Cryogenic Refrigerator, and Method for Cooling Down Cryogenic Refrigerator

This is a bypass continuation of International PCT Application No. PCT/JP2024/000833, filed on Jan. 15, 2024, which claims priority to Japanese Patent Application No. 2023-014704, filed on Feb. 2, 2023, which are incorporated by reference herein in their entirety.

Certain embodiments of the present invention relate to a cryocooler and a cool-down method for a cryocooler.

A cryocooler is used to cool various objects such as superconducting devices, measuring instruments, and samples used in a cryogenic environment.

According to an aspect of the present invention, there is provided a cryocooler including: a cold head including a first cooling stage and a second cooling stage cooled to a lower temperature than the first cooling stage; a heater thermally coupled to the first cooling stage; a first temperature sensor that measures a first temperature of the first cooling stage; and a controller configured to perform a cool-down operation of the cold head to cool the first cooling stage from an initial temperature to a first final target temperature and to cool the second cooling stage from the initial temperature to a second final target temperature which is lower than the first final target temperature. The controller is configured to, during the cool-down operation, acquire the first temperature of the first cooling stage from the first temperature sensor, and control the heater such that the first temperature of the first cooling stage follows an intermediate target temperature which is lower than the initial temperature and higher than the first final target temperature.

According to another aspect of the present invention, there is provided a cool-down method for a cryocooler, the cool-down method including: performing a cool-down operation of a cold head to cool a first cooling stage of the cold head from an initial temperature to a first final target temperature and to cool a second cooling stage of the cold head from the initial temperature to a second final target temperature which is lower than the first final target temperature; and during the cool-down operation, applying a heat load to the first cooling stage such that a temperature of the first cooling stage follows an intermediate target temperature which is lower than the initial temperature and higher than the first final target temperature.

In order to cool an object with the cryocooler, first, the cryocooler has to be activated to cool the cryocooler from an initial temperature to a target cryogenic temperature. Such initial cooling of the cryocooler is also referred to as cool-down. Since the cool-down is only a preparation for starting the cooling of the object, it is desirable that a time required for the cool-down is as short as possible.

It is desirable to shorten a cool-down time of a cryocooler.

Hereinafter, an embodiment for carrying out the present invention will be described in detail with reference to the drawings. In the description and the drawings, the same or equivalent components, members, and processes are denoted by the same reference numerals, and overlapping description is omitted as appropriate. The scale and the shape of each of parts illustrated in the drawings are set for convenience to make the description easy to understand, and are not to be interpreted as limiting unless stated otherwise. The embodiment is merely an example and does not limit the scope of the present invention. All features described in the embodiment or combinations thereof are not necessarily essential to the present invention.

are views schematically showing a cryocooleraccording to the embodiment. The cryocooleris, for example, a two-stage type Gifford-McMahon (GM) cryocooler.shows an appearance of the cryocooler, andshows an internal structure of the cryocooler.

The cryocoolercan provide cryogenic cooling for various applications. For example, the cryocoolermay be used to cool a superconducting coil of a superconducting magnet device. The superconducting magnet device can be mounted on a high magnetic field utilization device as a magnetic field source of, for example, a single crystal pulling-up device, a nuclear magnetic resonance (NMR) system, a magnetic resonance imaging (MRI) system, an accelerator such as a cyclotron, a high energy physics system such as a nuclear fusion system, or another high magnetic field utilization device (not shown) to generate a high magnetic field required for the high magnetic field utilization device.

The cryocoolerincludes a compressorand a cold head. The compressoris configured to collect a working gas of the cryocoolerfrom the cold head, pressurize the collected working gas, and supply the working gas to the cold headagain. The cold headis also called an expander. The working gas is also called a refrigerant gas, and other suitable gases may be used although a helium gas is typically used.

The cold headincludes a cryocooler cylinder, a displacer assembly, and a cryocooler housing. The cryocooler housingis coupled to the cryocooler cylinder, thereby forming a hermetic container that accommodates the displacer assembly. An internal volume of the cryocooler housingmay be connected to a low pressure side of the compressorand be maintained at a low pressure.

