Dilution refrigerator and dilution refrigeration method

Abstract

A dilution refrigerator comprises: a separator (50) that is disposed below a mixer (10) and separates helium-3 from a mixed liquid; a heat exchange unit (60) that is disposed between the mixer (10) and the separator (50), forms a counter flow between a dense phase mixed liquid which rises from the separator (50) to the mixer (10) and a dilute phase mixed liquid which flows down from the mixer (10) to the separator (50), and performs heat exchange between the dense phase mixed liquid and the dilute phase mixed liquid by means of the counter flow; a helium-4 circulation unit (40) that comprises a filter for separating helium-4 from the mixed liquid, separates the helium-4 from the dilute phase mixed liquid in the separator (50) by using the mechanocaloric effect, and returns the helium-4 to the dilute phase mixed liquid in the mixer (10); and an auxiliary refrigerator that achieves low temperature by means of reduced-pressure discharge of the helium-3. The auxiliary refrigerator cools the helium-4 which is returned to the mixer (10) by the helium-4 circulation unit (40).

Description

Dilution refrigerator and dilution refrigeration method

[0001] The present disclosure relates to a refrigerant-free dilution refrigerator and a dilution refrigeration method.

[0002] Helium 4 ( ４ He) and its isotope helium 3 ( ３ At temperatures below 0.86 K, the liquid mixture with Helium-3 separates into a dense phase (phase C) with a high concentration of Helium-3 and a dilute phase (phase D) with a low concentration of Helium-3. The mixing chamber of the dilution refrigerator maintains an extremely low temperature of 100 mK or less by transferring Helium-3 from the dense phase liquid mixture to the dilute phase liquid mixture.

[0003] A cryogen-free dilution refrigerator comprises a mechanical refrigerator, a fractionation chamber, a mixing chamber, and a heat exchanger. As mentioned above, a vacuum pump is also provided to continuously transfer 3He from the dense phase to the dilute phase. The vacuum pump is generally placed in the room temperature section, with its intake side connected to a pipe leading to the fractionation chamber and its exhaust side connected to a pipe leading to the mixing chamber. A closed circulation system consisting of the mixing chamber - heat exchanger - fractionation chamber - vacuum pump - (mechanical refrigerator heat exchanger) - heat exchanger - mixing chamber is formed, and 3He is circulated using the driving force of the vacuum pump. As 3He moves from the mixing chamber to the fractionation chamber, the mixing chamber also transfers 3He from the dense phase liquid mixture to the dilute phase liquid mixture within the mixing chamber. The fractionation chamber is connected to the mixing chamber via a heat exchanger. The fractionation chamber is depressurized by the vacuum pump to evaporate 3He, so that 3He moves from the dilute phase liquid mixture in the mixing chamber to the fractionation chamber. Because the vapor pressure of 3He in a mixture of 3He and 4He is more than 10 times greater than that of 4He, the vapor in the fractionation chamber is primarily 3He gas. The fractionation chamber loses latent heat as 3He evaporates, providing a cooling capacity of 0.7 to 1.0 K. The mechanical refrigerator cools 3He in the mechanical refrigerator heat exchanger so that the 3He flowing in from the vacuum pump in the room-temperature section can be returned to the mixing chamber. The capillary tube that introduces 3He passes from the mechanical refrigerator heat exchanger through a capillary tube with high flow resistance (impedance), through the fractionation chamber heat exchanger, and then connects to the heat exchanger. The 3He cooled in the mechanical refrigerator heat exchanger passes through a capillary tube with high flow resistance, where its temperature is further reduced by the Joule-Thomson effect (JT effect). After passing through the JT effect, the 3He is cooled in the fractionation chamber heat exchanger by the cooling capacity of the fractionation chamber. The heat exchange section exchanges heat between the helium 3 that is cooled by the mechanical refrigerator and returned to the mixing chamber, and the diluted phase of the mixed liquid (phase D liquid) that flows from the mixing chamber toward the fractionation chamber (see, for example, Patent Document 1).

