Abstract:
A cooling device ( 7 ) for re-liquefying cryogenic gases, comprising an outer jacket ( 8 ) which delimits a vacuum chamber ( 9 ), and a cryocooler cold head ( 10 ) installed therein, which has at least two cold stages ( 11, 12 ) and is at least partially surrounded by a radiation shield ( 13 ) is characterized in that at least two cold stages ( 11, 12 ) of the cold head ( 10 ) are separately individually connected in a heat-conducting manner to a heat-transferring device ( 14   a,    14   b ) which can be inserted into the neck or suspension tubes ( 3   a,    3   b ) of a cryostat ( 1 ) for keeping at least two different cryogenic liquids ( 18   a,    18   b ). The cooling device can be easily retrofitted into existing cryostat configurations, in particular, those containing superconducting magnets and without (or with minimum) adjustment to permit operation with no or little cryogen loss, even if several cryogens are used.

Description:
[0001]     This application claims Paris Convention priority of DE 10 2004 037 173.3 filed Jul. 30, 2004 the complete disclosure of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The invention concerns a cooling device for re-liquefying cryogenic gases, comprising an outer jacket which delimits a vacuum chamber, and a cryocooler cold head installed therein, which has at least two cold stages and is at least partially surrounded by a radiation shield. EP0905436, EP0905524, WO03036207, WO03036190, U.S. Pat. No. 5,966,944, U.S. Pat. No. 5,563,566, U.S. Pat. No. 5,613,367, U.S. Pat. No. 5,782,095, U.S. Pat. No. 2002/000283, US2003/230089 e.g. describe cooling of a superconducting magnet system with no or little cryogen loss using a cryocooler.  
         [0003]     The e.g. two-stage cold head of the cryocooler is usually installed in a separate sleeve assembly which is under vacuum (described e.g. in U.S. Pat. No. 5,613,367) or directly in the vacuum chamber of a cryostat (described e.g. in U.S. Pat. No. 5,563,566) such that the first cold stage of the cold head is rigidly connected to a radiation shield and the second cold stage is connected in a heat-conducting manner to the helium container either directly, or via a fixed thermal bridge, wherein the helium container holds the superconducting magnet in liquid helium. Through recondensation of the helium, which evaporates due to external heat input, on the cold contact surface in the helium container, the overall heat input into the helium container can be compensated for, thereby providing operation with no or little cryogen loss of the system. In an alternative manner, the cold head can be inserted into a neck tube which connects the outer vacuum sleeve of the cryostat to the helium container and is correspondingly filled with helium gas as described e.g. in US2002/0002830A1. The first cold stage of the two-stage cold head is in fixed thermal contact with a radiation shield and the second cold stage is freely suspended in the helium atmosphere to directly liquefy evaporating helium.  
         [0004]     These variants have certain disadvantages. The design and construction of the cryostat becomes more demanding and complex and installation of the additional sleeve which receives the cryocooler cold head generates additional heat input into the cold head. If an additional neck tube is used for the cold head, further heat is transferred into the helium container or cooler cold head due to thermal conduction in the helium gas column and in the tube wall and through possible convection flow in the helium gas. Moreover, fixed, rigid or even flexible thermal elements connected between the cold head and the cryostat can transfer cold head vibrations to the cryostat. Furthermore, in a temperature range below 10K, magnetic regenerator materials are usually used in the regenerator of the second stage of the cold head of cryocoolers such as pulse tube coolers or Gifford-McMahon coolers, and the regenerator may be relatively close to the magnetic center of the NMR magnet system. In consequence thereof, the regenerator must generally be shielded to prevent disturbance of the magnetic field at the location of the NMR sample and to prevent the function of the regenerator from being impaired. Finally, an unstable state occurs when the cryocooler fails, and the temperatures of cryostat components such as e.g. the radiation shield continuously change until a new balanced state is reached. In a magnet system for high-resolution nuclear magnetic resonance (NMR) spectroscopy, this can preclude NMR measurements, since the shim state of the magnet constantly changes or, in the worst case, the magnet runs dry and quenches.  
