Abstract:
The invention relates to a multistage low-temperature refrigeration machine with a first stage in the form of a displacer refrigerator ( 1 ) as well as at least one further stage in the form of a pulse tube refrigerator ( 25 ); in order to suppress vibrations it is proposed that a flexible component ( 45 ) designed to prevent the spread of vibrations be placed between the displacer refrigerator ( 1 ) and the pulse tube refrigerator ( 25 ).

Description:
FIELD OF THE INVENTION 
     The invention relates to a multistage low-temperature refrigeration machine with a first stage in the form of a displacer refrigerator as well as at least one further stage in the form of a pulse tube refrigerator. 
     BACKGROUND OF THE INVENTION 
     The term displacer refrigerator is to be understood as a Gifford McMahon, Sterling or similar refrigeration machine. Single-stage refrigeration machines of this kind have a working area with a displacer. The working area is connected in alternating fashion to a high pressure and a low pressure source of gas in such a manner that during the forced to-and-fro motion of the displacer, a thermodynamic cycle is performed. The working gas is cycled preferably through a regenerator (heat accumulator for pre-cooling of the entering gas) located within the displacer. During operation of the refrigeration machine, heat is removed at one of the two ends of the working area. With a single-stage refrigerator of this kind and with helium as the working gas, temperatures down to 10-30° K. may be generated. Displacer refrigerators offer the advantage of being relatively powerful and their theoretical fundamentals are well understood. Their disadvantage is the generation of vibrations caused by the mass of the displacer as it moves to-and-fro. 
     Also known are refrigeration machines, the operation of which is based on the principle of the pulse tube. These machines include an area with a fixed refrigerator in which in-flowing gas is cooled by exchanging the heat with the regenerator material, as well as a pulse tube into which the working gas from the area of the regenerator flows in and out periodically at one end (cold side). Connected to the other end (warm side) of the pulse tube, preferably through a constriction, is a sealed volume. By suitably selecting this throttle, the phase relationship between mass flow and pressure variation in the area of the pulse tube may be influenced for the purpose of attaining optimum performance. Besides the above approach detailed (referred to as a “Orifice Pulse Tube” design) other types of design such as (“Double Inlet”, “4-valve”) also exist that modify the phase relationship. The efficiency of refrigeration machines of these kinds is limited. These designs offer an advantage however, in that they do not generate any vibrations since they do not contain any moving parts. 
     Through a lecture on occasion of the “Cryogenic Engineering Conference” Columbus, Ohio, in July 1995 it is known to combine a displacement refrigerator with a pulse tube refrigerator. The displacer refrigerator forms the first stage and, the pulse tube refrigerator forms the second stage of a multistage low-temperature refrigeration machine. In order to ensure that the cold end of the displacer refrigerator and the warm end of the pulse tube refrigerator are at the same temperature, a thermal link made from a rigid copper panel is provided which is linked in a thermally well conducting manner to both ends of the refrigerators. Owing to this rigid link, vibrations produced by the displacer refrigerator spread to the pulse tube refrigerator. Thus, this known type of combined refrigeration machine is not suited for cooling vibration sensitive objects. 
     SUMMARY OF THE INVENTION 
     It is the task of the present invention to utilize the advantage of the pulse tube refrigerator in which vibrations are not generated, in connection with a combined refrigeration machine of the aforementioned kind, at least in the area of the second stage or subsequent stages. 
     This task is solved through the present invention by placing a flexible component which prevents the spread of vibrations between the first displacement refrigerator stage and the further stage which is designed as a pulse tube refrigeration stage. Through this measure, the vibrations from the displacement refrigerator can be kept away from the pulse tube refrigerator. The cold end of the further stage operating as a pulse tube refrigerator may thus be linked thermally to vibration sensitive objects or the like without any additional means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages and details of the present invention shall be explained by referring to the design examples presented schematically in the following figures: 
     FIGS. 1 and 2 illustrate combined refrigeration machines according to the present invention different feeds for the working gas leading to the pulse tube refrigerator, 
     FIG. 3 depicts insertion of a refrigeration machine according to the present invention in a cryostat which serves the purpose of cooling magnets with liquid helium, and 
     FIG. 4 illustrates a cryostat with a refrigeration machine according to the present invention in which the magnets are cooled directly. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The displacement refrigerator  1  presented in FIGS. 1 and 2 has a housing which consists of two housing sections  2  and  3 . Housing section  2  houses a cylindrical working area  4  for the displacer  6 . Located in displacer  6  is the regenerator  7 . 
