Patent Description:
In the related art, there is known a cryogenic refrigerator using a phenomenon in which heat is absorbed in a case where helium-<NUM> (3He) of a liquid is dissolved and diluted in helium-<NUM> (4He) of the liquid, called a dilution refrigerator. The dilution refrigerator can provide cryogenic cooling equal to or lower than <NUM>. An example of the dilution refrigerator is of a type in which a small-sized mechanical refrigerator, such as a Gifford-McMahon (GM) cryocooler, is mounted for precooling (for example, <CIT>). <CIT> discloses a pulse tube refrigerator including a heat exchanger, wherein the warm end heat exchanger is provided with a secondary cooling mechanism to improve efficiency of the pulse tube refrigerator. Moreover, there is known a pulse tube refrigerator including a phase control mechanism (for example, <CIT>).

For example, in advanced usage, such as cooling of a superconducting element of a quantum computer, there is a case where a dilution refrigerator that realizes cryogenic cooling of an order of millikelvin (mK) is used. In such cryogenic cooling, even vibration acceleration due to the operation of the GM cryocooler may turn into a heat source. Therefore, it has been proposed that, instead of the GM cryocooler, a pulse tube cryocooler that is operable with less vibration is employed as a precooling refrigerator of the dilution refrigerator.

In general, at the start of the dilution refrigerator, the precooling refrigerator is cooled from an ambient temperature (for example, a normal temperature of about <NUM>) to an intended cryogenic temperature (for example, a liquid helium temperature of about <NUM>). Such initial cooling is also called cool-down. Because the dilution refrigerator is a comparatively large-sized device, the heat capacity of a low-temperature section cooled by the precooling refrigerator tends to be increased, and a precooling refrigerator having a comparatively large cooling capacity (for example, a cooling capacity of more than <NUM> W at <NUM>) is desirably employed. For this reason, in cool-down, a comparatively large amount of heat corresponding to the cooling capacity of the precooling refrigerator is generated from an adiabatic compression process in a refrigeration cycle of the precooling refrigerator. The heat is radiated from a cold head high-temperature end of the precooling refrigerator to the outside. In a dilution refrigerator of existing design, a cold head high-temperature end of a GM cryocooler is exposed to outside air, and required heat radiation is obtained by natural convection cooling.

Note that the present inventors have recognized that, in a case where a pulse tube cryocooler is mounted as a precooling refrigerator in a dilution refrigerator, heat radiation in cool-down may be insufficient. There is a concern that a cold head high-temperature end of the pulse tube cryocooler is heated to a considerably high temperature (for example, higher than an environmental temperature by tens of °C) due to insufficient heat radiation, and due to the heating of the cold head high-temperature end of the pulse tube cryocooler, a time required for cooling down the precooling refrigerator is extended. Because the cool-down of the precooling refrigerator is only part of preparation work for cooling a desired object to be cooled to a cryogenic temperature by the dilution refrigerator, it is desirable that the required time is as short as possible.

An exemplary object of a certain aspect of the present invention is to promote heat radiation from a cold head of a pulse tube cryocooler.

According to a first aspect of the present invention, there is provided a pulse tube cryocooler as defined in claim <NUM>. It includes a cold head including a pulse tube and a radiator thermally coupled to a high-temperature end of the pulse tube, and a forced cooling device that is configured to forcedly cool the radiator in cool-down of the pulse tube cryocooler from an ambient temperature to a cryogenic temperature, wherein the forced cooling device includes a sensor that detects a state of the pulse tube cryocooler, an air-cooled or liquid-cooled cooler that cools the radiator, and a controller configured to determine whether or not the pulse tube cryocooler is in the cool-down, based on an output of the sensor, and operate the cooler in the cool-down.

According to a second aspect of the present invention, there is provided a method for cooling down a pulse tube cryocooler as defined in claim <NUM>. The pulse tube cryocooler includes a cold head including a pulse tube, a sensor detecting a state of the pulse tube cryocooler, an air-cooled or liquid-cooled cooler, and a radiator thermally coupled to a high-temperature end of the pulse tube. The method includes performing cool-down of the pulse tube cryocooler from an ambient temperature to a cryogenic temperature, detecting a state of the pulse tube cryocooler, determining whether or not the pulse tube cryocooler is in the cool-down, based on an output of the sensor, operating the cooler in the cool-down of the pulse tube cryocooler, and forcedly cooling the radiator in the cool-down of the pulse tube cryocooler.