         [0005]     One method for preventing some of these disadvantages while still realizing a partially loss-free cryogen system entails use of a device cooled by a cryocooler which can be used for re-liquefying one single evaporated cryogen. In a hitherto common cryostat arrangement for e.g. a superconducting magnet system, the magnet is usually installed into a container filled with liquid helium at 4.2 K. The helium container is generally surrounded by a boil-off-gas-cooled radiation shield and a further shield cooled with liquid nitrogen, such that the external heat input onto the helium container is minimized. Due to passive cooling by the evaporating cryogens, liquid helium and nitrogen must be refilled at certain intervals.  
         [0006]     JP11257770 and JP2000283578 propose inserting a heat-transferring device in the form of a heat tube into the existing neck or suspension tubes of a nitrogen container of a cryostat configuration, the heat tube being connected to the cold head of a cryocooler to re-liquefy evaporating nitrogen (see also Advances of Cryogenic Engineering, 45, 41-48). The liquefier is thereby directly flanged onto the cold head of the one-stage pulse tube cooler and consists of a thin tube in which the nitrogen vapor rises, is liquefied on a cold surface which is in contact with the cold head, and runs downwards along the tube wall. This very thin tube is surrounded by a vacuum sleeve in its upper region and can be directly inserted into a nitrogen neck tube or suspension tube to prevent or reduce evaporation of the nitrogen and nitrogen losses. The helium losses are not addressed, since only the nitrogen is re-liquefied.  
         [0007]     In a similar manner, re-liquefaction of helium only has also been carried out in a helium storage container using a two-stage cryocooler cold head.  
         [0008]     In both cases (nitrogen or helium liquefier), the cold head of the cryocooler is in an outer jacket which delimits a vacuum chamber. When multi-stage cryocoolers are used, parts of the cold head are usually surrounded by a radiation shield which is in contact with a cold stage (not the coldest stage) to ensure good insulation of the cold head against thermal radiation in the low temperature region.  
         [0009]     As already mentioned above, different conventional cryostat configurations which are used, in particular, for magnet systems in high-resolution nuclear magnetic resonance (NMR) spectroscopy have more than one cryogen. In addition to a container filled with liquid helium and holding the magnet, an additional radiation shield is e.g. provided which is cooled with liquid nitrogen. In this manner, one would have to use a separate helium liquefier and a separate nitrogen liquefier to reduce both the helium and nitrogen losses or to obtain loss-free operation. This would considerably increase the number of devices, the investment, and the operating costs.  
         [0010]     It is therefore the object of the present invention to provide a cooling device which, in an advantageous and straightforward manner, can be retrofitted to an existing cryostat configuration containing at least two cryogenic liquids, in particular, a cryostat configuration which comprises a superconducting magnet arrangement, to eliminate or strongly reduce loss of some or all existing cryogenic liquids relative to conventional devices.  
       SUMMARY OF THE INVENTION  
       [0011]     Departing from prior art and in accordance with the invention, this object is achieved in that at least two cold stages of the cold head of the cryocooler are each individually connected in a heat-conducting manner to a heat-transferring device which can be inserted into the neck or suspension tubes of a cryostat for keeping at least two different cryogenic liquids.  
         [0012]     A cooling device of this type offers the following advantages: Existing cryostat configurations, and in particular those which contain superconducting magnets can be retrofitted without (or with only minor) adjustments, to permit operation with no or little cryogen loss requiring only little extra hardware even if several cryogens are used. The cryostat must not be re-engineered. The additional heat input into the cryostat produced by the device is small and can be predicted quite precisely when properly engineered. The heat-transferring devices in which the cryogens are liquefied are designed such that they can be introduced in a contact-free manner into the neck or suspension tubes of the cryostat configuration. The evaporating gas is liquefied in a thermodynamically effective manner, since the vapor is not overheated and therefore need not be cooled down to the liquefying temperature. The cryocooler cold head is placed at such a distance from the magnetic center of a superconducting magnet arrangement in the cryostat that disturbances on the magnet arrangement caused by the magnetic regenerator material are less severe than if the cold head were directly integrated into the cryostat. Conversely, the function of the cryocooler is also less impaired by the magnetic field of the magnet arrangement. If the cryocooler fails or must be switched off for maintenance work, the cryostat configuration still functions, e.g. for cooling a superconducting magnet arrangement. This ensures high operational reliability. Moreover, the user can freely select the mode of operation (conventional or without cryogen loss).  
