GAS-DRIVEN CRYOCOOLER

Abstract

A gas-driven cryocooler is capable of executing an initial cooling for cooling from an initial temperature to a cryogenic temperature and a steady-state operation for maintaining the cryogenic temperature following the initial cooling. The gas-driven cryocooler includes a compressor, a cold head, and a flow restrictor that is connected between the compressor and the cold head and that operates to restrict a flow rate of a working gas to the cold head during the initial cooling, as compared with a flow rate during the steady-state operation.

Claims

1. A gas-driven cryocooler configured to execute an initial cooling for cooling from an initial temperature to a cryogenic temperature and a steady-state operation for maintaining the cryogenic temperature following the initial cooling, the gas-driven cryocooler comprising: a compressor; a cold head; and a flow restrictor that is connected between the compressor and the cold head and that operates to restrict a flow rate of a working gas to the cold head during the initial cooling, as compared with a flow rate during the steady-state operation.

2. The gas-driven cryocooler according to claim 1, wherein the compressor includes a discharge port and a suction port for the working gas, the cold head includes: a displacer that extends in an axial direction, a displacer cylinder that accommodates the displacer to be reciprocable in the axial direction and that forms an expansion chamber between the displacer and the displacer cylinder, a main pressure switching valve that alternately connects the expansion chamber to the discharge port and the suction port, a drive piston that extends from the displacer to a side opposite to the expansion chamber in the axial direction, a drive chamber that accommodates the drive piston to be reciprocable in the axial direction, and an auxiliary pressure switching valve that alternately connects the drive chamber to the discharge port and the suction port to generate a differential pressure between the expansion chamber and the drive chamber and reciprocate the drive piston in the axial direction by the differential pressure, and the flow restrictor is connected between the auxiliary pressure switching valve and the drive chamber and operates to restrict a flow rate of the working gas to the drive chamber during the initial cooling, as compared with a flow rate during the steady-state operation.

3. The gas-driven cryocooler according to claim 1, wherein the flow restrictor includes a flow control valve that opens at a smaller opening degree during the initial cooling, as compared with an opening degree during the steady-state operation.

4. The gas-driven cryocooler according to claim 3, wherein the flow control valve includes a gas pressure actuator that drives the flow control valve such that the flow control valve opens at a smaller opening degree during the initial cooling, as compared with an opening degree during the steady-state operation.

5. The gas-driven cryocooler according to claim 4, wherein the flow restrictor includes a gas volume thermally coupled to a low temperature portion of the cold head, the flow control valve includes a valve body that defines an opening degree of the flow control valve, and the gas pressure actuator includes a pressure chamber connected to the gas volume and drives the valve body in response to a pressure in the pressure chamber.

6. The gas-driven cryocooler according to claim 4, wherein the flow control valve includes a valve body that defines an opening degree of the flow control valve, and the gas pressure actuator includes a pressure chamber connected to the compressor and drives the valve body in response to a pressure in the pressure chamber.

7. The gas-driven cryocooler according to claim 4, wherein the flow control valve includes a valve body stopper that defines a maximum opening degree of the flow control valve by the gas pressure actuator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 is a view schematically showing a cryocooler according to an embodiment.

[0006] FIG. 2 is a view schematically showing the cryocooler according to the embodiment.

[0007] FIG. 3 is a schematic view showing another example of a flow restrictor that can be applied to the cryocooler according to the embodiment.

[0008] FIG. 4 is a schematic view showing another example of the cryocooler according to the embodiment.

[0009] FIG. 5 is a schematic view showing still another example of the flow restrictor that can be applied to the cryocooler according to the embodiment.

[0010] FIG. 6 is a schematic view showing still another example of the flow restrictor that can be applied to the cryocooler according to the embodiment.

DETAILED DESCRIPTION

[0011] A cryocooler is used in order to cool various target objects, such as a superconducting device, a measuring device, and a sample that are used in cryogenic temperature environments. In order to cool a target object with the cryocooler, it is necessary to first start the cryocooler and cool the cryocooler from an initial temperature, such as a room temperature, to a target cryogenic temperature. Such an initial cooling of the cryocooler is also referred to as cool-down. Following the cool-down, the cryocooler is operated to maintain a cryogenically cooled state. Through such a steady-state operation, the cryocooler can cool the target object.

[0012] In many cases, an amount of a working gas sealed in the cryocooler is defined such that the cryocooler can be operated at an optimal operating pressure in a steady-state operation, that is, in a cryogenically cooled state. At the start of the cool-down, the operating pressure of the cryocooler tends to be high, because a temperature of the cryocooler at that point in time is at the initial temperature that is much higher than a cryogenic temperature in the steady-state operation. Accordingly, the gas pressure for driving the displacer may also increase during the cool-down, particularly at the beginning of the cool-down. In a typical gas-driven cryocooler, the displacer is moved by the gas pressure until the displacer interferes with (for example, collides with) an end portion of a cylinder. The interference may cause noise (for example, a collision sound). As the gas pressure increases, the noise may also become more pronounced.

[0013] It is desirable to improve quietness of a gas-driven cryocooler.

[0014] Embodiments for carrying out the present invention will be described in detail below with reference to the drawings. In the description and the drawings, the same or equivalent components, members, and processes will be denoted by the same reference numerals, and overlapping descriptions will be omitted as appropriate. The scales and shapes of respective portions shown in the drawings are conveniently set for ease of description and are not to be interpreted as limiting unless otherwise specified. The embodiments are exemplary and do not limit the scope of the present invention. All of the features described in the embodiments and combinations thereof are not necessarily essential to the invention.

[0015] FIGS. 1 and 2 are schematic views showing a cryocooler 10 according to the embodiment. The cryocooler 10 is, for example, a gas-driven GM cryocooler.

[0016] The cryocooler 10 includes a compressor 12 that compresses a working gas (for example, a helium gas) and a cold head 14 that cools the working gas by adiabatic expansion. The compressor 12 includes a compressor discharge port 12a and a compressor suction port 12b. The compressor discharge port 12a and the compressor suction port 12b function as a high pressure source and a low pressure source of the cryocooler 10, respectively. The cold head 14 is also called an expander.

[0017] As will be described in detail below, the compressor 12 supplies the high pressure (PH) working gas from the compressor discharge port 12a to the cold head 14. The cold head 14 is provided with a regenerator 15 that pre-cools the working gas. The pre-cooled working gas is further cooled by expansion in the cold head 14. The working gas is recovered at the compressor suction port 12b through the regenerator 15. The working gas cools the regenerator 15 when passing through the regenerator 15. The compressor 12 compresses the recovered low pressure (PL) working gas and supplies the working gas to the cold head 14 again.

[0018] The cold head 14 shown in the drawing is a single-stage type. However, the cold head 14 may be a multi-stage type.

[0019] The cold head 14 includes an axially movable body 16 serving as a free piston driven by a gas pressure, and a cold head housing 18 that is hermetically configured and that accommodates the axially movable body 16. The cold head housing 18 supports the axially movable body 16 to be reciprocable in an axial direction and is configured as a pressure vessel for the working gas. Unlike a motor-driven GM cryocooler, the cold head 14 does not include a motor that drives the axially movable body 16, and a connection mechanism (for example, a Scotch yoke mechanism).

[0020] The axially movable body 16 includes a displacer 20 that extends in the axial direction (an up-down direction in FIG. 1, indicated by an arrow C) and that is reciprocable in the axial direction, and a drive piston 22 that is coaxially connected to the displacer 20 to drive the displacer 20 in the axial direction. The drive piston 22 is rigidly connected to the displacer 20 such that the displacer 20 reciprocates integrally with the drive piston 22 in the axial direction. The drive piston 22 has a smaller dimension than the displacer 20. An axial length of the drive piston 22 is shorter than that of the displacer 20, and a diameter of the drive piston 22 is also smaller than that of the displacer 20.

