Abstract:
A cryogenic cooling system ( 12 ) for cooling electromagnetic energy detectors ( 50 ). The cooling system ( 12 ) includes a first mechanism ( 18 ) that accommodates cryogen fluid in one or more spaces ( 58, 60 ). A second mechanism ( 16, 42 ) freezes the cryogen fluid in the one or more spaces ( 58, 60 ) adjacent to the electromagnetic energy detectors ( 50 ). In a specific embodiment, the electromagnetic energy detectors ( 50 ) comprise an infrared focal plane array ( 50 ). The second mechanism ( 16, 42 ) includes a heat exchanger ( 16 ) that is mounted separately from the first mechanism ( 18 ). The one or more spaces ( 58, 60 ) are fitted with three-dimensional cooling interface surfaces ( 62, 64 ). The three-dimensional cooling surfaces ( 62, 64 ) are implemented via a thermally conductive matrix ( 62, 64 ). The thermally conductive matrix ( 62, 64 ) is a copper metal matrix or carbon/graphite matrix, and the solid cryogen reservoir ( 18 ) is a beryllium reservoir ( 18 ). The solid cryogen reservoir ( 18 ) includes integrated mounting features ( 52, 54 ) for mounting the reservoir ( 18 ) to a missile housing and a surface for attaching the focal plane array ( 50 ) to the reservoir ( 18 ). The second mechanism ( 16, 42 ) includes a Joule-Thomson orifice ( 42 ) that employs the Joule-Thomson effect to cool the cryogen fluid to a solid state. The first mechanism ( 18 ) includes a selectively detachable cryogen canister that provides pressurized cryogen fluid to the heat exchanger ( 16 ). The heat exchanger ( 16 ) directs cooled pressurized cryogen fluid to the solid cryogen reservoir ( 18 ) and Joule-Thomson orifice ( 42 ) and is positioned remotely from the cryogen reservoir ( 18 ). In an illustrative embodiment, the heat exchanger ( 16 ) outputs cooled cryogen gas to plural solid cryogen reservoirs ( 18 ) to cool plural corresponding infrared focal plane arrays ( 50 ). A line cutter selectively detaches the gas canister and/or the heat exchanger ( 16 ) from the missile in response control signal from a computer. The computer generates the control signal after a predetermined amount of the cryogen fluid is present in the cryogen reservoir ( 18 ) or after a predetermined time interval.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to cooling systems. Specifically, the present invention relates to cryogenic cooling systems for cooling focal plane arrays. 
     2. Description of the Related Art 
     Cryogenic cooling systems are employed in various demanding applications including military and civilian active and remote sensing, superconducting, and general electronics cooling. Such applications often demand efficient, reliable, and cost-effective cooling systems that can achieve extremely cold temperatures below 80 degrees Kelvin. 
     Efficient cryogenic cooling systems are particularly important in sensing applications involving high-sensitivity infrared focal plane arrays of electromagnetic energy detectors (FPA&#39;s). An FPA may detect electromagnetic energy radiated or reflected from a scene and convert the detected electromagnetic energy into electrical signals corresponding to an image of the scene. To optimize FPA imaging performance, any FPA detector nonuniformities, such as differences in individual detector offsets, gains, or frequency responses, are corrected. Any spatial or temporal variations in temperature across the FPA may cause prohibitive FPA nonuniformities. 
     FPA&#39;s are often employed in missile targeting applications, where weight, size, and spatial and temporal uniformity of cryogenic cooling systems are important design considerations. An FPA must operate at stable cryogenic temperatures for maximum performance and sensitivity. 