         [0013]     In a particularly preferred embodiment of the inventive cooling device, at least one of the heat-transferring devices has a cavity which is connected to an open line, in particular a conduit. The cryogen which evaporates from the liquid tank of the cryostat is guided through the conduit to the cavity at the cold stage, where it is liquefied. The condensed matter then flows back through the conduit into the liquid tank of the cryostat. This heat-transferring device functions like conventional heat engineering heat pipes.  
         [0014]     In a further preferred embodiment of the invention, at least one of the heat-transferring devices comprises a metallic connection with excellent heat transferring properties, at the end of which the cryogen evaporated from the liquid tank of the cryostat is liquefied and subsequently flows back into the liquid bath of the liquid tank of the cryostat. The other end of this connection is flanged to a cold stage of the cold head of the cryocooler. Various combinations of the heat-transferring devices are possible. A metallic connection with excellent heat conducting properties may e.g. be flanged to the first cold stage of a two-stage cold head, while the second cold stage is connected to a conduit.  
         [0015]     In particular for high-resolution NMR methods, the cryocooler is advantageously a pulse tube cooler, since pulse tube coolers operate with extremely low vibration. Moreover, pulse tube coolers also provide reliable operation and require little maintenance.  
         [0016]     The cooling device may also be operated with a Gifford McMahon cooler. One disadvantage of this cryocooler compared to a pulse tube cooler are increased vibrations. This disadvantage can be overcome if soft sealing elements are provided between the cryocooler and the cryostat configuration, as is described below.  
         [0017]     In a particularly advantageous manner, at least one connecting line, which is open at both sides, is provided to connect the cold head of the cryocooler to at least one neck or suspension tube of the liquid tank containing the cryogen of lowest-boiling temperature and into which no heat-transferring device is inserted, wherein the line is in thermal contact with at least two cold stages of the cold head and may also contact a regenerator tube above the coldest cold stage, wherein the line terminates in the cavity mounted to the cold head after thermal contact with the coldest cold stage, or is guided along the metallic connection into the liquid tank. The gas in the line is cooled at the cold head of the cryocooler and liquefied at the coldest cold stage such that a flow is generated in the line through the neck or suspension tubes to the cooling device due to the resulting suction. The gas flow cools the neck or suspension tubes thereby ideally completely compensating for the heat input via the neck or suspension tubes. This circulating flow for the cooling neck or suspension tubes further reduces heat input into the cryostat.  
         [0018]     In a further development of this embodiment, a valve and/or a pump is provided in the connecting line between the neck or suspension tubes and the cold head to control the gas flow. The gas flow can be reduced or the optimum gas flow can be adjusted as required if e.g. the suction effect at the cold head is so large that the gas flow becomes greater than required for optimum cooling of the suspension or neck tubes.  
         [0019]     In an advantageous manner, helium can be liquefied at the coldest cold stage of the cold head at a temperature of 4.2 K or less to provide a plurality of possible applications in the region of low temperature. The helium loss and the refilling processes can be reduced or loss-free operation can be obtained if the cooling capacity of the cryocooler is sufficiently large.  
         [0020]     According to a further advantageous aspect, nitrogen can be liquefied at 77 K or less at a cold stage of the cryocooler cold head. With the use of the heat-transferring devices in a cryostat configuration having a container with liquid nitrogen, the nitrogen loss can be reduced or eliminated during operation if the cooling capacity of the cryocooler is sufficiently large.  
