Cryogenic cooling apparatus and cryogenic cooling method for cooling object to very low temperatures

Abstract: A coil unit (41) and a refrigeration unit (42) are positioned such that a second heat conductive member (55) disposed on an extendible wall (54) of a vacuum container (43) and a fourth heat conductive member (74) disposed on an extendible wall (73) of another vacuum container (60) face each other coaxially. In this state, the coil unit (41) and refrigeration unit (42) are relatively moved to approach each other, and thus the second heat conductive member (55) and fourth heat conductive member (74) come in contact. If the coil unit (41) and refrigeration unit (42) are further moved, the extendible wall (54) extends and consequently the second heat conductive member (55) comes in contact with a first heat conductive member (49). In addition, the extendible wall (73) contracts and consequently the fourth heat conductive member (74) comes in contact with a third heat conductive member (62). Thus, a superconducting coil (44) is thermally connected to the refrigeration unit (42) so that the superconducting coil is cooled.

Description: The present invention relates to a cryogenic cooling apparatus for cooling an object such as a superconducting coil. As is well known, recent development in refrigeration technology is remarkable. A small-sized refrigerator capable of efficiently attaining a temperature level of liquid helium has been developed, in particular, by virtue of the discovery of a cold accumulation element exhibiting high specific heat characteristics at very low temperatures. In fact, this type of refrigerator, in general, adopts a cold accumulation system represented by a Gifford-MacMahon refrigeration cycle or a Stirling refrigeration cycle. With the development of the refrigeration technology, a superconducting magnet apparatus of a refrigerator direct-cooling type has recently been developed wherein a superconducting coil housed in heat-insulating container is directly cooled by a cryogenic refrigerator. In the superconducting magnet apparatus of the refrigerator direct-cooling type, unlike a conventional dip-cooling type apparatus wherein a superconducting coil is dipped and cooled in liquid helium, there is no need to use a coolant. Thus, the handling of the apparatus is very easy and the system can be simplified and the operating cost reduced. However, the superconducting magnet apparatus of the refrigerator direct-cooling type has the following problems. In the superconducting magnet apparatus of the refrigerator direct-cooling type, in general, a superconducting coil and a thermal shield are housed in a vacuum container serving as a heat-insulating container, and a plurality of stages of cold accumulating refrigerators are disposed so that cooling stages are situated within the vacuum container. The lowest-temperature cooling stage of the cold accumulating refrigerator is thermally connected to the superconducting coil by a heat conductive member, and the cooling stage of a temperature different from the temperature of the lowest-temperature cooling stage is thermally connected to the thermal shield by another heat conductive member. Since the superconducting magnet apparatus of the above-described refrigerator direct-cooling type has the structure wherein the cold accumulating refrigerator is directly attached to the vacuum container containing the superconducting coil, the following problems are posed: (1) It is difficult to reduce the size of a so-called coil unit by reducing the size of the vacuum container because of the presence of the cold accumulating refrigerator. Thus, the coil unit is inevitably large, and the degree of freedom of installation and use is low. (2) In the cold accumulating refrigerator, as represented by the Gifford-MacMahon refrigeration cycle, a displacer containing a cold accumulator having at least one stage must be driven. Thus, occurrence of mechanical vibration is inevitable. Vibration of the cold accumulating refrigerator is transmitted to the superconducting coil, and due to the vibration of the superconducting coil, the uniformity of magnetic field produced by the coil is degraded. (3) An example of the cold accumulation element exhibiting high specific heat characteristics at very low temperatures is a cold accumulation element making use of abnormal magnetic specific heat caused by magnetic phase transition. This cold accumulation element is a magnetic element. If this magnetic cold accumulation element is built in the cold accumulator of the cold accumulating refrigerator situated near the superconducting coil, the symmetry of magnetic field produced by the superconducting coil is greatly disturbed. In addition, if the displacer containing the cold accumulator having the magnetic cold accumulation element is driven, the displacer will be inclined by an electromagnetic force caused between the magnetic field produced by the superconducting coil and the magnetic cold accumulation material. This will accelerate wear of the sealing member, etc., resulting a decrease in refrigeration performance of the refrigerator in a short time. (4) In the superconducting magnet apparatus of the refrigerator direct-cooling type, as compared to the dip-cooling type apparatus, the time needed to cool the superconducting coil from normal temperature down to a predetermined temperature is longer. In order to decrease this time, a refrigerator with a large capacity must be built in. As a result, the size of the magnet apparatus inevitably increases, and the feature of the refrigerator direct-cooling type apparatus cannot be exhibited. As has been described above, the superconducting magnet apparatus of refrigerator direct-cooling type has problems as regards the degree of freedom in installation and use, stability and reliability. US 5 430 423 relates to a superconducting magnet comprising a cryocooler cold-head, a sleeve assembly and a superconducting coil assembly. The sleeve assembly includes a flexible bellow and a sleeve tube which allows easy removal and replacing of the cryocooler cold-head. US 4 851 684 relates to a photon detector system which is cooled by a dewar. The radiation detector is vacuum-jacketed and is provided in a cryostat assembly. The detector and its vacuum jacket can be transferred between different cryostats. US 3 483 709 relates to a cold exchange system which is primarily intended for cooling gamma ray detectors. The cold exchange system may be disconnected from the detector. The present invention has been made in consideration of the above-described circumstances, and an object of the present invention is to provide a cryogenic cooling apparatus wherein the degree of freedom of installation and use of the cooling apparatus can be increased without deteriorating the reliability and stability, and the range of uses can be greatly increased. Another object of the present invention is to provide a cryogenic cooling method using such a cryogenic cooling apparatus. In order to achieve the above objects, the present invention provides a cooling apparatus for cooling an object, said apparatus comprising: a first vacuum container; a cooling source housed within the first vacuum container; a second vacuum container, provided separately from the first vacuum container, for accommodating the object; a cold accumulation layer provided in thermal contact with the object; and thermal connection and disconnection means for thermally connecting the cooling source onto the object when the object is to be cooled by the cooling source, and thermally disconnecting the cooling source frorn the object when the object is to be thermally insulated and operated. In a preferred embodiment, a cooling apparatus is provided which comprises a coil storing vacuum container; a