[0004] International Publication No. 2019 / 163978

[0005] "Experimental Techniques at Ultralow Temperatures," edited by the Institute of Low Temperature Physics and Engineering, Academy of Chemical Sciences of Ukraine, translated by Hideki Yayama and I. B. Berkutov, Kyushu University Press, October 1, 2000. "Physics at Ultralow Temperatures," by Yoshika Masuda, Nagoya University Press, December 1, 1987.

[0006] As described above, the heat exchange section of a dilution refrigerator exchanges heat between the high-temperature liquid (Helium 3) returning to the mixing chamber and the low-temperature liquid (D-phase liquid) leaving the mixing chamber, so that the liquid returning to the mixing chamber is cooled. The cooling capacity of a dilution refrigerator depends on lowering the temperature of the Helium 3 entering the mixing chamber as much as possible. Therefore, a structure that efficiently transfers heat from the high-temperature liquid to the low-temperature liquid has been sought. Currently, the mainstream heat exchange section has a structure in which a porous body made by sintering silver or copper particles of 1 μm or less is sintered and bonded to both sides of a silver or copper member to create a partition member. This structure has two flow paths separated by this partition member (see, for example, Non-Patent Documents 1 and 2).

[0007] Meanwhile, in the heat exchange section of a dilution refrigerator, the high-temperature liquid flow path and the low-temperature liquid flow path are separated by a partition wall. Therefore, even if the partition wall is made of a material with high thermal conductivity, such as copper or silver, there is a limit to how much heat exchange efficiency can be improved. In particular, at temperatures below 100 mK, a large thermal resistance occurs between the Helium-3 liquid, D-phase liquid, and metals such as silver or copper. This thermal resistance, called Kapitza resistance, is caused by a mismatch in acoustic impedance between low-density liquid helium and high-density metals, and is an unavoidable obstacle (see, for example, Non-Patent Documents 1 and 2).

[0008] Furthermore, the cooling capacity of the mixing chamber increases as the amount of 3He circulated increases, but as the amount of 3He circulated increases, the temperature of the 3He returning to the mixing chamber also increases. Therefore, even if the amount of 3He circulated is increased to increase the cooling capacity, there is a limit to how much the cooling capacity can be increased.

[0009] A dilution refrigerator according to one embodiment is a dilution refrigerator having a mixed liquid composed of 3He and 4He, which includes a mixer that forms an internal phase separation interface between a dilute phase mixed liquid and a dense phase mixed liquid, and which generates extremely low temperatures by moving the 3He across the phase separation interface. The dilution refrigerator comprises: a separator that is disposed below the mixer and separates 3He from the mixed liquid; a heat exchange unit that is disposed between the mixer and the separator and forms countercurrent flows between a dense phase mixed liquid that rises from the separator to the mixer and a dilute phase mixed liquid that flows down from the mixer to the separator, and performs heat exchange between the dense phase mixed liquid and the dilute phase mixed liquid by means of these countercurrent flows; a 4He circulating unit that has a filter that separates 4He from the mixed liquid and uses thermomechanical effect to separate 4He from the dilute phase mixed liquid in the separator and return the 4He to the dilute phase mixed liquid in the mixer; and an auxiliary refrigerator that generates a low temperature by decompressing and exhausting the 3He, and the auxiliary refrigerator cools the 4He that the 4He is returned to the mixer by the 4He circulating unit.