         [0021]     In an advantageous embodiment, a cold stage of the cold head, which is not the coldest, is connected in a heat-conducting manner to the radiation shield which, at least partially, surrounds the cold head. In this manner, the radiative heat input onto the colder components of the cold head is substantially reduced.  
         [0022]     It is moreover advantageous if the heat-transferring device comes to rest at least partially within the outer jacket of the cooling device, i.e. within the vacuum chamber. This is relevant in particular for that part of the heat-transferring device which is connected to the cold head of the cryocooler. This part of the heat-transferring device is thereby excellently insulated against heat conduction towards the outside.  
         [0023]     It is also very advantageous if the heat-transferring device is surrounded at least partially by a first tube in the region outside of the outer jacket. This tube thermally insulates the heat-transferring device. It may but must not have a constant diameter along its entire length. It may be more favorable in view of construction to select the smallest possible diameter for one part of the tube, and a larger diameter for the rest.  
         [0024]     In a preferred embodiment, the first tube which surrounds the heat-transferring device is open at one end, and that end is connected to the vacuum chamber of the outer jacket, while the other end is connected to the conduit or the metallic connection of the heat-transferring device in a gas-tight manner. If the vacuum chamber of the cooling device of this embodiment is evacuated, the part of the heat-transferring device surrounded by the first tube is also under vacuum. The heat-transferring device is then excellently insulated in this region against thermal conduction towards the outside.  
         [0025]     In another advantageous embodiment, the first tube surrounding the heat-transferring device is connected at both ends to the conduit or the metallic connection of the heat-transferring device in a gas-tight manner, and evacuated via a separate connection. The interior of the tube can thereby be evacuated and the part of the heat-transferring device, which is surrounded by the tube can be excellently insulated against thermal conduction towards the outside.  
         [0026]     The conduit or the metallic connection of the heat-transferring device can advantageously at least partially surround a further second tube which is connected in a heat-conducting manner to the radiation shield. This tube is disposed within the first tube to provide vacuum insulation, as described above. In this manner, the part of the heat-transferring device surrounded by the second tube is excellently insulated against thermal radiation towards the outside.  
         [0027]     With particular preference, the above-described tubes surrounding the heat-transferring device are flexible, at least in sections, and are preferably designed as a bellows.  
         [0028]     In a further favorable manner, the heat-transferring device is designed to be flexible, at least in sections, in particular as a bellows or in the form of wires plaited into strands. In this embodiment of the inventive cooling device, the heat-transferring device and the surrounding tubes are flexible to considerably facilitate installation thereof into the neck or suspension tubes of a cryostat configuration.  
         [0029]     In this connection, it is also advantageous if the heat-transferring device and the surrounding tubes can be connected to and disconnected from each other at at least one location using a gas-tight coupling. The coupling is designed such that the functionality of the heat-transferring device and the surrounding tubes is not impaired. This substantially, facilitates mounting of the cooling device to a cryostat configuration.  
         [0030]     In a further embodiment of the invention, the cooling device can be mounted to the cryostat for keeping cryogenic liquids either at the neck, at the suspension tubes, or on the outer jacket of the cryostat configuration.  
         [0031]     In an alternative and preferred manner, the cooling device may be mounted outside of the cryostat e.g. on the room ceiling or on a separate stand. In this case, the cryostat configuration does not have to bear the weight of the cooling device. This can increase the mechanical stability of the cryostat configuration.  
         [0032]     In this connection, a soft connecting element which does not transmit vibrations is advantageously provided as a seal between the cooling device and the cryostat. This ensures that—in particular for high-resolution NMR methods—none or only little disturbing vibrations of the cooling device are transferred to the cryostat configuration.  
         [0033]     Another possibility is mounting electric heaters to the cold stages of the cold head of the cryocooler. In case of surplus cooling capacity of the cryocooler, the heaters can be adjusted such that the cryocooler exactly compensates for the heat input into the different containers of the cryostat configuration.  