The cryocooler cylinderincludes a first cylinderand a second cylinder. As an example, the first cylinderand the second cylindereach are members having a cylindrical shape, and the second cylinderhas a smaller diameter than the first cylinder. The first cylinderand the second cylinderare coaxially disposed, and a lower end of the first cylinderis rigidly connected to an upper end of the second cylinder

The displacer assemblyincludes a first displacerand a second displacer. As an example, the first displacerand the second displacereach are members having a cylindrical shape, and the second displacerhas a smaller diameter than the first displacer. The first displacerand the second displacerare coaxially disposed.

The first displaceris accommodated in the first cylinder, and the second displaceris accommodated in the second cylinder. The first displacercan reciprocate in an axial direction along the first cylinder, and the second displacercan reciprocate in the axial direction along the second cylinder. The first displacerand the second displacerare connected to each other, and move integrally.

In the present specification, in order to describe a positional relationship between components of the cryocooler, for convenience of description, a side close to a top dead center of axial reciprocation of a displacer will be referred to as “up” and a side close to a bottom dead center will be referred to as “down”. The top dead center is a position of the displacer at which a volume of an expansion space is maximum, and the bottom dead center is a position of the displacer at which the volume of the expansion space is minimum. During an operation of the cryocooler, a temperature gradient occurs in which the temperature decreases from an upper side to a lower side in the axial direction, so the upper side can also be referred to as a high temperature side and the lower side as a low temperature side.

The first displaceraccommodates a first regenerator. The first regeneratoris formed by filling a tubular main body of the first displacerwith a metal mesh made of copper or the like or other suitable first regenerator materials. In addition, the second displaceraccommodates the second regenerator. The second regeneratoris formed by filling a tubular main body portion of the second displacerfor example, with a non-magnetic regenerator material such as bismuth, a magnetic regenerator material such as HoCu, or other suitable second regenerator materials. The second regenerator material may be formed in a granular shape.

The displacer assemblyforms an upper chamber, a first expansion chamber, and a second expansion chamberinside the cryocooler cylinder. The cold headincludes a first cooling stageand a second cooling stagefor heat exchange with a desired object or medium to be cooled by the cryocooler. The first cooling stageis fixed to a lower portion of the first cylinderto surround the first expansion chamber, and the second cooling stageis fixed to a lower portion of the second cylinderto surround the second expansion chamber. The upper chamberis formed between an upper end portion of the first displacerand an upper portion of the first cylinder. The first expansion chamberis formed between a lower end portion of the first displacerand the first cooling stage. The second expansion chamberis formed between a lower lid portion of the second displacerand the second cooling stage.

The first cooling stageand the second cooling stageare formed of, for example, a metal material such as copper or other materials having high thermal conductivity. The cryocooler cylinderis usually formed of a metal material having a lower thermal conductivity than the first cooling stageand the second cooling stage, for example, stainless steel.

The first regeneratoris connected to the upper chamberthrough a working gas flow pathformed in the upper end portion of the first displacer, and is connected to the first expansion chamberthrough a working gas flow pathformed in the lower end portion of the first displacer. The second regeneratoris connected to the first regeneratorthrough a working gas flow pathformed from the lower end portion of the first displacerto an upper end portion of the second displacer. In addition, the second regeneratoris connected to the second expansion chamberthrough a working gas flow pathformed in the lower lid portion of the second displacer

A first sealand a second sealmay be provided so that working gas flows between the first expansion chamber, the second expansion chamberand the room temperature chamberare guided to the first regeneratorand the second regenerator, rather than to a clearance between the cryocooler cylinderand the displacer assembly. The first sealmay be mounted on the upper end portion of the first displacerto be disposed between the first displacerand the first cylinder. The second sealmay be mounted on the upper end portion of the second displacerto be disposed between the second displacerand the second cylinder

In addition, the cold headincludes a pressure switching valveand a drive motor. The pressure switching valveis accommodated in the cryocooler housing, and the drive motoris attached to the cryocooler housing.

