Abstract:
A closed-cycle refrigerator provides cooling to extremely low temperatures, particularly in the range of 0.5 K to 2.0 K. A 4 K pulse-tube cryocooler cold head or G-M cryocooler cold head liquefies helium in a first cooling chamber at a pressure at approximately 1 atmosphere. Liquid helium flows from the first cooling chamber, through a Joule-Thomson valve, and into a second cooling chamber under a pressure differential created by a pump. Helium vapor extracted from the second cooling chamber by the pump is routed back to the first cooling chamber to be re-condensed. This closed-cycle design provides continuous cooling below 2 K. Cryocooler cold head cold sections have no physical contact with subsequent cooling elements, such as the first and second cooling chambers to reduce vibration transfer. In some embodiments the cryocooler cold head is connected to a vacuum chamber via a vibration damping coupler to further reduce vibration transfer.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims one or more inventions which were disclosed in Provisional Application No. 61/756211, filed Jan. 24, 2013, entitled “CLOSED CYCLE 1 K REFRIGERATION SYSTEM”. The benefit under 35 USC §119(e) of the United States provisional application is hereby claimed, and the aforementioned application is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention pertains to the field of cryorefrigeration. More particularly, the invention pertains to closed cycle cryorefrigerators for temperatures below about 4.2 Kelvin. 
         [0004]    2. Description of Related Art 
         [0005]    Liquid helium has a boiling point at 1 atm (atmosphere of pressure) of 4.2 K, and is by-product of natural gas production. Helium is found in two isotopes, helium-3 ( 3 He) and helium-4 ( 4 He). Helium-4 is by far the most abundant isotope, and constitutes over 99.9% of the helium on Earth. As such, liquid helium makes an ideal cooling medium for a wide range of low temperature devices and low temperature applications, such as particle detectors, Superfluid Helium Droplet Spectroscopy (SHeDS), Superconducting Quantum Interference Devices (SQUIDs), and construction of high accuracy gyroscopes, for example. 
         [0006]    Unfortunately, sources of natural gas that also contain helium are limited, and atmospheric helium migrates into space and is lost. The primary sources for helium production have been the United States, Russia, and a variety of other smaller producers across the globe. As helium is a limited resource, and world-wide demand has increased dramatically over several decades, the cost of this irreplaceable natural resource has also increased dramatically. According to a report by the United States Government Accounting Office (Urgent Issues Facing BLM&#39;s Storage and Sale of Helium Reserves, Feb. 14, 2013, incorporated herein by reference), the price of helium has tripled during the period from 2000 to 2012. 
         [0007]    In addition to cost considerations, helium is a critical natural resource that is a fundamental requirement for many technologies. According to the same GAO report, approximately 26% of helium consumption is directed at cryogenic applications such as superconducting magnets, basic science research, and industrial processing. Other major applications include: controlled atmospheres in manufacturing processes (22%), aerospace pressurization and purging operations (17%), and welding (17%), among others. 
         [0008]    Therefore it is clear that a shortage, or worse depletion, of world helium reserves could potentially have a dramatic negative effect on the production and use of critical technologies such as magnetic resonance imaging, fiber optics and semi-conductor manufacturing, space exploration, and military rockets, for example. 
         [0009]    In order to reduce the reliance of low temperature devices on a ready supply of helium, and reduce the boil-off rate of the helium contained in a device cryostat, cryocoolers, known in the art as “cold heads”, have been developed to remove heat directly from the helium in a cryostat, thereby reducing the rate at which it boils off to the ambient atmosphere, and extending the time required between refills. 
         [0010]    Cryocooler cold heads in common use operate on one of two principles: Gifford-McMahon cryocooler cold heads; and pulse-tube cryocooler cold heads. Both types of cryocooler are regenerative cryocollers, and generally operate on compression-expansion cycle and have a hot section outside the cryostat, and a cold section inside the cryostat. Further details of the operating principles of Gifford-McMahon cryocooler cold heads and pulse-tube cryocooler cold heads generally need not be elaborated for the purpose of this description. 
