Cryogenic refrigerator

In a cryogenic refrigerator, a valve switches between a flow passage of a low-pressure refrigerant gas and a flow passage of a high-pressure refrigerant gas. A motor drives the valve. The motor includes a rotor and a stator, the rotor located radially inward of the stator. A casing hermetically houses the rotor and the stator. The stator includes a back yoke and a magnetic member that acts as a magnetic path of an external magnetic field generated outside of the casing, the magnetic member located radially outward of and spaced apart from the back yoke. The magnetic member is hermetically housed in the casing.

RELATED APPLICATION

Priority is claimed to Japanese Patent Application No. 2014-177744, filed on Sep. 2, 2014, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cryogenic refrigerator, and more particularly, to a cryogenic refrigerator suitable for cooling a superconducting coil.

2. Description of the Related Art

A Gifford-McMahon (GM) refrigerator or a pulse tube refrigerator is known as a refrigerator that generates cryogenic temperature. Such a refrigerator includes a valve that switches a flow of a high-pressure working gas and a low-pressure working gas, and a motor that drives the valve. Such a refrigerator is used for cooling, for example, a superconducting coil that generates a strong magnetic field.

SUMMARY OF THE INVENTION

A purpose of the present invention is to provide a technology for reducing influence of an external magnetic field exerted on a motor provided with a cryogenic refrigerator.

According to an embodiment of the present invention, a cryogenic refrigerator includes: a valve that switches between a flow passage of a low-pressure refrigerant gas and a flow passage of a high-pressure refrigerant gas; and a motor that drives the valve. The motor includes a rotor and a stator, the rotor located radially inward of the stator, and a casing that hermetically houses the rotor and the stator. The stator includes a back yoke and a magnetic member that acts as a magnetic path of an external magnetic field generated outside of the casing, the magnetic member located radially outward of and spaced apart from the back yoke. The magnetic member is hermetically housed in the casing.

DETAILED DESCRIPTION OF THE INVENTION

Generally, a motor is used as a power for driving a valve in a cryogenic refrigerator. For example, such a cryogenic refrigerator may be used together with a device using superconductivity and may be used for cooling a superconducting coil.

In the case of using the cryogenic refrigerator for cooling the superconducting coil, if a magnet motor is employed for the motor as the power for driving the valve, a torque of the motor may be reduced due to the influence of a magnetic field generated by the superconducting coil that is to be cooled. This may adversely affect the operation of the GM refrigerator.

Therefore, the cryogenic refrigerator according to an embodiment uses a motor having a magnetic path to guide an external magnetic field, so as to isolate aback yoke of the motor from the external magnetic field.

First, an entire configuration of a cryogenic refrigerator according to an embodiment will be described.FIGS. 1 to 3are diagrams for explaining the cryogenic refrigerator according to an embodiment of the present invention. In the present embodiment, a Gifford-McMahon refrigerator (hereinafter referred to as a GM refrigerator10) will be described as an example of the cryogenic refrigerator. However, the cryogenic refrigerator according to an embodiment is not limited to the GM refrigerator. The present invention can be applied to any type of cryogenic refrigerator using a motor for driving a valve and can be applied to, for example, a pulse tube refrigerator.

The GM refrigerator10according to the embodiment includes a compressor1, a cylinder2, a housing3, a motor housing unit5, etc.

The compressor1recovers a low-pressure refrigerant gas from its suction side to which a low-pressure pipe1ais connected, compresses the low-pressure refrigerant gas, and supplies a high-pressure refrigerant gas to a high-pressure pipe1bconnected to the discharge side of the compressor1. For example, a helium gas may be used as the refrigerant gas, but the refrigerant gas is not limited thereto.

The GM refrigerator10according to the embodiment is a two-stage GM refrigerator. In the two-stage GM refrigerator10, the cylinder2has two sub-cylinders: a high-temperature side cylinder11and a low-temperature side cylinder12. A high-temperature side displacer13is inserted inside the high-temperature side cylinder11. Also, a low-temperature side displacer14is inserted inside the low-temperature side cylinder12.