[0021] The cold head housing 18 includes a displacer cylinder 26 that accommodates the displacer 20 to be reciprocable in the axial direction, and a piston cylinder 28 that accommodates the drive piston 22 to be reciprocable in the axial direction. A drive chamber 29 that receives the drive piston 22 is formed in the piston cylinder 28. The piston cylinder 28 is disposed coaxially with the displacer cylinder 26 and adjacent to the displacer cylinder 26 in the axial direction. Details will be described below, but a drive unit of the cold head 14 that is a gas-driven type includes the drive piston 22 and the piston cylinder 28. A volume of the piston cylinder 28 is smaller than that of the displacer cylinder 26. An axial length of the piston cylinder 28 is shorter than that of the displacer cylinder 26, and a diameter of the piston cylinder 28 is also smaller than that of the displacer cylinder 26.

[0022] The axial reciprocation of the displacer 20 is guided by the displacer cylinder 26. Usually, the displacer 20 and the displacer cylinder 26 are each a cylindrical member extending in the axial direction, and an inner diameter of the displacer cylinder 26 coincides with or is slightly larger than an outer diameter of the displacer 20. Similarly, the axial reciprocation of the drive piston 22 is guided by the piston cylinder 28. Usually, the drive piston 22 and the piston cylinder 28 are each a cylindrical member extending in the axial direction, and an inner diameter of the piston cylinder 28 coincides with or is slightly larger than an outer diameter of the drive piston 22.

[0023] Since the displacer 20 and the drive piston 22 are rigidly connected to each other, an axial stroke of the drive piston 22 is equal to an axial stroke of the displacer 20, and the two integrally move throughout the entire stroke. A position of the drive piston 22 with respect to the displacer 20 remains unchanged during the axial reciprocation of the axially movable body 16.

[0024] A first seal portion 32 is provided between the drive piston 22 and the piston cylinder 28. The first seal portion 32 is mounted on either the drive piston 22 or the piston cylinder 28 and slides on the other of the drive piston 22 or the piston cylinder 28. The first seal portion 32 is composed of, for example, a sealing member such as a slipper seal or an O-ring. The piston cylinder 28 is hermetically configured with respect to the displacer cylinder 26 by the first seal portion 32. Since the first seal portion 32 is provided, no direct gas circulation occurs between the piston cylinder 28 and the displacer cylinder 26 (that is, the drive chamber 29 and a room temperature chamber 36). An internal pressure of the piston cylinder 28 and an internal pressure of the displacer cylinder 26 can differ in magnitude.

[0025] The displacer cylinder 26 is partitioned by the displacer 20 into an expansion chamber 34 and the room temperature chamber 36. The displacer 20 forms the expansion chamber 34 between the displacer 20 and the displacer cylinder 26 at one end in the axial direction and forms the room temperature chamber 36 between the displacer 20 and the displacer cylinder 26 at the other end in the axial direction. The room temperature chamber 36 can also be called a compression chamber. The drive piston 22 extends from the displacer 20 to a side opposite to the expansion chamber 34 in the axial direction, that is, to a room temperature chamber 36 side. In addition, the cold head 14 is provided with a cooling stage 38 that is stuck to the displacer cylinder 26 so as to envelop the expansion chamber 34.

[0026] The regenerator 15 is incorporated into the displacer 20. The displacer 20 includes, in an upper lid portion thereof, an inlet flow path 40 through which the regenerator 15 communicates with the room temperature chamber 36. Additionally, the displacer 20 includes, in a cylinder portion thereof, an outlet flow path 42 through which the regenerator 15 communicates with the expansion chamber 34. Alternatively, the outlet flow path 42 may be provided in a lower lid portion of the displacer 20. In addition, the regenerator 15 includes an inlet retainer 41 that is inscribed in the upper lid portion and an outlet retainer 43 that is inscribed in the lower lid portion. The inlet retainer 41 and the outlet retainer 43 act as rectifiers for the working gas. A regenerator material may be, for example, a copper mesh. The retainer may be a coarser mesh than the regenerator material.

[0027] A second seal portion 44 is provided between the displacer 20 and the displacer cylinder 26. The second seal portion 44 is, for example, a slipper seal and is mounted on the cylinder portion or the upper lid portion of the displacer 20. Since a clearance between the displacer 20 and the displacer cylinder 26 is sealed by the second seal portion 44, there is no direct gas circulation (that is, no gas flow bypassing the regenerator 15) between the room temperature chamber 36 and the expansion chamber 34.

[0028] The working gas flows into the regenerator 15 from the room temperature chamber 36 through the inlet flow path 40. More accurately, the working gas flows into the regenerator 15 from the inlet flow path 40 through the inlet retainer 41. The working gas flows from the regenerator 15 into the expansion chamber 34 through the outlet retainer 43 and the outlet flow path 42. When the working gas returns from the expansion chamber 34 to the room temperature chamber 36, the working gas passes through a reverse path. That is, the working gas returns from the expansion chamber 34 to the room temperature chamber 36 through the outlet flow path 42, the regenerator 15, and the inlet flow path 40. The working gas that attempts to flow through the clearance by bypassing the regenerator 15 is blocked by the second seal portion 44.

[0029] The cold head 14 is installed in an orientation shown in the drawing at a job site where the cold head 14 is used. That is, the displacer cylinder 26 is disposed below in a vertical direction, and the piston cylinder 28 is disposed above in the vertical direction, so that the cold head 14 is installed in a vertical orientation. In this way, when the cold head 14 is installed in a posture in which the cooling stage 38 is directed downward in the vertical direction, the cryocooler 10 has highest cooling capacity. However, the disposition of the cryocooler 10 is not limited thereto. Conversely, the cold head 14 may be installed in a posture in which the cooling stage 38 is directed upward in the vertical direction. Alternatively, the cold head 14 may be installed in a horizontal orientation or in other orientations. A cooling operation is possible regardless of the posture in which the cold head 14 is installed.

[0030] An end of the reciprocating stroke of the displacer 20 on an expansion chamber 34 side is referred to as a bottom dead center of the displacer 20, and an end of the reciprocating stroke of the displacer 20 on the room temperature chamber 36 side is referred to as a top dead center of the displacer 20. The movement of the displacer 20 toward the top dead center may be referred to as upward movement, and the movement of the displacer 20 toward the bottom dead center may be referred to as downward movement. However, such terms do not limit the posture of the cold head 14.

[0031] When the displacer 20 moves in the axial direction, the expansion chamber 34 and the room temperature chamber 36 complementarily increase or decrease their volumes. That is, when the displacer 20 moves downward, the expansion chamber 34 becomes narrower and the room temperature chamber 36 becomes wider. The same applies to the reverse. Therefore, when the displacer 20 is located at the bottom dead center, the volume of the expansion chamber 34 is minimum (the volume of the room temperature chamber 36 is maximum). When the displacer 20 is located at the top dead center, the volume of the expansion chamber 34 is maximum (the volume of the room temperature chamber 36 is minimum). Since the drive piston 22 moves integrally with the displacer 20, the volume of the drive chamber 29 is maximum when the displacer 20 is located at the bottom dead center, and the volume of the drive chamber 29 is minimum when the displacer 20 is located at the top dead center.

[0032] Further, the cryocooler 10 includes a working gas circuit 52 that connects the compressor 12 to the cold head 14. The working gas circuit 52 is configured to generate a pressure difference between the piston cylinder 28 and the displacer cylinder 26 (that is, the drive chamber 29 and the expansion chamber 34). This pressure difference causes the axially movable body 16 to move in the axial direction. When the pressure of the displacer cylinder 26 is lower than that of the piston cylinder 28, the drive piston 22 moves downward, and accordingly, the displacer 20 also moves downward. Conversely, when the pressure of the displacer cylinder 26 is higher than that of the piston cylinder 28, the drive piston 22 moves upward, and accordingly, the displacer 20 also moves upward.