         [0011]    It is relevant to note however, that a single cryocooler cold head can only achieve temperatures down to approximately 10 K. However, in both Gifford-McMahon and pulse-tube cryocooler cold heads, the expansion systems can be ganged together in serial stages, such that the cooled gas of one stage may be used as a pre-cooler for a second stage, thus achieving a lower temperature. Two stage cold heads currently in use can achieve temperatures in the range of 2.5 K to 4.2 K. 
         [0012]    In many cases, the devices to which cryocooler cold heads are to be connected for cooling purposes are extremely sensitive to vibration. However, both Gifford-McMahon and pulse-tube type cryocooler cold heads produce vibrations which complicate their direct use in such applications. 
         [0013]    While in some applications a cryocooler cold head cools a helium bath which acts as a primary conductive heat transfer medium in which a coil, circuit or detector is immersed, in other applications they may be in thermal contact with substrates that are in direct thermal contact with magnet coil windings, detector systems, or electronics components. 
         [0014]    Cooling helium below its Lambda point (2.17 K for  4 He) results in a state change from a classical liquid state to a “superfluid” state. Superfluids are notable, and useful, for the fact that they exhibit zero viscosity and have infinite thermal conductivity. These properties can be utilized in a wide variety of commercial, experimental, and research applications. One interesting property of superfluid helium is that it forms what is known in the art as a “Rollin film”. A Rollin film is a result of the superfluid helium having zero viscosity, which in turn allows superfluid helium to migrate, or “creep”, along surfaces and coat a vessel it is contained in, for example, and other objects it comes in contact with to form a thin film. In one application, an electronics package in bath of superfluid  4 He will be surrounded by a Rollin film. Hence, rather than immersing an object in helium to cool it, cooling the object and the helium to 2.17 K or less will coat the entire object with helium and provide uniform and immediate heat transfer (due to infinite thermal conductivity in superfluid helium) from the object through the helium Rollin film. 
         [0015]    In some applications, temperatures below 2 K are obtained by pumping liquid helium through a Joule-Thomson valve. During the pumping process, the exhausted helium vapor is vented from a cryostat into the environment. This mode of operation requires costly periodic refills of the system&#39;s liquid helium. While regenerative refrigerators, such as GM and pulse tube cryocoolers, have been developed to reach low temperatures, in the range of 4 K or less, with helium-4, they are not able to produce temperatures below the helium-4 Lambda line near approximately 2.1 K. 
       SUMMARY OF THE INVENTION 
       [0016]    A closed-cycle refrigeration provides cooling to extremely low temperatures, particularly in the range of 0.5 K to 2.0 K. Furthermore, being a closed system, the closed-cycle refrigerator also addresses both the economic and resource conservation challenges previously existing in the prior art. Further, in some embodiments, the closed-cycle refrigerator decouples vibrations produced by cryocooler cold heads from low temperature devices. 
         [0017]    A 4 K pulse tube cryocooler cold head or Gifford-McMahon (G-M) cryocooler cold head liquefies helium in a first cooling chamber at a pressure of approximately 1 atm. Liquid helium flows from the first cooling chamber through a Joule-Thomson valve and into a second cooling chamber that is evacuated by a pump. The discharged gas from the pump is routed back to the first cooling chamber to be re-condensed. This design creates a closed-cycle refrigerator that provides continuous cooling below 2 K. 
         [0018]    The closed-cycle refrigerator also has an extra low vibration design. Cryocooler cold head cold sections of the closed-cycle refrigerator have no physical contact with subsequent cooling elements of the closed-cycle refrigerator, such as the first and second cooling chambers. In some embodiments the entire cryocooler cold head is connected to a vacuum chamber via a vibration damping coupler to further reduce the transfer of vibrations to other closed-cycle refrigerator elements. In other embodiments, the closed-cycle refrigerator cold sections are contained in a vacuum chamber as a cryostat, where a first radiation shield is thermally coupled to the first cooling chamber and a second radiation shield is thermally coupled to the first cooling chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0019]      FIG. 1  shows a schematic of the closed-cycle refrigerator. 
           [0020]      FIG. 2  shows a schematic of the closed-cycle refrigerator system with a vibration damping coupling between the cryocooler cold head and a vacuum chamber flange. 
