CA1190757A - Miniature cryogenic cooling system with split-phase dual compressor and phase-shifting device - Google Patents

Abstract

Abstract of the Disclosure A miniature cryogenic cooling system having low microphonics and thermophonics. The apparatus includes a cold finger incorporating primary and secondary working volumes and their associated regenerative beds. Both volumes are adapted to effect cooling. A displacer reciprocates within the cold finger under the influence of push-pull pressure waves from a 180°
opposed compressor assembly. The cold finger includes an arrangement for providing pneumatic cushioning of the recipro-cating displacer to control displacer slamming.

Description

1 , MINIATURE CRYOGENIC COOLING SYSTEM WITH SPLIT
I PHASE DURl COMPRESSOR AND PH~SE-~HIFTI~G DEVICE

Background of the Inv ntion Thi~ inven~ion rela~es in general to cryogenic cooling ~y~tem~ and more par~icularly ~o a miniature dual-split cryogenic system with the compressor ~ection ~eparated from the cooling ~ ection.
i : Miniature cryogenic oooling systems are known and widely u~ed t~ cool cry~tal~ used as radia~ion detectorsO
Cooling to cryogenic temperatures reduces the crystal lattice vibrations 60 aR ~0 improve the signal to noise ratio. A par-ticularly important application for such miniature cryogenic coolers is in the cooling of infrared detectors for use in nigh~
vision or heat seekiny devices. The~e systems are also uæeful for medical application~ where it i~ de~ired to de~troy ti~ue by means of freezing. Although the requirement~ for such miniature cryogenic sys~ems will vary depending on the end use of ~he , cooler, typical design csnsiderations are ~heir operating effi ciency, durability~ compactne~s, weight, microphonics charac-teristics ~generally vibration~ resulting rom the compressor motor, vibra~ions within the working fluid, or the physical impact of moving components in the ~ystem3, and thermophonics (which i~ noi~e resulting from thermo~radients within the ~ystem). For applicationg involving infrared ~ensors for air-borne devicz6, all of the~e design considerativn6 are importantO

Kn~wn cryvgenic coolin~ ~ystem~ fall into æeveral cate-gorie~. In one type the compre~sor and expander units form an 1 integral system opera-ting on stirling cycle. Because of the proximity of the compressor and expander sections, such integral systems are partlcularly susceptible to the effects of micro-phonics or mechanical vibra-tions. The sys-tems also tend to be relatively heavy and have typical operating lives of only 300-500 hours. Thus, integral sys-tems are not particularly effective especially for use in airborne systems.
~ s a way of isolating the compressor vibration from the expander (known as the "cold finger"), split stirling devices are known which separate the compressor system from the cold finger by conduits carrying the working fluid. A
typical design problem with such split systems is the acoustic noise generated by an oscillating displacer which is continually being accelerated and decelerated as it oscillates within the cold inger. Such noise is particularly troublesome in single-split stirling systems in which but a single conduit extends from a single compressor to the expander.
Dual-split systems have also been developed in an effort to overcome problems in controlling the motion

2~ of the displacer so as to m;n;m; ze microphonics. In such systems two conduits extend from two separate compressoxs to the expander. In general the pressure waves in each of the conduits are out of phase with one another so that a push-pull arrangement is effected. That is, in such a system the d-splacer is moved by alternately introducing high pressuxe on one side and a lower pressure on the other so as to reciprocate the displacer element. One such dual split cooler is disclosed by Durenec in U.S. Patent No. 4,092~833 which issued June 6, 1978. The Durenec system employs a