[0033] The working gas circuit 52 includes a valve portion 54. The valve portion 54 may be disposed adjacent to the piston cylinder 28 to be integrated with the cold head housing 18 and may be connected to the compressor 12 using a pipe. The valve portion 54 may be disposed outside the cold head housing 18 and may be connected to each of the compressor 12 and the cold head 14 using a pipe.

[0034] The valve portion 54 includes an expansion chamber pressure switching valve (hereinafter, also referred to as a main pressure switching valve) 60 and a drive chamber pressure switching valve (hereinafter, also referred to as an auxiliary pressure switching valve) 62. The main pressure switching valve 60 includes a main intake on-off valve V1 and a main exhaust on-off valve V2. The auxiliary pressure switching valve 62 includes an auxiliary intake on-off valve V3 and an auxiliary exhaust on-off valve V4.

[0035] The working gas circuit 52 includes a high pressure line 13a and a low pressure line 13b that connect the compressor 12 to the valve portion 54. The high pressure line 13a extends from the compressor discharge port 12a, branches midway, and is connected to the main intake on-off valve V1 and the auxiliary intake on-off valve V3. The low pressure line 13b extends from the compressor suction port 12b, branches midway, and is connected to the main exhaust on-off valve V2 and the auxiliary exhaust on-off valve V4.

[0036] Additionally, the working gas circuit 52 includes a main communication passage 64 and an auxiliary communication passage 66 that connect the cold head 14 to the valve portion 54. The main communication passage 64 connects the displacer cylinder 26 to the main pressure switching valve 60. The main communication passage 64 extends from the room temperature chamber 36, branches midway, and is connected to the main intake on-off valve V1 and the main exhaust on-off valve V2. The auxiliary communication passage 66 connects the piston cylinder 28 to the auxiliary pressure switching valve 62. The auxiliary communication passage 66 extends from the drive chamber 29, branches midway, and is connected to the auxiliary intake on-off valve V3 and the auxiliary exhaust on-off valve V4.

[0037] The main pressure switching valve 60 is configured such that the compressor discharge port 12a or the compressor suction port 12b selectively communicates with the room temperature chamber 36 of the displacer cylinder 26. In the main pressure switching valve 60, the main intake on-off valve V1 and the main exhaust on-off valve V2 are opened exclusively of each other. That is, the main intake on-off valve V1 and the main exhaust on-off valve V2 are prohibited from being opened simultaneously. The main intake on-off valve V1 and the main exhaust on-off valve V2 may be temporarily closed together.

[0038] The main exhaust on-off valve V2 is closed when the main intake on-off valve V1 is open. The working gas flows from the compressor discharge port 12a to the displacer cylinder 26 through the high pressure line 13a and the main communication passage 64. As mentioned above, the working gas flows from the room temperature chamber 36 to the expansion chamber 34 through the regenerator 15. In this way, the working gas having the high pressure PH is supplied from the compressor 12 to the expansion chamber 34, and the pressure in the expansion chamber 34 increases. Conversely, when the main intake on-off valve V1 is closed, the supply of the working gas from the compressor 12 to the expansion chamber 34 is stopped.

[0039] On the other hand, when the main exhaust on-off valve V2 is open, the main intake on-off valve V1 is closed. First, the working gas having the high pressure PH expands and is depressurized in the expansion chamber 34. The working gas flows from the expansion chamber 34 to the room temperature chamber 36 through the regenerator 15. The working gas flows from the displacer cylinder 26 to the compressor suction port 12b through the main communication passage 64 and the low pressure line 13b. In this way, the working gas having the low pressure PL is recovered from the cold head 14 to the compressor 12. When the main exhaust on-off valve V2 is closed, the recovery of the working gas from the expansion chamber 34 to the compressor 12 is stopped.

[0040] The auxiliary pressure switching valve 62 is configured such that the compressor discharge port 12a or the compressor suction port 12b selectively communicates with the piston cylinder 28. The auxiliary pressure switching valve 62 is configured such that the auxiliary intake on-off valve V3 and the auxiliary exhaust on-off valve V4 are opened exclusively of each other. That is, the auxiliary intake on-off valve V3 and the auxiliary exhaust on-off valve V4 are prohibited from being opened simultaneously. The auxiliary intake on-off valve V3 and the auxiliary exhaust on-off valve V4 may be temporarily closed together.

[0041] The auxiliary exhaust on-off valve V4 is closed when the auxiliary intake on-off valve V3 is open. The working gas flows from the compressor discharge port 12a to the piston cylinder 28 through the high pressure line 13a and the auxiliary communication passage 66. In this way, the working gas having the high pressure PH is supplied from the compressor 12 to the drive chamber 29, and the pressure in the drive chamber 29 increases. When the auxiliary intake on-off valve V3 is closed, the supply of the working gas from the compressor 12 to the piston cylinder 28 is stopped.

[0042] On the other hand, when the auxiliary exhaust on-off valve V4 is open, the auxiliary intake on-off valve V3 is closed. The working gas is recovered from the piston cylinder 28 to the compressor suction port 12b through the auxiliary communication passage 66 and the low pressure line 13b, and the drive chamber 29 is depressurized to the low pressure PL. When the auxiliary exhaust on-off valve V4 is closed, the recovery of the working gas from the piston cylinder 28 to the compressor 12 is stopped.

[0043] In this manner, the main pressure switching valve 60 alternately connects the expansion chamber 34 to the compressor discharge port 12a and the compressor suction port 12b and generates periodic pressure fluctuations of the high pressure PH and the low pressure PL in the expansion chamber 34. In addition, the auxiliary pressure switching valve 62 alternately connects the drive chamber 29 to the compressor discharge port 12a and the compressor suction port 12b and generates periodic pressure fluctuations of the high pressure PH and the low pressure PL in the drive chamber 29.

[0044] The auxiliary pressure switching valve 62 alternately connects the drive chamber 29 to the compressor discharge port 12a and the compressor suction port 12b so as to generate a differential pressure between the expansion chamber 34 and the drive chamber 29 to reciprocate the drive piston 22 in the axial direction by the differential pressure. Typically, the pressure fluctuations in the drive chamber 29 are generated in a substantially opposite phase to and in the same cycle as that of the pressure fluctuations in the expansion chamber 34. When the expansion chamber 34 has the high pressure PH, the drive chamber 29 has the low pressure PL, and the drive piston 22 can move the displacer 20 upward. When the expansion chamber 34 has the low pressure PL, the drive chamber 29 has the high pressure PH, and the drive piston 22 can move the displacer 20 downward. In this manner, the auxiliary pressure switching valve 62 controls the pressure of the drive chamber 29 such that the drive piston 22 drives the axial reciprocation of the displacer 20.

[0045] The valve portion 54 may employ a form of a rotary valve. In this case, a group of valves (V1 to V4) are incorporated into the valve portion 54 and are synchronously driven. The valve portion 54 is configured such that the valves (V1 to V4) are properly switched by rotational sliding of a valve disc (or a valve rotor) with respect to a valve main body (or a valve stator). The group of valves (V1 to V4) are switched in the same cycle during an operation of the cryocooler 10, whereby four on-off valves (V1 to V4) periodically change their open/closed states. The four on-off valves (V1 to V4) are each opened and closed in a different phase.

[0046] The cryocooler 10 may include a rotation drive source 56 connected to the valve portion 54 to rotate the valve portion 54. The rotation drive source 56 is mechanically connected to the valve portion 54. The rotation drive source 56 is, for example, a motor. However, the rotation drive source 56 is not mechanically connected to the axially movable body 16. Additionally, the cryocooler 10 may include a controller 58 that controls the valve portion 54. The controller 58 may control the rotation drive source 56.

