JP2689230B2 - Dilution refrigerator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

This invention relates to liquid helium ( ³ H
e, ⁴ He) and a dilution refrigerator for continuously obtaining an ultralow temperature of 1 to 10 ⁻³ K.

[0002]

As is well known, a mixed liquid of a liquid phase of ³ He and a liquid phase of ⁴ He separates into two phases at 0.8 K or less and contains 6.4% of ³ He at a low temperature. Dilute phase and 1 He ³ He
Coexistent with a rich phase containing 00%. And the rich phase of ³ He
When is dissolved (diluted) in the dilute phase, heat is absorbed from the outside, and as a result, an ultralow temperature of 1 to 10 ⁻³ K can be obtained. A refrigerator utilizing such a phenomenon is called a dilution refrigerator and has been put into practical use in recent years.

The principle of the dilution refrigerator is described, for example, in "Journal of the Physical Society of Japan" Vol. 37, No. 5 (1982), No. 409.
Pp. To 418 (Principle of ³ He- ⁴ He dilution refrigerator and problems in design I), pp. 595 to 600 ( ³ ) of "Journal of the Physical Society of Japan" Vol. 37, No. 7 (1982). He- ⁴ H
e The principle of the dilution refrigerator and design problems II) and the like are explained, but the principle configuration is shown in FIG.

In FIG. 2, the vacuum pump 1 is for forcedly circulating ³ He, and the gas ³ He sent from the vacuum pump 1 at a temperature of about 300 K is a liquid.
⁴ He is liquefied in a condenser (condenser) 3 which is in thermal contact with a 1K pot 2 which is depressurized and kept at about 1.3K, and is further sent to a heat exchanger 6 in a fractionator 5 via an impedance 4. To be This distiller 5 has ³
By utilizing the difference in the saturated vapor pressure of He and ⁴ He ^{4 He-3}
This is for selectively discharging ³ He from the mixed liquid of He, but ³ He sent from the condenser 3 is heat-exchanged in the heat exchanger 6 which is in thermal contact with the fractionator 5. And is cooled to about 0.5 to 0.7K. Moreover
³ He is cooled to about 100 mK in the heat exchanger 8 via the impedance 7 and sent to the mixer 9. In mixer 9, a dense phase of 100% ³ the He as described above, ³ the He is melted into ⁴ He ⁴ He-6.4% ³
Two phases are separated into a diluted phase of He, and the lower layer is a diluted phase ( ⁴ He-6.4% ³ He) and the upper layer is a rich phase (
³ He phase). And ³ He sent to the rich phase
When is dissolved in the dilute phase, it absorbs heat as described above, and is cooled to an ultralow temperature of the order of 10 mK. That is, since this mixer 9 serves as a cold head as a refrigerator, if the object to be cooled (sample) is held in this portion, the sample can be cooled to the order of 10 mK.

The concentration of ³ He in the dilute phase of the mixer 9 remains 6.4%, while the concentration of ⁴ He- ³ He in the fractionator 5 is kept.
From the mixed solution, only ³ He is gasified and discharged by the heat of saturated vapor pressure of ⁴ He and ³ He, so the ³ He concentration in the fractionator 5 is 0.5 to 0.7 K. %, And therefore the diluted phase in the mixer 9 and the mixed liquid in the fractionator 5 are
A concentration difference of ³ He occurs, so that ³ He is drawn into the fractionator 5 side from the dilute phase in the mixer 9 due to the concentration gradient between the two, and accordingly, in the mixer 9, 100% ³
The dissolution of ³ He from the concentrated He phase to the diluted phase occurs continuously. While the ³ He is drawn from the mixer 9 into the fractionator 5, the ³ He passes through the heat exchanger 8 and cools the ³ He on the forward path side.

[0006] In the fractionator 5, as already described, due to the difference in saturated vapor pressure, ³ H from the ⁴ He- ³ He mixed solution is mixed.
Only e evaporates and is exhausted by the vacuum pump 1 described above. The ³ He sucked by the vacuum pump 1 is sent to the condenser 3 again as described above.

As described above, in the dilution refrigerator, 10
Ultra low temperature of mK order can be obtained.