[0010] A dilution refrigeration method according to one embodiment is a dilution refrigeration method in which a mixed liquid is composed of 3helium and 4helium, a phase separation interface between a dilute phase mixed liquid and a dense phase mixed liquid is formed inside a mixer, and a cryogenic temperature is created by moving the phase separation interface in the 3helium, the method comprising: separating the 3helium from the mixed liquid in a separator arranged below the mixer; forming countercurrents in a heat exchanger arranged between the mixer and the separator, between a dense phase mixed liquid rising from the separator toward the mixer and a dilute phase mixed liquid flowing down from the mixer into the separator, and performing heat exchange between the dense phase mixed liquid and the dilute phase mixed liquid by the countercurrent flows; separating the 4helium from the dilute phase mixed liquid in the separator using a thermomechanical effect in a 4helium circulation unit equipped with a filter that separates the 4helium from the mixed liquid, and returning the 4helium to the dilute phase mixed liquid in the mixer; and cooling the 4helium that the 4helium that the 4helium circulation unit returns to the mixer using an auxiliary refrigerator that creates a low temperature by decompressing and exhausting the 3helium.

[0011] According to each of the above configurations, the dilute phase liquid mixture and the dense phase liquid mixture form countercurrent flows while directly contacting each other in the heat exchange section. In this heat exchange through direct contact between liquids, there is almost no difference in density between the two, so no acoustic impedance mismatch occurs, and the efficiency is approximately 10,000 times higher than heat exchange between helium liquid and metal. Therefore, the dense phase liquid mixture introduced into the mixer is easily cooled to a temperature approximately equal to that of the dilute phase liquid mixture discharged from the mixer or the dense phase liquid mixture in the mixer.

[0012] Furthermore, the temperature achieved by depressurizing and exhausting 3-helium is lower than the temperature achieved by depressurizing and exhausting 4-helium. The cooling of 4-helium by the auxiliary refrigerator uses this depressurization and exhausting of 3-helium. Therefore, even if it is required to increase the circulation rate of 4-helium, the temperature of the 4-helium can be easily cooled to the temperature of the mixer.

[0013] Furthermore, since 4He is not required as a separate cryogen, the dilution refrigerator is easier to handle and the size of the dilution refrigerator is prevented from increasing. In the above configuration, the 4He circulation unit may include a circulation pump, and the circulation pump may be configured to draw 4He from the separator by using a thermomechanical effect caused by raising the temperature of the circulation pump higher than that of the separator.

[0014] According to the above configuration, the heated helium 4 is cooled by the auxiliary refrigerator for helium 3. Therefore, the configuration in which heat is used to separate helium 4 or to circulate helium 4 is suitable for application.

[0015] The above configuration may further include a control unit that controls the operation of the circulation pump, and the control unit may drive the circulation pump so that any one selected from the group consisting of the temperature of the circulation pump, the pressure inside the circulation pump, and the amount of 4He circulated by the circulation pump remains constant.

[0016] According to the above configuration, the circulation of 4 helium is stabilized, and therefore the output temperature of the mixer is stabilized. In the above configuration, the auxiliary refrigerator may include a pot that stores 3 helium in liquid form and an outlet tube that discharges the 3 helium from the pot, and the 4 helium circulation unit may include a storage unit cooled by the auxiliary refrigerator that stores 4 helium introduced from the circulation pump and discharges the 4 helium into the mixer, and a circulation tube that connects the circulation pump to the storage unit and is arranged to exchange heat with the outlet tube.

[0017] Fig. 1 is a diagram showing the configuration of a dilution refrigerator together with its temperatures, and Fig. 2 is an operational diagram showing heat exchange in the dilution refrigerator.

[0018] A refrigerant-free dilution refrigerator will be described with reference to Figure 1. [Configuration of dilution refrigerator] As shown in Figure 1, the refrigerant-free dilution refrigerator comprises a mixer 10, a control unit 20, a helium-3 circulation unit 30, a helium-4 circulation unit 40, a separator 50, and a heat exchange unit 60. The helium-3 circulation unit 30 is an example of an auxiliary refrigerator. In Figure 1, the helium-4 flow path is indicated by a thick dashed line, and the helium-3 flow path, the dense phase liquid mixture flow path, and the dilute phase liquid mixture flow paths are indicated by thick solid lines.