The high-temperature side displacer13and the low-temperature side displacer14are connected to each other and are configured to be able to reciprocate in the cylinder axial direction inside the high-temperature side cylinder11and the low-temperature side cylinder12, respectively. A high-temperature side internal space15and a low-temperature side internal space16are formed inside the high-temperature side displacer13and the low-temperature side displacer14, respectively. The high-temperature side internal space15and the low-temperature side internal space16are filled with regenerator materials and function as a high-temperature side regenerator17and a low-temperature side regenerator18, respectively.

The high-temperature side displacer13located at the upper part is connected to a drive shaft36extending upward (in a Z1direction). This drive shaft36forms part of a scotch yoke mechanism32described later.

A gas flow passage L1is formed on a high-temperature end side (at an end portion on the side of the Z1direction) of the high-temperature side displacer13. Further, a gas flow passage L2that allows the high-temperature side internal space15to communicate with a high-temperature side expansion space21is formed on a low-temperature end side (at an end portion on the side of a Z2direction) of the high-temperature side displacer13.

The high-temperature side expansion space21is formed at an end portion on the low-temperature side of the high-temperature side cylinder11(end portion on the side of the direction indicated by an arrow Z2inFIG. 1). Further, an upper chamber23is formed at an end portion on the high-temperature side of the high-temperature side cylinder11(end portion on the side of the direction indicated by an arrow Z1inFIG. 1).

Further, a low-temperature side expansion space22is formed at an end portion on the low-temperature side inside the low-temperature side cylinder12(end portion on the side of the direction indicated by the arrow Z2inFIG. 1).

The low-temperature side displacer14is attached to a lower portion of the high-temperature side displacer13by a joint mechanism that is not illustrated. A gas flow passage L3that allows the high-temperature side expansion space21to communicate with the low-temperature side internal space16is formed at an end portion on the high-temperature side (end portion on the side of the direction indicated by the arrow Z1inFIG. 1) of this low-temperature side displacer14. Further, a gas flow passage L4that allows the low-temperature side internal space16to communicate with the low-temperature side expansion space22is formed at an end portion on the low-temperature side (end portion on the side of the direction indicated by the arrow Z2inFIG. 1) of the low-temperature side displacer14.

A high-temperature side cooling stage19is disposed at a position facing the high-temperature side expansion space21on an outer peripheral surface of the high-temperature side cylinder11. Further, a low-temperature side cooling stage20is disposed at a position facing the low-temperature side expansion space22on an outer peripheral surface of the low-temperature side cylinder12.

The above-mentioned high-temperature side displacer13and low-temperature side displacer14move in a vertical direction in the figure (in the directions of the arrows Z1and Z2) inside the high-temperature side cylinder11and the low-temperature side cylinder12, respectively, by means of the scotch yoke mechanism32.

As shown inFIG. 1, the housing3has a rotary valve40, etc., and the motor housing unit5houses a motor31.

The motor31, a driving rotary shaft31a, and the scotch yoke mechanism32form a drive unit. The motor31generates rotational driving force, and a rotary shaft (hereafter referred to as “driving rotary shaft31a”) that is connected to the motor31transmits the rotary motion of the motor31to the scotch yoke mechanism32. The driving rotary shaft31ais supported by a bearing60.

FIG. 2illustrates the scotch yoke mechanism32that is enlarged. The scotch yoke mechanism32has a crank33, a scotch yoke34, etc. This scotch yoke mechanism32can be driven by a driving means, for example, a motor31or the like.