         [0034]     The advantages of the inventive cooling device are particularly well utilized if they are part of a cryostat configuration.  
         [0035]     In a particularly advantageous manner, the cooling device serves to cool a superconducting magnet arrangement, in particular, a superconducting magnet arrangement which is part of a nuclear magnetic resonance apparatus, in particular, a magnetic resonance imaging (MRI) or nuclear magnetic resonance spectroscopy (NMR) apparatus.  
         [0036]     An electric heater can also be introduced into the liquid tank of a cryostat configuration provided with the inventive cooling device via a neck or suspension tube of at least one liquid tank. In case of surplus cooling capacity of the cryocooler cold head which is integrated in the cooling device, the pressure in the liquid containers can thereby be kept at a constant level above the surrounding pressure. It is, however, also feasible to control the power of the cryocooler via its operating frequency and/or the fill amount of working gas in the cryocooler.  
         [0037]     Further advantages of the invention can be extracted from the description and the drawings. The features mentioned above and below may be used individually or collectively in arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but have exemplary character for illustrating the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0038]      FIG. 1  shows a cryostat configuration with two liquid tanks for cryogenic liquids;  
         [0039]      FIG. 2   a  shows an inventive cooling device with heat-transferring devices having a cavity;  
         [0040]      FIG. 2   b  shows an inventive cooling device with heat-transferring devices comprising a metallic connection with excellent heat-transferring properties;  
         [0041]      FIG. 3  shows a cooling device installed in a cryostat according to  FIG. 2   a;    
         [0042]      FIG. 4  shows a cooling device according to the present invention which is installed in a cryostat, with a connecting line which connects the cold head of the cryocooler to a suspension tube of a liquid tank;  
         [0043]      FIG. 5   a  shows a cooling device in accordance with the present invention, which is mounted on the cryostat;  
         [0044]      FIG. 5   b  shows a cooling device in accordance with the present invention which is mounted to the room ceiling; and  
         [0045]      FIG. 5   c  shows a cooling device in accordance with the invention which is mounted to a stand. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0046]      FIG. 1  is a schematic illustration of a cryostat  1  with a magnet arrangement  5  as commonly used for MR applications. The cryostat  1  comprises a liquid tank  2   a  filled with helium which is connected via suspension tubes  3   a  to an outer jacket  4  of the cryostat  1  and contains a magnet arrangement  5 . A further liquid tank  2   b  is disposed about the liquid tank  2   a,  which contains nitrogen and is connected via the suspension tubes  3   b  to the outer jacket  4  of the cryostat  1 . The liquid tank  2   b  with nitrogen is in thermal contact with the suspension tubes  3   a.  A boil-off-gas-cooled radiation shield  6  is disposed between the two liquid tanks  2   a,    2   b  and is also in thermal contact with the suspension tubes  3   a.    
         [0047]      FIG. 2   a  shows an embodiment of an inventive cooling device  7 . The cooling device comprises an outer jacket  8  which delimits a vacuum chamber  9 , and a cold head  10  of a cryocooler disposed therein which comprises at least two cold stages  11 ,  12  and is at least partially surrounded by a radiation shield  13 . The cold stages  11 ,  12  of the cold head  10  are each connected in a heat-conducting manner to a heat-transferring device  14   a,    14   b.  The heat-transferring devices  14   a,    14   b  each have a cavity  15   a,    15   b,  wherein each cavity  15   a,    15   b  is connected to a respective conduit  16   a,    16   b.    
         [0048]      FIG. 2   b  shows an alternative embodiment of the inventive cooling device  7 , wherein the heat-transferring devices  14   a,    14   b  comprise connections  17   a,    17   b  with excellent heat conducting properties. These connections may be e.g. in the form of cold fingers which are generally designed as metal rods. Such a metal rod should have a maximum cross-sectional surface to ensure minimum temperature differences along the rod.  
