JP2835305B2 - Multi-stage refrigerator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multistage refrigerator for cooling an object to be cooled to a very low temperature, and more particularly to a multistage refrigerator for cooling a superconducting magnet to a very low temperature.

[0002]

2. Description of the Related Art Generally, a multistage refrigerator for cooling an object to be cooled to an extremely low temperature has a plurality of cooling ends. 2. Description of the Related Art Conventionally, a multi-stage refrigerator cools an object to be cooled to an extremely low temperature by a structure in which a first cooling end having a high minimum temperature and a second cooling end having a low minimum temperature are connected in series. The object to be cooled is disposed so as to be in contact with the second cooling end, and is cooled to a very low temperature. However, the second cooling end has a low minimum temperature, but has a low cooling capacity, so that the object to be cooled is cooled only by the second cooling end having a low cooling capacity, and there is a problem that it takes time to cool. . On the other hand, although the first cooling end has a high cooling capacity, the minimum attainable temperature is high, and it has been difficult to obtain an extremely low temperature.

For this reason, there has been proposed a multi-stage refrigerator having a structure in which a heat switch is disposed between a first cooling end and a second cooling end, and the first cooling end is thermally connected to an object to be cooled. (Fifth
2nd Annual Meeting of the Low Temperature Engineering and Superconductivity Society of Japan, Fall 1994 (P7)). The role of the thermal switch is as follows. In the initial stage of cooling, the object to be cooled and the first cooling end are connected by a heat switch, and the first cooling end having a high cooling capacity is used.
The object to be cooled is cooled faster. Next, when the temperature reaches the lowest attained temperature of the first cooling end, the heat switch is turned from the on state to the off state, and cooling is performed to an extremely low temperature only by the second cooled end having the lowest attained temperature.

The inside of the heat switch is configured such that the heat transfer surface on the first cooling end side and the heat transfer surface on the second cooling end side face each other with a gap. This switch is in a state in which nitrogen gas of 1 atm at room temperature is sealed. When the cooling is started, the object to be cooled is cooled from the first cooling end via the thermal switch.
The heat conduction inside the heat switch is due to the heat conduction of the nitrogen gas existing between the heat transfer surface on the first cooling end side and the heat transfer surface on the second cooling end side inside the heat switch. The object to be cooled is cooled, and the temperature of the heat switch is set to the boiling point of nitrogen (at 1 atm:
When the temperature falls below 77K), the nitrogen gas is liquefied and the inside of the thermal switch is brought into a vacuum state. As a result, the gap between the two heat transfer surfaces inside the heat switch is vacuum insulated, and the heat switch is turned off from the on state. Thereafter, cooling is performed to an extremely low temperature only by the second cooling end having the lowest lowest temperature.

[0005]

However, in the conventional thermal switch, the first cooling end has a minimum temperature of 30K.
Since the heat switch is turned off from the on state before reaching the vicinity, there is a problem that the cooling capacity of the first cooling end is not fully utilized. This is because the heat switch is turned off at about 70 K because the internal gas is nitrogen gas. In general, the specific heat of the object to be cooled in this temperature range is sufficiently larger than the specific heat near the lowest temperature 30K at the first cooling end. As a result, the object to be cooled having a considerable heat capacity must be cooled only by the second cooling end having a low cooling capacity, and there is a problem that the cooling rate is low.

[0006] By arranging a thermal switch between the first cooling end and the second cooling end, the cooling speed of the multi-stage cooler is improved, but the cooling capacity of the first cooling end is not fully utilized. However, the refrigerating speed is not satisfactory. Therefore, an object of the present invention is to provide a multi-stage refrigerator capable of efficiently and quickly cooling an object to be cooled by sufficiently utilizing a cooling capacity of a first cooling end having a high cooling capacity. is there.

[0007]

In order to achieve the above-mentioned object, the invention according to claim 1 of the present invention comprises a first cooling end 5 having a high minimum temperature and a high cooling capacity; A second cooling end 6 having a low temperature and a low cooling capacity is provided so as to be thermally connectable between the first cooling end 5 and the second cooling end 6. A multi-stage refrigerator 1 for thermally cooling an object 7 to be cooled to an extremely low temperature by thermally attaching the object 7 to the second cooling end 5, comprising: The thermal switch 4 is turned on by the vaporization of the gas sealed in the thermal switch and turned off by the liquefaction of the gas, and the gas reaches the first cooling end 5 at the lowest temperature. So as to have a boiling point between the temperature and the lowest attained temperature of the second cooling end 6. Enclosed in the switch, it is characterized in that the predetermined temperature is set between the.