According to the present invention, it is possible to promote heat radiation from the cold head of the pulse tube 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 will be represented by the same reference numerals, and overlapping description will be omitted as appropriate. The scale and the shape of each of parts shown 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 scope of the present invention is only defined by the appended claims. All features described in the embodiment or combinations thereof are not necessarily essential to the present invention.

<FIG> is a diagram schematically showing a cryogenic device <NUM> according to an embodiment of the present invention. The cryogenic device <NUM> is configured as a dilution refrigerator, and includes a pulse tube cryocooler <NUM> for precooling the dilution refrigerator. Though details will be described below, the pulse tube cryocooler <NUM> includes a cold head <NUM> and a forced cooling device <NUM>. A radiator <NUM> is provided at a high-temperature end of the cold head <NUM>, and the forced cooling device <NUM> is configured to forcedly cool the radiator <NUM> in cool-down of the pulse tube cryocooler <NUM> from an ambient temperature to a cryogenic temperature.

As shown in <FIG>, the cryogenic device <NUM> includes a vacuum chamber <NUM>, a first heat shield <NUM> and a second heat shield <NUM>, and a helium circulation circuit <NUM> that operates as a dilution refrigerator.

The vacuum chamber <NUM> is an adiabatic vacuum chamber that provides a cryogenic temperature vacuum environment suitable for the dilution refrigerator, and is also called a cryostat. The vacuum chamber <NUM> is a casing of the dilution refrigerator. In general, the vacuum chamber <NUM> has a cylindrical shape, and includes a top plate and a bottom plate having a substantially flat circular shape, and a cylindrical sidewall that connects the top plate and the bottom plate. The pulse tube cryocooler <NUM> is provided on, for example, the top plate of the vacuum chamber <NUM>. The vacuum chamber <NUM> is formed of, for example, a metallic material, such as stainless steel, or other suitable high strength materials to withstand ambient pressure (for example, atmospheric pressure).

To thermally protect a low-temperature section of the helium circulation circuit <NUM> from an external environment and radiant heat from the vacuum chamber <NUM>, the first heat shield <NUM> and the second heat shield <NUM> are disposed to surround the low-temperature section of the helium circulation circuit <NUM> in the vacuum chamber <NUM>. The heat shield is formed of, for example, a metallic material, such as copper, or other materials having high thermal conductivity. The first heat shield <NUM> is cooled to a first cooling temperature, for example, less than <NUM> (for example, about <NUM> to <NUM>), and the second heat shield <NUM> is disposed inside the first heat shield <NUM> and is cooled to a second cooling temperature lower than the first cooling temperature, for example, about <NUM> to <NUM>. For cooling, the first heat shield <NUM> is thermally coupled to a first cooling stage 114a of the pulse tube cryocooler <NUM>, and the second heat shield <NUM> is thermally coupled to a second cooling stage 114b of the pulse tube cryocooler <NUM>.

The helium circulation circuit <NUM> includes a pump <NUM> that circulates 3He gas. The pump <NUM> is, for example, a vacuum pump, and is disposed outside the vacuum chamber <NUM>, that is, in the ambient environment. 3He gas at an ambient temperature (for example, room temperature) delivered by the pump <NUM> is sent into an outward-side flow path 20a of the helium circulation circuit <NUM> through a trap <NUM>.

The outward-side flow path 20a is provided with a precooling heat exchanger <NUM>, a 3He condenser <NUM>, and main impedance <NUM>. The precooling heat exchanger <NUM> is thermally coupled to the second cooling stage 114b of the pulse tube cryocooler <NUM>, and cools 3He gas to the above-described second cooling temperature. The outward-side flow path 20a enters a return pipe <NUM> that is a part of an inward-side flow path 20b extending from a fractional distillation chamber <NUM> to the pump <NUM>, downstream of the precooling heat exchanger <NUM>, and the 3He condenser <NUM> and the main impedance <NUM> are disposed in the return pipe <NUM>. 3He gas cooled by the precooling heat exchanger <NUM> is condensed and liquefied by the 3He condenser <NUM> and the main impedance <NUM>.