The crank33is fixed to the driving rotary shaft31a. The crank33is configured such that a crank pin33bis provided at a position eccentric from a position where the driving rotary shaft31ais attached. Therefore, when the crank33is attached to the driving rotary shaft31a, the crank pin33bbecomes eccentric with respect to the driving rotary shaft31a. In this sense, the crank pin33bfunctions as an eccentric rotating body. The driving rotary shaft31amay be rotatably supported at a plurality of sites in a longitudinal direction thereof.

The scotch yoke34has a drive shaft36a, a drive shaft36b, a yoke plate35, a roller bearing37, etc. A housing space is formed inside the housing3. This housing space is formed as a gastight container having gastightness that houses the scotch yoke34, a rotor valve42of the rotary valve40described below, and so on. The housing space inside the housing3is hereinafter referred to as “gastight container4” in the present specification. The gastight container4communicates with the suction port of the compressor1via the low-pressure pipe1a. Therefore, the low pressure is always maintained within the gastight container4.

The drive shaft36aextends upward (in the Z1direction) from the yoke plate35. This drive shaft36ais supported by a sliding bearing38aprovided inside the housing3. Therefore, the drive shaft36ais configured to be movable in the vertical direction in the figure (in the directions of the arrows Z1and Z2in the figure).

The drive shaft36bextends downward (in the Z2direction) from the yoke plate35. This drive shaft36bis supported by a sliding bearing38bprovided inside the housing3. Therefore, the drive shaft36is also configured to be movable in the vertical direction in the figure (in the directions of the arrows Z1and Z2in the figure).

Since the drive shaft36aand the drive shaft36bare supported by the sliding bearing38aand the sliding bearing38b, respectively, the scotch yoke34is configured to be movable in the vertical direction (in the directions of the arrows Z1and Z2in the figure) inside the housing3.

It should be noted that a term “shaft direction” may be used to clearly express a positional relationship of the components of the cryogenic refrigerator in the present embodiment. The shaft direction is a direction in which the drive shaft36aand the drive shaft36bextend and conforms to the direction in which the high-temperature side displacer13and the low-temperature side displacer14move. For the sake of convenience, relative closeness to the expansion space or the cooling stage may be referred to as “lower” or “downward” and relative remoteness therefrom may be referred to as “upper” or “upward” in relation to the shaft direction. In other words, relative remoteness from the end portion of the low-temperature side may be referred to as “upper” or “upward,” and relative closeness thereto may be referred to as “lower” or “downward.” It should be noted that these expressions are irrespective of arrangement occurring when the GM refrigerator10is mounted. For example, the GM refrigerator10may be mounted while having the expansion space directed upward in the vertical direction.

A horizontally long window35ais formed on the yoke plate35. This horizontally long window35aextends in a direction that intersects with the direction in which the drive shaft36aand the drive shaft36bextend, for example, in an orthogonal direction (directions of arrows X1and X2inFIG. 2).

The roller bearing37is disposed inside this horizontally long window35a. The roller bearing37is configured to be rollable inside the horizontally long window35a. Further, a hole37ato be engaged with the crank pin33bis formed at a center position of the roller bearing37. The horizontally long window35apermits lateral movement of the crank pin33band the roller bearing37. The horizontally long window35aincludes an upper frame and a lower frame that extend in the lateral direction, and further includes a first side frame and a second side frame that extend in the shaft direction or the longitudinal direction at respective lateral end portions of the upper frame and the lower frame and that connect the upper frame with the lower frame.

When the motor31is driven such that the driving rotary shaft31arotates, the crank pin33brotates to draw a circle. With this movement, the scotch yoke34reciprocates in the directions of the arrows Z1and Z2in the figure. Concurrently, the roller bearing37reciprocates in the direction of the arrows X1and X2in the figure inside the horizontally long window35a.

The high-temperature side displacer13is connected to the drive shaft36bdisposed at a lower portion of the scotch yoke34. Therefore, when the scotch yoke34reciprocates in the directions of the arrows Z1and Z2in the figure, the high-temperature side displacer13and the low-temperature side displacer14connected thereto also reciprocate in the directions of the arrows Z1and Z2inside the high-temperature side cylinder11and the low-temperature side cylinder12, respectively.