           [0021]      FIG. 3  shows a schematic of the closed-cycle refrigerator system with a counter-flow heat exchanger between the first cooling chamber and the second cooling chamber. 
           [0022]      FIG. 4  shows a schematic of the closed-cycle refrigerator with a counter-flow heat exchanger and an adjustable Joule-Thompson valve located outside the second cooling chamber. 
           [0023]      FIG. 5  shows a schematic of the closed-cycle refrigerator with a counter-flow heat exchanger directly above the second cooling chamber. 
           [0024]      FIG. 6  shows a schematic of one embodiment of the closed-cycle refrigerator in conjunction with a cryostat including a vacuum chamber and two radiation shields. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    A closed-cycle refrigerator capable of producing temperatures down to 1 K or less, and doing so in an economic and reliable manner, has a number of practical applications. Integration of a 1 K closed-cycle refrigerator in a superconducting magnet allows for substantially 100% recovery and recycling of gaseous helium from the magnet cryostat. In other applications, the 1 K closed-cycle refrigerator can be used to cool a variety of detectors and low noise electronic circuitry. Further, when the working fluid is  4 He (Helium-4, an abundant isotope of helium), temperatures below 2.17 K (at approximately 1 atm) cause the  4 He to act as a superfluid. Superfluid  4 H has a number of applications in basic quantum mechanical research (superfluid  4 He is a Bose-Einstein condensate), and practical application in Superfluid Helium Droplet Spectroscopy (SHeDS) and construction of high accuracy gyroscopes, for example. 
         [0026]    Referring now to  FIG. 1  and  FIG. 2 , a schematic of a closed-cycle refrigerator capable of achieving temperatures of 1 K or less is shown. A cryocooler cold head  110  of the pulse-tube type is specifically shown. However, either the Gifford-McMahon or pulse-tube type cryocooler cold head can be equally employed, and this schematic representation is not meant to be limiting on the type of cryocooler cold head used. Henceforth, this cryocooler cold head  110  representation is intended to include both Gifford-McMahon and pulse-tube type cryocooler cold heads. 
         [0027]    The cryocooler cold head  110  has a hot section  116  and a cold section  115 . The hot section  116 , containing, for example, rotary valves, chambers, orifices, and other cryocooler cold head  110  elements, is outside the vacuum chamber  30 , in ambient atmosphere. The vacuum chamber  30  is only shown in relation to a flange  20 . One skilled in the art of cryocooler cold heads  110  will appreciate that vacuum chambers  30  may be constructed in a variety of configurations, and the operation of the closed-cycle refrigerator is not directly dependent on the specific configuration of vacuum chamber  30  employed. The hot section  116  of the cryocooler cold head  110  is connected to a helium compressor through high pressure  5  and low pressure  6  lines. 
         [0028]    The cold section  115  includes, in this example, a first stage having a first stage heat exchanger  112  and first stage tubes  12 , and a second stage having second stage tubes  11  and a second stage heat exchanger  111  having a helium condenser  113 . It will be understood that cryocooler cold heads with more stages can be used within the teachings of the invention. The cold section  115  is mounted in a first cooling chamber  40  that is also connected to a 4 K cooling station  41 , which is within a vacuum chamber  30 . The 4 K cooling station  41  is generally a plate of material with a high thermal conductivity, such as copper, aluminum, or similar material. Typical operating temperatures for the cryocooler cold head  110  are below 5 K, and preferably in the range 2.5 K to 4.5 K. 
         [0029]    Preferably, as shown in  FIG. 2 , the cryocooler cold head  110  is mounted on a flange  10 , which is in turn mounted to a vibration damper  15 , which in turn mates to the vacuum chamber flange  20 . Transfer of vibration from the cryocooler cold head  110  to the liquid helium in first cooling chamber  40  and the 4 K cooling station  41  is preferably minimized by this arrangement. 
         [0030]    A condenser  113  is attached to the lowest stage heat exchanger (here, second stage exchanger  111 ). Operation of the cryocooler cold head  110  produces a nominal operating temperature near 4 K at the condenser  113 . After precooling at the cryocooler cold head  110  first stage tubes  12 , first stage heat exchanger  112 , and second stage tubes  11 , gas condenses upon the condenser  113  and drips into the bottom  118  of the first cooling chamber  40 . 