The outward-side flow path 20a is further provided with a first heat exchanger <NUM>, sub-impedance <NUM>, and a second heat exchanger <NUM>, and the outward-side flow path 20a is connected to a mixing chamber <NUM> in front. The first heat exchanger <NUM> is provided in the fractional distillation chamber <NUM>, and the sub-impedance <NUM> and the second heat exchanger <NUM> are provided outside the fractional distillation chamber <NUM>. The second heat exchanger <NUM> is provided between the fractional distillation chamber <NUM> and the mixing chamber <NUM>, and is configured to exchange heat between a flow path entering the mixing chamber <NUM> and a flow path exiting the mixing chamber <NUM>.

Liquid 3He liquefied by the 3He condenser <NUM> and the main impedance <NUM> is delivered to the first heat exchanger <NUM>. The fractional distillation chamber <NUM> selectively extracts 3He from a 3He-4He mixed solution using a difference in saturated vapor pressure of 3He and 4He, and is maintained at a temperature of about <NUM> to <NUM>, for example. Liquid 3He delivered to the first heat exchanger <NUM> is cooled to a cooling temperature of the fractional distillation chamber <NUM> by heat exchange with a liquid in the fractional distillation chamber <NUM>. Liquid 3He is further cooled (for example, about 100mK) by the second heat exchanger <NUM> by way of the sub-impedance <NUM> and is delivered to the mixing chamber <NUM>.

Liquid helium of the mixing chamber <NUM> is separated into two phases of a dense phase of <NUM>% 3He and a dilute phase of 4He-<NUM>% 3He in which 3He is blended in 4He, and based on a difference in density, an upper phase is a dense phase (3He liquid) and a lower phase is a dilute phase (4He-<NUM>. <NUM>% 3He liquid). Heat absorption occurs in a case where 3He in a dense phase is blended in a dilute phase, and tens of mK or a lower temperature is generated. A desired object to be cooled is disposed in the mixing chamber <NUM>. In this manner, the dilution refrigerator can provide cryogenic cooling of an order of millikelvin (mK).

<FIG> is a diagram schematically showing the pulse tube cryocooler <NUM> according to the embodiment. Referring to <FIG> and <FIG>, in the embodiment, the pulse tube cryocooler <NUM> is a GM type four-valve two-stage pulse tube cryocooler, and includes a two-stage cold head <NUM>, a valve unit <NUM>, and a compressor <NUM>. The pulse tube cryocooler <NUM> is configured as a valve unit separation type in which the valve unit <NUM> is disposed to be separated from the cold head <NUM>.

The cold head <NUM> includes a first-stage pulse tube 110a, a first-stage regenerator 112a, the first cooling stage 114a, a second-stage pulse tube 110b, a second-stage regenerator 112b, the second cooling stage 114b, a top flange <NUM>, and the radiator <NUM>.

As shown in <FIG>, the cold head <NUM> is provided in the vacuum chamber <NUM> by attaching the top flange <NUM> to the vacuum chamber <NUM>. In many cases, the cold head <NUM> is provided to be attachable to and detachable from the top plate or the top of the vacuum chamber <NUM> in such a manner that a tube axis direction of pulse tubes (110a, 110b) matches a vertical direction, and the pulse tubes, the regenerators (112a, 112b), and the cooling stages (114a, 114b) are disposed in the vacuum chamber <NUM>. The cold head <NUM> may be provided in the vacuum chamber <NUM> in other postures and dispositions.

The first-stage pulse tube 110a and the first-stage regenerator 112a connect the top flange <NUM> to the first cooling stage 114a, and the second-stage pulse tube 110b and the second-stage regenerator 112b connect the top flange <NUM> to the second cooling stage 114b. The second-stage regenerator 112b is connected in series to the first-stage regenerator 112a. The two regenerators, the first-stage pulse tube 110a, and the second-stage pulse tube 110b are disposed in parallel with each other.