As shown in, the pressure switching valveincludes a high pressure valveand a low pressure valve, and is configured to generate periodic pressure fluctuations in the cryocooler cylinder. A working gas discharge port of the compressoris connected to the upper chambervia the high pressure valve, and a working gas suction port of the compressoris connected to the upper chambervia the low pressure valve. The high pressure valveand the low pressure valveare configured to open and close selectively and alternately (that is, such that when one is open, the other is closed). A high pressure (for example, 2 to 3 MPa) working gas is supplied from the compressorto the cold headthrough the high pressure valve, and a low pressure (for example, 0.5 to 1.5 MPa) working gas is collected from the cold headto the compressorthrough the low pressure valve. To facilitate understanding, a direction in which the working gas flows is indicated by arrows in.

The drive motoris provided to drive reciprocation of the displacer assembly. The drive motoris connected to a displacer drive shaftvia a motion conversion mechanismsuch as a Scotch yoke mechanism. The motion conversion mechanismis accommodated in the cryocooler housinglike the pressure switching valve. The displacer drive shaftextends from the motion conversion mechanisminto the upper chamberthrough the cryocooler housing, and is fixed to the upper end portion of the first displacer. A third sealis provided to prevent a leakage of the working gas from the upper chamberto the cryocooler housing(which may be maintained at a low pressure as described above). The third sealmay be mounted on the cryocooler housingto be disposed between the cryocooler housingand the displacer drive shaft.

When the drive motoris driven, a rotational output of the drive motoris converted into axial reciprocation of the displacer drive shaftby the motion conversion mechanism, and the displacer assemblyreciprocates in the cryocooler cylinderin the axial direction. In addition, the drive motoris connected to the high pressure valveand the low pressure valveto selectively and alternately open and close these valves.

When the compressorand the drive motorare operated, the cryocoolergenerates periodic volume fluctuations in the first expansion chamberand the second expansion chamberand pressure fluctuations of the working gas synchronized therewith, thereby forming a refrigeration cycle, and the first cooling stageand the second cooling stageare cooled to a desired cryogenic temperature. The first cooling stageis cooled to a first cooling temperature in a first temperature range, and the second cooling stageis cooled to a second cooling temperature in a second temperature range. The first temperature range may be, for example, a temperature range of about 30 K to about 80 K. The second temperature range is lower than the first temperature range, and may be, for example, a temperature range of about 3 K to about 20 K. The first cooling temperature and the second cooling temperature can be selected to various temperature values according to the use of the cryocooler. For example, in a case where the cryocooleris used to cool a superconducting coil, the second cooling temperature is typically about 4.2 K or lower.

In this embodiment, the cold headincludes a heaterthermally coupled to the first cooling stage. As shown in, the heatermay be mounted on a surface of the first cooling stageor inside the first cooling stage. The heateris configured to apply a heat load corresponding to an output thereof to the first cooling stageand heat the first cooling stage. The heatermay be a heating device such as an electric heater.

The cold headincludes a first temperature sensorand a second temperature sensor. The first temperature sensoris configured to measure a first temperature of the first cooling stageand to generate a first measured temperature signal Sindicating the first temperature. The second temperature sensoris configured to measure a second temperature of the second cooling stageand to generate a second measured temperature signal Sindicating the second temperature. As shown in, the first temperature sensormay be attached to the surface of the first cooling stageor to the inside of the first cooling stage. The second temperature sensormay be attached to a surface of the second cooling stageor to an inside of the second cooling stage.

In addition, a controllerthat controls the cryocooleris provided. The controllermay be configured to switch on and off the cryocooler(that is, on and off the compressorand the drive motorof the cold head).

The controlleris electrically connected to the first temperature sensorand the second temperature sensorto acquire the first measured temperature signal Sand the second measured temperature signal S. In addition, the controlleris electrically connected to the heaterto switch on and off the heaterand/or to control an output of the heater. As will be described later, the controllermay be configured to switch on and off the heaterand/or control the output of the heater, based on the measured temperature of the first temperature sensorand/or the second temperature sensor

In the shown example, the controlleris provided separately from the compressorand the cold headand is connected to the compressorand the cold head, but the controlleris not limited thereto. The controllermay be mounted on the compressor. Alternatively, the controllermay be provided in the cold head, for example, by being mounted on the drive motor.