         [0031]    An outlet  117  in the bottom of the first cooling chamber  40  allows withdrawal of condensed liquid, which is led to a second cooling chamber  60  and 1 K cooling station  61 . The 1 K cooling station is generally constructed as a plate of a material having a high thermal conductivity, including but not limited to, copper, aluminum, and other similar materials. 
         [0032]    As helium flows from the first cooling chamber  40  to the second cooling chamber  60 , the liquid helium flows through a J-T (Joule-Thomson) expansion valve  50  (J-T valves are also called “isenthalpic expansion valves”). As the helium passes through the J-T valve  50 , it experiences a negative pressure change, as the working pressure (−1 atm) of the first cooling chamber  40  is greater than the working pressure of the second cooling chamber  60 . Henceforth, the term “Joule-Thompson” (“J-T”) valve refers to any number of expansion valves that can effectively be used to expand a fluid for the purposes of cooling the fluid, including, but not limited to, needle valves, capillary tube arrays, and porous ceramic constructions, for example. 
         [0033]    The helium flowing from the first cooling chamber  40  is well below the helium Joule-Thomson inversion temperature (approximately 55 K) and therefore cools as it transitions to the second cooling chamber  60 . Depending on the pressure differential between the first cooling chamber  40  and the second cooling chamber  60 , the helium will be preferably cooled below the Lambda point of helium to a temperature in the range of 2.17 K (He-4 Lambda point) to 1 K or less. At this point, the helium ( 4 He) is a superfluid. 
         [0034]    Helium vapor is then pumped from the second cooling chamber  60  through passage  70  by the pump  80 , and is returned to the first cooling chamber through lines  100  for re-condensing, completing the closed cycle of the refrigerator. In some preferred embodiments, the pump  80  is of the oil-free dry type. 
         [0035]    At the start of operation, the closed cycle refrigerator is charged by opening a charging valve  91  to allow helium gas from a supply  90  of helium stored in a tank or dewar into the first cooling chamber  40 , where it is condensed to liquid helium. Once the first cooling chamber  40  accumulates enough liquid helium, the charging valve  91  is closed, and the supply  90  of helium is no longer needed. Then, the pump  80  is turned on to generate a vacuum in the second cooling chamber  60  and circulate helium from the second cooling chamber  60  through passages  70  and  100  back to the first cooling chamber  40  and cryocooler cold head  110  for re-condensing. 
         [0036]    As the liquefaction process of the cryocooler cold head  110  within the first cooling chamber  40  takes place at a working pressure of approximately 1 atm, flow between the first cooling chamber  40  and the second cooling chamber  60  is driven by a pressure differential created by the pump  80 . In other words, in the process of transferring helium from the second cooling chamber back to the cryocooler cold head  110  and first cooling chamber  40 , a pressure below 1 atm is created in the second cooling chamber  60 , and liquid helium flows from the first cooling chamber  40 , through the J-T valve  50 , to the second cooling chamber  60 . 
         [0037]    Thus, the closed-cycle refrigerator creates a closed loop for refrigeration below the helium Lambda point. In addition to achieving very low temperatures, mechanical isolation of the cryocooler cold head  110  from the first cooling chamber  40  and second cooling chamber  60  via the vibration damper coupling  15  ( FIG. 2 ) minimizes mechanical vibrations created by the cryocooler cold head compression/expansion mechanisms from being transferred to the first cooling chamber  40  and second cooling chamber  60 . Elimination of vibration can be important in some applications such as high resolution detectors and precision gyroscopes, as external vibration can negatively impact their performance. 
         [0038]    The superfluid helium present in the second cooling chamber  60  can be applied to cool devices in thermal contact  120   a  with the 1 K cooling station  61 , and/or, devices located inside  120   b  the second cooling chamber  60 . 