As shown in <FIG>, a low-temperature end of the first-stage regenerator 112a communicates with a low-temperature end of the first-stage pulse tube 110a, and a low-temperature end of the second-stage regenerator 112b communicates with a low-temperature end of the second-stage pulse tube 110b. The first cooling stage 114a is provided at the low-temperature ends of the first-stage pulse tube 110a and the first-stage regenerator 112a, and the second cooling stage 114b is provided at the low-temperature ends of the second-stage pulse tube 110b and the second-stage regenerator 112b. The first cooling stage 114a and the second cooling stage 114b are formed of, for example, a metallic material, such as copper, and other materials having high thermal conductivity.

The radiator <NUM> is thermally coupled to high-temperature ends of the first-stage pulse tube 110a and the second-stage pulse tube 110b. The radiator <NUM> is fixed to the top flange <NUM> on an opposite side of the cooling stages. The radiator <NUM> is formed of, for example, aluminum or an aluminum alloy. Alternatively, the radiator <NUM> may be formed of, for example, a metallic material, such as copper, or other materials having high thermal conductivity.

In the example shown in the drawing, end surfaces of the high-temperature ends of the first-stage pulse tube 110a and the second-stage pulse tube 110b are in contact with a bottom surface of the radiator <NUM> or only the high-temperature ends of the pulse tubes slightly penetrate the radiator <NUM>. Note that greater portions of the pulse tubes may be disposed in the radiator <NUM>.

For example, at most <NUM>/<NUM> of a total length of the first-stage pulse tube 110a in an axial direction may extend inside the radiator <NUM>. With this, a <NUM>/<NUM> portion or less on a high-temperature side of the length in the axial direction of the first-stage pulse tube 110a is disposed in the radiator <NUM>, and a remaining <NUM>/<NUM> portion or more is disposed in the vacuum chamber <NUM>. A high-temperature section of the first-stage pulse tube 110a disposed in the radiator <NUM> can actively exchange heat with outside air through the radiator <NUM>, and this may advantageously act on cooling of a cold head high-temperature end.

As required, along with the first-stage pulse tube 110a or in place of the first-stage pulse tube 110a, at most <NUM>/<NUM> of a total length of the second-stage pulse tube 110b in an axial direction may extend inside the radiator <NUM>.

The radiator <NUM> is disposed outside the vacuum chamber <NUM> and is exposed to the ambient environment, and can come into contact with outside air. The radiator <NUM> is cooled by the forced cooling device <NUM> described below. In the radiator <NUM>, as described below, a radiating fin may be formed to increase a surface area (heat exchange area).

The valve unit <NUM> includes main pressure switching valves (V1, V2), first-stage sub-pressure switching valves (V3, V4), and second-stage sub-pressure switching valves (V5, V6). Typically, the valve unit <NUM> is configured in a form of a rotary valve in which the main pressure switching valves, the first-stage sub-pressure switching valves, and the second-stage sub-pressure switching valves are incorporated. Accordingly, the valve unit <NUM> includes the rotary valve and a valve motor that rotates the rotary valve.

The main pressure switching valves (V1, V2) are connected to the high-temperature end of the first-stage regenerator 112a by a regenerator communication passage <NUM>, the first-stage sub-pressure switching valves (V3, V4) are connected to the high-temperature end of the first-stage pulse tube 110a by a first-stage pulse tube communication passage 120a, and the second-stage sub-pressure switching valves (V5, V6) are connected to the high-temperature end of the second-stage pulse tube 110b by a second-stage pulse tube communication passage 120b. The main pressure switching valves (V1, V2) operate to alternately connect the first-stage regenerator 112a and the second-stage regenerator 112b to a discharge port and a suction port of the compressor <NUM>, the first-stage sub-pressure switching valves (V3, V4) operate to alternately connect the first-stage pulse tube 110a to the discharge port and the suction port of the compressor <NUM>, and the second-stage sub-pressure switching valves (V5, V6) operate to alternately connect the second-stage pulse tube 110b to the discharge port and the suction port of the compressor <NUM>.