A gas having a boiling point between the lowest temperature of the first cooling end and the lowest temperature of the second cooling end is sealed in the heat switch so that the first cooling end reaches the lowest temperature. After that, the heat switch is turned off from the on state, so that the high cooling capacity of the first cooling end can be used to the end and cooling can be performed efficiently.

Conventionally, if the object to be cooled is thermally connected to a temperature lower than the lowest temperature of the first cooling end, the cooling effect of the second cooling end is hindered, and the temperature lower than the lowest temperature of the first cooling end. This breaks the conventional concept that the heat switch cannot be cooled to extremely low temperatures. The reason is that the gas pressure decreases as the temperature decreases, and the thermal conductivity of the gas decreases,
It is considered that the reason is that the heat entering from the first cooling end is reduced before the switch is turned off.

It is preferable that the boiling point of the gas is lower than the lowest temperature at the first cooling end and higher than an intermediate temperature between the lowest temperature at the first cooling end and the lowest temperature at the second cooling end. It is preferable to increase the cooling rate of the object to be cooled by using a gas having a boiling point in this region. Further, the container 2 is a vacuum container, and the inside is kept at a vacuum in order to suppress heat intrusion from the outside.

Further, the invention according to claim 2 is characterized in that, in the structure of the invention according to claim 1, the gas is replaced with neon gas or hydrogen gas. By using a neon gas or a hydrogen gas as a gas having a boiling point between the lowest temperature of the first cooling end 5 and the lowest temperature of the second cooling end 6, the object to be cooled can be reduced as compared with the conventional nitrogen gas. It is preferable to increase the cooling rate of the steel.

Further, according to a third aspect of the present invention, the minimum temperature of the first cooling end 5 and the minimum temperature of the second cooling end 6 are set by setting a pressure at which the gas is sealed in the heat switch to a predetermined pressure. The boiling point is adjusted to a predetermined temperature between the temperature and the temperature. By adjusting the boiling point of the gas, it becomes possible to set an optimum temperature at which the thermal switch 4 is turned off from an on state.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a layout diagram of a heat switch in a multistage refrigerator of the present invention, and FIG. 2 is a diagram showing a structure of the heat switch of the present invention. The structure of the multistage refrigerator of the present invention will be described. As shown in FIG. 1, a multi-stage refrigerator 1 of the present invention is configured such that most of a refrigerator main body 1 a protrudes into a container 2. This container 2 is usually formed as a vacuum container. The cooler main body 1a in the container 2
Has a first cooling end 5 and a second cooling end 6. The first cooling end 5 has a structure in which a first cooling plate 5a and a first cooling unit 21a are connected, and the second cooling end 6 has a structure in which a first cooling plate 6a and a second cooling unit 22a are connected. . The first cooling plate 5 a is connected to the radiation shield 3 to form a cooling space 8. The second cooling end 6 is disposed in the cooling space 8, and the object to be cooled 7 is connected to the second cooling plate 6 a of the second cooling end 6. The heat switch 4 is disposed so as to be connected to the first cooling plate 5a and the second cooling plate 6a. A gas having a boiling point between the lowest temperature at the first cooling end 5 and the lowest temperature at the second cooling end 6 is sealed in the heat switch.

As shown in FIG. 2, the thermal switch 4 of the present invention comprises two upper and lower pure copper blocks 10a and 10b. A plurality of cylindrical upper fins 11a made of pure copper are concentrically connected to the upper pure copper block 10a by welding or the like. A plurality of cylindrical lower fins 11b made of pure copper are concentrically connected to the lower pure copper block 10b by welding or the like, and a thin stainless steel pipe 12 (SUS316) is concentrically connected to the outside thereof. Yes,
The length of the stainless steel tube 12 is equal to the length of the plurality of pure copper cylindrical fins 1 of the upper and lower two pure copper blocks 10a and 10b.
It is longer than the length of 1a, 11b. In the thermal switch 4 of the present invention, the upper and lower two pure copper blocks 10a and 10b are connected to form a closed space with an outer stainless steel tube. In this space, the upper fins 11a and the fins 11b are arranged concentrically and alternately. The heat transfer surfaces of the upper fin and the lower fin are opposed to each other with a gap.