superconducting coil stored in the coil storing vacuum container; a first heat conductive member thermally connected to the superconducting coil within the coil storing vacuum container; a first extendible wall formed to constitute a part of the wall of the coil storing vacuum container, situated normally at a location away from the first heat conductive member, and displaced toward the first heat conductive member when pushed toward the first heat conductive member; a second heat conductive member disposed on the first extendible wall and put in contact with the first heat conductive member when the first extendible wall has moved toward the first heat conductive member by a predetermined distance, thereby constituting a heat conduction path reaching the superconducting coil; a cooling source vacuum container; a cooling source having a cooling stage situated within the cooling source vacuum container; a third heat conductive member thermally connected to the cooling stage within the cooling source vacuum container; a second extendible wall formed to constitute a part of the wall of the cooling source vacuum container, situated normally at a location away from the third heat conductive member, and displaced toward the third heat conductive member when pushed toward the third heat conductive member; and a fourth heat conductive member disposed on the second extendible wall. When the superconducting coil is cooled down to a critical temperature or below, the coil storing vacuum container and cooling source vacuum container are relatively moved to approach each other. Thereby, the first extendible wall and second extendible wall are displaced to constitute a mechanical-contact-type heat conduction path comprising the third heat conductive member, fourth heat conductive member, second heat conductive member and first heat conductive member. The superconducting coil is cooled via this heat conduction path. In this cryogenic cooling apparatus, the coil storing vacuum container, which stores the superconducting coil, and the cooling source vacuum container are constituted completely separately from the beginning. Thus, when the superconducting coil is cooled down to the critical temperature or below, the vacuum within the coil storing vacuum container as well as the cooling source vacuum container is not lost. The extendible walls, which constitute parts of the heat conduction path, are provided as portions of the walls of both vacuum container. The extendible walls are forcibly moved to constitute the mechanical-contact-type heat conduction path extending from the cooling stage of the cooling source to the superconducting coil. After the superconducting coil has been cooled to the critical temperature or below, the coil storing vacuum container is separated from the cooling source vacuum container. In the above cryogenic cooling apparatus, a liquid coolant source such as a liquid helium bath can be used as the cooling source. Accordingly, the time needed to cool the superconducting coil to a predetermined temperature can be shortened. Needless to say, depending on conditions, the superconducting coil can be cooled by one of selectable refrigerators having different cooling performances. A plurality of superconducting coils housed within coil storing vacuum containers can be successively cooled by a single cooling source in a time-sequential manner. It is also possible to cool the superconducting coil in a place different from an installation site, and then carry the superconducting coil to the installation site. Since the cooling source can selectively be connected to and disconnected from the cooling source, the coil unit can be designed independently of the cooling source and the size of the coil unit can be reduced. In the above cryogenic cooling apparatus, the cooling source is separated after the superconducting coil has been cooled. Thus, the temperature of the superconducting coil increases gradually. The rate of increase in temperature is determined by the thermal capacity of the superconducting coil. Accordingly, in order to keep the temperature of the superconducting coil below the critical temperature for a long time, it is preferable to have the superconducting coil put in thermal contact with a cold accumulation layer having a high specific heat at or less than the critical temperature of the superconducting coil. In order to make the superconducting coil continuously generate a stable magnetic field, it is necessary to provide power leads for supplying power and a permanent current switch as original structural parts. The power leads and control wires for the permanent current switch constitute heat entrance paths from the outside. To solve this problem, it is advantageous to provide conductor paths constituting the power leads for the superconducting coil and the control wires of the permanent current switch, which are electrically connected to each other when the mechanical-contact-type heat conduction path extending from the cooling stage of the cooling source to the superconducting coil is constituted. In this case, after the superconducting coil has been cooled and set in the permanent current mode, the cooling source is separated. Thereby, the power leads and control wires can be completely separated from the superconducting coil. Therefore, it is possible to prevent heat from entering via the power leads and control wires. In this cryogenic cooling apparatus, the degree of freedom of installation and use is high and the cooling source can be completely separated during operation. Accordingly, the size of the coil unit can be reduced, the stability of the generated magnetic field enhanced, and the range of uses increased. In a second aspect, the present invention provides a cooling method for a cooling apparatus comprising a first vacuum container; a cooling source housed within the first vacuum container; and a second vacuum container, provided separately from the first vacuum container, for accommodating an object to be cooled; and a cold accumulation layer provided in thermal contact with the object, said method comprising the steps of: thermally connecting the cooling source and the object to cool the object by means of the cooling source; and thermally disconnecting the cooling source and the object to thermally insulate the object, when operated. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a cross-sectional view of a part of a cryogenic cooling apparatus for a superconducting magnet, according to a first embodiment of the present invention; Fig. 2 is a cross-sectional view of a part of a coil unit of the cryogenic cooling apparatus according to the first embodiment; FIG. 3 is an enlarged view of a coupling portion between the coil unit and a refrigeration unit of the cryogenic cooling apparatus according to the first embodiment; FIG. 4A is a view taken along line 4A-4A in FIG. 3; FIG. 4B is a view taken along line 4B-4B in FIG.3; FIG. 4C is a cross-sectional view taken along line 4C-4C in FIG. 3; FIG. 5 is a circuit diagram in a mode in which the coil unit and the refrigeration unit are coupled; FIG. 6 is a circuit diagram in a mode in which the coil unit and the refrigeration unit are separated; FIG. 7 is a cross-sectional view of a part of a cryogenic cooling apparatus for a superconducting magnet, according to a second embodiment of the present invention; FIG. 8 is a view showing a state in which a coil unit and refrigeration unit are thermally separated; FIG. 9 is a view showing a thermal switch; FIG. 10 is a cross-sectional view of a coupling portion between a coil unit and a refrigeration unit of a cryogenic cooling apparatus according to a third embodiment of the invention; FIG. 11 is a cross-sectional view of the coupling portion between the coil unit and refrigeration