limiting the maximum displacement of cap 24. The pin 
26 may be bonded, such as by soldering, to the cap 24 after assembly of 
the coupler. Stud 28, cap 24 and pin 26 are all made of a corrosion 
resistant, high thermal conductivity metal which has the appropriate 
structural characteristics. This might include high purity nickel (i.e. 
99.5% pure, or better), silver, or gold alloys. Spring 30 comprises a 
corrosion resistant metal, e.g., cadmium plated steel, having relatively 
low spring tension. 
The thermal coupler comprising parts 24, 26 28 and 30 is designed such that 
air gaps 50, 52 and 54 between the inner wall 60 of coldwell 18 and cap 
24, and between cap 24 and stud 28 are 0.0005 inches wide, or less. The 
diameter of the stud 28 is selected to maximize contact area with 
coldfinger 22, in order to reduce thermal gradients between the coupler 
and coldfinger, and to improve coupler performance. In addition, the inner 
diameter of the cap 24 is selected such that the thickness of radial end 
portions of cap 24 minimize thermal gradients between coldwell 18 and 
coldfinger 22, while providing necessary structural integrity. In one 
embodiment, the thickness of cap 24 is uniform along radial and axial 
surfaces. 
The lengths of stud 28 and cap 24 are selected to maximize the overlap of 
cap 24 and stud 28 surfaces adjoining gap 54, thus providing maximum 
thermal transfer radially across gap 54, and providing thermal transfer 
between cap 24 and surface 62 of coldwell 18 for the full range of 
accumulated tolerances caused by manufacturing errors and thermal effects. 
Stud 28 is mounted on the coldfinger 22 using a high thermal conductivity 
bonding method (e.g., soldering), or in an alternate embodiment may 
comprise an extension of coldfinger 22. When the coldfinger 22 with 
coupler is inserted into coldwell 18, spring 30 extends, placing the cap 
24 into contact with surface 62 of coldwell 18. If necessary, thermal 
grease or some other thermally conductive material may be placed in gap 56 
in order to maximize thermal transfer. If such grease or material is not 
used, then gap 56 would be eliminated such that surface 62 of coldwell 18 
and cap 24 are in physical contact.