[0047] In a certain embodiment, the group of valves (V1 to V4) may employ a form of a plurality of individually controllable valves. Each of the valves (V1 to V4) may be an electromagnetic on-off valve. In this case, the rotation drive source 56 is not provided, and each of the valves (V1 to V4) is electrically connected to the controller 58. The controller 58 may control the opening and closing of each of the valves (V1 to V4).

[0048] In addition, the cold head 14 may include a temperature sensor 68 that measures a temperature of the low temperature portion (for example, the cooling stage 38) and that outputs a measured temperature signal indicating the measured temperature. The controller 58 may be electrically connected to the temperature sensor 68 to acquire the measured temperature signal from the temperature sensor 68.

[0049] FIG. 1 shows a state in which the displacer 20 is located at the bottom dead center, and FIG. 2 shows a state in which the displacer 20 is located at the top dead center.

[0050] In this embodiment, a design referred to as a collar bumper is employed in order to prevent the interference between the displacer 20 and the end portion of the displacer cylinder 26 and to reduce vibration and noise during the operation of the cryocooler 10. The cold head 14 includes a collar 70 and a collar chamber 72 divided into an upper section 72a and a lower section 72b by the collar 70. The collar 70 is rigidly connected to the displacer 20 to reciprocate together with the displacer 20 and constitutes a part of the axially movable body 16. As will be described below, the reciprocating stroke of the collar 70 in the collar chamber 72 defines the reciprocating stroke of the displacer 20.

[0051] The displacer cylinder 26 includes a cylinder flange 26a that defines a cylinder upper opening. The cylinder flange 26a extends outward in a radial direction from an axial upper end of the displacer cylinder 26. The cold head housing 18 includes a top plate 30 and a sleeve 73. The piston cylinder 28 and the sleeve 73 are fixed to the top plate 30, and the valve portion 54 is mounted on the top plate 30. The cylinder flange 26a is connected to the top plate 30 via the sleeve 73. The sleeve 73 is disposed outside the piston cylinder 28 to surround the piston cylinder 28.

[0052] The collar 70 includes a tubular main body 70a and a collar upper end 70b. The main body 70a has an outer diameter substantially the same as that of the displacer 20 and extends upward from the room temperature chamber 36 side of the displacer 20. An inner diameter of the main body 70a is larger than an outer diameter of the piston cylinder 28. The collar upper end 70b exists outward of the outer diameter of the displacer 20. The collar chamber 72 is divided into the upper section 72a and the lower section 72b by the collar upper end 70b. The collar chamber 72 communicates with the room temperature chamber 36. When the displacer 20 reciprocates inside the displacer cylinder 26, the collar 70 reciprocates in the collar chamber 72 without frictional contact with the displacer cylinder 26 and the piston cylinder 28. The collar 70 does not come into frictional contact with an inner peripheral surface of the sleeve 73.

[0053] Additionally, the cold head 14 includes an upper bumper 74 provided in the upper section 72a to mitigate interference between the displacer 20 and the displacer cylinder 26 when the displacer 20 is located at the top dead center. The upper bumper 74 is installed on an upper surface of the collar chamber 72 and includes an upper cushioning material 74a and an upper retainer 74b. The upper bumper 74 is attached to, for example, the sleeve 73. The upper cushioning material 74a is, for example, a resin-made annular member such as an O-ring and is sandwiched between the upper surface of the collar chamber 72 and the upper retainer 74b. The upper retainer 74b is formed of, for example, a resin material. The upper retainer 74b need not be provided.

[0054] The upper bumper 74 comes into contact with the collar 70 when the displacer 20 is located at the top dead center, and prevents the displacer 20 and the displacer cylinder 26 from colliding with each other on the room temperature chamber 36 side. The collar upper end 70b engages with the upper bumper 74 inside the collar chamber 72 before the displacer 20 collides with the piston cylinder 28 when the displacer 20 moves upward. In this case, the collar upper end 70b comes into contact with the upper retainer 74b, and the upper cushioning material 74a is compressed to absorb an impact.

[0055] The cold head 14 includes a lower bumper 76 provided in the lower section 72b to mitigate interference between the displacer 20 and the displacer cylinder 26 when the displacer 20 is located at the bottom dead center. The lower bumper 76 is installed on a lower surface of the collar chamber 72 and includes a lower cushioning material 76a and a lower retainer 76b. The lower bumper 76 is attached to, for example, the cylinder flange 26a. The lower bumper 76 may be attached to the sleeve 73. The lower cushioning material 76a is, for example, a resin-made annular member such as an O-ring and is sandwiched between the lower surface of the collar chamber 72 and the lower retainer 76b. The lower retainer 76b is formed of, for example, a resin material. The lower retainer 76b need not be provided.

[0056] The lower bumper 76 comes into contact with the collar 70 when the displacer 20 is located at the bottom dead center, and prevents the displacer 20 and the displacer cylinder 26 from colliding with each other on the expansion chamber 34 side. The collar upper end 70b engages with the lower bumper 76 inside the collar chamber 72 before the displacer 20 collides with the displacer cylinder 26 on the expansion chamber 34 side when the displacer 20 moves downward. In this case, the collar upper end 70b comes into contact with the lower retainer 76b, and the lower cushioning material 76a is compressed to absorb an impact.

[0057] The upper section 72a communicates with the room temperature chamber 36. A first clearance 78a is formed between an outer peripheral surface of the piston cylinder 28 and an inner peripheral surface of the collar 70, thereby allowing the working gas to flow between the room temperature chamber 36 and the upper section 72a through the first clearance 78a.

[0058] The lower section 72b communicates with the upper section 72a. A second clearance 78b is formed between the inner peripheral surface of the sleeve 73 and an outer peripheral surface of the collar upper end 70b, thereby allowing the working gas to flow between the upper section 72a and the lower section 72b through the second clearance 78b. However, when the displacer 20 is located at the bottom dead center, the collar upper end 70b comes into contact with the lower bumper 76, and communication between the lower section 72b and the upper section 72a through the second clearance 78b is blocked. When the displacer 20 is located at the top dead center, the collar upper end 70b comes into contact with the upper bumper 74, and the communication between the lower section 72b and the upper section 72a through the second clearance 78b is blocked. Therefore, when the displacer 20 is at an intermediate position between the top dead center and the bottom dead center, the lower section 72b communicates with the room temperature chamber 36 through the upper section 72a, and the working gas can flow between the room temperature chamber 36 and the lower section 72b. In addition, since the lower section 72b is sealed by the second seal portion 44, the lower section 72b does not communicate with the expansion chamber 34.

[0059] Additionally, the cold head 14 includes a communication passage 80 that ensures the communication between the upper section 72a and the lower section 72b when the displacer 20 is located at the bottom dead center. The communication passage 80 is formed in the collar 70 such that the upper section 72a communicates with the lower section 72b in a state in which the collar upper end 70b is in contact with the lower bumper 76. The communication passage 80 is formed to penetrate the collar 70 (for example, the collar upper end 70b) from the upper section 72a to the lower section 72b, and at least one communication passage 80 need only be provided in a circumferential direction. As shown in the drawing, in a case where the collar upper end 70b extends outward in the radial direction from the main body 70a of the collar 70, the communication passage 80 is formed in the collar upper end 70b at a position radially inward of the lower bumper 76. The communication passage 80 may be formed to penetrate the main body 70a of the collar 70.

[0060] The first clearance 78a, the second clearance 78b, and the communication passage 80 act as flow path resistances. Therefore, when the displacer 20 reciprocates, the upper section 72a and the lower section 72b can each generate a gas spring force. As the displacer 20 moves upward, the collar upper end 70b also moves upward, thereby narrowing the upper section 72a. In this case, the gas in the upper section 72a is compressed, and the pressure increases. The pressure in the upper section 72a acts downward on the upper surface of the collar upper end 70b. Therefore, the upper section 72a generates a gas spring force that resists the upward movement of the collar 70 and the displacer 20. Similarly, when the displacer 20 moves downward, the lower section 72b generates a gas spring force that resists the downward movement of the collar 70 and the displacer 20. The upper section 72a and the lower section 72b may be referred to as an upper gas spring chamber and a lower gas spring chamber, respectively. The gas spring force is helpful in reducing vibration and noise that may occur when the collar 70 comes into contact with the upper bumper 74 and the lower bumper 76.