unit of the cryogenic cooling apparatus according to the third embodiment; FIG. 12 is a cross-sectional view of the coupling portion between the coil unit and refrigeration unit of the cryogenic cooling apparatus according to the third embodiment; and FIG. 13 is an enlarged view of a coupling portion between a heat conductive member of the coil unit of the cryogenic cooling apparatus according to the third embodiment and a heat conductive member of the refrigeration unit. Embodiments of the present invention will now be described with reference to the accompanying drawings. <First Embodiment> FIG. 1 is a cross-sectional view of a part of a cryogenic cooling apparatus for a superconducting magnet, according to a first embodiment of the present invention. The cryogenic cooling apparatus generally comprises a coil unit 41 and a refrigeration unit 42 separated from the coil unit 41. The coil unit 41, as shown in FIGS. 1 and2, comprises an annular vacuum container 43 functioning as a heat insulating container formed of a nonmagnetic material such as stainless steel, a superconducting coil 44 (cooling object) housed within the vacuum container 43, and a thermal shield 45 disposed between the superconducting coil 44 and vacuum container 43 so as to surround the superconducting coil 44. The superconducting coil 44 is formed of an Nb-Ti alloy wire or an Nb 3 Sn wire and is supported by heat insulating support means (not shown). A heat conductive member 46 formed of a material with good heat conductivity such as copper, aluminum or aluminum nitride is thermally connected to a lower end face (in FIGS. 1 and 2) of the superconducting coil 44. End portions of the superconducting coil 44 are connected to both ends of a permanent current switch 47 provided on the heat conductive member 46 serving as a support. A cold accumulation element having a high specific heat at temperatures lower than a critical temperature of the superconducting coil 44, for example, a cold accumulation layer 48 mixed with particles of Er 3 Ni, is provided around the superconductive coil 44 in the state in which the cold accumulation element is put in thermal contact with the superconducting coil 44. A part 46a of the heat conductive member 46 projects toward a side wall of the vacuum chamber 43, and a part 45a of the thermal shield 45 projects coaxially with the part 46a. The parts 46a and 45a constitute a first heat conductive member 49. The part 46a serves as a first heat conductive portion and the part 45a serves as a second heat conductive portion. A heat insulating member 50 and an electric insulating member (fiber reinforced plastic) 51 are provided between the parts 46a and 45a, as shown in FIG. 3. Power lead portions 52a and 52b connected to both ends of the permanent current switch 47 and control wire portions 53a and 53b for controlling the permanent current switch 47 are buried in the electric insulating member 51. The end portions of the power lead portions 52a and 52b and the end portions of the control wire portions 53a and 53b are exposed to face the side wall of the vacuum container 43, as shown in FIg. 4A. A bellows-type extendible wall 54 is disposed on the side wall of the vacuum container 43 in a position facing the first heat conductive member 49. The extendible wall 54 is normally situated in a position away from the first heat conductive member 49. Although the extendible wall 54 is not moved due to a degree of vacuum within the vacuum container 43, it is displaced toward the first heat conductive member 49 when an external force of a predetermined level or more is applied. A second heat conductive member 55 is provided on the extendible wall 54 in a position facing the first heat conductive member 49. The second heat conductive member 55, as shown in FIG. 3, comprises a first heat conductive portion 56 formed of a material with good heat conductivity, which is situated to face the end face of the part 46a of the first heat conductive member 49, and a second heat conductive portion 57 formed of a material with good heat conductivity, which is situated to face the part 45a. The first heat conductive portion 56 and second heat conductive portion 57 are electrically insulated from each other and from the extendible wall 54 by means of electrical insulators. Power lead portions 58a and 58b and control wire portions 59a and 59b are provided in an electrically insulated state between the first heat conductive portion 56 and the second heat conductive portion 57, as shown in FIG. 4B, too, such that the power lead portions 58a and 58b and control wire portions 59a and 59b face the end portions of the power lead portions 52a and 52b and the end portions of the control wire portions 53a and 53b provided in the first heat conductive member 49. On the other hand, the refrigeration unit 42 comprises a vacuum container 60, a cold accumulating refrigerator (cooling source) 61 situated to extend inside and outside the vacuum container 60 such that cooling stages are located within the vacuum container 60, and a third heat conductive member 62 thermally connected to the cooling stage of the cold accumulating refrigerator 61. In this embodiment, the cold accumulating refrigerator 61 is constituted by a two-stage expansion type Gifford-MacMahon refrigerator. In the cold accumulating refrigerator 61, copper mesh, etc. is used as a cold accumulation element in a first-stage cold accumulator, and a magnetic cold accumulation element such as Er 3 Ni, which makes use of abnormal magnetic specific heat due to magnetic phase transition, is used as a cold accumulation element in a second-stage cold accumulator. By the use of these cold accumulating elements, cold of about 50K is generated by a first-stage cooling stage 63 and cold of about 4K is generated by a second-stage cooling stage 64. In FIG. 1, reference numeral 65 denotes a motor for reciprocally moving the displacer containing the cold accumulators connected serially in two stages, and numeral 66 denotes a compressor for compressing and sucking a coolant gas. The third heat conductive member 62, as shown in FIG. 3, too, comprises a first heat conductive portion 67 formed of a material with good heat conductivity and having one end thermally connected to the second cooling stage 64, and a second heat conductive portion 68 formed of a material with good heat conductivity and having one end thermally connected to the first cooling stage 63 and the other end situated coaxially with the first heat conductive portion 67. The first heat conductive portion 67 and second heat conductive portion 68 are insulated from each other by means of an electrical insulator (thermal insulator). As is shown in FIG. 4C, power lead portions 70a and 70b and control wire portions 71a and 71b are provided in an electrically insulated state between the first heat conductive portion 67 and second heat conductive portion 68. As is shown in FIGS. 1 and 3, end faces of the third heat conductive member 62 having the above structure are opposed to the side wall of the vacuum container 60. An end portion of each of the power lead portions 70a and 70b and control wire portions 71a and 71b is led to the outside via an associated bushing 72 hermetically penetrating the wall of the vacuum container 60, as shown in FIG. 1. A bellows-type extendible wall 73 is disposed on the side wall of the vacuum container 60 in a position facing the third heat conductive member 62. The extendible wall 73 is normally situated in a position away from the third heat conductive member 62. Although the extendible wall 73 is not moved due to a degree of vacuum within the vacuum container 60, it is displaced toward the third heat conductive member 62 when an external force of a predetermined level or more is applied. A fourth heat conductive member 