         [0049]     The conduits  16   a,    16   b  can be inserted into the suspension tubes  3   a,    3   b  of the liquid tanks  2   a,    2   b  of a cryostat  1 .  FIG. 3  shows an inventive cooling device  7  in the installed state. The conduits  16   a,    16   b  are located in the cryogen vapor above the liquid surface of the cryogens  18   a,    18   b  disposed in the liquid tanks  2   a,    2   b.  The heat-transferring devices  14   a,    14   b  are each connected in a heat-conducting manner to a cold stage  11 ,  12  of the cryocooler ( FIGS. 2   a,    2   b  and  3 ). The cryogens  18   a,    18   b  evaporated from the liquid tanks  2   a,    2   b  of the cryostat  1  are guided through the conduits  16   a,    16   b  into the cavity  15   a,    15   b  on the respective cold stage  12 ,  11  where the cryogens  18   a,    18   b  are condensed and are thereby liquefied and subsequently flow back through the conduits  16   a,    16   b  into the liquid tanks  2   a,    2   b  of the cryostat  1 . The helium vapor can also be liquefied at the end of a metallic connection  17   a,    17   b  which is in contact with the cold head  10  and has excellent heat conducting properties ( FIG. 2   b ).  
         [0050]     The cryogen  18   b  with higher boiling temperature from the liquid tank  2   b  is thereby liquefied on the first cold stage  11  of the cold head  10  while the cryogen  18   a  with a lower boiling temperature is liquefied at the second, colder cold stage  12  of the cold head  10 . The invention also comprises cooling devices with a multi-stage cold head  10  such that, in principle, any number of cryogens, corresponding to the number of the cold stages of the cold head  10 , can be liquefied.  
         [0051]     The heat-transferring devices  14   a,    14   b  are surrounded by a first tube  19   a,    19   b  to insulate them from thermal input, the first tube being connected to the vacuum chamber  9  of the outer jacket  8  of the cooling device  7  and can be evacuated together with the vacuum chamber  9  ( FIGS. 2   a,    2   b ). To improve thermal insulation from the external heat radiation, a second tube  20  is disposed within the first tube  19   a  which is connected in a heat-conducting manner to the radiation shield  13 . The diameter of the first tube  19   b  varies along its length in  FIGS. 2   a,    2   b  and  FIG. 3 . It may be necessary to reduce the diameter of the tube at the closed end such that it can be inserted into the suspension tube  3   b  of the liquid tank  2   b  in a contact-free manner. A bellows provides a flexible connection between the first tube  19   b  and the outer jacket  8  of the cooling device  7 . A bellows may also be interposed between the first tube  19   a  and the outer jacket  8  and in a section of the second tube  20 . The metallic connection  17   a,    17   b  shown in  FIG. 2   b  can be made flexible through flexible connecting elements  21   a,    21   b  (such as e.g. wire plaited into strands). In case of surplus cooling capacity of the cryocooler, additional heaters (not shown) can be mounted to the cold stages  11 ,  12  of the cold head  10  of the cryocooler. Alternatively or additionally, in case of surplus cooling capacity of the cryocooler, the pressure in the liquid tanks  2   a,    2   b  for the cryogens  18   a,    18   b  can be kept constant using heaters  22   a,    22   b  which are disposed in the liquid tanks  2   a,    2   b  and which are e.g. inserted via remaining free neck or suspension tubes  3   c,    3   d.    