[0019] The mixer 10 is connected to the 4H helium circulation section 40 by the third circulation tube 14. The mixer 10 is connected to the heat exchange section 60 by the first heat exchange tube 16. The mixer 10 is connected to the separator 50 via the 4H helium circulation section 40. The mixer 10 is connected to the separator 50 via the heat exchange section 60. The separator 50 is connected to the 4H helium circulation section 40 by the first circulation tube 54. The separator 50 is connected to the heat exchange section 60 by the second heat exchange tube 62.

[0020] The mixer 10 is disposed above the separator 50. The separator 50 is disposed below the mixer 10 and the heat exchange section 60. The mixer 10 is adjusted to form a phase separation interface between a dense phase liquid mixture SC1 (see FIG. 2) and a dilute phase liquid mixture SD1 (see FIG. 2) inside the mixer 10. In the mixer 10, the helium 3 concentration in the dense phase liquid mixture SC1 is approximately 100%. In the mixer 10, the helium 3 concentration in the dilute phase liquid mixture SD1 is approximately 6.4%. The mixer 10 transfers helium 3 from the dense phase liquid mixture SC1 to the dilute phase liquid mixture SD1. Through the phase transfer of helium 3, the mixer 10 maintains the output temperature T11 of the mixer 10 at an extremely low temperature of 100 mK or less.

[0021] The 3He circulation unit 30 includes a 3He pot 31 (0.6K pot), an inlet tube 32, an outlet tube 33, and a vacuum pump 34. The 3He pot 31 is connected to the external vacuum pump 34 via the inlet tube 32 and the outlet tube 33. The 3He pot 31 stores the 3He introduced from the inlet tube 32 by the evacuation of the vacuum pump 34. The 3He pot 31 discharges the helium 3 stored in the 3He pot 31 from the outlet tube 33 by the evacuation of the vacuum pump 34. The vacuum pump 34 maintains the output temperature T31 of the 3He pot 31 at 0.6K.

[0022] The mechanical refrigerator 35 is a pulse tube refrigerator, a GM refrigerator, or the like. The mechanical refrigerator 35 is a second auxiliary refrigerator that cools the inside of the dilution refrigerator to 4 K. The separator 50 is thermally connected to the helium-3 pot 31 via a heat conduction plate 36. The separator 50 is cooled to the output temperature T31 of the pot 31. In the separator 50, helium-4 is extracted by a circulation pump 41, and helium-3 in excess of the two-phase equilibrium concentration is precipitated from a dense phase liquid mixture SC5 (see FIG. 2). The separator 50 generates heat through the separation of helium-3 and generates a dilute phase liquid mixture SD5 (see FIG. 2). The separator 50 is adjusted to form a phase separation interface between the dense phase liquid mixture SC5 and the dilute phase liquid mixture SD5 within the separator 50.

[0023] The separator 50 causes the dense phase mixed liquid SC5 to float together with the separated helium 3 to the heat exchange section 60. The dense phase mixed liquid SC5 that floats to the heat exchange section 60 has been cooled to 0.6 K, which is the output temperature T31 of the pot 31.

[0024] [Helium 4 Circulation System] The helium 4 circulation unit 40 includes a first circulation tube 54, a circulation pump 41, a helium 4 storage unit 42, and a second circulation tube 43 connecting the circulation pump 41 and the storage unit 42.

[0025] The first circulation tube 54 includes a super leak filter. The super leak filter separates helium-4 from the dilute phase liquid mixture SD5. The super leak filter is, for example, a porous material with connecting pores having a pore size of 1 μm or less. Helium-4 becomes superfluid at temperatures of 2.17 K or less. Superfluid helium-4 passes through the connecting pores without friction, while viscous helium-3 does not pass through the connecting pores. The first circulation tube 54 separates helium-4 from the dilute phase liquid mixture SD5 delivered from the separator 50 and introduces the separated helium-4 into the circulation pump 41.