3~ compressor with pistons opposed at a 180 degree angle. In 1 this system, however, the compressor pistons are of different areas so that the pressure waves in the individual conduits are of different magnitudes. In addition, -there is no suggestion in the Durenec patent of using the split phas~
relationship to decelerate and reverse the direction of motion of the displacer in a way to prevent contact of the displacer with the walls of the housing. Such contact with the housing or "slamming" not only causes additional microphonics but also audible acoustic noise. Furthermore 1~ this Durenec reference n~ither teach~s nor suggests using the cooling effects of a second working volume -to precool the working fluid destined for the main workin~ volume at the tip o~ the cold finger~ In addition, the known split phase coolers have short operating lives.
Other known coolers are described in U.S. Patent Nos. 4,090,859 to Hanson which issued May 23, 1978; 4,078~389 to Bamberg which issued March 14, 1978; 3,523,427 to Simpson which issued August 11~ 1970; and 4/206~609 to Durenec which issued June 10, 1980.
~ It is there~ore a principal object of this invention to pro~ide a miniature cryogenic system which is hiyhly efficient, compact and characterized by low level o~ micro-phonics and thermophonics.
' Another principal object is -to provide such a system operating on a dual split "compound" stirling cycle which has a comparatively long operating life.
A further object of the invention is to provide a cryogenic cooler employing a co~pressor assembly with two isolot~d compressors and producing a split-phase relationship of 180 degrees.

;l A still fur~her object of the invention i~ ~o provide a 6y5tem having all of the foreyolng adv 3.ntag~s and w~ich i~ manu-~actured of conven~ional ma~erials in a rela~ively simplP a.nd ~traightforward way.
, Summary c: f ~he Inventioll 'lThe miniature cryogenic ~ystem according ~o the pre~nt invention includes; two com1?ressor~ with two outlets. ~irst and second condults containing a working fluid are connected to the : compressor ou~lets. The compres~or~ pressurize the working fluid in a ~plit~phase relation~hip. These conduits are connected to a cryogenic cold finger a~sembly, including an elongate housing . having a cryoyenically cooled end and an internal abutment dividing the housin~ in~o irst and second interior compartments.
The f irst compartment extends from the cooled end to the abut-ment. A diæplacer is disposed slidingly within the first: com-partment or longitudinal reciprocating motion therein and crea~es a first (or primary) working volume between the coole~
end and the displacer and create~ a ~cond (or ~econdary) working I volume between ~he displacer and the abutmen~0 A first regenerative bed is isposed within the displacer and the housing has a firæt inle~ adapted for providing fluid communication between ~he first conduit and the fir~
I interior compartment, The displacer has first and ~econd port mean~ so ~s to provide fl~id communlcation through the first regensrative bed ke~ween the fir~t inlet and the first working volume. In addi~ion, a second regenerative bed i~ disposed within the second interior compartment within the housing. The housing also has a 6econd inlet w~ich provide~ 1uid com~

4--. !
munication ~hrough the econd regenexative bed between the ~econd conduit and the second working volumeO ~ean~ are provided to develop tran ient cu6hioning overpr~essur~s in he fix~t and 6eeond working volumes as the displacer reciprocates under the push-pull influence of the spli~-phase working fluid flow from the compre~or ~o that cooling is effec~ed in the fir~ and second working volumes and ~lammin~ i6 controlledO

The di~placer i~ guided for longitudinal reciprocating I motion within the housing by &liding 6eals. In a preferred embodiment the fir6t conduit i5 coiled around the exterior of the hvusing adjacent ~o the second workin~ volume to ef~ect a pre-cooling of ~he working fluid carried in the fir~t conduitO In thi~ embodiment the abutment compri~es a web extending acro s the ! interior of the hou~ing and includes an opening to provide fluid communication ~etween the fir~t ~nd 6econd interior compartments.
A plate is provided for 6ecuring the second regenerative bed in a fixed location within the second interior compartment, and a heat exchanger i6 disposed b~ween this plate and this abutment. A

preferred heat exchanger includes a plurality of longitudinally-~paced annular discs.