     Conventionally, a cooling fluid is applied to the FPA via a cooling interface. Heat is transferred to the cooling fluid from the FPA. The heated fluid is then expelled from the missile or re-cooled via a heat exchanger integrated into the FPA. The cooling fluid requires a heavy and bulky FPA cooling interface and heat exchanger, which are attached to the FPA mounting assembly. Consequently, the FPA assembly must have additional mechanical support to secure the interface, heat exchanger, and cooling fluid. The bulky components and additional support hardware may require additional cooling, which increases demands placed on the cooling system. The bulky support structure, conventionally thought to improve temperature stability, may conduct excess heat from the warm missile body into the FPA, thereby reducing system cooling efficiency. Furthermore, the additional bulky mechanical FPA support hardware may cause alignment problems with the on board optical or infrared system during installation and operation, thereby increasing installation and operating costs. In addition, missile maneuvering may cause the cooling liquid to slosh in the cooling interface, creating undesirable temperature instabilities. 
     Alternatively, Joule-Thompson cycle coolers are employed. A Joule-Thomson cycle cooler typically applies a regulated flow of cold gas over the infrared FPA. However, Joule-Thompson cycle coolers require undesirably expensive and bulky compressed gas canisters that must remain on the missile, aircraft, or other system. The additional weight increases the overall operating costs and reduces maneuvering capability and range of the accompanying system. Furthermore, excessive shock or vibration environments from missile maneuvering may interrupt gas flow, thereby creating potentially prohibitive temperature instabilities, resulting in reduced missile performance. 
     To address size and cost issues associated with using gas canisters, compressors, or other heat exchangers, more advanced construction materials are under continual development. In addition, researchers are attempting to design FPA&#39;s with reduced cooling requirements. Unfortunately, this has matured slowly and does not promise satisfactory solutions for high performance applications in the foreseeable future. 
     Hence, a need exists in the art for an efficient cryogenic cooling system for uniformly cooling an infrared FPA. There exists a further need for a cryogenic cooling system that efficiently employs a solid cryogen to cool an FPA with minimal weight and size impact. 
     SUMMARY OF THE INVENTION 
     The need in the art is addressed by the cryogenic cooling system for cooling electromagnetic energy detectors of the present invention. In the illustrative embodiment, the inventive system is adapted to cool infrared focal plane arrays. The system includes a first mechanism for accommodating cryogen fluid in one or more spaces. A second mechanism freezes the cryogen fluid in the one or more spaces adjacent to the electromagnetic energy detectors. 
     In a more specific embodiment, the electromagnetic energy detectors comprise one or more focal plane arrays. The second mechanism includes a heat exchanger that is mounted separately from the first mechanism. The one or more spaces are fitted with three-dimensional cooling interface surfaces. The first mechanism includes a solid cryogen reservoir having a thermally conductive matrix for implementing the three-dimensional cooling surfaces. The thermally conductive matrix is a copper, graphite, or beryllium matrix, and the solid cryogen reservoir is a beryllium reservoir. 
     The solid cryogen reservoir includes one or more mounting features for mounting the reservoir and has a surface for mounting the focal plane array on the reservoir. The second mechanism includes a mechanism for employing the Joule-Thomson effect (also called the Joule-Kelvin effect) to cool the cryogen fluid to a liquid state. The first mechanism includes a selectively detachable cryogen canister for providing pressurized cryogen fluid to the heat exchanger. 
     In an illustrative embodiment, the heat exchanger outputs cooled cryogen gas to plural solid cryogen reservoirs to cool plural corresponding infrared focal plane arrays. The cryogenic cooling system is mounted on or within a missile system. The cryogenic cooling system is connected to a cryogen canister and a heat exchanger for providing the cryogen fluid to a cryogen reservoir with three-dimensional cooling surfaces. A Joule-Thomson orifice employs the Joule-Thomson effect to create the cryogen fluid output from the heat exchanger. 
     The heat exchanger, which is positioned separately from the reservoir, employs a conduit to direct the fluid to the cryogen reservoir. An additional mechanism selectively detaches the gas canister and/or the heat exchanger from the missile after a predetermined amount of the fluid is collected within the cryogen reservoir or after a predetermined time interval. 