By the way, a simple dilution refrigerator utilizing such a principle of the dilution refrigerator was published in the United Kingdom, "Cryogenics" 1993 Vol 33, N.
o9, p.923-925, "One-day dilu"
"ion refrigerator". This simple dilution refrigerator is also shown in p15 to p20 "Trial manufacture of simple dilution refrigerator" of "Low Temperature Center Tayori" No. 16 (January 1993) published by the University of Tokyo Low Temperature Center.

FIG. 3 shows a schematic configuration of the simple refrigerator according to the above proposal.

In FIG. 3, liquid helium 14 as a refrigerant is injected into a cylindrical outer container 12 having a bottom and surrounded by an outer vacuum heat insulating layer 10, and an upper side wall of this outer container 12 is provided. A liquid helium decompression port 16 is provided. An inner container 18 having a bottomed cylindrical shape is immersed in the liquid helium 14 in the outer container 12. A vacuum heat insulating layer 20 is provided on the wall portion of the lower portion of the inner container 18 (the portion immersed in the liquid helium 14), and the ³ He discharge port 2 is provided near the upper end of the inner container 18.
1 is formed.

Liquid helium (liquid phases 23 and 25, which will be described later) is injected into the lower portion of the inner container 18, and the column and the vacuum exhaust pipe 22 are introduced into the liquid helium from above.
The hollow plunger 24 is immersed in the suspended state. Further, at an intermediate position of the strut / vacuum exhaust pipe 22 (above the plunger 24 and below the liquid level 23A), a good heat of copper or the like is provided so that the strut / vacuum exhaust pipe 22 vertically penetrates. A heat conduction block 26 made of a conductive material is fixed, and a ³ He passage 27 that vertically penetrates the heat conduction block 26 is formed in the heat conduction block 26. Due to the arrangement of the plunger 24 and the heat conduction block 26 as described above, a portion of the inner container 18 below the heat conduction block 26 is located above the plunger 24 and above the plunger 24. It will be divided into the fractionation chamber 40 below the conduction block 26. There is a gap 42 between the outer peripheral surface of the plunger 24 and the inner peripheral surface of the inner container 18.
The mixing chamber 38 and the fractionating chamber 40 communicate with each other, and the liquid surface 23A is located in the fractionating chamber 40. Furthermore, the heat conduction block 26 is in thermal contact with the inner surface of the inner container 18 via a copper spring member (not shown) or the like, but at least a part of the periphery of the heat conduction block 26 has the heat conduction block. There is an unavoidable gap 41 that connects the upper space and the lower space (distillation chamber 40) of 26.

Also, inside the inner container 18,
^{The 3} He supply pipe 28 is inserted. The ³ He supply pipe 28 is guided downward in the inner container 18 and is connected to a coil-tube-shaped condenser (condenser) 30 integrally incorporated in the heat conduction block 26.
The lower outlet side of 0 is connected to a coil-tube fractionating chamber heat exchanger 34 via a pipe 32. The fractionating chamber heat exchanger 34 is immersed in the liquid helium (liquid phase 23) in the fractionating chamber 40. Further, the lower exit side of the fractionating chamber heat exchanger 34 is connected to a heat exchanger 36 arranged in a gap 42 between the plunger 24 and the inner wall surface of the inner container 18,
Further, the lower end of the heat exchanger 36 is provided with a discharge port 44 which is introduced into the mixing chamber 38 and discharges ³ He into the mixing chamber 38. In addition, with the above-mentioned ³ He outlet 21
^A vacuum pump 46 is interposed between the ³ He supply pipe 28 and the inside of the inner container 18.