Next, the operation of the thermal switch will be described. The heat switch of the present invention is a gas conduction type switch in which a gas having a boiling point between the lowest temperature of the first cooling end and the lowest temperature of the second cooling end is sealed in a space enclosed by a stainless steel tube. is there. The upper and lower heat conduction is performed through a gap between the upper fin and the lower fin using the internal gas as a heat conduction medium. The outermost thin stainless steel tube has negligible thermal conductivity when the thermal switch is off. The switching of the on / off state of the thermal switch occurs at the boiling point of the charged gas. When the temperature of the heat switch is equal to or higher than the boiling point of the gas, heat conduction using the gas as a heat medium is turned on.

When the lowest temperature at the first cooling end is 30K and the lowest temperature at the second cooling end is 4K,
An example will be described in which a gas having a boiling point between 30 K and 30 N, for example, neon gas at room temperature and 4 atm is used. In the initial stage of cooling, the neon gas is in a gaseous state, the heat switch can transmit heat (the heat switch is in an on state), and the object to be cooled is cooled at the first cooling end. Since the first cooling end has a higher cooling capacity than the second cooling end, cooling can be performed faster than when the heat switch is not used. The neon gas of 4 atm at room temperature becomes 0.4 atm at the lowest temperature (30K) at the first cooling end,
The boiling point of neon gas changes from 27K to 24K,
It is between 4 and 30K.

Further, as the cooling progresses, the temperature of the heat switch becomes lower than the lowest temperature (30 K) of the first cooling end, and when the temperature reaches the boiling point of the neon gas, the gas liquefies, and the inside of the heat switch becomes a vacuum to transfer heat. After that, the object to be cooled is cooled to a very low temperature only at the second cooling end thereafter.

It is preferable that the boiling point of the gas is lower than the lowest temperature at the first cooling end and higher than an intermediate temperature between the lowest temperature at the first cooling end and the lowest temperature at the second cooling end. The cooling rate of the object to be cooled can be increased by reducing the temperature range in which the object to be cooled below the lowest temperature at the first cooling end and the first cooling end are thermally connected.

It is also preferable that the boiling point of the gas is adjusted by changing the pressure of the gas sealed in the heat switch, and an optimum temperature at which the heat switch is turned on from off is set. In this embodiment, neon gas is supplied
The atmosphere was sealed, but if the atmosphere was sealed at 10 atm, it would be 1 atm at 30K and the boiling point of neon gas would be 27K. The on / off state of the thermal switch can be changed from 24K to 27K. As a result, in the region below the lowest temperature of the first cooling end, the object to be cooled is cooled from the first cooling end to the second cooling end.
Switching to the cooling end can be performed quickly, and the cooling effect of the multi-stage refrigerator can be promoted.

[0020]

Next, as shown in FIG. 3, a pure copper braided wire was connected to a pure copper second cooling plate attached to the lower end of the thermal switch and the second cooling end, and compared with another embodiment of the system configuration. An example will be described. In this embodiment, 24 pure copper braided wires having a cross-sectional area of 10 mm ² were used.

[0021] The radiation shield is sewn into a cylindrical aluminum cylinder.
80 parts insulation are superimposed. The radiation shield is connected to the first cooling end;
It is cooled to 30K by the first cooling end. The radiant heat for the object to be cooled is 0.074 for the emissivity of aluminum, 0.011 for the emissivity of the object to be cooled, and 30 K for the radiation shield temperature.
It is estimated to be 1.4 mW when the temperature of the object to be cooled is 4K.

The minimum temperature of the multistage refrigerator used was 30K at the first cooling end, 4K at the second cooling end, and the cooling capacity was 25W at 50K at the first cooling end, and 4.0W at the second cooling end. 3
K is 0.5 W. The object to be cooled has an outer diameter of 200 mm, an inner diameter of 150 mm, a height of 300 mm, and a weight of 40.6 k.
g of superconducting magnet was used. This superconducting magnet is NbTi
Lines are used. The object to be cooled is attached to the second cooling end via a second cooling plate made of pure copper having a thickness of about 15 mm. An indium sheet is interposed between the second cooling end and the second cooling plate to improve heat transfer. Further, an indium sheet is interposed between the superconducting magnet and the second cooling plate, and between the heat switch and the second cooling plate.

The thermal switch used has the structure shown in FIG.
The specifications are shown below. -Upper and lower pure copper blocks: thickness 20 mm, diameter 70.2 mm-Stainless steel pipe (SUS316): length 100 mm, inner diameter 57.2 mm outer diameter 58 mm, wall thickness 0.4 mm-pure copper fins Upper fins (four pieces): Each inner diameter is 6, 14, 22, 32 mm 80 mm in length, 1 mm in thickness Lower fins (four pieces): Each inner diameter is 10, 18, 28, 38 mm 80 mm in length, 1 mm in thickness Interval of each fin: 1.2 mm up and down The pure copper blocks and the pure copper fins were connected by Tig welding, and the connection between the upper and lower pure copper blocks and the stainless steel tube was fixed using stainless steel solder.