In order to enhance ~he pneumatic cu6hioning, a phase ! shifting plug is di~posed on the lower end of the displacer adja-cent to the web a~d located 80 as to eeal t11~ opening in the abutment when ~he displacer iE closely spaced from the web. This plug can be a pad of resilliant material or a ~pring disposed within a coverlng that i8 located nd 6tructured to cover the hole in the abutmerlt~ By ~ealing the opening in the web, the phaæe relation~hip of the pressures in the working volume~ i5 .
~hif~ed ~o as ~o provide a cu hioning effect which deceler te~
the displacer in a controlled fa~hionO

In another embodiment the ,abu~ment comprises a soiid member which ex~ends ~ub~tan~ially the leng~h of the ~econd interior compartment, and the second regenera~ive bed is located between ~he ~olid member and the hou~ingO ~ heat exchanger, including longitudinally spaced-apart di~cs 1anking the 6econd worXiny volume i5 adap~ed for æecuring the 6econd regenerative bed in a fixed loeati~n within ~he 6econd interior compartment.
In this embodiment a hea~ radiation 6hield i8 provided for surrounding the hou~ing in a 6paced relation~hip and extending from the cooled end of the housing toward the 6econd worXing volumeO The radiation shield reduces the heat load falling on~o the fir~t working volume. The bottom of ~he di~placer functions as its own phas~-shiting device. W~en the displacer moves down-ward, it gradually closes off ~he fluid passage~ between th~ heat exchanger discs in the second working volume.

Brief Description of the Drawing The invention disclosed herein will be better u~der~ood wi~h reference to th~ following figure~ of ~he drawing of which:

I Fig. 1 is a vertical 6ectional Yiew of a cold fin~er a~sembly according to this invention ~uitable for operation in a dual æplit "compound" 6tirling cycle.

Fig. 2 is a cross-seetional view of another embodiment of such a cold finyer~

~ig. 3 is a top pl~n vi~w in partial horizontal ~ec~ion of a compre~ or ~ui~ed for u~e with the cold fingers of Figs, 1 1 and 2.
Fig. 4 is a side elevational view in partial section of the compressor.
Fig~ S is a pressure versus time diagram o~ the pressure waves produced by the compressor.
Fig. 6 is a side eleva~ional view in partial vertical cross section of a displacer disclosed herein.
Fig. 7 is a side elevational view in partial vertical cross section of the displacer disclosed herein.
13 Fig. 8 i~ a side elevational view in partial cross section showing an alternate embodiment of a phase shifting plug for use with the displacer7 De'scr'iption'o'f'the Preferred Embodiments The same reference numerals will be used to identify like components in the various figures of the drawing.
With reference first to Fig, 1, shown is a cold finger assembly lQ, including a housing 12 made of a material having a low therm~l conductivity such as stainless steel. Disposed within the housing 12 is an abu~ment 14 which divides the in~erior of the housing 12 into two interior compar~ments 16 and 18. The compartment 16 has a substantially circularr cylindrical cross section. A displacer 20 is adapted for reciprocating longitudinal motion within the compartment 16. The displacer 20 is formed of a material such as nylon~
The displacer ~0 is hollow and contains a first bed of regenerative material 22 comprising, for example, copper screens or copper spheres. The displacer 20 also includes entrance ports 5i~