In the simple dilution refrigerator as described above, liquid helium (normal ⁴ He) is injected into the space between the inner surface of the outer container 12 and the outer surface of the inner container 18, and The inside of the space is decompressed from the helium decompression port 16 and maintained at a low temperature of about 1K. Therefore, this portion corresponds to the 1K pot 2 in FIG. 2 and contributes to cooling the heat conduction block 26 to 1 to 1.3K.
On the other hand, the inside of the fractionation chamber 40 of the inner container 18 is filled with the liquid phase 23 composed of ⁴ He- 1% ³ He so that the liquid surface 23A is located in the middle of the fractionation chamber 40, while the mixing chamber 38 is 100
% ³ filled with the liquid phase 25 consisting of a dense phase and a ^{4 He-6.4%} 3 He dilute phase of He. In this state, ³ He
There is guided to the condenser 30 via a ³ He supply pipe 28 by the vacuum pump 46, the heat-conducting block 26 is ³ He liquefied is cooled to approximately 1.3K. Liquefied ³ He
Is further cooled through the fractionating chamber heat exchanger 34 and the heat exchanger 36, and is discharged from the discharge port 44 into the mixing chamber 38. In the mixing chamber 38, as already described for the mixer 9 in FIG. 2, the discharged ³ He dissolves in the upper 100% ³ He rich phase, and a part of the rich phase ³ He becomes lower. ⁴ He-6.4% Soluble in a dilute phase of ³ He. At this time, heat absorption occurs and an ultralow temperature of the order of 10 mK is obtained.

On the other hand, the mixing chamber 38 communicates with the fractionating chamber 40.
Therefore, in the dilute phase in the mixing chamber 38, ^ThreeHe is the fractionation chamber 40
However, this fractionating chamber 40 has a low temperature of 1 K or less.
Because^ThreeHe and^FourDue to the large difference in He saturated vapor pressure
hand^ThreeOnly He evaporates and this gas phase^ThreeHe is a heat conduction block
Of 26^ThreeAbove the inner container 18 through the He passage 27
From the space^ThreeBy the vacuum pump 46 through the He discharge port 21
Is exhausted. Along with this, the liquid in the fractionation chamber 40
In helium^ThreeSince the He concentration will be as low as 1%,
Of the room 40^ThreeHe concentration (about 1%) and dilute phase in mixing chamber 38
In^ThreeMix by concentration gradient with He concentration (6.3%)
From the lean phase in chamber 38^ThreeHe atoms are guided to the separation chamber 40
You. Further, by this, in the dilute phase in the mixing chamber 38,^ThreeHe
As the concentration decreases,^ThreeFrom 100% He rich phase
Continuously^ThreeHe will melt into the dilute phase.

In this way, ³ He is continuously circulated, and the melting of ³ He into the dilute phase in the mixing chamber 38 continuously maintains an ultralow temperature of the order of 10 mK.

[0016]

When a simple dilution refrigerator as shown in FIG. 3 was further tested for practical use, the following problems were found.

That is, in FIG. 3, the heat conducting block 26 is in thermal contact with the inner surface of the inner container 18 via the spring member made of a heat conducting material such as copper as already described. There is a gap 41 between at least a part of the inner container 18 and the inner surface of the inner container 18. By the way, ⁴ He in liquid phase
Is a liquid phase having a superfluidity called HeII at about 1 to 1.3 K, and due to this superfluidity, ⁴ He in the liquid phase 23 in the fractionating chamber 40 becomes a liquid surface 23A. From the inner wall of the inner container 18 to form a thin film and rise,
It passes through the above-mentioned void 41 around the heat conduction block 26 and further rises above the heat conduction block 26 as a thin film. Here, as the thin layer of superfluid helium as a thin film rises along the inner surface of the inner container 18 from the liquid surface 23A in the fractionating chamber 40, the temperature thereof gradually rises and the critical temperature at which superfluidity is not exhibited. The thin layer reaches the position (about 2.19K), and rises to the position A in FIG. 3, for example.

As described above, liquid ⁴ H is used as superfluid helium.
If e rises along the inner surface of the inner container 18 to a position above the heat transfer block 26, the temperature rises to a certain extent in the vicinity thereof, so that the saturated vapor pressure of ⁴ He is much higher than that in the fractionation chamber 40. Therefore, ⁴ He gas evaporates from the superfluid ⁴ He thin layer on the inner surface of the inner container 18. This ⁴ He gas is mixed with ³ He evaporated from the liquid surface 23A of the fractionating chamber 40, and is exhausted and collected together with ³ He from the ³ He discharge port 21 by the vacuum pump 46. In this case, the ³ He concentration in the gas collected by the vacuum pump 46 becomes low, so that the ³ He concentration in the gas sent from the vacuum pump 46 again via the ³ He supply pipe 28 also decreases. As a result, there is a problem in that the cooling capacity by dilution refrigeration is lowered and a sufficient ultra-low temperature cannot be obtained.