A valve mechanism will be described now. The GM refrigerator10according to the embodiment uses the rotary valve40as the valve mechanism.

The rotary valve40switches between the flow passage of the low-pressure refrigerant gas and the flow passage of the high-pressure refrigerant gas. The rotary valve40is driven by the motor31. The rotary valve40functions as a supply valve that guides a high-pressure refrigerant gas discharged from the discharge side of the compressor1to the upper chamber23of the high-temperature side displacer13and also functions as an exhaust valve that guides the refrigerant gas from the upper chamber23to the suction side of the compressor1.

This rotary valve40has a stator valve41and a rotor valve42as shown inFIG. 3as well as inFIG. 1. The stator valve41has a flat stator-side sliding surface45, and the rotor valve42also has a flat rotor-side sliding surface50. When this stator-side sliding surface45and the rotor-side sliding surface50are brought into surface contact with each other, the refrigerant gas is prevented from leaking.

The stator valve41is fixed inside the housing3by a fixing pin43. When the stator valve41is fixed using this fixing pin43, the rotation of the stator valve41is restricted.

The rotor valve42is rotatably supported by a rotor valve bearing62. An engaging hole (not illustrated) to be engaged with the crank pin33bis formed on an opposite-side end surface52located on the side of the rotor valve42opposite to the rotor-side sliding surface50. A tip portion of the crank pin33bprojects from the roller bearing37in a direction of an arrow Y1when the crankpin33bis inserted into the roller bearing (seeFIG. 1).

The tip portion of the crank pin33bprojecting from the roller bearing37is engaged with the engaging hole formed on the rotor valve42. Therefore, the rotor valve42rotates in synchronization with the reciprocation of the scotch yoke34when the crank pin33brotates (eccentrically rotates).

The stator valve41has a refrigerant gas supply hole44, an arc-shaped groove46, and a gas flow passage49. The refrigerant gas supply hole44is connected to the high-pressure pipe1bof the compressor1and is formed such that the refrigerant gas supply hole44penetrates a center portion of the stator valve41.

The arc-shaped groove46is formed on the stator-side sliding surface45. The arc-shaped groove46has an arc shape that centers the refrigerant gas supply hole44.

The gas flow passage49is formed through both the stator valve41and the housing3. One end portion of the gas flow passage49on the valve is open into the arc-shaped groove46to form an opening48. The gas flow passage49has a discharge port47that is open on the side surface of the stator valve41. The discharge port47communicates with the part of the gas flow passage49inside the housing. Further, the other end portion of the gas flow passage49inside the housing is connected to the high-temperature side expansion space21via the upper chamber23, the gas flow passage L1, the high-temperature side regenerator17, and so on.

The rotor valve42has an oval-shaped or elongate groove51and an arc-shaped hole53.

The oval-shaped groove51is formed on the rotor-side sliding surface50such that the oval-shaped groove51extends in the radial direction from the center of the rotor-side sliding surface50. The arc-shaped hole53penetrates the rotor valve42from the rotor-side sliding surface50to the opposite-side end surface52and is connected to the gastight container4. The arc-shaped hole53is formed such that the arc-shaped hole53is positioned on the same circumference as the arc-shaped groove46of the stator valve41.

A supply valve is formed of the refrigerant gas supply hole44, the oval-shaped groove51, the arc-shaped groove46, and the opening48. Further, an exhaust valve is formed of the opening48, the arc-shaped groove46, and the arc-shaped hole53. In the present embodiment, cavities that exist inside the valve such as the oval-shaped groove51and the arc-shaped groove46may be collectively referred to as a valve internal space.