     The novel design of the present invention is facilitated by the second mechanism, which freezes cryogen in a cooling interface adjacent to a focal plane array. Freezing the cryogen enables remote positioning of the heat exchanger relative to the cooling interface. The cooling interface and accompanying focal plane array assembly no longer require mounting of the heat exchanger in the same assembly to increase the temperature stability of the focal plane array. The frozen cryogen in combination with the efficient solid cryogen cooling interface of the present invention provides sufficient temperature stability. Consequently, costs, cooling inefficiencies, and sensor alignment problems associated with conventional cooling systems are avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a missile system employing a solid cryogen infrared Focal Plane Array (FPA) cooling system constructed in accordance with the teachings of the present invention. 
     FIG. 2 is a perspective view showing the heat exchanger and solid cryogen cooling interfaces of the solid cryogen cooling system of FIG.  1 . 
     FIG. 3 is a perspective view showing an alternative embodiment of a cryogen cooling interface and accompanying FPA assembly of FIG.  2 . 
    
    
     DESCRIPTION OF THE INVENTION 
     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
     FIG. 1 is a diagram of a missile system  10  employing a solid cryogen infrared Focal Plane Array (FPA) cooling system  12  constructed in accordance with the teachings of the present invention. For clarity, various well-known components, such as power supplies, actuators, heat exchanging coils, explosives compartments, and so on, have been omitted from the figures. However, those skilled in the art with access to the present teachings will know which components to implement and how to implement them to meet the needs of a given application. 
     The cooling system  12  includes a pressurized cryogen gas canister  14 , a heat exchanger  16 , a solid cryogen FPA cooling interface  18 , temperature sensors  20 , a missile computer  22 , and a line cutter  24 . The heat exchanger  16  is connected to the pressurized cryogen gas canister  14  via a high-pressure input line  26  that is connected to an electrically controlled valve  28  at the output of the gas canister  14 . An exhaust output line  30  is connected to the heat exchanger  16  at one end and to a flange  32  at the opposite end. The exhaust flange  32  is attached to a wall of the missile  10  so that exhaust gases may escape from the missile  10 . The exhaust output line  30  includes a flexible bellows  34  to provide mechanical and thermal isolation of the heat exchanger  16  from exhaust flange  32  and body of the missile  10 . The flexible exhaust bellows  34  may or may not include a pump depending on the demands of a given application. 
     The heat exchanger  16  is connected to the solid cryogen FPA cooling interface  18  via a flexible output pressure line  36  and an input exhaust line  38 . A sensor suite  40  is mounted on the FPA cooling interface  18 . One or more temperature sensors  20  provide temperature input to the missile computer  22 , which may send control signals to the line cutter  24  and to the electronically actuated cryogen canister valve  28 . 
     In operation, the missile computer  22  activates the cooling system  12  by opening the electrically controlled valve  28  via a control signal sent thereto. Pressurized cryogen gas is then transferred to the heat exchanger  16  via the high-pressure line  26 . The pressurized cryogen gas passes through various heat exchanging coils or other heat-exchanging mechanisms in the heat exchanger  16  before being transferred to the FPA cooling interface  18  via the output pressure line  36 . On the initial pass, the pressurized cryogen gas is not optimally cooled by the heat exchanger  16 , since cooling exhaust gasses have not yet been generated to facilitate cooling of the input cryogen gas. 
     The pressurized cryogen gas entering the FPA cooling interface  18  passes through a Joule-Thomson orifice  42 , where the gas is depressurized as it enters the FPA cooling interface  18 . Due to the Joule-Thomson effect, the depressurized gas passing through the FPA cooling interface  18  becomes sufficiently cold to enter a saturated state and liquefy. Any liquid cryogen that is caught in the FPA cooling interface  18  eventually freezes. Remaining gas that has not liquefied in the cooling interface  18  is directed back through the heat exchanger  16  via the input exhaust line  38 . 