The present invention has been made in view of the above circumstances. The simple dilution refrigerator as shown in FIG. 3 is improved so that it rises from the liquid surface in the fractionation chamber to the inner surface of the inner container by superfluidity. it is an essential object to prevent deterioration of the ³ He concentration in the recovered gas due to evaporation of ⁴ He gas from a thin layer of the ⁴ He.

[0020]

In order to solve the above-mentioned problems, in the dilution refrigerator according to the invention of claim 1, the space above the heat conduction block is isolated from the inner wall of the inner container, and ³ He gas At the time of recovery, the ⁴ He gas was not sucked in at that part.

Specifically, in the dilution refrigerator of the first aspect of the invention, the heat conduction block and the plunger are vertically spaced from above in a bottomed cylindrical inner container which is cooled from outside by liquid helium. The space below the heat transfer block inside the inner container is divided into an upper distilling chamber of the plunger and a lower mixing chamber of the plunger, and the space is inserted into the heat transfer block. Penetrates up and down
^{A 3} He passage is formed, and a ³ He supply pipe is introduced into the inner vessel. The ³ He supply pipe is connected to a condenser incorporated in the heat conduction block, and a heat exchanger is installed in the fractionation chamber. Is provided with a heat exchanger between the plunger and the inner surface of the inner container, the outlet side of the condenser is connected in that order to the heat exchanger in the fractionating chamber, the heat exchanger in the plunger position, and the lower end of the heat exchanger of the plunger position is not directed to the mixing chamber, the the tip has a discharge port is formed for discharging the ³ He in the mixing chamber, yet above the said heat conductive block, ³ He In a dilution refrigerator in which a ³ He discharge port for discharging gas is formed, a sealed tube is inserted into the inner container concentrically with the inner container from above, and the lower part of the wall of the sealed pipe is Outer peripheral edge of heat conduction block It is characterized in that the ³ He discharge port is formed in the upper part of the hermetically sealed tube.

In the dilution refrigerator according to the second aspect of the present invention, the space above the heat conduction block is isolated from the inner wall of the inner container as described above, and at the same time, ⁴ He is as much as possible in the fractionation chamber. It was designed so that it would not be inhaled.

[0023] Specifically, the dilution refrigerator of the invention of claim 2 is the dilution refrigerator of claim 1, wherein the passage is provided at the lower end of the ³ He passage vertically passing through the heat conduction block. An extension pipe is provided for extending downward into the fractionation chamber, and a lower end of the extension pipe is provided with a lower end in the fractionation chamber and a peripheral wall portion is provided with a cover for expanding downwardly. It is characterized by that.

[0024]

The principle of the dilution refrigeration operation of the dilution refrigerator of the present invention is the same as that of the simple dilution refrigerator of FIG. 3 already described. Especially, in the dilution refrigerator according to the invention of claim 1, the space above the heat conduction block in the inner container is isolated from the space adjacent to the inner surface of the inner container (to be precise, the heat conduction block is moved up and down). Penetrates ³ H
e It is isolated except the passage part) and ³ He
Since the discharge port is provided above the closed tube, liquid ⁴ H is generated by superfluid flow from the liquid level of liquid helium in the fractionation chamber.
e becomes a thin layer and rises above the heat conduction block along the inner surface of the inner container, and even if ⁴ He evaporates in that portion, the ⁴ He gas is directly in the space above the heat conduction block.
Not mixed with ³ He. Therefore ³ He
There is little risk that the concentration of ³ He in the recovered gas sucked by the vacuum pump from the outlet will be lowered by the incorporation of ⁴ He gas.