The opposed area of the pure copper fins of the thermal switch is 4.22 × 10 ² m ² . When the internal gas is nitrogen gas, the heat conduction when the heat switch is on is about 27 W when the temperature difference between the upper and lower pure copper blocks is 30K. When the heat switch is off, the heat conduction between the upper and lower pure copper blocks is 36 m for the stainless steel tube.
The radiation between W and the fin is estimated to be 0.054 mW.
As a result, the thermal conductivity in the off state of the thermal switch is so low as to be negligible.

The container of the multistage refrigerator is evacuated to 10 ^-3 Torr or less at room temperature by a turbo molecular pump. At the final stage of cooling, the degree of vacuum in the container reaches the order of 10 ^-6 Torr.
The degree of vacuum was measured using a cathode vacuum gauge.

FIGS. 4 to 8 show the cooling test results of the superconducting magnet using the multistage refrigerator of FIG. FIG. 4 is a diagram showing cooling characteristics when a heat switch is not used. FIG.
Is nitrogen gas at 4 atm, FIG. 6 is neon gas at 4 atm, FIG.
FIG. 4 is a diagram showing cooling characteristics when nitrogen gas at 4 atm is sealed as room gas at room temperature and cooled. FIG. 8 is a diagram showing a temperature change at both ends of the thermal switch when neon gas of 4 atm is used.

4 and 5, when the object to be cooled (superconducting magnet) is cooled from room temperature to 4K, the required cooling time without using a heat switch is 21 hours.
The cooling time required when using 4 atmospheres of nitrogen gas as the internal gas of the heat switch is 18 hours, which is reduced by 3 hours. Comparing the required cooling time up to about 70K, the required cooling time when using a nitrogen gas is 5 hours, while the required cooling time when not using a heat switch is 19 hours. It can be seen that the cooling object (superconducting magnet) is cooled following the first cooling end.

In the embodiment of the present invention, when neon gas of 4 atm is used as the internal gas of the heat switch (FIG. 6), the cooling time is 14 hours, which is different from the comparative example (FIG. 5) using nitrogen gas. Compared to this, the time is further reduced by 4 hours.
Comparing the required cooling time to about 70K, it is 5 hours, which is almost the same as the comparative example. However, the time required for cooling up to 4K thereafter is 13 hours longer than that of the comparative example (FIG. 5).
In this embodiment (FIG. 7), the time is reduced to 9 hours and 69%.
This is because the heat switch does not turn off until the boiling point of neon (27 K at 1 atm), so that the first power with high capacity to the end
It can be cooled at the cooling end.

In this embodiment, the neon gas of 4 atm at room temperature becomes 0.4 atm at the lowest temperature (30K) at the first cooling end, and the boiling point of the neon gas changes from 27K to 24K. FIG. 8 shows temperature changes at the upper end (first cooling end side) and the lower end (second cooling end side) of the thermal switch when 4 atm of neon gas is used. The temperature at the lower end of the thermal switch sharply drops from around 26K, and there is a large thermal gradient between the upper and lower sides.
From this, it is estimated that the thermal switch changed from the on state to the off state (neon gas is liquefied) at about 26K.

Conventionally, if the object to be cooled is thermally connected to a temperature lower than the lowest temperature of the first cooling end, the cooling effect of the second cooling end is hindered and the temperature lower than the lowest temperature of the first cooling end is hindered. It was thought that the heat switch could not be cooled to cryogenic temperatures. However, the present embodiment has demonstrated that the heat switch can be cooled to a very low temperature because the cooling effect of the second cooling end is greater than the heat conduction from the first cooling end to the object to be cooled. .

In another embodiment of the present invention, the cooling time required when using 4 atm of hydrogen gas as the internal gas of the heat switch (FIG. 7) is 16 hours, and a comparative example using nitrogen gas (FIG. 5) ) Is further reduced by 2 hours. The hydrogen gas at 4 atm at room temperature has a pressure of 0.4 atm at the lowest temperature (30K) at the first cooling end, and the boiling point of the hydrogen gas changes from 20K to 18K.