     Unlike conventional systems, the heat exchanger  16  is remotely positioned relative to the cryogen FPA cooling interface  18  and accompanying sensor suite  40 . This facilitates mounting of the sensor suite  40  and accompanying FPA&#39;s via smaller, lighter, and more cost-effective mounting structures. 
     By connecting the heat exchanger  16  to the FPA cooling interface  18  via the flexible pressure line  36  and the exhaust line  38  having a flexible coupling  44  included therein, the motion and vibration of the relatively heavy heat exchanger  16  is isolated from the sensor suite  40  and accompanying FPA cooling interface  18 . Consequently, abrupt missile maneuvers that move the heavy heat exchanger  16  are less likely to disrupt operations of the sensor suite  40 . The flexible exhaust coupling  44  may include a pump to facilitate circulation of exhaust gases in the cooling system  12 . 
     Cold cryogen exhaust gas returning from the FPA cooling interface  18  cools incoming pressurized gas in the heat exchanger  16 . This process raises the temperature of the exhaust gas, which is directed out of the missile  10  via the output exhaust line  30 . 
     Some depressurized cryogen gas passing through the FPA cooling interface  18  eventually liquefies and then freezes in the FPA cooling interface  18 . After cessation of gas flow, the internal pressure of the FPA cooling interface  18  decreases, enabling the liquid to boil and causing the cryogen in the FPA cooling interface  18  to freeze. The solidified cryogen improves temperature stability across the cooled FPA&#39;s in the sensor suite  40 . The temperature remains relatively constant in time and position across the surface of a cooled FPA. By employing the special cryogen FPA cooling interface  18  to cool an FPA via solid cryogen, both temporal and spatial temperature stability are enhanced. This may significantly enhance the operation of the FPA and accompanying sensor suite  40 . This may also simplify nonuniformity correction circuitry and algorithms required to compensate for FPA detector nonuniformities. 
     In the present specific embodiment, the cryogen gas is Argon. However, other types of cryogen gas, such as Krypton, Nitrogen, Neon, or Hydrogen, may be employed without departing from the scope of the present invention. 
     Strategically positioned temperature sensors  20  enable software running on the missile computer  22  to determine when the cooling interface  18  has reached a desired temporal and spatial temperature stability and/or uniformity. The software running on the missile computer  22  then actuates the line cutter  24 , which cuts the input pressure line  26 , enabling the pressurized cryogen gas canister  14  to release from the missile  10 . The missile  10  continues flying as frozen cryogen in the FPA cooling interface  18  continues to efficiently cool the FPA&#39;s in the sensor suite  40 . 
     Hence, the missile computer  22  runs software to actuate the line cutter  24  when the solid cryogen FPA cooling interface  18  reaches a predetermined temperature and/or spatial and temporal temperature stability and uniformity as determined via the temperature sensors  20 . Those skilled in the art with access to the present teachings may easily construct this software without undue experimentation. 
     Those skilled in the art will appreciate that various modules shown in FIG. 1 may be omitted or replaced with other types of modules without departing from the scope of the present invention. For example, the temperature sensors  20  for determining when to actuate the line cutter  24  may be replaced with a timer or mechanical mechanism to determine when to actuate the line cutter  24 . Furthermore, the missile computer  22 , the electrically controlled nozzle  28 , and/or the line cutter  24  may be omitted in various applications, such as those that do not require the release of the pressurized cryogen gas canister  14  from the missile  10 . In addition, some applications may demand that the heat exchanger  16  be released from the missile  10  along with the pressurized cryogen gas canister  14  when sufficient solid cryogen forms in the FPA cooling interface  18 . In this implementation, line cutters may be employed to cut the input exhaust line  38  and the output pressure line  36 . 