In the case of the dilution refrigerator of the second aspect of the invention,
The ³ He gas that evaporates from the liquid level in the fractionation chamber is extended from the lower open end of the recovery cover to the extension pipe and ³ He of the heat transfer block.
³ He from inside the sealed tube above the heat conduction block through the passage
Although it is sucked to the outside through the discharge port, the peripheral wall of the recovery cover expands downward as described above, so ⁴ He gas fell from the top to near the liquid level in the fractionation chamber. Even in this case, the recovery cover can prevent the ⁴ He gas from being sucked in, and can recover the ³ He gas from a wide range of the liquid surface. Therefore, it is possible to more reliably prevent the concentration of ³ He in the recovered gas from decreasing due to the incorporation of ⁴ He gas.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the overall construction of a dilution refrigerator according to an embodiment of the present invention. In FIG. 1, the same elements as those of the known simple dilution refrigerator shown in FIG. 3 are designated by the same reference numerals, and the description thereof will be omitted.

In FIG. 1, a hollow tubular closed tube 50 is inserted into the inner container 18 from above.
The closed tube 50 has a diameter slightly smaller than that of the inner container 18, and the inner container 18 and the support / vacuum exhaust pipe 22.
On the other hand, they are concentric with each other and surround the column / vacuum exhaust pipe 22. The lower end of the closed tube 50 is integrally joined to the outer peripheral edge portion of the heat conduction block 26, and the joined portion between the lower end of the closed tube 50 and the heat conduction block 26 is airtight. Further, a ³ He exhaust port 21 is formed in the upper part of the sealed tube 50, and this ³ He exhaust port 21 is shown in FIG.
It is guided to the vacuum pump 46 in the same manner as in the known example. The upper portion of the closed tube 50 is detachably and airtightly fixed to the flange portion 18A at the upper end of the inner container 18 via the flange portion 50A. Further, the upper end of the closed tube 50 is closed by a lid 52 so as to be openable and closable, and the lid 52 is vertically penetrated by a column / vacuum exhaust pipe 22 and a ³ He supply pipe 28. The closed tube 50 can be inserted into and removed from the inner container 18. Therefore, the closed tube 50 includes the heat conduction block 26, the support / vacuum exhaust tube 22, the plungers 24, ³
The He supply pipe 28, the condenser 30, and the heat exchangers 34 and 36 are integrally inserted into and removed from the inner container 18.

Further, the heat conduction block 26 is formed with a ³ He passage 27 which vertically penetrates it similarly to the known example shown in FIG. 3, and the passage is formed at the lower end of the ³ He passage 27. An extension pipe 54 that extends to the inside of the fractionation chamber 40 and projects into the fractionation chamber 40 is provided. And this extension pipe 54
A recovery cover 56, whose peripheral wall portion expands downward, is provided at the lower end of the umbrella-like shape. The recovery cover 56 is made so that its lower open end (enlarged end) is slightly smaller than the inner diameter of the inner container 18, and its lower open end is slightly immersed below the liquid surface in the fractionation chamber 40. ing.

In the above-described embodiment, as in the known example of FIG. 3, only ³ He gas evaporates from the liquid surface 23A in the fractionation chamber 40, and this ³ He gas is recovered by the recovery cover 56 and the extension pipe 54. , ³ He passage 27, and the inner space of the sealed tube 50 in that order, from the ³ He discharge port 21 to the vacuum pump 46.
Is sucked and exhausted by.

Here, as described above, from the liquid surface 23A of the fractionating chamber 40, a thin layer of superfluid helium of liquid ⁴ He rises along the inner wall surface of the inner container 18 and the superfluid helium thin The layer reaches a position above the heat conduction block 26,
In addition, ⁴ He is vaporized and vaporized in the relatively high temperature part of the upper part.
Although gas is generated, since the space above the heat transfer block 26 is surrounded and sealed by the sealing tube 50,
^{The 4} He gas is prevented from being directly mixed with the ³ He gas in the space above the heat conduction block 26. Therefore, the ³ He outlet 21 is sucked by the vacuum pump 46,
It is possible to prevent the concentration of ³ He in the exhaust gas from decreasing.