The controlleris implemented as a hardware configuration by elements and circuits including a central processing unit (CPU) and a memory of a computer, and as a software configuration by a computer program or the like. However, in the drawings, the configurations are illustrated as functional blocks implemented through cooperation therebetween. Those skilled in the art may understand that these functional blocks can be implemented in various forms by combining hardware and software.

The cryocoolercan perform a steady operation and a cool-down operation prior to the steady operation. The cool-down operation is an operation mode of the cryocoolerin which the cold headis rapidly cooled from an ambient temperature (for example, room temperature) to a desired cryogenic temperature as preparation for the steady operation, and can also be referred to as initial cooling of the cryocooler. The steady operation is an operation mode of the cryocoolerin which the cryocoolermaintains a state of being cooled to the cryogenic temperature by the cool-down operation.

Therefore, an object to be cooled by the cryocooleris cooled from the ambient temperature to a cryogenic temperature at which the object to be cooled can operate by the cool-down operation of the cryocooler, and then operates while being maintained at the cryogenic temperature by the steady operation of the cryocooler. For example, in the case of the superconducting magnet device, the superconducting coil is cooled to a cryogenic temperature equal to or lower than a superconducting transition temperature from the ambient temperature by a cool-down operation. Thereafter, during the steady operation of the cryocooler, the superconducting coil cooled to the cryogenic temperature is powered and capable of generating a strong magnetic field.

In the cool-down operation, the cold headis operated to cool the first cooling stagefrom an initial temperature to a first final target temperature and to cool the second cooling stagefrom the initial temperature to a second final target temperature which is lower than the first final target temperature. The controlleris configured to perform a cool-down operation of the cold head. The initial temperature may be the ambient temperature (for example, room temperature) of the cryocooler. The first final target temperature may be selected from the above-described first temperature range, and the second final target temperature may be selected from the above-described second temperature range. The first final target temperature and the second final target temperature may be set in advance and stored in the controller.

Incidentally, the present inventor has recognized that there is a case where the first cooling stageis cooled more rapidly than the second cooling stageduring the cool-down operation of the cold head, and in a case where such pre-cooling of the first cooling stageoccurs, the cooling of the second cooling stageis delayed, resulting in an increase in time required for the cool-down.

In general, a cooling capacity in the first cooling stageis larger than a cooling capacity in the second cooling stagethat is cooled to a lower temperature. In addition to such a basic difference in cooling capacity, a cooling rate of each of the first cooling stageand the second cooling stageis also influenced by a difference in weight (more accurately, heat capacity) of an object to be cooled in each cooling stage. Therefore, in a case where an object to be cooled by the second cooling stageis heavier than an object to be cooled by the first cooling stage, the time required for cooling of the second cooling stageis longer than the time required for cooling the first cooling stage. That is, as illustrated in, during the cool-down operation, a first temperature Tof the first cooling stageis cooled and lowered earlier than the second temperature Tof the second cooling stage.

In addition, a density of the working gas (for example, helium gas) in the first expansion chamberis increased due to the pre-cooling of the first cooling stage. The increase in the density of the working gas promotes accumulation of the working gas supplied to the cold head, in the first expansion chamber, and has an effect of absorbing the working gas into the first expansion chamber, so to speak. That is, this means that the amount of the working gas distributed to the second expansion chamberby passing through the first expansion chamberis reduced. Therefore, in a case where the first cooling stageis cooled prior to the second cooling stage, the cooling capacity of the second cooling stagemay be lowered due to the increase in the density of the working gas in the first expansion chamber. In this manner, there is a concern that the cool-down time is further extended.

In a case where the cryocooleris used to cool the superconducting magnet device, the superconducting coil is cooled by the second cooling stage. A cooling load of the second cooling stagetends to be larger than a cooling load of the first cooling stage. Therefore, the problem of an increase in the cool-down time described above is more likely to become apparent.