     A method adapted for use with the missile  10  and accompanying cryogenic cooling system  12  includes the following steps: 
     1. Launch the missile  10 . 
     2. Open valve  28  to release pressurized cryogen gas from the cryogen gas canister  14  to the heat exchanger  16 . 
     3. Employ the heat exchanger  16  to cool the incoming pressurized gas. 
     4. Depressurize the gas via a Joule-Thomson orifice  42  to release a freezing fluid in a solid cryogen cooling interface  18  having an integrated infrared FPA that is mounted remotely relative to the heat exchanger  16 . 
     5. Collect any resulting liquefied fluid in the solid cryogen interface  18  adjacent to an IR FPA, directing any remaining cold gaseous fluid (exhaust gas) back through the heat exchanger  16 . 
     6. Use the cold exhaust gas to cool incoming pressurized gas in the heat exchanger  16  before expelling the cool exhaust gas from the missile  10 . 
     7. After a predetermined amount of liquid cryogen is accumulated in the solid cryogen interface  18 , cut the pressure line  26  to the cryogen gas canister  14 . 
     8. Release the cryogen gas canister from the missile  10  and allow the liquid cryogen to boil, thereby cooling the cryogen to a solid. 
     Those skilled in the art will appreciate that some of the above steps may be omitted or interchanged with other steps without departing from the scope of the present invention. For example, the electrically controlled valve  28  at the output of the cryogen gas canister  14  may be opened before the missile  10  is launched, and steps 7 and 8, wherein the cryogen gas canister  14  is released from the missile  10  may be omitted in some applications. 
     FIG. 2 is a perspective view showing the heat exchanger  16  and two exemplary solid cryogen cooling interfaces  18  of the solid cryogen cooling system  12  of FIG.  1 . The input pressure line  26  is split into two output pressure lines  36  within the heat exchanger  16 . The output pressure lines  36  are fed to corresponding solid cryogen FPA cooling interfaces  18 . 
     The heat exchanger  16  may be adapted to accommodate several cooling interfaces. To accommodate a third cooling interface (not shown), the input pressure line  26  is separated into three output pressure lines, and the additional line goes to the third cooling interface. In the present specific embodiment, each cooling interface  38  has a separate return exhaust line  38 . The separate exhaust lines  38  feed cold exhaust gasses back to the heat exchanger  16  to cool incoming pressurized gas before being expelled from the missile  10  of FIG. 1 via the flexible bellows  34  and flange  32  of the output exhaust line  30 . The flex couplings  44  on the input exhaust lines  38  help isolate vibrations and movement of the heat exchanger from focal plane arrays  50  integrated with the cooling interfaces  18 . 
     Remotely positioning the heat exchanger  16  from the cryogen cooling interfaces  18  allows the single heat exchanger  16  to accommodate plural cooling interfaces  18 . This results in substantial size, weight, and cost savings, as fewer parts are required, which results in fewer installation, mounting, and FPA alignment problems. The ability to remotely position the heat exchanger  16  relative to the cooling interfaces  18  is facilitated by the use of solid cryogen, which is collected in the cooling interfaces  18 . When disposed in the cooling interfaces  18 , the solid cryogen provides sufficient spatial (or volumetric) and temporal temperature stability across the infrared FPA&#39;s  50  to obviate the need to incorporate the massive heat exchanger  16  into the FPA mounting assembly and cooling interface  18 . The efficient design of the cooling interfaces  18  enables the cooling interfaces  18  to act as both infrared FPA mounting assemblies and cooling interfaces. The FPA assemblies  50  are integrated with the cooling interfaces  18 . 
     The cooling interfaces  18  each include two side mounting features  52  and a front mounting feature  54  to facilitate stabilizing the cooling interfaces  18  within the body of the missile  10  of FIG.  1 . In the present specific embodiment, the mounting features  52  and  54  are constructed from the same block of material as the cooling interfaces  18 . The mounting features may be fitted with thermal insulation to prevent heat from transferring from the missile body to the cooling interfaces  18 . Hence, the solid cryogen cooling interfaces  18  efficiently integrate mounting features  52  and  54  and surfaces for mounting the FPA&#39;s  50  into single pieces  18 . 
