Abstract:
A three-stage pulse-tube cryocooler, in which the third stage pulse tube is arranged below the second stage pulse tube, with a gas flow conduit between the second stage pulse tube heat exchanger and the cold end of the second stage regenerator. The design of the invention is much simpler than a conventional three-stage parallel pulse tube cooler, requiring only two pulse tubes at the warm (room temperature) end and two reservoirs, with a corresponding reduction in the number of associated orifices, passages, etc. In effect, this provides a three stage cryocooler with a two-stage warm end design by putting the second and third stage pulse tubes in series, with a gas flow passage providing gas flow between the second and third stages for gas expansion and refrigeration. The three-stage design allows an intermediate temperature connection between the temperatures of the first and third stages, for applications which require three cooling temperatures.

Description:
REFERENCE TO RELATED APPLICATIONS 
   This application claims an invention which was disclosed in Provisional Application No. 60/579,800, filed Jun. 15, 2004, and entitled “Three-Stage Pulse Tube Cryocooler”. 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 
   1. Field of the Invention 
   The invention pertains to the field of cryorefrigeration. More particularly, the invention pertains to a multi-stage pulse tube cryocooler. 
   2. Description of Related Art 
   Typical closed-cycle regenerative cryocoolers include the Stirling, Gifford-McMahon and pulse tube types, all of which provide cooling through the alternating compression and expansion of a working fluid, with a consequent reduction of its temperature. Stirling and Gifford-McMahon regenerative cryocoolers use displacers to move a working fluid (usually helium) through their regenerators. The noise and vibration induced by the displacer creates problems, and the wear of the seals on the displacer require periodic maintenance and replacement. 
   Therefore, it is highly desirable to invent cryorefrigeration devices that generate less vibration and less acoustic noise than prior art cryocoolers. It is also desirable to decrease the number of moving parts used in cryorefrigeration devices and to significantly increase the required maintenance intervals and reliability. Pulse tube cryocoolers are a known alternative to the Stirling and Gifford-McMahon types, which differ from these in that pulse-tube cryocoolers do not use a mechanical displacer. 
   A pulse tube is essentially an adiabatic space wherein the temperature of the working fluid is stratified, such that one end of the tube is warmer than the other. A pulse tube cryocooler operates by cyclically compressing and expanding a working fluid in conjunction with its movement through heat exchangers. Heat is removed from the system upon the expansion of the working fluid in the gas phase. 
   As used herein, a “stage” in a cryocooler is a location in the cooler at which gas expansion and refrigeration occurs, and at which a thermal load may be attached at a “cooling station”. 
   Prior art single-stage valved pulse tube cryocoolers generally include a pulse tube, a rotary valve to generate the oscillating compression-expansion cycle, a reservoir to contain the expanding working fluid gas, orifices for the movement and phasing of the gas between the reservoir or buffer volume and the rest of the system, and a regenerator for absorbing heat temporarily and reversibly. Single stage pulse tube cryocoolers are generally capable of reaching temperatures above 20K., and achieving lower temperatures has in the past required staging of the pulse tubes. U.S. Pat. No. 3,237,421 to Gifford and other prior art publications disclose multistage pulse tube cryocoolers. U.S. Pat. No. 5,295,355, a 1994 patent issued to Zhou, et al, shows a single-stage multi-bypass refrigerator. 
   Prior art two-stage pulse tube cryocoolers generally include, in addition to the foregoing components, a first-stage pulse tube, a first-stage regenerator, a second-stage pulse tube, a second-stage regenerator and first and second cooling stages. FIG. 2 of U.S. Pat. No. 6,378,312, issued to the present inventor, shows a two-stage cryocooler. 
   US Published application 2003/0163996 shows a cryocooler having two pulse tubes 108 and 120 and regenerator 106. A “heat intercept” 202 (FIG. 2) connects second stage pulse tube 120 and regenerator 106, but there is no flow passage in this connector. Because there is no gas passage, no gas expansion or refrigeration occurs at this point, so this invention is not a true “three stage” cooler, as the term is defined and used herein. 
   In a 1997 paper in  Cryogenics  (vol. 37, No. 12, pp. 857-863), entitled “Experimental study of staging method for two-stage pulse tube refrigerators for liquid  4 He temperatures”, the present inventor, plus Thummes and Heiden, described several embodiments of two-stage cryocoolers. The cooler shown in figure (c) is an attempt by the inventor to increase efficiency in the second stage by adding a gas bypass orifice between the second stage pulse tube and the second stage regenerator to control phasing of gas flow within the second stage. There is no heat exchanger at the location of the bypass, so that there is no cooling station located at this bypass, therefore this is a two-stage, not a three-stage cooler. 
   Three-stage cryocoolers are also known in the prior art which include all of the parts of a two-stage pulse tube cryocooler, plus a third reservoir, a third-stage regenerator, and a separate third-stage pulse tube in parallel with the first- and second-stage tubes. 