The first-stage pulse tube communication passage 120a may be provided with, for example, a first-stage flow adjustment element 122a, such as an orifice, and the second-stage pulse tube communication passage 120b may be provided with a second-stage flow adjustment element 122b.

The pulse tube cryocooler <NUM> may be provided with a first-stage buffer line 124a that connects a first-stage buffer volume 126a to the high-temperature end of the first-stage pulse tube 110a via a first-stage buffer orifice 128a, and a second-stage buffer line 124b that connects a second-stage buffer volume 126b to the high-temperature end of the second-stage pulse tube 110b via a second-stage buffer orifice 128b. The first-stage buffer line 124a may be connected to the first-stage pulse tube communication passage 120a between the first-stage pulse tube 110a and the first-stage flow adjustment element 122a, and the second-stage buffer line 124b may be connected to the second-stage pulse tube communication passage 120b between the second-stage pulse tube 110b and the second-stage flow adjustment element 122b. In <FIG>, for convenience, the buffer lines are not shown.

Because the valve unit <NUM> is disposed to be separated from the cold head <NUM>, the valve unit <NUM> is connected to the high-temperature ends of the first-stage regenerator 112a, the first-stage pulse tube 110a, and the second-stage pulse tube 110b by pipes. The pipes may be, for example, flexible pipes, such as flexible hoses, or may be rigid pipes.

The radiator <NUM> is provided with, for example, fluid couplings <NUM>, such as self-sealing couplings, to which the pipes, that is, the regenerator communication passage <NUM>, the first-stage pulse tube communication passage 120a, and the second-stage pulse tube communication passage 120b are connected. As shown in <FIG> and <FIG>, the fluid couplings <NUM> may be provided on, for example, a top surface of the radiator <NUM>. To prevent or reduce an influence of a temperature increase of the radiator <NUM> on the fluid coupling <NUM>, a heat insulator may be interposed between the fluid couplings <NUM> and the radiator <NUM>. The heat insulator may be, for example, a plate made of engineering plastic.

Because the GM type four-valve pulse tube cryocooler itself has been well known, further description of each component of the pulse tube cryocooler <NUM> will be omitted.

With such a configuration, the pulse tube cryocooler <NUM> can generate PV work at the low-temperature end of the pulse tube by appropriately delaying a phase of displacement oscillation of a gas element (also called gas piston) in the pulse tube with respect to pressure oscillation of working gas, and can cool the cooling stage to an intended cooling temperature. The first cooling stage 114a may be cooled to the first cooling temperature, for example, less than <NUM> (for example, about <NUM> to <NUM>), and the second cooling stage 114b may be cooled to the second cooling temperature lower than the first cooling temperature, for example, about <NUM> to <NUM>.

However, at the start of the dilution refrigerator, the pulse tube cryocooler <NUM> is rapidly cooled from the ambient temperature (for example, a normal temperature of about <NUM>) to the intended cryogenic temperature (that is, the above-described first and second cooling temperatures). After such cool-down is completed, the pulse tube cryocooler <NUM> transits to a normal cooling operation to maintain the reached cooling temperature.

In the cool-down, a comparatively large amount of heat corresponding to the cooling capacity of the pulse tube cryocooler <NUM> is generated from an adiabatic compression process in a refrigeration cycle of the pulse tube cryocooler <NUM>. In particular, because the dilution refrigerator is a comparatively large-sized device, the heat capacity of a low-temperature section, such as the first heat shield <NUM>, the second heat shield <NUM>, and the helium circulation circuit <NUM>, tends to be increased, and an amount of heat that is generated in the cool-down to cool the low-temperature section is also increased. The heat is radiated from the cold head high-temperature end of the pulse tube cryocooler <NUM>, that is, the radiator <NUM> to the outside.

As described at the beginning of the present specification, the present inventors have recognized that, in a case where the pulse tube cryocooler <NUM> is mounted as a precooling refrigerator in a dilution refrigerator, heat radiation from the cold head <NUM> in the cool-down is insufficient, and the radiator <NUM> may be heated to a considerably high temperature (for example, higher than <NUM> to <NUM>).