     The cooling interfaces  18  include Joule-Thomson orifices  42 , which release pressurized gas from the output pressure lines  36  into the cooling interfaces  18 . As the cryogen gas exits the pressure lines  36  and passes into the interfaces  18  via the Joule-Thomson orifices  42 , the gas depressurizes sufficiently to initiate partial liquefaction of the cryogen gas in the interfaces  18 . Some of the liquefied cryogen is caught in the cooling interfaces  18  where it accumulates. After cessation of gas flow, the pressure in the cooling interfaces  18  is reduced, which allows a portion of the stored cryogenic liquid to boil, thereby cooling the remaining liquid until it freezes. 
     Each cooling interface  18  has a first cooling section  58  in fluid communication with a second cooling section  60 . Input cryogen gas is released from the Joule-Thomson orifice  42  into the first cooling section  58  before passing to the second cooling section  60 . For illustrative purposes, top surfaces of the cooling interfaces  18  are removed. In an actual implementation, the first section  58  and second section  60  are enclosed in the cooling interfaces  18 . In this realization, all cryogen gas entering the Joule-Thomson orifice  42  and not liquefying in the cooling sections  58  and  60  is transferred via the exhaust line  38  back to the heat exchanger  16 . One skilled in the art will appreciate that the exhaust gases exiting from the second cooling section  60  may pass directly out of the missile without passing first through the heat exchanger  16 . This may further simplify the configuration of the exhaust line  38  and allow the pressure within the cooling interface  18  to be reduced, thereby allowing the operating temperature of the solid cryogen to be reduced. While the utilization of the high pressure cryogenic gas may be less efficient in this implementation, a lower ultimate operating temperature may be achieved. The lost efficiency may be partially regained by adding a second cryogen gas flow to the heat exchanger  16 . This gas flow passes through a Joule-Thompson orifice (not shown) within the heat exchanger  16  and provides cooling of the cryogen gas passing through the heat exchanger  16  and then to the cooling interfaces  18  via the pressure lines  36 . 
     The first cooling section  58  represents a partial indentation in a cooling interface housing  66 . The first cooling section  58  is formed in the cooling interface housing  66  opposite the FPA  50 , which is mounted on a reverse side of the cooling interface housing  66 . The first cooling section  58  includes several pins  62 , which are integral to the beryllium cooling interface housing  66 . The pins  62  are strategically positioned and shaped to promote efficient thermal transfer between cryogen passing through the first cooling section  58  and the beryllium cooling interface housing  66 . The pins  62  form a metal matrix with plural spaces or compartments formed between the pins  62 . The plural compartments expand the thermally conductive surface area of the first section  58  and facilitate efficient cooling of the FPA  50 . The exact number of pins and the sizes and shapes of the pins  62  are application-specific and may be determined by one skilled in the art with access to the present teachings to meet the needs of a given application. 
     The second cooling section  60  receives cold cryogen gas from the first cooling section  58 . Cryogen gas that has not been trapped as frozen or liquefied cryogen in the first section  58  flows into the second section  60 . The second section  60  includes plural flanges  64  designed to optimize thermal transfer between liquid and solid cryogen freezing in the second section  60  and the FPA  50 . The plural compartments formed between the flanges  64  ensure that sufficient surface area of the cooling interface  18  contacts the frozen cryogen to achieve optimum FPA temperature stability. The relatively large volume of the second section  60  promotes long hold time and temperature stability of the FPA  50 . Those skilled in the art will appreciate that the second section  60  may be omitted in some applications without departing from the scope of the present invention. 