The reason why the cooling time of the neon gas is shorter than that of the hydrogen gas is that the lowest temperature at the first cooling end (30 ° C.).
K) In the following, this is due to the fact that the boiling point of the neon gas is closer to the lowest temperature (30K) at the first cooling end than the boiling point of the hydrogen gas. It is preferable to adjust the boiling point of the gas by changing the pressure of the gas to set an optimum temperature at which the heat switch is turned off.

[0033]

As described above, according to the first aspect of the present invention, a gas having a boiling point between the lowest temperature at the first cooling end and the lowest temperature at the second cooling end, By enclosing in the container, the heat switch is turned off after the first cooling end reaches the minimum temperature, so that the higher cooling capacity of the first cooling end can be used until the end. It can be cooled efficiently. By using a heat switch that uses a gas with a boiling point between the lowest temperatures of the cooling ends as the internal gas, it is necessary to cool the object to be cooled from normal temperature to 4K compared to a conventional heat switch using nitrogen gas. The time can be reduced to about 77%, in particular, the required cooling time from 70K to 4K can be reduced to about 69%.

According to a second aspect of the present invention, the neon gas or the hydrogen gas is used as the gas having a boiling point between the lowest temperature at the first cooling end and the lowest temperature at the second cooling end, thereby improving the efficiency. It can cool well.

According to a third aspect of the present invention, the pressure between the lowest temperature of the first cooling end and the lowest temperature of the second cooling end is set by setting a pressure at which the gas is sealed in the heat switch to a predetermined pressure. The temperature can be adjusted to the predetermined boiling point to set an optimum temperature at which the heat switch is turned off.

[Brief description of the drawings]

FIG. 1 is a layout view of a heat switch in a multistage refrigerator of the present invention.

FIG. 2 is a diagram showing a structure of a thermal switch of the present invention.

FIG. 3 is a layout view of a heat switch in another multi-stage refrigerator of the present invention.

FIG. 4 is a diagram showing cooling characteristics when a thermal switch is not used.

FIG. 5 is a diagram showing cooling characteristics when nitrogen gas at 4 atm is used as an internal gas.

FIG. 6 is a diagram showing cooling characteristics when a neon gas of 4 atm is used as an internal gas.

FIG. 7 is a diagram showing cooling characteristics when hydrogen gas at 4 atm is used as an internal gas.

FIG. 8 is a diagram showing a temperature change at both ends of a thermal switch when neon gas of 4 atm is used.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Multistage refrigerator 1a Refrigerator main body 2 Container (vacuum container) 3 Radiation shield 4 Heat switch 5 1st cooling end 5a 1st cooling plate 6 2nd cooling end 6a 2nd cooling plate 7 Cooled object 8 Cooling space 9 Pure copper braided wire 10a Pure copper block on 10b Pure copper block under 11a Upper fin 11b Lower fin 12 Stainless steel pipe 21a First cooling unit 21b Second cooling unit

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tsuyoshi Komon 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi, Hyogo Kobe Steel Ltd. Inside Kobe Research Institute (72) Inventor Satoshi Ito, Nishi-ku, Kobe-shi, Hyogo 1-5-5 Takatsukadai Kobe Steel, Ltd. Kobe Research Institute (56) References JP-A-8-128742 (JP, A) JP-A 63-70053 (JP, A) (58) Survey Field (Int.Cl. ⁶ , DB name) F25B 9/00

Claims (3)

(57) [Claims] 1. A first cooling end having a high minimum attainment temperature and a high cooling capacity; a second cooling end having a low minimum attainment temperature and a low cooling capacity; A heat switch that is provided so as to be thermally connectable between the two, and switches from a thermal connection on to a thermal connection off at a predetermined temperature, wherein the object to be cooled is thermally attached to the second cooling end to be cooled. A multi-stage refrigerator for cooling an object to an extremely low temperature, wherein the thermal switch turns on a thermal connection by vaporization of a gas sealed therein, and turns off a thermal connection by liquefaction of the gas. The gas changes the pressure sealed in the heat switch.
By this, the minimum temperature at the first cooling end and the second cooling temperature
It has a boiling point of the predetermined temperature between the lowest temperature at the reject end
A multi-stage refrigerator characterized by being adjusted so that: 2. The multi-stage refrigerator according to claim 1, wherein the gas is neon gas or hydrogen gas. 3. The gas according to claim 1, wherein the gas comprises neon gas,
The pressure sealed in the thermal switch at room temperature is 4 atm or less.
Multistage refrigerator according to claim 1 Symbol mounting, characterized in that a top.