In a dilution refrigerator of existing design, a GM cryocooler is used as a precooling refrigerator. In the GM cryocooler, in general, a pressure switching mechanism, such as a rotary valve, is incorporated at a high-temperature end of a cold head. The pressure switching mechanism of the cold head is connected to a compressor by a pipe exclusively for supply and a pipe exclusively for collection of working gas. For this reason, working gas heated at the cold head high-temperature end flows from the cold head to the compressor in one direction through a collection pipe. Along with the working gas flow, a comparatively large amount of heat can be carried away from the cold head to the compressor. In the GM cryocooler, in addition to natural convection cooling of the cold head high-temperature end, such working gas collection flow advantageously acts on heat radiation.

However, in the pulse tube cryocooler <NUM>, the valve unit <NUM> is separated from the cold head <NUM>. The separated valve unit <NUM> is connected to the cold head <NUM> by pipes as described above. All of the regenerator communication passage <NUM>, the first-stage pulse tube communication passage 120a, and the second-stage pulse tube communication passage 120b that connect the valve unit <NUM> and the cold head <NUM> are bidirectional flow paths of working gas. That is, in these flow paths, working gas inflow from the compressor <NUM> to the cold head <NUM> and working gas outflow from the cold head <NUM> to the compressor <NUM> backward alternately occur, and working gas flows in a reciprocating manner. Because the working gas flow is not in one direction, the amount of heat that is carried away from the radiator <NUM> by working gas may be made comparatively small.

As a result, in a case where the amount of heat that is generated in the cool-down exceeds a heat radiation amount by natural convection cooling, a large temperature increase may occur in the cold head high-temperature end of the pulse tube cryocooler <NUM>. Due to the temperature increase in the cold head high-temperature end of the pulse tube cryocooler <NUM>, there is a concern that a cool-down time of the pulse tube cryocooler <NUM> is extended. Because the cool-down of the pulse tube cryocooler <NUM> is only part of preparation work for cooling a desired object to be cooled to a cryogenic temperature by the dilution refrigerator, it is desirable that the required time is as short as possible.

According to the present invention, the pulse tube cryocooler <NUM> includes the forced cooling device <NUM>. The forced cooling device <NUM> is configured to forcedly cool the radiator <NUM> in the cool-down of the pulse tube cryocooler <NUM> from the ambient temperature to the cryogenic temperature.

As an example, the forced cooling device <NUM> includes an air-cooled cooler that is configured to provide an air flow <NUM> for cooling to the radiator <NUM>, for example, a cooling fan <NUM>. The cooling fan <NUM> is disposed close to or adjacent to the radiator <NUM> to blow air into the radiator <NUM> (or to suck air around the radiator <NUM>).

Therefore, according to the embodiment, it is possible to promote heat radiation from the high-temperature end of the cold head <NUM> of the pulse tube cryocooler <NUM> by forced cooling of the radiator <NUM> using the cooling fan <NUM>. With this, it is possible to suppress an excessive temperature increase of the high-temperature end of the cold head <NUM> and an increase in cool-down time due to the temperature increase.

The cooling fan <NUM> may be disposed to be separated from the cold head <NUM>. That is, the cooling fan <NUM> may not be mounted on the cold head <NUM> and may be disposed away from the cold head <NUM>. With such a configuration, vibration that may be caused by the cooling fan <NUM> in the operation of the cooling fan <NUM> is prevented from being transmitted to the low-temperature section of the dilution refrigerator through the cold head <NUM>. In this case, the cooling fan <NUM> may be supported by a support structure that supports the dilution refrigerator on a floor surface, such as a support frame. Alternatively, the cooling fan <NUM> may be supported by a dedicated support structure that is provided separately from the support structure of the dilution refrigerator to dispose the cooling fan <NUM> near the radiator <NUM>.

<FIG> is a diagram schematically showing another example of the high-temperature end of the cold head <NUM> of the pulse tube cryocooler <NUM> according to the embodiment. As shown in <FIG>, the cooling fan <NUM> may be mounted on the cold head <NUM>. The cooling fan <NUM> is attached to a bracket <NUM> provided on the top flange <NUM> of the cold head <NUM> and is disposed adjacent to the radiator <NUM>. With the operation of the cooling fan <NUM>, it is possible to provide an air flow for cooling to the radiator <NUM>.