In the GM refrigerator10configured as above, the scotch yoke34reciprocates in the Z1and Z2directions when the rotational driving force of the motor31is transmitted to the scotch yoke mechanism32via the driving rotary shaft31awhile causing the scotch yoke mechanism32to be driven. Due to this movement of the scotch yoke34, the high-temperature side displacer13and the low-temperature side displacer14reciprocate between a bottom dead center LP and a top dead center UP inside the high-temperature side cylinder11and the low-temperature side cylinder12, respectively.

Before the high-temperature side displacer13and the low-temperature side displacer14reach the bottom dead center LP, the exhaust valve closes. Then the supply valve opens. In other words, a refrigerant gas flow passage is formed via the refrigerant gas supply hole44, the oval-shaped groove51, the arc-shaped groove46, and the gas flow passage49.

Therefore, the high-pressure refrigerant gas from the compressor1starts filling the upper chamber23. Subsequently, the high-temperature side displacer13and the low-temperature side displacer14pass the bottom dead center LP and start moving upward, and the refrigerant gas passes the high-temperature side regenerator17and the low-temperature side regenerator18from the upper side to the lower side, filling the high-temperature side expansion space21and the low-temperature side expansion space22, respectively.

When the high-temperature side displacer13and the low-temperature side displacer14reach the top dead center UP, the supply valve closes. At the same time or subsequently, the exhaust valve opens. In other words, a refrigerant gas flow passage is formed via the gas flow passage49, the arc-shaped groove46, and the arc-shaped hole53.

Due to this, the high-pressure refrigerant gas expands inside the high-temperature side expansion space21and the low-temperature side expansion space22, thereby generating cold and cooling the high-temperature side cooling stage19and the low-temperature side cooling stage20. Further, a low-temperature refrigerant gas that has generated cold flows from the lower side to the upper side while cooling the regenerator materials inside the high-temperature side regenerator17and the low-temperature side regenerator18and then flows back to the low-pressure pipe la of the compressor1.

Then, before the high-temperature side displacer13and the low-temperature side displacer14reach the bottom dead center LP, the exhaust valve closes, and the supply valve opens, ending one cycle. By repeating the cycle of compression and expansion of the refrigerant gas in this manner, the high-temperature side cooling stage19and the low-temperature side cooling stage20of the GM refrigerator10are cooled to a cryogenic temperature. The high-temperature side cooling stage19and the low-temperature side cooling stage20of the GM refrigerator10conduct the cold generated by the expansion of the refrigerant gas inside the high-temperature side expansion space21and the low-temperature side expansion space22to the outside of the high-temperature side cylinder11and the low-temperature side cylinder12, respectively.

According to the embodiment as described above, the GM refrigerator10generates cold by converting the driving force of the drive unit such as the motor31to reciprocating movement of the high-temperature side displacer13and the low-temperature side displacer14. Thereby, the temperature of the low-temperature side cooling stage20becomes a cryogenic temperature of approximately 4K.

As an example of the cooling target of the GM refrigerator10according to the embodiment, there is a superconducting coil. Generally, the superconducting coil is used for generating a strong magnetic field. Therefore, when the GM refrigerator10is used for cooling the superconducting coil, the motor31also experiences the magnetic field generated by the superconducting coil.

FIG. 4is a diagram schematically illustrating the internal configuration of the motor31according to the embodiment. The motor31includes a rotor70, a stator71, a magnetic member72, a driving rotary shaft31a, a bearing61, and a casing73that hermetically houses these members. In the motor31according to the embodiment, the stator71is disposed around the rotor70. That is, the rotor70is provided inside the stator71in the radial direction, and the driving rotary shaft31apenetrates the center of the rotor70. Although details will be described below, the magnetic member72is disposed outside the stator71in the radial direction.

FIGS. 5A and 5Bare diagrams for explaining the flows of the magnetic field in the inside of the motor31according to the embodiment.