1 24 which can be seen more clearly with re~erence to Figs. 6 and 7~ Exi-~ por-ts 26 are also provided in the upper end of the displacer 20. The displacer 20 has a length which is shorter than the length of the interior compartment 16 so tha-t the dis-placer 20 crea-tes within the interior compartment 16 a first (or primary) wor~ing volume 28 and a second (or secondary~ working volume 30 in a working fluid ~o be described hereinafter~ As will become clearl the working volume 28 effects a cooling to cryogenic temperatures of the cold end 32 o~ the housing 12. The cold end 32 may be made of a material having a very high thermal conductivity such as copper for more efective heat transfer from a thermal load (e.g. - an infraxed detector) in contact with the cold end 32O
A second bed of regenerative material 34 is included within the interior compartment 18 and held in place by a plate 36. The plate 36 is preferably made of a material having high thermal conductivity such as copper. A series of longitudinally spaced-apart discs 38 are located between the plate 36 and the abutment 14 and are made of a material having a high thermal 2~ conductivity so as to serve as an effective heat exchanger.
A first conduit 40 encircles the housing 12 adjacent to the second working volume 30 and communicates with the interior compartment 16 at its terminal end 42. A second conduit 44 communicates with the interior compartment 18.
Referring now to Figs. l, 2, 3, and 4, a compressor assembly 50 includes a structural housing 52 with a circular cylindrical cavity 54 therein. A piston assembly 56 includes longitudinally spaced~apart and mutually isolated pistons 58 and 3~

1 ,' 60 which are riyidly connected to one ano~her by connecting member 6- The conneeting member 62 i~cludes a 310t 64 which engages a pin 66. The pin 66 is eccen~rically mounted on a disc 68 driven by a m~or 70u As the disc 68 rQtates, the pin 66 follow~ ~he circular path indicated by the dot~ed circle 72.
Thu6, ~s the pin 66 traveræe~ the circ~lar pa~h ~2, the piston assembly 66 will reciprocate longitudinally within the cavity 54~
As will be understood by ~hose 6killed in the art, w~en a working , ~luid such as helium i8 introduced into the cavi~y 54, the l pressure of the working fluid will al~ernately increase and decrea~e as the piston assembly 56 reciprocate~. Furthermore, as shown in ~igO 5, the pres~ure varia~ions in the conduits 40 and 44 will be 180 degrees out of phase, ~ince the pis~ons 58 and ~0 are rigidly linked together. Thus, w~en the pis~on assembly 56 i in the position as 6hown in Fig. 3, the pressure in conduit 40 will be relatively high and the pressure 44 will be relatively . l~w. This relationship reverses as the pi~ton a~sembly 56 moves toward the right.

The compres or ~ ~embly 50 disclosed herein is par-ticularly well adapted for use with ~ cryogenic cooler. Because the pistons 58 and 60 are of the same diameter and mass, they are balanced about the drive point ~o that mechanical vibration is minimized. In addi~ion, the " cotch yoke" design with its short moment arm and the linear mDtion of the pi~ton assembly 56 result in low ~eal wear and hence long operating life~ Furthermore, the prior art did not recognize the desirability or possibility of using a 180 oppo~ed compressor in which t~e pi6ton8 were of the ~ame ~ize~ This i8 the case becau~e the prior art did not con-ceive o the phase shifting ~rrangement disclosed herein whi~h _g_ 1 provides pneuma~ic cushioning fsr contr~lla~ly d eelerating the di~placer so a~ ~o decrea~e ~lamming. ~his d~ign re6ult~ in a ompre~Ror which is very comRack and one whlch is ~imple to fabricate.

,' The operation of the compressor a~sembly 50 in onjunc-¦ tion with the cold finger 10 will now he described~ As sta~e~
! above, a~ the pis~on assembly 56 reciprocates, pres~ure waves are created in ~he conduits 40 and 44, w~ich are 180 degre~s out of ~ phase with one another. The working fluid in the conduit 40 such 1 as helium circula~es around ~he extexior of the housing 12 and enters the first interior compartment 16 at the end 42 of the ~onduit 40. RecauRe of the arr~ngement of sliding ~eal~ 80 and 8~ which guide the motion of the displacer 20, th~ working 1uid which enters the interior compar~ment 16 passes in~o the interior o~ the displacer 20 through the por~ 24 and travels through the regenerative bed 22 and finally through the ports 26 into the first working volume 280 Similarly, the working fluid in the conduit 44 enters the Eecond interior compartment 18, passes through the second regenerative bed 34 past the plate 36 and the heat exchanger discs 38 and from there into interior compartmen~
16 through an opening 84 in the abu~ment 14 to crPate the working volume 30. ~ecau6e of the ~eals 80 and 82, the working volumes 28 and 30 with their working fluid remain separated and isolated from one another.