   SUMMARY OF THE INVENTION 
   The invention comprises a three-stage pulse-tube cryocooler, in which the third stage pulse tube is arranged below the second stage pulse tube, with a gas flow conduit between the second stage pulse tube heat exchanger and the cold end of the second stage regenerator. The design of the invention is much simpler than a conventional three-stage parallel pulse tube cooler, requiring only two pulse tubes at the warm (room temperature) end and two reservoirs, with a corresponding reduction in the number of associated orifices, passages, etc. 
   In effect, this provides a three stage cryocooler with a two-stage design by putting the second and third stage pulse tubes in series, with a gas flow passage providing gas flow between the second and third stages for gas expansion and refrigeration. The three-stage design allows an intermediate temperature connection between the temperatures of the first and third stages, for applications which require three cooling temperatures. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  shows a schematic diagram of a three-stage pulse-tube cryocooler of the invention. 
       FIGS. 2   a - 2   d  show alternative embodiments of the gas passage, as a detail of the area from  FIG. 1  enclosed in the dot-dash oval labeled  2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A cryocooler is a heat pump that pumps heat from one or more cooling loads (not shown) to a heat sink, and thus to the ambient environment. Referring to  FIG. 1 , a three-stage cryorefrigeration apparatus of the invention includes a first cooling stage  90  and a second cooling stage  100 . In prior art three-stage cryocoolers, the third stage pulse tube would be in parallel with the first two stages&#39; pulse tubes. 
   In contrast, the third stage of the cryocooler of the invention adds a third stage  110 , with its pulse tube  22  in series with the second stage pulse tube  10  (shown as underneath the second stage, in  FIG. 1 ). Each stage cools to a lower temperature than the preceding stage, and cooling loads may be connected to one or more of cooling stations  14 ,  19  and  21 , depending on the temperature required by the load. This provides a three stage cryocooler with two pulse tubes at the room temperature end by arranging the second- and third-stage pulse tubes in series, with gas flow passage  24  providing a passage for gas flow between the cold ends of the second stage regenerator  16  and the second stage pulse tube  10 , thus causing gas expansion and refrigeration to occur at this stage. 
   As an example, in a cryocooler built according to the teachings of the invention, the first cooling stage  90  has a first stage temperature at the first stage cooling station  14  of between 40 K and 75 K, the second stage temperature of the second cooling stage  100  at the second stage cooling station  19  is about 15 K to 25 K, and the third cooling stage  110  has a third cooling stage temperature at the third cooling station  21  of about 2 K or less to 10 K. Of course, it will be understood by those skilled in the art that the exact temperature ranges given in this description and the drawing are for example, only, and the cooler could be built for other temperatures as would be required by the particular application in which the cooler will be employed. The cryocooler of the invention will be described in greater detail below. 
     FIG. 1  shows a “working fluid compressor and pressure controller”  1 , providing an oscillating flow of working fluid  2  (for example, Helium) under pressure. It will be understood by one skilled in the art that the compressor/controller  1  could be a “Gifford-MacMahon (G-M) type” arrangement, in which a rotating valve alternately connects the cooler to a source of gas from a compressor and to an exhaust line back to the compressor. This causes an oscillating flow first from the compressor to the regenerator, then back from the regenerator to the exhaust. Typically, two gas-flow conduits would be provided from the compressor package, one for gas supply and one for exhaust. This arrangement is shown and explained in the inventor&#39;s U.S. Pat. No. 6,378,312, which is incorporated herein by reference. Alternatively, the compressor/controller  1  could be a “Stirling-type” valveless pressure wave generator, in which there is one conduit connecting the cooler to the compressor/controller, and the oscillating flow is in the form of pressure waves travelling back and forth along the single conduit. 
   One or more reservoirs (here shown as two reservoirs  6  and  7 ), flow channels and orifices  8 , provide phasing of gas flows and connections to the pulse tubes and regenerators, as will be described in greater detail below. 
   The first-stage regenerator  13  is typically filled with a stack of screens which acts as a thermal sponge, alternately absorbing heat from the working fluid and rejecting the absorbed heat back to the working fluid as the pressure oscillates. First-stage pulse tube  12  is a thin-walled tube of a lower thermal conductivity material, such as stainless steel. If desired, a heat exchanger can be included at the cold end of the first stage regenerator, as is shown at  18  for the second stage regenerator. 
   First-stage pulse tube  12  has heat exchangers  11  and  15 , preferably of copper, at its hot and cold ends, respectively. These are thermally coupled to the heat sink  25  and first stage cooling station  14 , respectively, and may also act as flow straighteners for the gas flow in the pulse tube. Gas passage  23  connects the cold end of the first stage pulse tube  12  to the cold end of the first stage regenerator  13  through heat exchanger  15 , providing the gas expansion and refrigeration for this stage. It should be noted here that while  FIG. 1  shows the second stage pulse tube  10  passing through the first-stage cooling station  14 , this is done for convenience of fabrication, and is not required. The first stage cooling station  14  may be made so as to avoid the second-stage pulse tube  10 , or the second-stage pulse tube may be thermally isolated from the first-stage cooling station by making the hole through which it passes large enough to provide isolation, or by provision of insulation around the tube. 