74 is provided on the extendible wall 73 in a position facing the third heat conductive member 62. The fourth heat conductive member 74 has substantially the same diameter and structure as the second heat conductive member 55. Specifically, the fourth heat conductive member 74, as shown in FIG. 3, comprises a first heat conductive portion 75 formed of a material with good heat conductivity, which is situated to face the end face of the first heat conductive portion 67 of third heat conductive member 62, and a second heat conductive portion 76 formed of a material with good heat conductivity, which is situated to face the second heat conductive portion 68. The first heat conductive portion 75 and second heat conductive portion 76 are electrically insulated from each other and from the extendible wall 73 by means of electrical insulators. Power lead portions 77a and 77b (only 77a shown) and control wire portions 78a and 78b (only 78a shown) are provided in an electrically insulated state between the first heat conductive portion 75 and the second heat conductive portion 76 such that the power lead portions 77a and 77b and control wire portions 78a and 78b face the end portions of the power lead portions 70a and 70b and the end portions of the control wire portions 71a and 71b provided in the third heat conductive member 62. A method of thermally connecting the refrigeration unit and the coil unit will now be described. The coil unit 41 and refrigeration unit 42 are positioned so that the second heat conductive member 55 provided on the extendible wall 54 of vacuum container 43 may coaxially face the fourth heat conductive member 74 provided on the extendible wall 73 of vacuum container 60. In this state, the coil unit 41 and refrigeration unit 42 are relatively moved to approach each other. Then, the second heat conductive member 55 comes into contact with the fourth heat conductive member 74. If the coil unit 41 and refrigeration unit 42 are further moved to approach each other, the extendible wall 54 extends and thus the second heat conductive member 55 comes in contact with the first heat conductive member 49, as shown in FIG. 1. In addition, the extendible wall 73 contracts and thus the fourth heat conductive member 74 comes in contact with the third heat conductive member 62. In this state, a mechanical contact type first heat conduction path 80 is constituted by the first heat conductive portions 67, 75, 56 and 46a of the third heat conductive member 62, fourth heat conductive member 74, second heat conductive member 55 and first heat conductive member 49, and a mechanical contact type second heat conduction path 81 is constituted by the second heat conductive portions 68, 76, 57 and 45a. In addition, power leads 82 and 83 are constituted by mutual contact among the power lead portions 70a, 77a, 58a and 52a and among the power lead portions 70b, 77b, 58b and 52b. The mechanical contact type heat conduction path constitutes thermal switch wherein heat is conducted by putting heat conduction members in contact with each other and heat is insulated by separating them. The dimensions of the respective parts and the positional relationship among them are determined so that control wires 84 and 85 may be constituted by mutual contact among the control wire portions 71a, 78a, 59a and 53a and among the control wire portions 71b, 78b, 59b and 53b. In the above structure, the superconducting coil 44 is cooled down to a critical temperature or below and subsequently shifted to a permanent current mode, in the following manner. First, the operation of the cold accumulating refrigerator 61 of the refrigeration unit 42 is started. Then, as shown in FIG. 1, the coil unit 41 and refrigeration unit 42 are mechanically coupled, thereby constituting the aforementioned first and second heat conduction paths 80 and 81. At this time, the entrance of a gap 86 between the extendible walls 54 and 73 may be sealed and the gap 86 may be evacuated. Since the first and second heat conduction paths 80 and 81 have been constituted, the heat of the superconducting coil 44 is absorbed by the second cooling stage 64 of cold accumulating refrigerator 61 via the first heat conduction path 80 and the heat of the thermal shield 45 is absorbed by the first cooling stage 63 via the second heat conduction path 81. If a predetermined time period has passed, the superconducting coil 44 is cooled to about 4K which is below a critical temperature, and the thermal shield 45 is cooled to about 50K. In this state, an electric current of a predetermined level is supplied over the control wires 84 and 85, and the permanent current switch 47 is turned off. Then, a current is supplied to the superconducting coil 44 via the power leads 82 and 83, while the current is increased at a predetermined rate. When the magnitude of the current supplied to the superconducting coil 44 has reached a target value, the current supplied over the control wires 84 and 85 is reduced to zero and the permanent current switch 47 is turned on. Then, the current supplied over the power leads 82 and 83 is decreased at a predetermined rate and reduced to zero. Thereby, a permanent current continues to flow in the superconducting coil 44. Subsequently, the coil unit 41 is mechanically decoupled from the refrigeration unit 42. As a result, the extendible wall 54 of the coil unit 41 contracts, and the second heat conductive member 55 is separated from the first heat conductive member 49, as shown in FIGS. 2 and 3. On the other hand, the extendible wall 73 of the refrigeration unit 42 extends and, as shown in FIG. 3, the fourth heat conductive member 74 is separated from the third heat conductive member 62. As described above, the heat conduction members are thermally separated, and the superconducting coil is thermally insulated. FIG. 5 shows the connection among the superconducting coil 44, permanent current switch 47, power leads 82 and 83 and control wires 84 and 85 in the state in which the coil unit 41 is mechanically coupled to the refrigeration unit 42. FIG. 6 shows the connection in the state in which the coil unit 41 is mechanically decoupled from the refrigeration unit 42. In the state shown in FIG. 6, the permanent current continues to flow in the superconducting coil 44. Since the cooling source for cooling the superconducting coil 44 is completely separated, the temperature of the superconducting coil 44 tends to increase gradually. In this embodiment, however, the cold accumulation layer 48 having a high specific heat at temperatures equal to or less than the critical temperature of the superconducting coil 44 is situated in thermal contact with the superconducting coil 44, the superconducting coil 44 can be maintained at temperatures equal to or less than the critical temperature over a long time period. As has been described above, the coil unit 14 and refrigeration unit 42 are constructed as completely separated units, and only when the superconducting coil 44 needs to be cooled, both are coupled (thermal connection) while the vacuum state is maintained. And then, the coil unit 14 and refrigeration unit 42 are separated (thermally separated) if the superconducting coil 44 is insulated. Accordingly, the coil unit 41 can be designed independently of the refrigeration unit 42, and the size of the coil unit 41 can be reduced. With the above structure, a plurality of coil units 41 each containing the superconducting coil 44 can be successively cooled by a single refrigeration unit 42 in a time-sequential manner. Besides, the superconducting coil 44 can be cooled or supplied with power by one of refrigeration units 42 having different refrigeration performances which can be selected in accordance with modes, e.g. pre-cooling mode, power supply mode, etc. It is also