[0061] Incidentally, the cryocooler 10 can execute an initial cooling and a steady-state operation following the initial cooling. The initial cooling is an operating mode for the cold head 14 to rapidly cool from an initial temperature to a cryogenic temperature when the cryocooler 10 is started, and the steady-state operation is an operating mode for the cold head 14 to maintain the cryogenically cooled state obtained through the initial cooling. The initial cooling can be called cool-down as mentioned above. The initial temperature may be an ambient temperature (for example, room temperature). The cold head 14 is cooled to a standard cooling temperature through the initial cooling and is maintained within an allowable temperature range of the cryogenic temperature including the standard cooling temperature in the steady-state operation. The standard cooling temperature may vary depending on the application and settings of the cryocooler 10 and may be, for example, about 10 K to 20 K, or 10 K or lower. The standard cooling temperature is typically about 4.2 K or lower, for example, in a cooling application for a superconducting device.

[0062] An amount of the working gas sealed in the cryocooler 10 may be defined such that the cryocooler 10 can be operated at an optimal operating pressure in the steady-state operation, that is, in the cryogenically cooled state. During the initial cooling, as the temperature decreases from the initial temperature to the cryogenic temperature, a density of the working gas inside the cold head 14 increases. Accordingly, the amount of the working gas accumulated inside the cold head 14 increases, and so to speak, the working gas is absorbed from the working gas circuit 52 into the cold head 14. Consequently, as the cooling of the cold head 14 progresses, the pressure of the working gas circulating through the working gas circuit 52 gradually decreases. As a result, the operating pressure of the cryocooler 10 in the steady-state operation is optimized.

[0063] From another perspective, the operating pressure of the cryocooler 10 at the start of the initial cooling tends to be high, because the temperature of the cryocooler 10 at that point in time is at the initial temperature that is much higher than the cryogenic temperature in the steady-state operation. Accordingly, the differential pressure between the drive chamber 29 and the expansion chamber 34, that is, the gas pressure for driving the displacer 20, may also be high during the initial cooling, particularly at the beginning of the initial cooling. In the typical gas-driven cryocooler, the displacer 20 may be moved by the gas pressure until the displacer 20 interferes with (for example, collides with) the end portion of the displacer cylinder 26. The interference may cause noise (for example, a collision sound). As the gas pressure increases, the noise may also become more pronounced.

[0064] In order to address such an issue, the cryocooler 10 includes a flow restrictor 100 connected between the compressor 12 and the cold head 14. The flow restrictor 100 operates to restrict a flow rate of the working gas to the cold head 14 during the initial cooling, as compared with that during the steady-state operation. In this manner, the flow rate of the working gas to the cold head 14 during the switching between the intake process and the exhaust process of the cryocooler 10 in the initial cooling can be reduced, and the rise of the gas pressure for driving the displacer 20 can be delayed. As a result, the quietness of the cryocooler 10 can be improved. For example, the movement speed of the displacer 20, and consequently the kinetic energy, can be reduced, thereby suppressing the collision sound of the displacer 20 against the displacer cylinder 26.

[0065] In this embodiment, the flow restrictor 100 is connected between the auxiliary pressure switching valve 62 and the drive chamber 29 and operates to restrict the flow rate of the working gas to the drive chamber 29 during the initial cooling, as compared with that during the steady-state operation. In this manner, the flow rate of the working gas to the drive chamber 29 can be reduced, and the pressure rise in the drive chamber 29 can be delayed, thereby delaying the rise of a driving force of the drive piston 22 caused by the gas pressure. As a result, the quietness of the cryocooler 10 can be improved.

[0066] The flow restrictor 100 includes a flow control valve 102 that opens at a smaller opening degree during the initial cooling, as compared with that during the steady-state operation. In this manner, the flow control valve 102 can restrict the flow rate of the working gas to the drive chamber 29 during the initial cooling, as compared with that during the steady-state operation. The flow control valve 102 is provided in the auxiliary communication passage 66 and is connected in series between the auxiliary pressure switching valve 62 and the drive chamber 29. The flow control valve 102 may be, for example, a flow control valve having any configuration with a variable opening degree, such as a needle valve and a variable orifice.

[0067] In order to control the opening degree of the flow control valve 102, the flow restrictor 100 may include the controller 58. The controller 58 may acquire the operating state of the cryocooler 10 (that is, whether the cryocooler 10 is executing the initial cooling or the steady-state operation), and control the flow control valve 102 based on the acquired operating state.

[0068] In order to acquire the operating state of the cryocooler 10, the controller 58 may use the temperature of the cryocooler 10. For example, the controller 58 may refer to the measured temperature of the temperature sensor 68 provided on the low temperature portion (for example, the cooling stage 38) of the cold head 14. When the measured temperature is higher than a predetermined target temperature (for example, the above-mentioned standard cooling temperature) for the initial cooling, the controller 58 may determine that the cryocooler 10 is executing the initial cooling. When the measured temperature is lower than the target temperature for the initial cooling, the controller 58 may determine that the cryocooler 10 is executing the steady-state operation.

[0069] In order to acquire the operating state of the cryocooler 10, the controller 58 may use the pressure of the cryocooler 10. For example, the controller 58 may refer to a measured pressure of a pressure sensor (not shown) that measures a high pressure (for example, a pressure of the compressor discharge port 12a) of the cryocooler 10 or a low pressure (for example, a pressure of the compressor suction port 12b) of the cryocooler 10. The differential pressure between the high pressure and the low pressure of the cryocooler 10 may be referred to. The measured pressure correlates with the cooling temperature of the cryocooler 10. Therefore, when the measured pressure is higher than a predetermined pressure threshold value, the controller 58 may determine that the cryocooler 10 is executing the initial cooling. When the measured pressure is lower than the predetermined pressure threshold value, the controller 58 may determine that the cryocooler 10 is executing the steady-state operation.

[0070] Alternatively, the controller 58 may be configured to measure an elapsed time from the start of the initial cooling or may acquire the operating state of the cryocooler 10 based on the elapsed time. A required time for the initial cooling can be empirically or experimentally identified. Therefore, when the measured elapsed time is shorter than a predetermined time, the controller 58 may determine that the cryocooler 10 is executing the initial cooling. When the measured elapsed time is longer than the predetermined time, the controller 58 may determine that the cryocooler 10 is executing the steady-state operation.

[0071] The controller 58 may set the flow control valve 102 to a first opening degree when the cryocooler 10 is executing the initial cooling, and may set the flow control valve 102 to a second opening degree when the cryocooler 10 is executing the steady-state operation. The first opening degree is smaller than the second opening degree. In this manner, the opening degree of the flow control valve 102 can be switched according to the operating state of the cryocooler 10.

[0072] The controller 58 may change the opening degree of the flow control valve 102 from the first opening degree to the second opening degree in a stepwise or continuous manner, based on the measured temperature (or the measured pressure). That is, the higher the measured temperature (or the measured pressure) is, the smaller the opening degree of the flow control valve 102 may be. Similarly, the controller 58 may change the opening degree of the flow control valve 102 from the first opening degree to the second opening degree in a stepwise or continuous manner, based on the elapsed time. That is, the shorter the elapsed time is, the smaller the opening degree of the flow control valve 102 may be.

[0073] The opening degree of the flow control valve 102 may be controlled manually. The flow control valve 102 may include an operation portion for adjusting the opening degree, and the opening degree may be changed by operating the operation portion. In this manner, the flow control valve 102 may operate to open at a smaller opening degree during the initial cooling, as compared with that during the steady-state operation.