   The second stage  100  of the cryocooler is made up of the second stage pulse tube  10  and second stage regenerator  16 . Second-stage pulse tube  10  is connected at its hot end to the heat sink  25  and at its cold end to the second stage cooling station  19 . Second-stage regenerator  16  is connected at its cold end to the second-stage cooling station  19 , and at its hot end to the cold end of first-stage regenerator  13 , for gas flow between the regenerators. The second stage pulse tube  10  and second-stage regenerator  16  are connected together at their cold ends by gas flow expansion passage  24 , which allows gas flow between the second-stage pulse tube and second-stage regenerator for expansion and refrigeration at the second stage. 
   Optionally, the gas flow in passage  24  may be controlled by an orifice for restricting gas flow, as shown in  FIG. 1 . 
     FIGS. 2   a  through  2   d  show alternative embodiments of the gas passage  24  which might be used within the teachings of the invention. The area shown in these figures is the area of  FIG. 1  enclosed in dot-dash oval labeled  2 . 
   Three differing embodiments of the gas passage  24  having fixed configurations are a tube  30  ( FIG. 2   a ), a constriction in the tube  32  ( FIG. 2   b ), and a capillary tube  34  ( FIG. 2   d ). Alternatively, a valve  33  ( FIG. 2   c ) could be provided, which would allow adjustment of the gas passage  24 . 
   As in first stage pulse tube  12 , the hot end of the second stage pulse tube  10  preferably has a heat exchanger  9  at its hot end which is thermally coupled to heat sink  25 , and (preferably) a heat exchanger  26  (possibly in the form of a screen region) at its cold end which is thermally coupled to the second stage cooling station  19 . It is also possible to have a screen region at  26  which is not a heat exchanger (for example, being made of nylon), which acts as a flow straightener. 
   The second stage regenerator  16  is also filled with a regeneration material, such as the screens used in the first stage regenerator, or a lower-temperature material such as lead shot or rare-earth spheres as shown in U.S. Pat. No. 5,186,765. The second stage regenerator may also have a heat exchanger  18  at its cold end, coupled to the second stage cooling station  19 . Either or both of heat exchangers  18  and  26  may be omitted if desired. 
   The second stage load is coupled to cooling station  19 . It will be understood by one skilled in the art that the location of this connector, and the associated gas passage  24 , will be determined by the desired temperature for this stage, between the temperatures of the first and third stages. 
   The third stage  110  of the cryocooler comprises the third stage pulse tube  22  and third stage regenerator  20 . The third stage regenerator  20  may be filled with the same material as the second stage regenerator  16 , or some other material having better low-temperature characteristics. The hot end of the third stage pulse tube  22  is directly below, and in gas communication with, the cold end heat exchanger  26  of the second stage pulse tube  17 . The cold end of the third stage pulse tube may preferably have a heat exchanger  27  thermally coupled to the third stage cooling station  21 . The hot end of the third stage regenerator  20  is directly below, and in gas communication with, the cold end of the second stage regenerator  16 . The cold end of the third stage regenerator  20  is connected to the cold end of the third stage pulse tube  22  by passage  28 . If desired, a heat exchanger can be included at the cold end of the third stage regenerator, as is shown at  18  for the second stage regenerator. 
   It will be understood that this could be extended to a four- or higher-stage design by the addition of more expansion passages like passage  24 , with corresponding cooling stations. 
   In operation, compressor/controller  1  delivers an oscillating flow of working fluid  2  (usually helium), under pressure, to provide an alternating mass flow throughout the pulse tube cryocooler. The alternating pressure and mass flow produced by compressor/controller  1  constitutes pressure/volume (PV) work, causing regenerator  13  to pump heat from the cooling load to the heat sink  25 , where the heat is ultimately rejected. The result of this heat pumping action is to lower the temperature of the cooling load. Meanwhile, the PV work travels down pulse tube  12 , where it is rejected as heat to the heat sink  25 . One or more reservoirs (shown in  FIG. 1  as two reservoirs  6  and  7 ), through flow passages and orifices  8 , provide phasing control for the gas flow. 
   The lower-temperature second-stage pulse tube  10  is in parallel with the first-stage pulse tube  12 . In operation, compressor  1  supplies a continuous pressure wave to first stage regenerator  13 . After providing cooling in the first-stage regenerator  13 , the pressure wave provides further cooling in second-stage regenerator  16 , with the cold end of second-stage pulse tube  10  and second-stage regenerator  16  being in thermal contact with a cooling load (not shown) at second stage cooling station  19 . The pressure wave continues through passage  24  to the pulse tube  10 , and the PV work is rejected as heat to the heat sink  25 . 
   The lowest-temperature third-stage pulse tube  22  is connected in series with the cold end of second-stage pulse tube  10 . After providing cooling in first-stage regenerator  13  and second-stage regenerator  16 , the pressure wave provides further cooling in third-stage regenerator  20 , with the cold end of third-stage pulse tube  22  and regenerator  20  in thermal contact with a cooling load (not shown) at third-stage cooling station  21 . The pressure wave continues through passage  28  to third-stage pulse tube  22 , and the PV work is ultimately rejected as heat to the second stage cooling station. 
   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.