possible to cool the coil unit 41 and set it in a permanent current mode in a place different from an installation site, and then carry the coil unit 41 to the installation site. The range of uses of this cooling apparatus is very wide. Since the refrigeration unit 42 can be separated from the coil unit 41 in the normal operation mode, the coil unit 41 can be used in the condition free from vibration or noise. In the above-described embodiment, the cold accumulating refrigerator 61 is used in the refrigeration unit 42. However, a liquid coolant such as liquid helium may be used as a refrigerant source. In this case, the superconducting coil 44 can be cooled to the critical temperature or below in a shorter time period. In the above-described embodiment, the cold accumulation layer 48 mixed with the particles of magnetic cold accumulation element is provided on the superconducting coil 44. However, a container filled with at least one selected from the group consisting of helium, hydrogen, neon, nitrogen and argon may be provided on the superconducting coil 44 as a cold accumulating layer. In the above embodiment, conductors are provided to form a pair of power leads and a pair of control wires. However, the first to fourth heat conductive members may be also used as one of the power leads and one of the control wires. In this embodiment, the first heat conductive member 49 and second heat conductive member 55 are mechanically coupled to constitute the heat transmission paths. The method of contacting the first and second heat conductive members 49 and 55 with each other is not limited to this. Specifically, it should suffice if the first heat conductive member 49 and second heat conductive member 55 can be thermally separated. For example, a thermal switch may be used. In this case, a gas is sealed between the first and second heat conductive members 49 and 55, thereby effecting heat conduction. When the gas is sealed between the first and second heat conductive members 49 and 55, the thermal switch is turned on and the heat conduction path is constituted between the superconducting coil and refrigerator. On the other hand, if the gas is exhausted and a vacuum is created, the thermal switch is turned off and the heat conduction path between the superconducting coil and refrigerator is cut off. <Second Embodiment> A cryogenic cooling apparatus according to a second embodiment of the present invention will now be described with reference to FIGS. 7 and 8. The second embodiment differs from the first embodiment with respect to the construction of the heat conductive path. The other parts of the second embodiment are common to those of the first embodiment shown in FIG. 1. The common parts are denoted by like reference numerals, and a description thereof is omitted. FIG. 7 shows a state wherein a coil unit 41 and a refrigeration unit 42 are thermally connected, and FIG. 8 shows a state wherein the coil unit 41 and refrigeration unit 42 are thermally separated and the coil unit 41 is thermally insulated. A heat conductive member 55 formed of a material with good heat conductivity constitutes a part of the wall of a vacuum container 43 accommodating a superconducting coil 44. The heat conductive member 55 has a flange portion 96. A portion 46a of a heat conductive member 46 formed of a material with good heat conductivity, which is thermally connected to the superconducting coil 44, is formed so as to extend to the vicinity of the wall of the vacuum container 43. A portion 45a of a thermal shield 45, too, is formed so as to extend to the vicinity of the wall of the vacuum container 43. A gas-type thermal switch 90a is provided between the heat conductive member 55 and the portion 46a of heat conductive member 46. The thermal switch 90a is provided to thermally connect the heat conductive member 55 and the portion 46a of heat conductive member 46. Another gas-type thermal switch 90b is provided between the heat conductive member 55 and the portion 45a of thermal shield 45. The thermal switch 90b is provided to thermally connect the heat conductive member 55 and the portion 45a of thermal shield 45. On the other hand, a heat conductive member 74 formed of a material with good heat conductivity constitutes a part of the wall of a vacuum container 60 accommodating a refrigerator 61. The heat conductive member 74 has a flange portion 97. An end portion of a heat conductive member 67 formed of a material with good heat conductivity, which is thermally connected to a second cooling stage 64, is formed so as to extend to the vicinity of the wall of the vacuum container 60. An end portion of a heat conductive member 68 formed of a material with good heat conductivity, which is thermally connected to a first cooling stage 63 is formed so as to extend to the vicinity of the wall of the vacuum container 60. In FIGS. 7 and 8, reference numeral 98 denotes bolts for putting the flange portions 96 and 97 into mechanical contact with each other and fastening the same. The fastening of the bolts 98 improves contact between the heat conductive members 55 and 74, enhancing heat conduction with less contact thermal resistance. A gas-type thermal switch 90c is provided between the heat conductive member 74 and end portion of heat conductive member 67. The thermal switch 90c is provided to thermally connect the heat conductive member 74 and the end portion of heat conductive member 67. Another gas-type thermal switch 90d is provided between the heat conductive member 74 and an end portion of heat conductive member 68. The thermal switch 90d is provided to thermally connect the heat conductive member 74 and the end portion of heat conductive member 68. These thermal switches 90a to 90d are gas-type thermal switches for performing thermal connection and disconnection by supplying and exhausting a heat conductive gas into and from the insides of the thermal switches, as shown in FIG. 9. FIG. 9 shows a detailed structure of each of the thermal switches 90a to 90d. Specifically, the thermal switches 90a to 90d are gas-pressure type thermal switches wherein a heat conductive gas supply/exhaust device 95 supplies/exhausts a heat conductive gas such as helium gas via a pipe 94 in/from a cylinder 93 defined at both ends by heat conductive plates 91 and 92, thereby to switch on/off thermal conduction. A number of projection plates 91a and 91b project from the heat conductive plates 91 and 92 within the cylinder 93, so as to face each other at a small distance in a comb-like arrangement. When a helium gas is supplied via the pipe 94 from the supply/exhaust device 95 and sealed in the cylinder 93, heat conducts between both heat conductive plates 91 and 92 by virtue of the heat conduction of helium gas. Thus, the thermal switch is turned on. When the helium gas is exhausted and the thermal switch is evacuated, heat conduction between the heat conductive plates 91 and 92 is stopped and the thermal switch is turned off. The thermal switch, 90a to 90d, shown in FIG. 9 is a gas-pressure type thermal switch which performs a switching operation by controlling the pressure of the heat conductive gas within the switch. However, the thermal switch, which can be used in the present invention, is not limited to this type. For example, a mechanical thermal switch may be used. The mechanical thermal switch is provided with a driving mechanism for moving first and second heat conductive members relative to each other. The first and second heat conductive members are mechanically moved and the contact state/non-contact state is switched. When the first and second heat conductive members are put in contact with each other, the mechanical thermal switch effects heat conduction ("switch on"). When the first and second heat conductive members are mechanically separated