As the compres~or assembly 50 operates, t.he pressure in wvrking volume 2~ is rela~ively high when the pressure in the working volume 30 is relatively l~wer and vice versa in a cycli-cal manner~ Becau~ of this cyclically varying pressure dif-.~ O -7~

1 ferential, the displacer 20 reciprocates within the first interior compartmen~ 16. As is understood by those skilled in the thermodynamic art, as ~he displacer 20 reciprocates in a cyclical fashion, temperatures within both oE the working volumes 28 and 30 decrease. Tha~ is, the energy expended results in cooling by the same means as in the well known ~hermodynamic stirling cycle.
CooIing in both working volumes 28 and 30 is advanta-geous for two reasons. First, because the conduit 40 encircles the housing 12 adjacent to the wor~ing volume 30, cooling in the working volume 30 will cool the working fluid in the conduit 40 before it enters the interior compar~ment 16. Thus, when the working fluid reaches the ~orking volume 28~ i~ is in a precooled condltion so that the overall cooling capability of the cold finger 10 is improvedr Second, the thermal gradient across the longitudinal length of the displacer 20 is reduced. Such a reduction in the thermal gradient ameliorates thermophonics.
A significant aspect of the present invention is that the cold finger structure, in combination with the split phase pressure waves applied to the inlets provides for pneumatic cushioning of the displacer so that physical contact between the displacer and the cold end 32 and the abutme~t 14 is eliminated or controlled. An important factor producing this cushioning is the fluid flow impedances through the regenerative beds 22 and 34. That is, because o~ the nature of the flow paths, as the displacer rapidly approachas the end 32, for example, pressure builds up in the working volume 28 so as to slow and cushion the displacer before it comes into contact with the end 32.

3~

3~

1 , This ef~et i~ enhanced by ~he la~eral loca~ion o the por~ 26 unlike an end location a~ taught irl the prior art. The lateral location also improv~ the heat tr~n~fer characteristics a~ the cold end 32.

The important embodiment ill~strated in ~ig. 6 and 7 enhanc~ thi~ pneumatic cu~hioning effec ~ A p~ase ~hifting plug 90 of a re~ilian material is disposed on the lower surface of the displacer 20. As the di~plac~r 20 moves downwardly, the plug 90 cover the op~ning 84 sealing off the working volume 30 , thereby enhancing the transient pressure buildup BO a~ ~o de~elerate the displacer 20~ By ~ealing the opening 84, the pha~e relation~hip of the pressures in the two working volumes has thus been ~hifted from the ~ubstantially 180D out-of-phase condition. Fig. 8 illustrates yet another embodiment adapted for providing enhanc~d pneumatic cushionîng. In ~his embodiment a ~prill9 92 i8 at~ached to th~ lower surface of the displacer 20 and covered by a metallic material 94~ A~ with the embodiment of Fig. 6 and 7~ when the material 94 reache6 the abutment 14, the opening 84 is ~ealed off so a~ to enhance the buildup of pressure. The Rpring ~2 restore6 the covering to 6ubstantially the same po~ition with each cycle.