[0026] The circulation pump 41 discharges the separated helium 4 from the separator 50. The circulation pump 41 is equipped with a heater 41H. The heater 41H utilizes the thermomechanical effect (fountain effect) to draw the helium 4 into the circulation pump 41 through the first circulation tube 54. That is, the circulation pump 41 raises the temperature of the helium 4 above that of the separator 50 to the extent that the helium changes from superfluid to normal flow. The circulation pump 41 draws the superfluid helium 4 through the first circulation tube 54 so as to equalize the chemical potential of the helium 4 between the separator 50 and the circulation pump 41 by the amount that the concentration of the superfluid helium 4 has been reduced.

[0027] The reservoir 42 is thermally connected to the pot 31 (0.6 K pot) via the heat conduction plate 36. The reservoir 42 introduces 4He from the circulation pump 41 through the second circulation tube 43. The reservoir 42 stores 4He and cools it to 0.6 K. The reservoir 42 introduces the stored 4He into the dilute phase liquid mixture SD1 in the mixer 10. The third circulation tube 14 is equipped with a super leak filter. The super leak filter prevents backflow of the 3He contained in the dilute phase liquid mixture SD1 in the mixer 10. As a result, the 4He circulation unit 40 circulates only 4He.

[0028] As described above, the 3He circulation unit 30 circulates 3He using the external vacuum pump 34, while maintaining the temperature of the 3He pot 31 at 0.6 K, which is the output temperature T31. The second circulation tube 43, which connects the circulation pump 41 and the reservoir 42, circulates around the outlet tube 33 of the 3He circulation unit 30. Because 3He vapor flows into the outlet tube 33 at 0.6 K, the 3He circulation unit 30 absorbs the exhaust heat of the 4He circulation unit 40 from the 4He circulation unit 40 through heat exchange between the 3He in the outlet tube 33 and the 4He in the second circulation tube 43. The 3He circulation unit 30 further cools the 4He filtered by the 4He circulation unit 40 to 0.6 K, which is the output temperature T31.

[0029] The control unit 20 controls the operation of the circulation pump 41. The control unit 20 monitors any one selected from the group consisting of the temperature of the circulation pump 41, the pressure inside the circulation pump 41, and the flow rate of 4He delivered from the circulation pump 41. The control unit 20 controls the output of the heater 41H so that the monitored value remains constant. In this way, the control unit 20 stabilizes the amount of 4He circulated between the separator 50 and the mixer 10. The control unit 20 then stabilizes the cooling capacity of the dilution refrigerator.

[0030] The heat exchange unit 60 includes a cold plate 61 and heat exchange tubes 16, 62. The heat exchange tubes 16, 62 exchange heat between the cooled dilute phase mixed liquid SD6 (derived from the dilute phase mixed liquid SD1 flowing down from the mixer 10) flowing from the mixer 10 toward the separator 50 and the dense phase mixed liquid SC6 flowing from the separator 50 toward the mixer 10. The heat exchange unit 60 includes a cold plate 61 between the heat exchange tubes 16, 62. The cold plate 61 also exchanges heat between the dilute phase mixed liquid SD1 flowing down from the mixer 10 and the dense phase mixed liquid SC6 floating up to the mixer 10. As a result of the heat exchange in the heat exchange tubes 16, 62, the temperature of the cold plate 61 at the intermediate position becomes 0.1 to 0.2 K, which is the output temperature T61.