and set in non-contact state, the mechanical switch renders heat conductive non-effective ("switch off"). A method of putting the coil unit and refrigeration unit into thermal contact in the cooling apparatus having the above structure will now be described. The refrigerator 61 is driven and then the temperatures of the first and second cooling stages 63 and 64 approach predetermined values. At this time instant, a helium gas is supplied into the thermal switches 90a to 90d and these switches are turned on. At least one of the coil unit 41 and refrigerator 42 is moved to put the heat conductive members 55 and 74 in contact with each other. Then, the flanges portions 96 and 97 are fastened by means of bolts 98. Since the heat conductive members 55 and 74 are put in close contact with each other and the thermal resistance is decreased, a good cooling operation can be performed. The first cooling stage 63 is thermally connected to the thermal shield 45 in the following order of thermal connection: first cooling stage 63 -> heat conductive member 68 -> thermal switch 90d -> heat conductive member 74 -> heat conductive member 55 -> thermal switch 90b -> the portion of thermal shield 45 -> thermal shield 45. On the other hand, the second cooling stage 64 is thermally connected to the superconducting coil 44 in the following order of thermal connection: second cooling stage 64 -> heat conductive member 67 -> thermal switch 90c -> heat conductive member 74 -> heat conductive member 55 -> thermal switch 90a -> heat conductive member 46 -> superconductive coil 46. When a sufficient time period has passed since the thermal conduction was effected, the temperature of the thermal shield 45 becomes substantially equal to that of the first cooling stage 63 (about 40 K) and the temperature of the superconducting coil 44 becomes substantially equal to that of the second cooling stage 64 (about 4 K). After the thermal shield 45 and superconducting coil 44 have been cooled to target temperatures, the helium gas within the thermal switches 90a to 90d is exhausted and the thermal switches 90a to 90d are turned off. In particular, when the thermal switches 90a and 90b have been turned off, the thermal shield 45 and superconducting coil 44 are completely thermally separated and insulated from the outside of the vacuum container 43. Subsequently, as shown in FIG.8, the heat conductive members 55 and 74 are mechanically separated and the coil unit 41 is put out of contact with the refrigerator 42 and thermally insulated from the refrigerator 42. If the heat conductive members 55 and 74 are mechanically separated, they are also thermally separated. The work of mechanically separating the heat conductive members 55 and 74 is equal in operational effect to the work of turning off a mechanical thermal switch which may be provided between the heat conductive members 55 and 74. Thereafter, in the state in which the superconducting coil 44 is separated from the refrigeration unit 42, the superconducting coil 44 is kept cooled during a cooling time period determined by the heat capacity of the superconducting coil itself and the radiation heat shield effect of the thermal shield 45. The cooling time period can be remarkably increased by increasing the number of thermal shields. For example, the superconducting coil 44 can be cooled for several to several tens of days, or several months. In the second embodiment, the heat conductive members 55 and 74 are mechanically separable and one of the coil unit 41 and refrigeration unit 42 is movable so that a mechanical thermal switch is theoretically provided between the heat conductive members 55 and 74. However, a gas-type thermal switch may be provided between the heat conductive members 55 and 74. Inversely, the gas-type thermal switches 90a to 90d may be replaced with mechanical thermal switches. Besides, the thermal switches 90c and 90d, provided on the refrigeration unit 42 side, may be dispensed with, if the driving of the refrigerator 61 is started at the time of cooling the coil unit 41 and the driving of the refrigerator 61 is stopped when the coil unit 41 has been completely cooled. <Third Embodiment> A cryogenic cooling apparatus according to a third embodiment of the present invention will now be described. In the first and second embodiments, the superconducting coil is cooled substantially via parts of the walls of vacuum containers. In the second embodiment, as shown in FIG. 10, a coil unit 41a and a refrigeration unit 42a are provided with vacuum valves 101 and 102. Using the vacuum valves 101 and 102, a heat conductive member 103 of the coil unit 41a is put in mechanical contact with a heat conductive member 104 of the refrigeration unit 42a, thereby constituting heat conduction paths. The second embodiment is the same as the first embodiment with respect to the other structural features. A vacuum container 105 of the coil unit 41a and a vacuum container 106 of the refrigeration unit 42a are provided with flanges 121 and 122. At least one of the vacuum container 105 of coil unit 41a and the vacuum container 106 of refrigeration unit 42a is provided with an extendible wall 107 which constitutes a part of the container 105 and/or container 106. In FIg. 10, the container 107 is provided with the extendible wall 107. As is shown in FiG. 11, the coil unit 41a and refrigeration unit 42a are connected by means of the flanges 121 and 122. Then, as shown in FIG. 12, the vacuum valves 101 and 102 are released and in this state the coil unit 41a and refrigeration unit 42a are moved to approach each other. In accordance with the movement, the extendible wall 107 contracts and the heat conductive member 103 of coil unit 41a and the heat conductive member 104 of refrigeration unit 42a come into mechanical contact with each other, thereby constituting a heat conduction path. The coil unit 41a and refrigeration unit 42a can be separated by the reverse procedure. With the above structure, the superconducting coil can be cooled while the vacuum in the coil unit 41a and refrigeration unit 42a is maintained. In this embodiment, too, when the mechanical-contact type heat conduction path extending from the refrigeration unit 42a to coil unit 41a is formed, it is desirable to provide conductors which are electrically connected to constitute power leads of the superconducting coil and control wires of the permanent current switch. In the present embodiment, the heat conductive member 103 of coil unit 41a and the heat conductive member 104 of refrigeration unit 42a are put in direct contact with each other. The contact faces of the heat conductive members 103 and 104 may be plated with gold or mirror-finished. Besides, as is shown in FIG. 13, a distal end portion of the heat conductive member 103 may be formed in the shape of a male screw, and a distal end portion of the heat conductive member 104 may be formed in the shape of a female screw, so that the heat conductive member 103 may be engaged in the heat conductive member 104. Thereby, the contact area between the heat conductive members 103 and 104 increases and, as a result, the efficiency of heat conduction between the heat conductive members 103 and 104 is enhanced. Although Nb-Ti alloy wires and Nb 3 Sn wires have been mentioned as examples of the material of the superconducting coil 44, oxides (high temperature) of La, Y, Bi, Tl, Pb and Hg may be used. The cryogenic cooling apparatus of the present invention is applicable to MRIs, NMRs linear motor-cars, single-crystal drawing apparatuses, etc. As has been described above, according to the present invention, the degree of freedom of installation and use of the cooling apparatus can be increased without deteriorating the reliability and stability, and the range of uses can be greatly increased.