         [0052]      FIG. 4  shows an advantageous variant of the inventive cooling device, wherein a free neck or suspension tube  3   c  of the cryostat  1  is connected, via a line  23  which is open on both sides, after thermal contact with the cold stages  11 ,  12  of the cold head  10 , to the cavity  15   a  and therefore also to the liquid tank  2   a.  A connection of this type can also be realized with several free neck or suspension tubes  3   c.  The lines from the suspension tubes  3   c  are initially combined into one line  23 . This line  23  is then guided through the outer jacket  8  of the cooling device  7  which contains the cold head  10  and is thermally contacted using the heat exchanger  24   b,    24   a  with at least two cold stages  11 ,  12  of the cold head  10  and possibly also with the regenerator tube  25  above the coldest cold stage  12  e.g. by wrapping it around the regenerator tube  25 . After contact with the coldest cold stage  12 , the line  23  terminates in the cavity  15   a  mounted to the cold head  10  or is guided along the metallic connection  17   a  into the liquid tank  2   a  for the cryogen  18   a  (helium). The gas in the line  23  is cooled by the cold head  10  and liquefied at the coldest cold stage  12 , thereby generating a flow in the line  23  through the suspension tube  3   c  towards the cooling device  7  due to the resulting suction. The heated gas flow cools the suspension tube  3   c,  whereby in the ideal case, the heat input is completely compensated or at least reduced via the suspension tube  3   c.  The overall cooling capacity of the cryocooler slightly decreases due to the additional load. The gain due to the reduced heat input is larger than the loss in cooling power. In particular for systems with massive neck or suspension tubes  3   c,  a cryocooler with lower power can thereby be used. The heat-transferring devices  14   a,    14   b  (heat tubes or cold fingers) may be made from two or more parts, which permits separation thereof using gas-tight couplings (not shown). This facilitates installation and-disassembly. The line  23  has a valve  26  and a pump  27  to control the gas flow through the line  23  and thereby adjust an optimum gas flow. The line  23  may be provided with such a device (valve  26  or pump  27 ) or such devices can be completely omitted. In the embodiment of  FIG. 4  and also in the embodiment of  FIG. 3 , heaters  22   a,    22   b  are provided in the liquid tank  2   a,    2   b.  For reasons of clarity, the connections are omitted in  FIG. 4 .  
         [0053]      FIGS. 5   a  through  5   c  show various possibilities for fixing the cooling device  7 . The vacuum container which contains the cold head  10  of the cryocooler can either be directly mounted on the outer jacket  4  of the cryostat  1  as shown in  FIG. 5   a  or externally e.g. on the room ceiling  28  ( FIG. 5   b ) or on a separate-stand  29  ( FIG. 5   c ). A seal  30  must be used for mounting onto the cryostat  1 . In case of external suspension only further sealing elements  31   a,    31   b  are used between the vacuum chamber  9  and the outer jacket  4  of the cryostat  1  with the consequence that no or only minimum vibrations of the cryocooler are transferred to the cryostat  1 . This is particularly favorable if the cooling device  7  is used for cooling a cryostat configuration which contains a superconducting magnet arrangement  5 , in particular, if the superconducting magnet arrangement  5  is part of a nuclear magnetic resonance apparatus, in particular magnetic resonance imaging (MRI) or magnetic resonance spectroscopy (Nuclear Magnetic Resonance, NMR). The inventive cooling device therefore allows high-resolution NMR methods.  
         [0054]     In summary, a cooling device is provided which permits retrofitting to existing cryostat configurations, and in particular such configurations which contain superconducting magnets without (or with only minor) adjustments to permit, in a straightforward manner, operation with no or little cryogen loss even if several cryogens are used.  
       LIST OF REFERENCE NUMERALS  
       [0000]    
       
           1  cryostat  
           2   a,   2   b  liquid tanks  
           3   a,b,c,d  suspension tubes  
           4  outer jacket  
           5  magnet arrangement  
           6  radiation shield of the cryostat  
           7  cooling device  
           8  outer jacket of the cooling device  
           9  vacuum chamber  
           10  cold head  
           11  first cold stage  
           12  second cold stage  
           13  radiation shield of the cooling device  
           14   a,b  heat-transferring device  
           15   a,b  cavity  
           16   a,b  conduit  
           17   a,b  connection  
           18   a,b  cryogen  
           19   a,b  first tube  
           20  second tube  
           21   a,b  connecting element  
           22   a,b  heater  
           23  open line  
           24   a,b  heat exchanger  
           25  regenerator tube  
           26  valve  
           27  pump  
           28  room ceiling  
           29  stand  
           30  seal  
           31   a,b  sealing elements