The radiator <NUM> includes radiating fins <NUM> to increase a heat exchange area with the air flow. The radiating fins <NUM> extend in the axial direction of the pulse tube. Accordingly, as shown in the drawing, the radiating fins <NUM> protrude upward from a bottom plate of the radiator <NUM> in contact with the top flange <NUM>. The cooling fan <NUM> is disposed obliquely upward of the radiating fins <NUM>. Therefore, the cooling fan <NUM> blows an air flow toward slits between the radiating fins <NUM> (or sucks an air flow from the slits), and as a result, the cooling fan <NUM> can promote heat exchange of the air flow of the radiating fins <NUM>, and can effectively cool the radiator <NUM>. From the same viewpoint, the cooling fan <NUM> may be disposed upward of the radiating fin <NUM>.

In the example shown in the drawing, one cooling fan <NUM> is provided on one side of the radiator <NUM>. Instead of this, a plurality of cooling fans <NUM> may be provided around the radiator <NUM>. For example, a set of cooling fans <NUM> may be disposed on both sides of the radiator <NUM>. Alternatively, four cooling fans <NUM> may be disposed around the radiator <NUM> at intervals of <NUM> degrees.

The forced cooling device <NUM> may be configured to stop the forced cooling of the radiator <NUM> after the cool-down. With such a configuration, in a normal cooling operation (that is, in cooling of the desired object to be cooled by the dilution refrigerator) after the cool-down of the pulse tube cryocooler <NUM> is completed, the forced cooling device <NUM> is not operated, and vibration transmission from the forced cooling device <NUM> to the low-temperature section of the dilution refrigerator cannot occur. Therefore, it is possible to restrain a bad influence of vibration on the cooling performance of the dilution refrigerator.

Referring to <FIG> again, the forced cooling device <NUM> includes a sensor that detects the state of the pulse tube cryocooler <NUM>, for example, a temperature sensor <NUM>, and a controller <NUM> that is configured to determine whether or not the pulse tube cryocooler <NUM> is in the cool-down, based on an output of the sensor and operate the cooler (for example, the cooling fan <NUM>) in the cool-down.

The controller <NUM> is realized by elements or circuits including a CPU or a memory of a computer in terms of a hardware configuration and is realized by a computer program and the like in terms of a software configuration, but is shown in the drawing as appropriate as a functional block that is realized by cooperation of the hardware configuration and the software configuration. It will be understood by those skilled in the art that the functional block can be realized in a variety of manners by a combination of hardware and software.

The temperature sensor <NUM> is provided in the cold head <NUM>, specifically, for example, the radiator <NUM>. The controller <NUM> may be configured to compare a measured temperature (in this case, a measured temperature of the radiator <NUM>) of the cold head <NUM> measured by the temperature sensor <NUM> with a temperature threshold, and operate the cooler in a case where the measured temperature of the cold head <NUM> exceeds the temperature threshold. The temperature threshold may be set based on the temperature of the radiator <NUM> assumed in the cool-down of the pulse tube cryocooler <NUM> or can be set as appropriate based on empirical knowledge of a designer, an experiment or a simulation by the designer, or the like.

In this case, the temperature sensor <NUM> is connected to the controller <NUM> in a communicable manner to transmit measured temperature data indicating the measured temperature to the controller <NUM>. The controller <NUM> receives the measured temperature data from the temperature sensor <NUM> and compares the measured temperature with the temperature threshold. The controller <NUM> operates the cooling fan <NUM> in a case where the measured temperature exceeds the temperature threshold. That is, the controller <NUM> starts the cooling fan <NUM> by switching the cooling fan <NUM> from off to on. On the other hand, the controller <NUM> does not start the cooling fan <NUM> (remains the cooling fan <NUM> off) in a case where the measured temperature does not exceed the temperature threshold.