Yet another em~odiment of a cold finger assembly 100 adapted for operation with the compressor a sembly 50 ~f Fig. 3 i5 ~h~wn in Fig. 2~ In thi embodiment a housing including 6ec-tion~ 102 and 103 i5 6eparated into interior compartmen~ 104 and 106 by ~ solid abu~nent lOB. The di~placer 110 i8 adapted ~or reciprocal movement wi~hin the in~erior compartment 104. As in the embodiment of Fig~ he di~pla2er 110 i8 hollow and filled 1 with a bed of regenerative material such as ~mall copper æpheres.
. Likewise the annul~r region between ~he housing ~sction 103 and the ~olid abutment 108 is also filled with a bed of regener~tive material~ ~lso lncluded in this embodiment i5 a heat radia~ion shield 111 which surrounds the housing 6ec~.ion 102 from ~he cold end 112 t~ a 1uid inlet 114. The radiation ~hield 111 i8 made of a highly thermally conductive material such as copper ~nd is effective in keeping radiant energy away from the cooled end 112.
As in the embodiment of Fig. 1, ~wo worXing volume~ 116 and 118 are crea~ed within the housing 102. ~lso a5 in the case of the embodiment of FigO 1, longitudinally ~paced-apar~ circular di~cs 120 are provided ~o enhance heat ~ransfer. A second inlet 122 communicates wi~h the interior compartment 106. The inle s 114 and 122 are connected to a compressor such a~ the compressor assembly 5~ illu6tr~ed in Fig. 30 It will thus be appreciated that the displacer 110 will reciprocate in ~he internal compart-. ment 104 u~der the influence of "push-pull" pressure waves from the compres~or assembly 50. Such reciprocativn causes cooling both in ~he primary working volume 116 and in the ~econdary ' working vol~m~ 118.

; Disposed ad3acen~ ~o the working volume 118 and in a hea~ exchange relation with the discs 120 are thermally c~nduc-tive masses 130 and 132. A suitable material is copper. As the working volume 118 i~ cooled, the cooliny effect i~ transmit~ed through the therm211y conductive material 130 and 132 ~o as to e~fect a precooling of th~ working fluid coming into the int~rior compartment 104 through ~he inlet 114. As di~cussed with the earlier ~mbodim~n~, this arrangemen not ~nly effects a pre-cooling of ~he working fluid bu~ al~o substantially reduces the ~`

1 thermal gradient acro ~ ~he length o~ ~he di~placer llO which i~
effective to control unwan~ed thermophoni~s. ThusO becaue~ of the thermally conducting ma~erial 130 and 132, i~ i~ unnece~ary that the inle~ 113 be coiled around the ou~side of the assembly lQ0.

A signifieant aspect of the embodiment of Fig. 2 i~ ~
that as ~h~ di~placer travels downwardly, it sequentially comes abreast of the di6cs 120 ~ Because of the close t:alerance between these di~cs and the di6pl~cer 110, the fluid flow pa~h i~
progressively reduced a~ each of the di6cs is encountered. When the displacer xeaches the lowermost disc, the 1OW i8 completely cut off<, Thi8 progres~ive diminution of ~low thus provides for the controlled deceleration of the displacer by way of pneumatic cushioning. In ~his way, slamming is eff~ctiv~ly controlled by this phase shifting means.

It is thus ~een that the object~ of this invention have been achieved in ~hat there has been di~closed a miniature cryogenic cooling ~ystem which has the very desirable charac-teristics of low microphonics and low thermophonics. These attributes are accomplished because the cold finger employ5 pneumatic cushioning to prevent contact between the reciprocating displacer and it~ enclo~ure. In addi~ion, precooling of the I working fluid in a second working volume reduces thermal gradi-ent~ which ~ub~tan~ially ameliorate the problem of thermophonics.
The two compre~sors of the compre ~or a~semb~y which drive the reciprocating displacer are of a ~imple construction becau e ~he two pi~ton~ reciproc~te colinearly. This results in a compressor assembly which is much ~impler to manufac~ure than known ~14~

~o~

compres~ors which have pi8ton8 deployed at angles other ~han 180 degree~ with respect o one another or havlng piston6 of dif-ferent ~ize~ In addition becau~e ~:he com~r*~sor as~embly -disclo~ed herein use~ pi~ong of the ~ame ~ize, ~he compre sor assembly i8 self-balancing.