Claims: A cooling apparatus for cooling an object, said apparatus comprising: a first vacuum container (60); a cooling source (61) housed within the first vacuum container; a second vacuum container (43), provided separately from the first vacuum container, for accommodating the object; a cold accumulation layer (48) provided in thermal contact with the object; and thermal connection and disconnection means (45a, 46, 46a, 54, 55, 67, 68, 73, 74, 90a, 90b, 90c, 90d, 96, 97, 101, 102, 103, 104, 107) for thermally connecting the cooling source onto the object when the object is to be cooled by the cooling source, and thermally disconnecting the cooling source from the object when the object is to be thermally insulated and operated. The cooling apparatus according to claim 1, characterized in that the thermal connection and disconnection means thermally connects and disconnects the cooling source and the object in the state in which the insides of the first and second vacuum containers are set in a vacuum state. The cooling apparatus according to claim 1, characterized in that said thermal connection and disconnection means comprises a first heat conductive member (74) with heat conductivity, which forms a part of the first vacuum container, and a second heat conductive member (55) with heat conductivity, which forms a part of the second vacuum container, and wherein the cooling source and the object are thermally connected via the first and second heat conductive members with, so that the cooling source and the object can be thermally connected and disconnected in the state in which the insides of the first and second vacuum containers are set in a vacuum state. The cooling apparatus according to claim 1, characterized in that said thermal connection and disconnection means includes at least one thermal switch means (90a, 90b, 90c, 90d, 55, 74), and wherein when the object is to be cooled by the cooling source, the thermal switch means is turned on to thermally connect the cooling source and the object, and when the object is to be thermally insulated, the thermal switch means is turned off to thermally disconnect the cooling source and the object. The cooling apparatus according to claim 1, characterized in that said thermal connection and disconnection means includes: a first heat conductive member (74) with heat conductivity, which forms a part of the first vacuum container; a second heat conductive member (55) with heat conductivity, which forms a part of the second vacuum container; a first thermal switch means (90a) provided between the object and the first heat conductive member with heat conductivity; and a second thermal switch means (55, 74) provided between the first and second heat conductive members with heat conductivity. The cooling apparatus according to claim 5, characterized in that said second thermal switch conducts heat by holding the first and second heat conductive members with heat conductivity in contact with each other, and stops heat conduction by holding the first and second heat conductive members with heat conductivity out of contact with each other when said the first and second vacuum containers are moved relative to each other. The cooling apparatus according to claim 1, characterized in that said thermal connection and disconnection means includes: a first heat conductive member (67) thermally connected to the cooling source; a second heat conductive member (46) thermally connected to the object; a third heat conductive member (74) provided on a part of a wall of the first vacuum container; and a fourth heat conductive member (55) provided on a part of a wall of the second vacuum container, whereby the object is cooled by putting the first to fourth heat conductive members in thermal contact with each other, and the object is thermally insulated by thermally disconnecting the second and fourth heat conductive members. The cooling apparatus according to claim 1, characterized in that said thermal connection and disconnection means includes: a first heat conductive member (67) thermally connected to the cooling source; a second heat conductive member (46) thermally connected to the object; a third heat conductive member (74) provided on a part of a wall of the first vacuum container; a fourth heat conductive member (55) provided on a part of a wall of the second vacuum container; and thermal switch means (90a, 46, 55) for thermally connecting and disconnecting the second and fourth heat conductive members, whereby the thermal switch means is turned on to thermally connect the first to fourth heat conductive members to each other, thereby to cool the object, and to thermally disconnect the second and fourth heat conductive members, thereby to thermally insulate the object. The cooling apparatus according to claim 1, characterized in that said thermal connection and disconnection means includes: a first heat conductive member (67) thermally connected to the cooling source; a second heat conductive member (46) thermally connected to the object; a third heat conductive member (74) provided on a part of a wall of the first vacuum container; a fourth heat conductive member (55) provided on a part of a wall of the second vacuum container; first thermal switch means (90a, 46, 55) for thermally connecting and disconnecting the second and fourth heat conductive members; and second thermal switch means (55, 74) for thermally connecting and disconnecting the third and fourth heat conductive members. The cooling apparatus according to claim 1, characterized in that said thermal connection and disconnection means includes: a first heat conductive member (67) thermally connected to the cooling source; a second heat conductive member (46) thermally connected to the object; a first extendible wall (73) provided on a part of the first vacuum container; a second extendible wall (54) provided on a part of the second vacuum container; a third heat conductive member (74), disposed on the first extendible wall, for conducting heat of the first heat conductive member when the third heat conductive member is put in contact with the first heat conductive member; and a fourth heat conductive member (55), disposed on the second extendible wall, for conducting heat from the third heat conductive member to the second heat conductive member when the fourth heat conductive member is put in contact with the second heat conductive member and the third heat conductive member. The cooling apparatus according to claim 1, characterized in that said thermal connection and disconnection means includes: a first heat conductive member (67) thermally connected to the cooling source; a second heat conductive member (46) thermally connected to the object; a first vacuum valve (102) for maintaining a vacuum in the first vacuum container; and a second vacuum valve (101) for maintaining a vacuum in the second vacuum container, wherein the first vacuum valve and second vacuum valve are opened such that the first vacuum container and second vacuum container communicate with each other while maintaining the vacuum state in the first vacuum container and second vacuum