         [0039]    Referring now to  FIGS. 3-5 , additional elements may be added to the embodiments previously described in  FIGS. 1-2  to improve overall cooling efficiency of the closed-cycle refrigerator. The operation of the closed-cycle refrigerator remains substantially unchanged from that described herein in relation to  FIGS. 1-2 , and identical reference numbers refer to the same components. However, a counter-flow heat exchanger  320  is added at the top of the second cooling chamber  60 . 
         [0040]    In these embodiments, liquid helium is drawn from the first cooling chamber  40  through a port  117  in the bottom  118  of the first cooling chamber  40 , and passes through a first channel of the counter-flow heat exchanger  320  before passing through the J-T valve  50 . Colder helium vapor inside the second cooling chamber  60  is simultaneously drawn past a second channel of the counter-flow heat exchanger  320  for return to the cryocooler cold head  110  cold section  115  and second cooling chamber  40 . Thus, the liquid helium flowing inside the counter-flow heat exchanger  320  from the first cooling chamber  40  is pre-cooled before reaching the J-T valve  50 . 
         [0041]    As shown in  FIGS. 3-4 , the counter-flow heat exchanger  320  can be an independent element located in the flow path between the first cooling chamber  40  and the J-T valve  50 . In some embodiments, shown in  FIGS. 3-4 , the J-T valve  50  is located outside the second cooling chamber  60 . In alternate embodiments, shown in  FIGS. 5-6 , the first channel of the counter-flow heat exchanger  320  is located in a volume of the second cooling chamber  60 . In these embodiments, the second cooling chamber  60  defines the second channel of the counter-flow heat exchanger  320 , and the J-T valve  50  may be located outside the second cooling chamber  60  ( FIG. 5 ), or inside the second cooling chamber  60  ( FIG. 6 ). 
         [0042]    In some embodiments, shown in  FIG. 4 , an adjustable J-T valve  50  is used, and a low thermal conductive tube  330  is provided to allow variation of flow through the J-T valve from the room temperature side of the closed-cycle refrigerator for the purpose of temperature optimization in the second cooling chamber  60 . 
         [0043]    Referring to  FIG. 6 , the closed-cycle refrigerator is shown in relation to a vacuum chamber  30  and radiation shields  300 ,  310  that would preferably be included in a practical implementation of the closed-cycle refrigerator described herein. Cold components are contained within a vacuum vessel  30  to minimize convective heat transfer from the ambient environment to the low temperature components of the closed-cycle refrigerator. 
         [0044]    Radiation shields  300 ,  310  are preferably incorporated to minimize radiant heat transfer from the environment to the closed-cycle refrigerator. A 4 K radiation shield  310  surrounds the second cooling chamber  60 , J-T valve  50 , and associated helium transfer lines, and is in thermal contact with the first cooling chamber  40 , being coupled to the 4 K cooling station  41  in some embodiments. A 50 K radiation shield  300  surrounds the 4 K radiation shield and part of the first cooling chamber  40 , and is in thermal contact with the first cooling chamber  40 . The point of thermal contact between the 50 K radiation shield  300  and the first cooling chamber  40  is preferably near the cryocooler cold head first stage heat exchanger  112 . 
         [0045]    As with other embodiments described herein, the embodiments shown in  FIGS. 3-6  produce superfluid helium in the second cooling chamber  60  that can be applied to cool devices in thermal contact  120   a  with the 1 K cooling station  61 , and/or, devices located inside  120   b  the second cooling chamber  60 . 
         [0046]      FIGS. 3-6  also show a cryocooler cold head  110  configuration in which moving parts, such as a motor and rotary valve assembly  7 , are separated from the cryocooler cold head  110  by a bi-directional flow line  8 . This physical separation provides an added level of vibration damping within critical areas of the closed-cycle refrigerator, for example, the second cooling chamber  60 . 
         [0047]    The closed-cycle refrigerator described herein can provide cooling temperatures down to 1 K or below and an almost vibration free environment. Further, compared to multi-stage cryocooler cold heads, the closed-cycle refrigerator is more reliable and less costly as it has almost no additional moving parts. Furthermore, in contrast to prior art devices, the closed-cycle refrigerator can achieve temperatures below approximately 2 K with no loss of helium to the ambient environment, thus providing a solution that is more cost effective and conservative of a limited natural resource. 
         [0048]    Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments is not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.