Therefore, in this embodiment, a cool-down method for the cryocoolerincludes performing the cool-down operation of the cold headand applying a heat load to the first cooling stagesuch that the temperature of the first cooling stagefollows an intermediate target temperature during the cool-down operation. As described above, by performing the cool-down operation, the first cooling stageis cooled from the initial temperature to the first final target temperature, and the second cooling stageis cooled from the initial temperature to the second final target temperature which is lower than the first final target temperature. The intermediate target temperature is lower than the initial temperature and higher than the first final target temperature. The heateris used to apply a heat load to the first cooling stage.

In this way, the first temperature of the first cooling stageis temporarily maintained at or near the intermediate target temperature during the cool-down operation of the cold head. That is, the first cooling stageis intentionally maintained at a temperature which is higher than the first final target temperature, and as a result, the cooling rate of the first cooling stageis reduced. Due to such an intentional cooling delay of the first cooling stage, an imbalance in the cooling rate between the first cooling stageand the second cooling stagecan be eliminated or alleviated, and thus the increase in the cool-down time described above can be coped with.

is a flowchart illustrating the cool-down method according to the embodiment. This method is performed by the controller. As shown in, first, the first temperature Tof the first cooling stageand the second temperature Tof the second cooling stageare measured (S). The first measured temperature signal Sis output from the first temperature sensorto the controller, and the second measured temperature signal Sis output from the second temperature sensorto the controller. The controllerreceives the first measured temperature signal Sfrom the first temperature sensor, and acquires the first temperature Tfrom the first measured temperature signal S. In addition, the controllerreceives the second measured temperature signal Sfrom the second temperature sensor, and acquires the second temperature Tfrom the second measured temperature signal S.

Next, a condition for ending the cool-down is determined (S). The condition for ending the cool-down is that the first temperature Tof the first cooling stageis equal to or lower than the first final target temperature and the second temperature Tof the second cooling stageis equal to or lower than the second final target temperature. The controlleris configured to compare the first temperature Tto the first final target temperature, compare the second temperature Tto the second final target temperature, and determine whether or not the condition for ending the cool-down is satisfied, based on comparison results.

In a case where the condition for ending the cool-down is satisfied (Yes in S), the controllerends the cool-down operation. In this case, as described above, the cryocoolermay transition to the steady operation following the end of the cool-down operation.

In a case where the condition for ending the cool-down is not satisfied (No in S), the cool-down operation is continued. In this case, the controllermay compare the second temperature Tof the second cooling stageto a predetermined temperature threshold Tx (S). This comparison is performed to determine whether or not the second cooling stageis sufficiently cooled toward the second final target temperature. For example, the temperature threshold Tx may be determined in advance to be a temperature value close to the first final target temperature of the first cooling stage, and may be, for example, 50 K or lower. The temperature threshold Tx may be equal to or lower than the first final target temperature. The temperature threshold Tx may be higher than the second final target temperature.

In a case where the second temperature Tof the second cooling stageexceeds the above-described temperature threshold Tx (No in S), as will be described later, the controllercontrols the heatersuch that the first temperature Tof the first cooling stagefollows an intermediate target temperature Tm.

In order to control the heater, the controllerfirst sets the intermediate target temperature Tm (S). The intermediate target temperature Tm is selected from a temperature range which is lower than the initial temperature of the cool-down and higher than the first final target temperature.

Plain text: The controllermay be configured to change the intermediate target temperature Tm during the cool-down operation. The cooling rate of the first cooling stageduring the cool-down operation can be controlled by changing the intermediate target temperature Tm.

For example, the controllermay be configured to set the intermediate target temperature Tm based on the second temperature Tof the second cooling stage. In this way, the intermediate target temperature Tm can be set to be close to the second temperature T, and as a result, it is possible to avoid the first temperature of the first cooling stagelargely deviating from the second temperature Tof the second cooling stage.

In this case, the intermediate target temperature Tm may be set to coincide with the second temperature Tof the second cooling stage, or may be set to a temperature which is higher than the second temperature T(for example, a temperature value obtained by adding a margin value to the second temperature T). The margin value may be, for example, a value within 10 K. In this way, it is possible to prevent the first temperature of the first cooling stagefrom falling below the second temperature Tof the second cooling stage, or to reduce such a probability.