FIG. 5Ais a diagram schematically illustrating the cross-section when the motor31according to the embodiment is cut out by a plane perpendicular to the driving rotary shaft31a, and is a cross-sectional view taken along line A-A ofFIG. 4. As shown inFIG. 5A, the stator71includes an annular back yoke71aand a plurality of teeth71bformed inside the back yoke in the radial direction. The magnetic member72is disposed at a position spaced apart from the back yoke71ain the outside of the back yoke71ain the radial direction. As in the stator71or the rotor70, the magnetic member72is hermetically housed inside the casing73.

In the example shown inFIG. 5A, the back yoke71aand the magnetic member72are directly connected to each other via a connecting member76. More specifically, the stator71, the magnetic member72, and the connecting member76are formed of a laminated steel plate member, and each layer constituting the laminated steel plate member is integrally formed by performing a punching process to include the back yoke71a, the teeth71b, and the magnetic member72. Thereby, the back yoke71aand the magnetic member72are fixed such that the relative position therebetween is unchanged. Therefore, it can be considered that the magnetic member72constitutes part of the stator71.

FIG. 5Bis a diagram illustrating an external magnetic field74and an internal magnetic field75in the motor31. InFIG. 5B, dashed lines represent the flow of the external magnetic field74generated in the outside of the casing73. Thick solid lines represent the flow of the internal magnetic field75that causes the driving force of the motor31. The external magnetic field74is a magnetic field generated by, for example, the superconducting coil that is the cooling target of the GM refrigerator10. InFIG. 5B, the illustration of the casing73is omitted in order to avoid being complicated.

As shown inFIG. 5B, the internal magnetic field75of the motor31forms a loop-shaped magnetic path via the back yoke71a, the teeth71b, and the rotor70. Since the back yoke71aand the magnetic member72are separated from each other, the internal magnetic field75of the motor31is substantially blocked from the magnetic member72.

As shown inFIG. 5B, the magnetic member72becomes a magnetic path of the external magnetic field74generated in the outside of the casing73. Therefore, most of the external magnetic field74is induced to the magnetic member72and is blocked from the back yoke71a. As such, the external magnetic field74does not almost interfere with the internal magnetic field75of the motor31. That is, it is possible to prevent the external magnetic field74of the motor from being affected on the output torque of the motor31.

FIGS. 6A and 6Bare diagrams for explaining the flows of a magnetic field in the inside of a motor according to a comparative example of the embodiment.

FIG. 6Ais a diagram schematically illustrating the cross-section when the motor according to the comparative example is cut out by a plane perpendicular to a driving rotary shaft and is a diagram corresponding toFIG. 5A. As shown inFIG. 6A, in the motor according to the comparative example, a back yoke71a, teeth71b, and a rotor70are housed in a casing73. However, the motor according to the comparative example does not include a magnetic member72unlike the motor31according to the embodiment.

FIG. 6Bis a diagram illustrating an external magnetic field74and an internal magnetic field75in the motor according to the comparative example. As shown inFIG. 6B, in the motor according to the comparative example, the external magnetic field74passes through the back yoke71athat is the magnetic path of the internal magnetic field75. Therefore, the external magnetic field74interferes with the internal magnetic field75and may be a factor that reduces the output torque of the motor. If the output torque of the motor is less than a torque required for reciprocating movement of a high-temperature side displacer13and a low-temperature side displacer14, the GM refrigerator10may not normally operate. The magnetic member72included in the motor31according to the embodiment can prevent such an external magnetic field74from interfering with the operation of the motor31. InFIG. 6B, the illustration of the casing73is omitted in order to avoid being complicated, as inFIG. 5B.

The following returns to the description ofFIG. 5. A region77between the back yoke71aand the magnetic member72may be filled with a non-magnetic material. For example, a metal such as stainless steel, copper, aluminum, and the like, or a resin such as G-FRP, epoxy, and the like can be used. From the viewpoint of the weight reduction, the resin is preferable. Alternatively, the region77may be a hollow space. In this case, it is preferable that the region77communicate with the above-described gastight container4. Since the gastight container4communicates with the suction port of the compressor1via the low-pressure pipe1a, the region77is also a space that communicates with the flow passage of the low-pressure refrigerant gas.