     In the case of a pneumatic drive system, the displacer  6  is equipped with a drive piston  8 , the corresponding cylinder  9  of which is accommodated in a guide bushing  10  which in turn seals off the working area  4  in the direction of housing section  3 . The guide bushing  10  is equipped with bore holes for distributing the high and low pressure gas controlled by a turning valve into the control volume ( 9 ) as well as into the actual working area. The bore holes  11  open out into the working area  4  and serve the purpose of supplying this area with the working gas. The bore hole  13  opens into a cross bore  14  which is linked via an annular groove  15  to the outside wall of the guide bushing  10 . The low pressure side is connected via this link to the valve controller. Two additional bore holes  12 , indicated by dashed lines, are used to pneumatically drive the displacer  6 . The additional bore holes  12  extend through planes which differ from the plane of FIGS. 1 and 2 so that they do not cross each other, this being indicated by the dashed lines. 
     A control motor  16  located within the housing section  3  actuates a control valve  18 , via a motor shaft  17 . This control valve  18  serves the purpose of supplying the; and working gas, preferably helium, at high and at low pressures to the different bore holes  12  in a manner which is basically known. The working gas is run in a cycle  23  outside refrigerator  1  through the line  22  with the compressor  21 . The high pressure connection  19  at the refrigerator  1  is linked with the high pressure side of the compressor  21 , and the low pressure connection  20  is linked to the low pressure side of the compressor  21 . 
     The pulse tube refrigerator  25  according to this embodiment comprises a pulse tube  26 , having a warm end to which the gas volume  27  is connected with the constriction  28 . A cold flange located in the area of a cold end of the pulse tube  26  is designated as  29 . The gas is supplied to the pulse tube  26  via line  31  in which the regenerator  32  is located. 
     In the design example according to FIG. 1, the gas supplied to the pulse tube  26  is taken from the cycle  23  for the working gas with the compressor  21 . The gas supply line  31  opens into two separate lines  34  and  35 , each which are equipped with a control valve  36  and  37 , respectively. The line  34  is linked to the high pressure side of the compressor  21 . The control valve  36  is arranged such that the working gas is allowed to flow through the lines  34  and  31  to the pulse tube  26 . The line  35  is linked to the low pressure side of the compressor  21 . The control valve  37  is arranged such that gas flowing in the reverse direction is allowed to flow through lines  31  and  35  into the cycle  23  with the working gas. 
     In order to be able to pre-cool the working gas flowing through the pulse tube  26 , two heat exchangers  41  and  42  are provided. The first heat exchanger  41 , preferably of the regenerative type, is interspersed by the working gas flowing to-and-fro. The gas flowing back from the pulse tube  26  into the cycle  23  for the working gas pre-cools the gas flowing in the direction of the pulse tube  26 . The second heat exchanger  42  is linked via a thermal link  43  to the cold end of the displacer refrigerator  1 . In the heat exchanger  42 , the gas flowing in the direction of the pulse tube refrigerator  25  is cooled down to the temperature of the cold end of displacer refrigerator  1 . 
     In the design example according to FIG. 2, the gas supply line  31  opens into the area of the cold end of the displacer refrigerator  1  that is into the working area  4 . The working gas is supplied to the pulse tube refrigerator  25  directly from the cold section of the displacer refrigerator  1 . Compared to the design according to drawing FIG. 1, this design has the advantage of being simpler, but with the disadvantage that the cycle frequency (high pressure/low pressure switching) for the displacer machine and the pulse tube section is always identical, which can be a hindrance in attaining an optimum refrigeration capacity at both stages. 