container, and the first heat conductive member and second heat conductive member are thermally connected. The cooling apparatus according to claim 1, characterized in that said cooling source is a refrigerator having a cooling stage. The cooling apparatus according to claim 1, characterized in that said cooling source is a coolant contained in said first vacuum container. The cooling apparatus according to claim 1, characterized in that said object is a superconducting coil. The cooling apparatus according to claim 14, characterized by further comprising current leads (52a, 52b, 58a, 58b, 70a, 70b, 77a, 77b, 82, 83) for supplying a current to the superconducting coil. The cooling apparatus according to claim 15, characterized in that said current leads are provided on the thermal connection and disconnection means, and the current leads are electrically connected in the state in which the thermal connection and disconnection means is set in a thermal connection state. The cooling apparatus according to claim 3, characterized in that said object is a superconducting coil, and said first heat conductive member with heat conductivity, which constitutes the part of the first vacuum container, and said second heat conductive member with heat conductivity, which constitutes the part of the second vacuum container, are provided with electrical conductive portions (58a, 77a) with electrical conductivity for supplying a current to the superconducting coil. The cooling apparatus according to claim 1, characterized in that said cold accumulation layer includes a cold accumulation element having a high specific heat at temperatures near a cooling temperature of the object. The cooling apparatus according to claim 1, characterized in that said cold accumulation layer includes a container filled with at least one coolant selected from among the group consisting of helium, hydrogen, neon, nitrogen and argon. The cooling apparatus according to claim 1, characterized by further comprising a thermal shield (45) provided within the second vacuum container so as to surround the object. The cooling apparatus according to claim 20, characterized in that the cooling source is a refrigerator having first and second cooling stages (63, 64) with different target temperatures, the first cooling stage (63) with the higher target temperature cooling the thermal shield, the second cooling (64) with the lower target temperature cooling the object. The cooling apparatus according to claim 21, characterized in that said thermal connection and disconnection means includes: a first heat conductive member (67) thermally connected to the second cooling stage; a second heat conductive member (46) thermally connected to the object; a third heat conductive member (74) provided on a part of a wall of the first vacuum container; a fourth heat conductive member (55) provided on a part of a wall of the second vacuum container; a fifth heat conductive member (68) thermally connected to the first cooling stage; and a sixth heat conductive member (49) thermally connected to the thermal shield, where the first, second, third and fourth heat conductive members are thermally connected to cool the object, the third, fourth, fifth and sixth heat conductive members are thermally connected to cool the object, and the second and fourth heat conductive members are thermally disconnected to thermally insulate the object. A cooling apparatus according to claim 1, wherein the cooling source is provided by: a refrigerator (46) contained in the first vacuum container and having a cooling stage; the thermal connection and disconnection means comprising: a first heat conductive member (68) with heat conductivity, which constitutes a part of the first vacuum container; and a second heat conductive member (49) with heat conductivity, which constitutes a part of the second vacuum container, wherein when the object is to be cooled by the cooling stage, the cooling stage and the first heat conductive member with heat conductivity are thermally connected, the second heat conductive member with heat conductivity and the object are thermally connected, and the first and second heat conductivity members with heat conductivity are thermally connected, thereby thermally connecting the cooling stage and the object, and when the object is thermally insulated, the object and the second heat conductive member with heat conductivity are thermally disconnected, and the first and second heat conductive members with heat conductivity are thermally disconnected, thereby thermally insulating the object from the outside of the second vacuum container. The cooling apparatus according to claim 23, characterized in that said object is a superconducting coil, and said first heat conductive member with heat conductivity, which constitutes the part of the first vacuum container, and said second heat conductive member with heat conductivity, which constitutes the part of the second vacuum container, are provided with electrical conductive portions (58a, 77a) with good electrical conductivity for supplying a current to the superconducting coil. A cooling method for a cooling apparatus comprising: a first vacuum container (60); a cooling source (61) housed within the first vacuum container; and a second vacuum container (43), provided separately from the first vacuum container, for accommodating an object to be cooled; and a cold accumulation layer (48) provided in thermal contact with the object, said method comprising the steps of: thermally connecting the cooling source and the object to cool the object by means of the cooling source; and thermally disconnecting the cooling source and the object to thermally insulate the object, when operated. A cooling method according to claim 25, the cooling apparatus further comprising: a first heat conductive member (74) with heat conductivity, which constitutes a part of the first vacuum container; and a second heat conductive member (55) with heat conductivity, which constitutes a part of the second vacuum container, wherein the step of thermally connecting the cooling source and the object comprises: thermally connecting the cooling source and the first heat conductive member with heat conductivity; thermally connecting the second heat conductive member with heat conductivity and the object; and thermally connecting the first and second heat conductivity members with heat conductivity, thereby cooling the object by means of the cooling source; and wherein the step of thermally disconnecting the cooling source and object comprises: thermally disconnecting the object and the second heat conductive member with heat conductivity; and thermally disconnecting the first and second heat conductive members with heat conductivity, thereby thermally insulating the object.

Inventors: CHANDRATILLEKE ROHANA, HATAKEYAMA HIDEO, KURIYAMA TORU, NAKAGOME HIDEKI, OHTANI YASUMI, TAKAHASHI MASAHIKO, CHANDRATILLEKE, ROHANA, HATAKEYAMA, HIDEO, KURIYAMA, TORU, NAKAGOME, HIDEKI, OHTANI, YASUMI, TAKAHASHI, MASAHIKO
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1997
Main classification: F25D19
IPC codes: F25D19, H01F6, F17C13