In this way, the controller <NUM> regards a state in which the radiator <NUM> is heated compared to the ambient temperature (specifically, a case where the measured temperature of the radiator <NUM> is higher than the temperature threshold), as the pulse tube cryocooler <NUM> being in the cool-down, and operates the forced cooling device <NUM> to forcedly cool the radiator <NUM>. On the other hand, the controller <NUM> regards a state in which the radiator <NUM> is cooled to about the ambient temperature (specifically, a case where the measured temperature of the radiator <NUM> is lower than the temperature threshold), as the end of the cool-down of the pulse tube cryocooler <NUM> ends, and stops the operation of the forced cooling device <NUM>. After the end of the cool-down, as described above, the pulse tube cryocooler <NUM> transits to the normal cooling operation. In such a manner, in the normal cooling operation of the pulse tube cryocooler <NUM>, the forced cooling device <NUM> is stopped, and the occurrence of vibration by the operation is prevented.

The temperature sensor <NUM> may be provided in the first cooling stage 114a or the second cooling stage 114b, instead of being provided in the radiator <NUM>. The temperature sensor <NUM> may be provided in the first heat shield <NUM> or the second heat shield <NUM>. Also in such a manner, the controller <NUM> can determine whether or not the pulse tube cryocooler <NUM> is in the cool-down, based on the output of the temperature sensor <NUM>. The temperature sensor <NUM> may be provided a cooling stage (not shown) of the cryogenic device <NUM> (dilution refrigerator) or an object to be cooled.

The sensor that detects the state of the pulse tube cryocooler <NUM> may be a pressure sensor that measures working gas pressure of the cold head <NUM> or a sensor that measures power consumption of the compressor <NUM>, in place of the temperature sensor <NUM>. It is assumed that the working gas pressure or the power consumption increases in the cool-down compared to the normal cooling operation. Therefore, the controller <NUM> can determine whether or not the pulse tube cryocooler <NUM> is in the cool-down, based on the output of such a sensor.

<FIG> is a diagram schematically showing another example of the forced cooling device <NUM> of the pulse tube cryocooler <NUM> according to the embodiment. The forced cooling device <NUM> may include a liquid-cooled cooler, and an internal flow path <NUM> through which a coolant (for example, cooling water) flows may be formed in the radiator <NUM>. Also in such a manner, similarly to the cooling fan <NUM>, it is possible to forcedly cool the radiator <NUM>.

For example, a coolant <NUM> is supplied from a coolant source (not shown) to the internal flow path <NUM> of the radiator <NUM>. In the internal flow path <NUM>, the coolant <NUM> cools the radiator <NUM> by heat exchange with the radiator <NUM>. The coolant (schematically represented by an arrow <NUM>) that is discharged from the internal flow path <NUM> may be returned to the coolant source, cooled again, and supplied to the radiator <NUM>.

In the above-described embodiment, although a case where the pulse tube cryocooler <NUM> is the four-valve type has been described as an example, the pulse tube cryocooler <NUM> may have other forms, such as a double inlet type. The pulse tube cryocooler <NUM> is not limited to the GM type, and may be a Stirling type pulse tube cryocooler. The pulse tube cryocooler <NUM> is not limited to a two-stage type, and may be a single-stage type, or a three-stage type or other multi-stage pulse tube cryocoolers.

In the above-described embodiment, although a case where the pulse tube cryocooler <NUM> is applied to the dilution refrigerator has been described as an example, the pulse tube cryocooler <NUM> may be applied in other applications in which the pulse tube cryocooler <NUM> is mounted as a precooling refrigerator in other forms of cryogenic refrigerators.

Claim 1:
A pulse tube cryocooler (<NUM>) comprising:
a cold head (<NUM>) including a pulse tube (110a, 110b) and a radiator (<NUM>) thermally coupled to a high-temperature end of the pulse tube (110a, 110b); and
a forced cooling device (<NUM>) that is configured to forcedly cool the radiator (<NUM>) in cool-down of the pulse tube cryocooler (<NUM>) from an ambient temperature to a cryogenic temperature,
wherein the forced cooling device (<NUM>) includes
a sensor that is configured to detect a state of the pulse tube cryocooler (<NUM>),
an air-cooled or liquid-cooled cooler that is configured to cool the radiator (<NUM>), and
a controller (<NUM>) configured to determine whether or not the pulse tube cryocooler (<NUM>) is in the cool-down, based on an output of the sensor, and operate the cooler in the cool-down.