     The first section  58  and the second section  60  accommodate three-dimensional surfaces formed by the pins  62  and the absorber flanges  64 , respectively. The absorber flanges  64  may be replaced with another type of absorbent structure, such as a thermally conductive matrix or mesh absorbent, without departing from the scope of the present invention. For the purposes of the present discussion, a three dimensional surface is a surface that includes a plurality of surface dips, grooves, contours, or compartments for expanding the surface area over that of a substantially flat surface. 
     The cooling sections  58  and  60  and the infrared FPA  58  are positioned so that cold incoming cryogen gas initially cools the first section  58 , thereby cooling the FPA  50  first before being warmed by other features. This improves the efficiency of the cryogen cooling interface  18  by ensuring that the first section  58 , which is adjacent to the FPA  50 , remains at a spatially and temporally stable cryogenic temperature at or below 80 degrees Kelvin. 
     The second section  60  of the cooling interfaces  18  eventually contain solid cryogen. Unlike gas or liquid cooling systems, the solid cryogen does not slosh in response to missile maneuvers. Consequently, the cooling interface  18  can provide stable cryogenic temperatures to the FPA, which are stable in time and uniform across a given volume near the FPA  50 . 
     The flanges  64  are brazed to the body of the second section  60 . Copper mesh or graphite fiber may be used. The fibrous nature of the material prevents separation of the liquid and wicking material from the housing. A material should be chosen that can be joined to the housing of the cooling interfaces  18 . The material should wick the liquid cryogen efficiently to prevent it from being expelled out of the exhaust lines  38  when high-volume cryogen gas flow is occurring. 
     Those skilled in the art will appreciate that the copper pins  62  in the first cooling section  58  and the cooling flanges  64  in the second section  60  may be replaced with other features without departing from the scope of the present invention. For example, the pins  62  in the first cooling section  58  may be replaced with a sintered or foamed metal matrix, such as a metallic sponge, constructed via a sintering or a metal or graphite foaming process. The pins  62  may represent a copper metal matrix or a carbon/graphite matrix. 
     Employing frozen cryogen in the efficient solid cryogen cooling interfaces  18  to cool the infrared FPA&#39;s  50  allows the heat exchanger  16  to be positioned remotely from the FPA&#39;s  50  and corresponding mounting structure  66 . The large mass of the heat exchanger  16  is no longer required to increase the temperature stability of the FPA&#39;s  50 , since the frozen cryogen trapped in the efficient cooling interfaces  18  provides sufficient temperature stability. Unlike the heat exchanger  16 , which is connected to the cooling interfaces  18  and FPA assemblies  58  only via the pressure lines  36  and exhaust lines  38  and is mounted separately from the cooling interfaces  18 , conventional systems employ one heat exchanger for each FPA to be cooled. In these systems, a heat exchanger is mounted to each FPA assembly and/or cooling interface. This increases installation and parts costs and may create sensor alignment difficulties. 
     FIG. 3 is a perspective view showing an alternative embodiment of a cryogen cooling interface  18 ′ and accompanying FPA assembly  68 . The operation of the cryogen cooling interface  18 ′ is similar to the operation of the cryogen cooling interfaces  18  of FIG. 2 with the exception that the pins  62  of FIG. 2 are replaced with vertically oriented rectangular cooling plates  70 , and the infrared FPA  50  of FIG. 2 is replaced with a more elaborate infrared FPA assembly  68 . Furthermore, the mounting features  52  and  54  of FIG. 2 are omitted from the cooling interface  18 ′ of FIG.  3 . The rectangular cooling plates  70  form various volumetric sections  76  to promote heat transfer away from the FPA assembly  68 . 
     The FPA assembly  68  includes an additional saw-toothed fitting  72  designed to mate with and help stabilize a corresponding FPA assembly support structure  74 . In addition, the ridged fitting  72  promotes the conduction of heat away from FPA assembly  68  to the cooling interface  18 ′. The FPA assembly  68  is efficiently integrated with cooling interface  18 ′ to provide excellent temperature stability. 
     Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications, and embodiments within the scope thereof. 
     It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. 
     Accordingly,