[0074] The operation of the cryocooler 10 will be described. When the displacer 20 is located at the bottom dead center or in the vicinity thereof, the intake process of the cryocooler 10 is started. The main intake on-off valve V1 is opened, and the main exhaust on-off valve V2 is closed. The working gas is supplied from the compressor discharge port 12a to the displacer cylinder 26 of the cold head 14 through the main intake on-off valve V1, and the expansion chamber 34 and the room temperature chamber 36 are at the high pressure PH. The exhaust of the piston cylinder 28 is performed simultaneously with the intake into the expansion chamber 34. The auxiliary intake on-off valve V3 is closed, and the auxiliary exhaust on-off valve V4 is opened. The working gas is discharged from the piston cylinder 28 to the compressor suction port 12b through the auxiliary exhaust on-off valve V4, and the drive chamber 29 is depressurized to the low pressure PL.

[0075] Therefore, in the intake process, a driving force generated by the differential pressure (PHPL) between the drive chamber 29 and the expansion chamber 34 acts upward on the drive piston 22. As a result, the displacer 20 moves together with the drive piston 22 from the bottom dead center toward the top dead center. In this way, as the volume of the expansion chamber 34 increases, the expansion chamber 34 is filled with a high pressure gas.

[0076] The collar 70 moves upward together with the displacer 20. The collar 70 comes into contact with the upper bumper 74 before the displacer 20 collides with a high temperature end portion (for example, the piston cylinder 28) of the displacer cylinder 26. The upper cushioning material 74a is compressed to absorb the impact. While the collar 70 moves upward, the upper section 72a communicates with the room temperature chamber 36 through the first clearance 78a, and the lower section 72b communicates with the upper section 72a through the second clearance 78b and the communication passage 80. Therefore, the upper section 72a and the lower section 72b are at the high pressure PH, similar to the room temperature chamber 36.

[0077] When the displacer 20 is located at the top dead center or in the vicinity thereof, the exhaust process of the cryocooler 10 is started. The main exhaust on-off valve V2 is opened, and the main intake on-off valve V1 is closed. The high pressure gas is expanded and cooled in the expansion chamber 34. The expanded gas is recovered to the compressor suction port 12b through the room temperature chamber 36 while cooling the regenerator 15. The expansion chamber 34 and the room temperature chamber 36 are at the low pressure PL. The intake into the piston cylinder 28 is performed simultaneously with the exhaust from the expansion chamber 34. The auxiliary exhaust on-off valve V4 is closed, and the auxiliary intake on-off valve V3 is opened. The working gas is supplied from the compressor discharge port 12a to the piston cylinder 28 through the auxiliary intake on-off valve V3, and the pressure in the drive chamber 29 increases to the high pressure PH.

[0078] Therefore, in the exhaust process, a driving force generated by the differential pressure (PHPL) between the drive chamber 29 and the expansion chamber 34 acts downward on the drive piston 22. As a result, the displacer 20 moves together with the drive piston 22 from the top dead center toward the bottom dead center. In this way, as the volume of the expansion chamber 34 is reduced, a low pressure gas is discharged.

[0079] The collar 70 moves downward together with the displacer 20. The collar 70 comes into contact with the lower bumper 76 before the displacer 20 collides with a low temperature end portion of the displacer cylinder 26. The lower cushioning material 76a is compressed to absorb the impact. While the collar 70 moves downward, the upper section 72a communicates with the room temperature chamber 36 through the first clearance 78a, and the lower section 72b communicates with the upper section 72a through the second clearance 78b and the communication passage 80. Therefore, the upper section 72a and the lower section 72b are at the low pressure PL, similar to the room temperature chamber 36.

[0080] Incidentally, the typical gas-driven cryocooler of the collar bumper type does not include the communication passage 80, unlike the above-mentioned embodiment. In this case, when the collar 70 is located at the bottom dead center, the working gas having the low pressure PL may be sealed in the lower section 72b. In this state, when the pressure in the upper section 72a increases to the high pressure PH at the start of the intake process, the collar upper end 70b may be pressed against the lower bumper 76 by the differential pressure (PHPL). This differential pressure may hinder the upward movement of the displacer 20.

[0081] However, the cryocooler 10 according to the embodiment includes the communication passage 80 formed in the collar 70 to ensure the communication between the upper section 72a and the lower section 72b when the displacer 20 is located at the bottom dead center. Therefore, even when the collar 70 is located at the bottom dead center and the collar upper end 70b is in contact with the lower bumper 76, the lower section 72b communicates with the upper section 72a through the communication passage 80. The lower section 72b is not sealed. Since the differential pressure that may occur between the upper section 72a and the lower section 72b is reduced or eliminated through the communication passage 80, the upward movement of the displacer 20 is not hindered. Therefore, the displacer 20 can move from the bottom dead center toward the top dead center.

[0082] The cryocooler 10 cools the cooling stage 38 by repeating such a refrigeration cycle (that is, a GM cycle). As a result, the cryocooler 10 can cool an object to be cooled (not shown) thermally coupled to the cooling stage 38.

[0083] Since the cryocooler 10 is a collar bumper type, the contact between the collar 70 and the bumper (74, 76) can prevent the interference (for example, collision) between the displacer 20 and the displacer cylinder 26, thereby reducing vibration and noise.

[0084] In this embodiment, as mentioned above, the flow control valve 102 is provided between the auxiliary pressure switching valve 62 and the drive chamber 29, and the opening degree of the flow control valve 102 is smaller during the initial cooling, as compared with that during the steady-state operation. Consequently, the flow rate of the working gas from the drive chamber 29 to the compressor suction port 12b through the auxiliary exhaust on-off valve V4 can be restricted at the start of the intake process, thereby delaying the pressure drop to the low pressure PL in the drive chamber 29. Since such a flow restriction is not performed in the main communication passage 64, the pressure increase to the high pressure PH in the expansion chamber 34 and the room temperature chamber 36 is not delayed. In this way, during the initial cooling, the rise of the driving force acting on the drive piston 22 can be delayed, as compared with that during the steady-state operation. The movement speed of the displacer 20, and consequently the kinetic energy, can be reduced, thereby suppressing the collision sound of the displacer 20 at the top dead center and improving the quietness of the cryocooler 10 in the initial cooling.

[0085] Additionally, when the exhaust process is started, the flow rate of the working gas from the compressor discharge port 12a to the drive chamber 29 through the auxiliary intake on-off valve V3 can be restricted, thereby delaying the pressure increase to the high pressure PH in the drive chamber 29. In the main communication passage 64, the pressure drop to the low pressure PL in the expansion chamber 34 and the room temperature chamber 36 is not delayed. In this way, during the initial cooling, the rise of the driving force acting on the drive piston 22 can be delayed, as compared with that during the steady-state operation. The movement speed of the displacer 20, and consequently the kinetic energy, can be reduced, thereby suppressing the collision sound of the displacer 20 at the bottom dead center and improving the quietness of the cryocooler 10 in the initial cooling.

[0086] On the other hand, in the steady-state operation, the opening degree of the flow control valve 102 is increased, and the flow restriction by the flow restrictor 100 is released. A sufficient flow rate of the working gas can be ensured in the auxiliary communication passage 66 to optimize the cooling capacity of the cryocooler 10.

[0087] FIG. 3 is a schematic view showing another example of the flow restrictor 100 that can be applied to the cryocooler 10 according to the embodiment. The flow restrictor 100 shown in FIG. 3 may be applied to the cryocooler 10 shown in FIGS. 1 and 2. Therefore, the flow restrictor 100 may be connected between the compressor 12 and the cold head 14, for example, between the auxiliary pressure switching valve 62 and the drive chamber 29, similar to the above-mentioned embodiment. The flow restrictor 100 may operate to restrict the flow rate of the working gas to the cold head 14 (for example, the drive chamber 29) during the initial cooling, as compared with that during the steady-state operation.