.
j While this invention ha~ been describad wi~h reference to its preferred embodimen~ hould be under~tood that variations and modification~ will occur to tho~e ~killed in the art in view of the foregoing description. It i~ intended that all ~uch variation~ and modification~ be included within the scope of th~ appended claim~.

What i~ claimed i~:

Claims (17)

1. In a miniature cryogenic system having two compressors with two outlets, first and second conduits carrying a working fluid, each conduit connected to one of the outlets, said working fluid pressurized in a split-phase relationship by said compressors, wherein the improvement comprises a cold finger assembly including:
an elongate housing having a cryogenically cooled end and an internal abutment dividing said housing into first and second interior compartments, said first compartment extending from said cooled end to said abutment;
a displacer disposed slidingly within said first com-partment for longitudinal reciprocating motion therein and creating a first working volume between said cooled end and said displacer and creating a second working volume between said displacer and said abutment;
a first regenerative bed disposed within said displacer;
a first inlet means located in said housing adapted for providing fluid communication between said first conduit and said first interior compartment;
first and second port means in said displacer providing fluid communication through said first regenerative bed between said first inlet means and said first working volume;
a second regenerative bed disposed within said second interior compartment;
1. Claim 1 cont'd.
   a second inlet means in said housing providing fluid communication through said second regenerative bed between said second conduit means and said second working volume;
   and means to develop transient cushioning overpressures in said first and second working volumes as said displacer reciprocates alternately toward said cooled end and toward said abutment under the push-pull influence of said split-phase working fluid flow from said compressors whereby cooling is effected in said first and in said second working volumes.
2. 2. The system of claim 1 further including sliding seal means disposed between said housing and said displacer to guide said displacer for longitudinal reciprocating motion within said first interior compartment.
3. 3. The system of claim 1 wherein said first conduit is coiled around the exterior of said housing adjacent said second working volume to effect a precooling of the working fluid carried in said first conduit.
4. 4. The system of claim 1 wherein said abutment comprises a web extending across the interior of said housing and includes an opening to provide fluid communication between said first and second interior compartments.
5. 5. The system of claim 1 further including plate means for securing said second regenerative bed in a fixed location within said second interior compartment and heat exchanger means disposed between said plate and said abutment.
6. 6. The system of claim 5 wherein said heat exchanger means comprises a plurality of longitudinally spaced annular discs.
7. 7. The system of claim 2 wherein said sliding seal means comprises a set of resilient sealing members disposed on opposite sides of said first port means in said displacer to block substantially any fluid flow past said sealing means.
8. 8. The system of claim 1 wherein said abutment comprises a solid member that extends substantially the length of said second interior compartment and said second regenerative bed is located between said solid member and said housing.
9. 9. The system of claim 8 further including heat exchanger means adapted for securing said second regenerative bed in a fixed location within said second interior compartment.
10. 10. The system of claim 9 wherein said heat exchanger means comprises longitudinally spaced discs flanking said second working volume.
11. 11, The system of claim 1 further comprising a radiation shield surrounding said housing in a spaced rela-tionship and extending from said cooled end toward said second working volume.
12. 12. The system of claim 4 further including phase-shifting plug means disposed on the lower end of said displacer adjacent to said web and located to seal said opening when said displacer is closely spaced from said web.
13. 13. The system of claim 12 wherein said plug means comprises; a pad of resilient material.
14. 14. The system of claim 12 wherein said plug means comprises a spring disposed within a covering.
15. 15. The system of claim 1 wherein said compressors are adapted for providing a 180 degree phase relationship in said conduits .
16. 16. The system of claim 1 wherein said compressors comprise a cylindrical cavity within a housing and a pair of spaced-apart, rigidly connected pistons adapted for reciprocating motion within said cavity to create a 180 degree split-phase relationship in said working fluid.
17. 17 . The system of claim 10 wherein said longitudinally spaced discs cooperate with said displacer to restrict the flow of said working fluid progressively as said displacer comes abreast of said discs sequentially as it approaches the bottom of its travel.