     In order to avoid the spread of vibrations produced by the displacer refrigerator  1  to the pulse tube refrigerator  25 , the line  31 , in the case of both design examples, is equipped with a flexible component  45 . This component may be, for example, a section of metallic (stainless steel) corrugated tube. Alternately, a hose section made from plastic may be employed. In the design example according to FIG. 1 thermal link  43  can also be made to be flexible so as to prevent the spread of vibrations. 
     In the case of a third alternative, not shown, the two machines  1  and  25  are operated with separate compressors. For example, by employing a linear compressor for the pulse tube machine, the valve control can be omitted. As in the preceding, the vibrations may be decoupled by means of a flexible component. 
     FIGS. 3 and 4 depict application examples of two cryostats equipped with a refrigeration machine according to the present invention. The cryostats the purpose of cooling superconducting magnets  52 . Other items where cooling with liquid helium or direct cooling may be employed are, for example: 
     superconducting feeders and wires, 
     superconducting (Josephson) switching components, 
     sensors which need to be cooled (because they are superconducting or to reduce noise), 
     electronics components which need to be cooled (to reduce noise), 
     cryopump arrangements. 
     In the design examples presented, the magnets  52  which need to be cooled are arranged in a circle within the housing of the cryostat  53  and surround a central analysis chamber  54 . Located between the magnets  52  and the outer housing of the cryostat  53  is a thermal screening stage formed by a thermal radiation shield  55 . 
     In the design example according to FIG. 3, the magnets  52  are accommodated in a tank  56  of a circular cross section filled with a refrigerant, preferably helium. The tank&#39;s helium filling port  57  is equipped with a safety valve  58 . Is the task of the presently described refrigeration machines  1 ,  25  to maintain the helium in the tank at a temperature of about 4.2° K. (boiling point of the refrigerant), so as to prevent the refrigerant from evaporating or to recover any evaporated refrigerant by means of condensation. To enable this tanks, the cold end of the pulse tube  26  is thermally coupled via a thermal link  59  to the filling port  57 . The coupling point  60  is located in the immediate vicinity of the opening of the filling port  57  in tank  56 , so that it is located below the surface of the liquid helium. The cold end of the displacer refrigerator  1  is equipped with a cold flange  62  which is thermally coupled to the thermal radiation shield  55  so that it attains a temperature of 30 to 100 K. The pulse tube refrigerator  25  is supplied with gas from the cold end of the working area  4  of the displacer refrigerator  1  (FIG. 2) so that the temperature at both ends is approximately equal. The gas supply line  31  leading from the cold end of the displacer refrigerator  1  to the warm end of the pulse tube refrigerator  25  is a metallic section of highly flexible corrugated hose and thus forms the desired flexible coupling  45 . 
     In the design example according to FIG. 4, a helium tank is not present. The cold end of the pulse tube refrigerator  25  is thermally linked directly via a cold flange  29  to the magnets  52 . In particular, this application makes sense when the superconducting material of the magnets  52  permits higher temperatures (5 to 10 K). In this design, the pulse tube refrigerator  25  is supplied with gas from the cold end of the working area  4  of the displacer refrigerator  1 . The gas supply line  31  consisting of a section of corrugated hose, forming the flexible coupling  45 . 
     In the design example according to FIG. 4, also the warm end of the pulse tube refrigerator  25  is also coupled to the thermal radiation shield  55  (via thermal link  63 ) in addition to the cold end of the displacer refrigerator  1 . The formation of equal temperatures at both these ends is thus facilitated. For this purpose, also the regenerator  32 , with its side facing away from the pulse tube  26 , is thermally linked to the thermal link  63 . 
     In order to prevent the spread of vibrations generated by the displacer refrigerator  1  via the thermal radiation shield  55  to the pulse tube refrigerator, an additional flexible coupling  64  is provided between the thermal link  63  and the thermal radiation shield. The coupling  64  comprises metallic straps  65 , preferably made of copper, which are in good thermal contact with the thermal radiation shield  55  and a flange  66  at the thermal link  63 .