[0088] As shown in FIG. 3, the flow restrictor 100 may include a first flow control valve 102a and a second flow control valve 102b that are connected in parallel. Both the first flow control valve 102a and the second flow control valve 102b are provided in the auxiliary communication passage 66. The first flow control valve 102a may be an on-off valve (for example, a solenoid valve). The second flow control valve 102b may be an orifice, for example, a fixed orifice.

[0089] The controller 58 may acquire the operating state of the cryocooler 10 (that is, whether the cryocooler 10 is executing the initial cooling or the steady-state operation), and control the first flow control valve 102a based on the acquired operating state. The controller 58 may close the first flow control valve 102a when the cryocooler 10 is executing the initial cooling, and may open the first flow control valve 102a when the cryocooler 10 is executing the steady-state operation.

[0090] In this manner, in the initial cooling, the first flow control valve 102a is closed, thereby restricting the flow rate of the working gas in the auxiliary communication passage 66. That is, in the initial cooling, the working gas in the auxiliary communication passage 66 flows only through the second flow control valve 102b. In the steady-state operation, the first flow control valve 102a is opened, and the flow restriction in the initial cooling is released. The working gas can pass through the first flow control valve 102a and the second flow control valve 102b.

[0091] In this manner as well, the flow restrictor 100 can reduce the flow rate of the working gas to the drive chamber 29 during the initial cooling, as compared with that during the steady-state operation, thereby delaying the pressure rise in the drive chamber 29. Consequently, the quietness of the cryocooler 10 can be improved. In addition, as compared with the opening degree control of the flow control valve 102 described with reference to FIGS. 1 and 2, the on-off control of the first flow control valve 102a can be advantageous in that the controller 58 can be implemented with a simpler configuration.

[0092] The first flow control valve 102a may be an on-off valve that can be manually operated.

[0093] In addition, the first flow control valve 102a may be a flow control valve whose opening degree is controllable. The second flow control valve 102b may be a flow control valve whose opening degree is controllable. When necessary, an additional orifice or a third flow control valve may be provided upstream and/or downstream of the first flow control valve 102a and/or the second flow control valve 102b.

[0094] FIG. 4 is a schematic view showing another example of the cryocooler 10 according to the embodiment. The cryocooler 10 is, for example, a gas-driven GM cryocooler. The cryocooler 10 includes the compressor 12, the cold head 14, and the flow restrictor 100.

[0095] The cryocooler 10 shown in FIG. 4 has the same configuration as the cryocooler 10 shown in FIGS. 1 and 2 except for a specific configuration of the flow restrictor 100 to be described below. Therefore, the flow restrictor 100 may be connected between the compressor 12 and the cold head 14, for example, between the auxiliary pressure switching valve 62 and the drive chamber 29, similar to the above-mentioned embodiment. The flow restrictor 100 operates to restrict the flow rate of the working gas to the cold head 14 (for example, the drive chamber 29) during the initial cooling, as compared with that during the steady-state operation.

[0096] The flow restrictor 100 includes the flow control valve 102 and a gas volume 104. The flow control valve 102 includes a gas pressure actuator 106 that drives the flow control valve 102 such that the flow control valve 102 opens at a smaller opening degree during the initial cooling, as compared with that during the steady-state operation.

[0097] The flow control valve 102 includes a valve body 108 that defines an opening degree of the flow control valve 102, and a valve housing 110 that accommodates the valve body 108. The valve body 108 includes a tip part 108a and a base end part 108b extending from the tip part 108a. The valve housing 110 includes, on an inside thereof, an orifice 110a that is connected to the auxiliary communication passage 66 and that forms a part of the auxiliary communication passage 66, and a pressure chamber 110b isolated from the orifice 110a by an elastically deformable partition wall 112 (for example, a bellows). The tip part 108a of the valve body 108 is accommodated in the orifice 110a, and the base end part 108b of the valve body 108 is accommodated in the pressure chamber 110b. The opening degree of the flow control valve 102 is defined by the tip part 108a of the valve body 108 at the orifice 110a. The valve body 108 is connected to the valve housing 110 by the partition wall 112 and is movable with respect to the valve housing 110 by the deformation of the partition wall 112. The opening degree of the flow control valve 102 is changed by the movement of the valve body 108 with respect to the valve housing 110. The flow control valve 102 may be, for example, a needle valve, as shown in the drawing.

[0098] The gas volume 104 is thermally coupled to the low temperature portion (for example, the cooling stage 38) of the cold head 14. The gas volume 104 may be attached to the low temperature portion of the cold head 14 to be cooled by the low temperature portion of the cold head 14. The gas volume 104 is filled with, for example, a predetermined gas such as nitrogen gas. The gas that fills the gas volume 104 may be a gas that is condensed (that is, liquefied or solidified) at the cooling temperature of the cryocooler 10 (for example, the above-mentioned standard cooling temperature). The gas volume 104 is connected to the pressure chamber 110b so as to allow the gas to flow between the gas volume 104 and the pressure chamber 110b. For example, the gas volume 104 may be connected to the pressure chamber 110b by a connection flow path 105 such as a pipe.

[0099] The pressure chamber 110b of the valve housing 110 and the gas volume 104 constitute a part of the gas pressure actuator 106. The gas pressure actuator 106 drives the valve body 108 in response to the pressure in the pressure chamber 110b. The gas pressure actuator 106 is configured to open the flow control valve 102 at the first opening degree in the initial cooling and to open the flow control valve 102 at the second opening degree in the steady-state operation. The first opening degree is smaller than the second opening degree.

[0100] Therefore, the gas volume 104 and the pressure chamber 110b are filled with a gas in advance such that the pressure in the pressure chamber 110b at the initial temperature when the initial cooling is started is higher than the pressure of the orifice 110a (that is, the operating pressure of the working gas of the cryocooler 10 in the auxiliary communication passage 66). In this manner, the valve body 108 can move forward from the pressure chamber 110b toward the orifice 110a such that the flow control valve 102 is set to the first opening degree.

[0101] During the initial cooling, the cold head 14 is cooled, and the gas volume 104 is also cooled by the cold head 14. As the gas inside the gas volume 104 is reduced in volume by cooling, the pressure in the pressure chamber 110b correspondingly decreases. In a case where the gas inside the gas volume 104 is condensed by cooling, the pressure in the pressure chamber 110b further decreases. As a result, the pressure in the pressure chamber 110b becomes lower than the pressure at the orifice 110a, and as schematically shown by an arrow 114 in FIG. 4, the valve body 108 moves backward from the orifice 110a toward the pressure chamber 110b while deforming the partition wall 112. Accordingly, the opening degree of the flow control valve 102 increases from the first opening degree to the second opening degree.

[0102] In this manner, the gas pressure actuator 106 can drive the valve body 108 to minimize the opening degree of the flow control valve 102 at the beginning of the initial cooling, while increasing the opening degree of the flow control valve 102 as the initial cooling progresses. Therefore, in this embodiment as well, the flow restrictor 100 operates to restrict the flow rate of the working gas to the drive chamber 29 during the initial cooling, as compared with that during the steady-state operation. In this manner, the flow rate of the working gas to the drive chamber 29 can be reduced, and the pressure rise in the drive chamber 29 can be delayed, thereby delaying the rise of the driving force of the drive piston 22 caused by the gas pressure. As a result, the quietness of the cryocooler 10 can be improved, similar to the above-mentioned embodiment.

[0103] The flow restrictor 100 described with reference to FIG. 4 can be passively operated using the temperature decrease of the cold head 14 in the initial cooling. Unlike the flow restrictor 100 described with reference to FIGS. 1 and 2, it can be advantageous in that control using the controller 58 is not required.