In the GM refrigerator10according to the embodiment, since the region77between the back yoke71aand the magnetic member72is hollow, the volume of the part of the GM refrigerator10where the low-pressure refrigerant gas exists increases. The inventors of the present application have conducted the experiments and found that the coefficient of performance (COP) of the GM refrigerator10was improved by increasing the volume of the part of the GM refrigerator10where the low-pressure refrigerant gas existed.

FIG. 7is a diagram schematically illustrating the relationship between the volume of the part where the low-pressure refrigerant gas exists and the coefficient of performance in a tabular form. The inventors of the present application has conducted the experiments of increasing the volume of the part where the low-pressure refrigerant gas existed in the GM refrigerator10in which the temperature of the high-temperature side cooling stage19was 41.23 [K], the temperature of the low-temperature side cooling stage20was 3.96 [K], and the coefficient of performance was 0.832. Specifically, when the volume of the part where the low-pressure refrigerant gas existed was increased 2.25 times, the temperature of the high-temperature side cooling stage19was improved to 39.8 [K], the temperature of the low-temperature side cooling stage20was improved to 3.935 [K], and the coefficient of performance was improved to 0.872.

From the above experiments, the performance of the GM refrigerator10can be improved by communicating the hollow region77between the back yoke71aand the magnetic member72with the gastight container4.

As described above, the GM refrigerator10according to the embodiment can reduce the influence of the external magnetic field74that is exerted to the motor31provided in the GM refrigerator10. Further, the performance of the GM refrigerator10can be improved by communicating the hollow region77between the back yoke71aand the magnetic member72with the gastight container4.

While the present invention has been described based on the embodiment, the embodiment is merely illustrative of the principles and applications of the present invention. Additionally, many variations and changes in arrangement may be made in the embodiment without departing from the spirit of the present invention as defined by the appended claims.

First Modification

FIGS. 8A and 8Bare diagrams illustrating a motor31according to a modification of the embodiment. Specifically,FIG. 8Ais a diagram schematically illustrating the internal configuration of the motor31according to the modification.FIG. 8Bis a diagram schematically illustrating the cross-section when the motor31according to the modification is cut out by a plane perpendicular to the driving rotary shaft31a, and is a cross-sectional view taken along line A-A ofFIG. 8A.

As shown inFIGS. 8A and 8B, the motor31according to the modification also includes a magnetic member72. However, in the motor31according to the modification, the magnetic member72and the back yoke71aare not directly connected to each other, unlike the motor31according to the embodiment. Instead, in the motor31according to the modification, the magnetic member72is connected to the back yoke71avia the casing73. Thereby, the back yoke71aand the magnetic member72are fixed such that the relative position therebetween is unchanged. As compared to the motor31according to the embodiment, since the connecting member76is not present in the motor31according to the modification, the volume of the part where the low-pressure refrigerant gas exists is increased. Therefore, there is the effect that can further improve the performance of the GM refrigerator10.

Second Modification

In the above, the two-stage GM refrigerator10has been described as an example of the cryogenic refrigerator. In addition, the present invention can be used in a single-stage GM refrigerator or a three-stage GM refrigerator. Also, the invention can also be applied to a case where a pulse tube refrigerator is used as the cryogenic refrigerator. That is, the motor may be adopted for the driving force of the valve that switches the flow passage of the low-pressure refrigerant gas and the flow passage of the high-pressure refrigerant gas. For example, in a case where such a pulse tube refrigerator is used for cooling of the superconducting coil, the magnetic field generated by the superconducting coil may influence the operation of the motor. In such a case, by adopting the motor31with the above-described magnetic member72, it is possible to reduce the influence of the external magnetic field that is exerted to the driving force of the motor.