Further, ⁴ H evaporated on the inner surface of the inner container 18
Although a part of the e gas descends to reach the fractionation chamber 40, most of the liquid level 23A of the liquid phase 23 in the fractionation chamber 40 is covered by the recovery cover 56, and therefore the liquid level 23A The ⁴ He is not mixed with the ³ He evaporating gas, and almost only the ³ He gas is recovered.
It is sucked upward from the inside through the extension pipe 54 and the ³ He passage 27. Although the recovery cover 56 is provided in the embodiment, even if the recovery cover 56 is not provided, as described above, the presence of the closed tube 50 may reduce the concentration of ³ He gas in the recovered gas. It can be prevented to a considerable extent. Therefore, the invention of claim 1 defines the case where the recovery cover 56 is not provided, and the claim 2 defines the case where the recovery cover 56 is provided.

The shape of the recovery cover 56 is not limited to the one having an arcuate cross section as shown in FIG.
Or various things such as a short cylindrical shape are conceivable,
The point is that the lower end side should be enlarged. Further, it is desirable that the lower opening end of the recovery cover 56 is slightly immersed below the liquid surface of the fractionating chamber 40 as described above, but in some cases, it may be located above the liquid surface without being immersed. However, some effect can be obtained.

[0033]

Effects of the Invention According to the dilution refrigerator of the present invention, ⁴ from a thin layer of superfluid helium rose to Tsutai inner surface of the inner container
It is possible to effectively prevent the He vaporized gas from being mixed with the ³ He gas vaporized in the fractionation chamber to lower the ³ He concentration in the recovered gas, and therefore, the recovered gas is sent again into the refrigerator for circulation. Since the ³ He concentration in the gas at that time can be kept sufficiently high, there is a possibility that the dilution refrigeration performance will deteriorate due to the decrease in the ³ He concentration in the circulating gas and a sufficient ultra-low temperature cannot be obtained. It is possible to prevent, stably, reliably and sufficiently obtain an ultralow temperature.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a dilution refrigerator according to an embodiment of the present invention.

FIG. 2 is a block diagram showing the principle of a dilution refrigerator.

FIG. 3 is a schematic diagram showing an example of a conventional simple dilution refrigerator.

[Explanation of symbols]

18 Inner Container 21 ³ He Discharge Port 24 Plunger 26 Heat Conduction Block 27 ³ He Passage 28 ³ He Supply Pipe 30 Condenser 34 Fractional Chamber Heat Exchanger 36 Heat Exchanger 38 Mixing Chamber 40 Fractional Chamber 44 Discharge Port 46 Vacuum Pump 50 Closed pipe 54 Extension pipe 56 Recovery cover

Claims (2)

(57) [Claims] 1. A heat-conducting block and a plunger are inserted vertically from above into a cylindrical inner container having a bottom, which is cooled by liquid helium from the outside. The lower space is also divided into an upper distilling chamber of the plunger and a lower mixing chamber of the plunger, and the heat transfer block is formed with a ³ He passage vertically passing therethrough. Within
^{A 3} He supply pipe is guided, the ³ He supply pipe is connected to a condenser incorporated in the heat conduction block, and a heat exchanger is provided in the fractionation chamber and between the plunger and the inner surface of the inner container. A heat exchanger is provided at the outlet side of the condenser is connected in that order to the heat exchanger in the fractionating chamber and the heat exchanger in the plunger position, and the lower end of the heat exchanger in the plunger position is the mixing chamber. And a discharge port for discharging ³ He is formed in the tip of the mixing chamber, and a ³ He discharge port for discharging ³ He gas is formed above the heat conduction block. In the dilution refrigerator configured as described above, a sealed tube is concentrically inserted from the upper side into the inner container, and a tube wall lower portion of the sealed tube is airtightly integrated with an outer peripheral edge portion of the heat transfer block. Is bound to
Further, the dilution refrigerator in which the ³ He discharge port is formed above the closed tube. 2. An extension pipe is provided at the lower end of the ³ He passage that vertically penetrates the heat conduction block, and an extension pipe is provided to extend the passage downward into the fractionation chamber, and at the lower end of the extension pipe, The dilution refrigerator according to claim 1, wherein a lower end of the distilling chamber is opened, and a recovery cover whose peripheral wall portion expands downward is provided.