[0104] FIG. 5 is a schematic view showing still another example of the flow restrictor 100 that can be applied to the cryocooler 10 according to the embodiment. As shown in FIG. 5, the flow control valve 102 may include a valve body stopper 116 that defines a maximum opening degree of the flow control valve 102 by the gas pressure actuator 106. In this manner, the opening degree of the flow control valve 102 can be kept constant during the steady-state operation. This can lead to stabilization of the cooling capacity during the steady-state operation of the cryocooler 10.

[0105] The valve body stopper 116 may be provided on the valve housing 110 to come into contact with the valve body 108 and hold the valve body 108 when the flow control valve 102 is set to the second opening degree. As shown in the drawing, the valve body stopper 116 may be provided in the pressure chamber 110b. When the flow control valve 102 is set to the first opening degree, the valve body 108 is separated from the valve body stopper 116 by the forward movement from the pressure chamber 110b toward the orifice 110a.

[0106] The position of the valve body stopper 116 with respect to the valve housing 110 may be adjustable such that the maximum opening degree of the flow control valve 102 can be adjusted as necessary.

[0107] FIG. 6 is a schematic view showing still another example of the flow restrictor 100 that can be applied to the cryocooler 10 according to the embodiment. The flow restrictor 100 may be connected between the compressor 12 and the cold head 14, for example, between the auxiliary pressure switching valve 62 and the drive chamber 29, similar to the above-mentioned embodiment. The flow restrictor 100 operates to restrict the flow rate of the working gas to the cold head 14 (for example, the drive chamber 29) during the initial cooling, as compared with that during the steady-state operation.

[0108] The flow restrictor 100 includes the flow control valve 102. The flow control valve 102 includes the gas pressure actuator 106 that drives the flow control valve 102 such that the flow control valve 102 opens at a smaller opening degree during the initial cooling, as compared with that during the steady-state operation.

[0109] The flow control valve 102 includes the valve body 108 that defines the opening degree of the flow control valve 102, and the valve housing 110 that accommodates the valve body 108. The valve body 108 includes the tip part 108a and the base end part 108b extending from the tip part 108a. The valve housing 110 includes, on the inside thereof, the orifice 110a that is connected to the auxiliary communication passage 66 and that forms a part of the auxiliary communication passage 66, and the pressure chamber 110b isolated from the orifice 110a. The tip part 108a of the valve body 108 is accommodated in the orifice 110a, and the base end part 108b of the valve body 108 is accommodated in the pressure chamber 110b. The opening degree of the flow control valve 102 is defined by the tip part 108a of the valve body 108 at the orifice 110a. The valve body 108 is connected to the valve housing 110 by an elastic member 113 such as a spring and is movable with respect to the valve housing 110. The opening degree of the flow control valve 102 is changed by the movement of the valve body 108 with respect to the valve housing 110. The flow control valve 102 may be, for example, a needle valve, as shown in the drawing.

[0110] In the flow restrictor 100 shown in FIG. 6, the pressure chamber 110b of the valve housing 110 is connected to the compressor 12, instead of the gas volume 104 shown in FIG. 4. More specifically, the pressure chamber 110b includes a first chamber 118 and a second chamber 120 that are isolated from each other. The second chamber 120 has a lower pressure than the first chamber 118. Therefore, for example, the first chamber 118 is connected to the compressor discharge port 12a, and the second chamber 120 is connected to the compressor suction port 12b. The second chamber 120 is disposed between the first chamber 118 and the orifice 110a. The second chamber 120 is isolated from the orifice 110a.

[0111] The valve body 108 is driven in response to a differential pressure between the first chamber 118 and the second chamber 120. When the temperature of the cryocooler 10 is relatively high, such as at the beginning of the initial cooling, the differential pressure between the pressure (high pressure PH) of the compressor discharge port 12a and the pressure (low pressure PL) of the compressor suction port 12b is relatively large. Therefore, the differential pressure between the first chamber 118 and the second chamber 120 acts to compress the elastic member 113, thereby allowing the valve body 108 to move forward from the pressure chamber 110b toward the orifice 110a such that the flow control valve 102 is set to the first opening degree.

[0112] During the initial cooling, as mentioned above, the operating pressure of the cryocooler 10 gradually decreases. Accordingly, the differential pressure between the first chamber 118 and the second chamber 120 is also reduced, and a restoring force of the elastic member 113 is also added, thereby allowing the valve body 108 to move backward from the orifice 110a toward the pressure chamber 110b. As a result, the opening degree of the flow control valve 102 increases from the first opening degree to the second opening degree.

[0113] In this manner, the gas pressure actuator 106 can drive the valve body 108 to minimize the opening degree of the flow control valve 102 at the beginning of the initial cooling, while increasing the opening degree of the flow control valve 102 as the initial cooling progresses. Therefore, in this embodiment as well, the flow restrictor 100 operates to restrict the flow rate of the working gas to the drive chamber 29 during the initial cooling, as compared with that during the steady-state operation. In this manner, the flow rate of the working gas to the drive chamber 29 can be reduced, and the pressure rise in the drive chamber 29 can be delayed, thereby delaying the rise of the driving force of the drive piston 22 caused by the gas pressure. As a result, the quietness of the cryocooler 10 can be improved, similar to the above-mentioned embodiment.

[0114] The flow restrictor 100 described with reference to FIG. 6 can also be passively operated using the temperature decrease of the cold head 14 in the initial cooling, similar to the flow restrictor 100 shown in FIG. 4. Unlike the flow restrictor 100 described with reference to FIGS. 1 and 2, it can be advantageous in that control using the controller 58 is not required.

[0115] Instead of connecting the second chamber 120 to the compressor suction port 12b, the second chamber 120 may be open to the atmosphere. In this manner as well, since the first chamber 118 is connected to the compressor discharge port 12a, the differential pressure between the first chamber 118 and the second chamber 120 can be ensured.

[0116] Additionally, the valve body stopper 116 described with reference to FIG. 5 may be applied to the flow control valve 102 shown in FIG. 6. In this case, the valve body stopper 116 may be provided in the first chamber 118 of the pressure chamber 110b.

[0117] Hereinabove, the present invention has been described based on the examples. The present invention is not limited to the above-described embodiment, and various design changes are possible, and it will be understood by those skilled in the art that various modification examples are possible and such modification examples also fall within the scope of the present invention. Various features described in relation to a certain embodiment are also applicable to other embodiments. A new embodiment resulting from combination will have the effects of each of the embodiments that are combined.

[0118] The above-mentioned embodiment has been described with the example in which the flow restrictor 100 is connected between the auxiliary pressure switching valve 62 and the drive chamber 29, but the present invention is not limited thereto. The flow restrictor 100 may be connected between the main pressure switching valve 60 and the cold head 14 (for example, the room temperature chamber 36). In this manner as well, the flow rate of the working gas to the cold head 14 can be restricted during the initial cooling, as compared with that during the steady-state operation, and the rise of the gas pressure for driving the displacer 20 can be delayed. As a result, the quietness of the cryocooler 10 can be improved.

[0119] The above-mentioned embodiment has been described with the example in which the cryocooler 10 is a collar bumper type, but the present invention is not limited thereto. That is, the cryocooler 10 need not include the collar 70, the upper bumper 74, and the lower bumper 76, and the flow restrictor 100 according to the embodiment may be applied to such a cryocooler 10.

[0120] The above-mentioned embodiment has been described with the example in which the cryocooler 10 is a gas-driven GM cryocooler, but the present invention is not limited thereto. The flow restrictor 100 according to the embodiment can also be applied to, for example, other gas-driven cryocoolers such as a Solvay cryocooler.

[0121] The present invention has been described using specific terminology based on embodiments, but the embodiments merely show one aspect of the principles and applications of the present invention, and many modification examples and changes in disposition are allowed within the scope that does not depart from the concept of the present invention defined in the claims.

[0122] It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the invention. Additionally, the modifications are included in the scope of the invention.