[0031] [Operation of Dilution Refrigerator] As shown in Figure 2, the density of the dilute phase liquid mixture SD1 is higher than the density of the dense phase liquid mixture SC1. Therefore, in the dilution refrigeration method, when helium-4 is introduced from the helium-4 circulation section 40 into the mixer 10, the dilute phase liquid mixture SD1 overflows from the mixer 10. The dilute phase liquid mixture SD1 overflowing from the mixer 10 flows down through the heat exchange section 60 toward the separator 50 as the dilute phase liquid mixture SD6. Meanwhile, when the dilute phase liquid mixture SD1 flows down from the mixer 10 into the heat exchange section 60, the dense phase liquid mixture SC6 rises from the heat exchange section 60 to the mixer 10. In the mixer 10, the concentration of helium-3 in the dilute phase liquid mixture SD1 is 6.4%. In the mixer 10, the concentration of helium-3 in the dense phase liquid mixture SC1 is nearly 100%. In separator 50, the concentration of helium 3 in dilute phase liquid mixture SD5 is approximately 30%. In separator 50, the concentration of helium 3 in dense phase liquid mixture SC5 is approximately 85%. In heat exchange section 60, helium 3 moves from dense phase liquid mixture SC6 to dilute phase liquid mixture SD1 during heat exchange, and the concentration of helium 3 in dilute phase liquid mixture SD6 changes from 6.4% to 30%. The concentration of helium 3 in dense phase liquid mixture SC6 changes from 85% to 100%.

[0032] As a result, the dilute phase mixed liquid SD6 and the dense phase mixed liquid SC6 flow in opposite directions within the heat exchange section 60. That is, the dilute phase mixed liquid SD6 and the dense phase mixed liquid SC6 form countercurrent flows while directly contacting each other within the heat exchange section 60. The dilute phase mixed liquid SD6 flowing from the mixer 10 toward the separator 50 and the dense phase mixed liquid SC6 flowing from the separator 50 toward the mixer 10 exchange heat through direct liquid-to-liquid contact, with the dilute phase mixed liquid SD6 breaking down into minute droplets. Heat exchange through direct liquid-to-liquid contact is approximately 10,000 times more efficient than heat exchange through contact between a liquid and a metal such as silver. Therefore, the dense phase mixed liquid SC6 introduced into the mixer 10 is likely to be cooled to a temperature approximately equal to that of the dilute phase mixed liquid SD6 discharged from the mixer 10 or the dense phase mixed liquid SC1 within the mixer 10.

[0033] In this way, the temperature of the dense phase liquid mixture SC6 introduced from the separator 50 into the mixer 10 becomes approximately equal to the output temperature T11 of the mixer 10 due to direct heat exchange between the liquids. When the temperature of the dense phase liquid mixture SC6 introduced into the mixer 10 and the output temperature T11 are approximately equal, the cooling capacity of the mixer 10 is roughly proportional to the amount of 4 helium circulated. In this case, since the 4 helium discharged from the circulation pump 41 is cooled by the 3 helium circulating section 30, even if the temperature of the circulation pump 41 is increased to increase the amount of 4 helium circulated, the 4 helium introduced into the mixer 10 is cooled to the output temperature T31 through heat exchange with the 3 helium pot 31. The amount of 4 helium circulated by the mixer 10 is maintained at a high value as monitored by the control section 20.

[0034] According to the above embodiment, the following effects can be obtained: (1) The dilute phase liquid mixture SD6 and the dense phase liquid mixture SC6 form countercurrent flows while coming into direct contact with each other in the heat exchange section 60. Therefore, the dense phase liquid mixture SC6 introduced into the mixer 10 is likely to be cooled to a temperature approximately equal to that of the dilute phase liquid mixture SD6 discharged from the mixer 10 or the dense phase liquid mixture SC1 in the mixer 10.

[0035] (2) The temperature achieved by decompression and exhaust of 3-helium is lower than the temperature achieved by decompression and exhaust of 4-helium. The 3-helium circulation unit 30 uses decompression and exhaust of 3-helium to cool the circulating 4-helium. Therefore, even if it is required to increase the circulation rate of 4-helium, the temperature of the 4-helium can be easily cooled to the output temperature T11 of the mixer 10. Furthermore, because 4-helium is not required as a separate cryogen, handling of the dilution refrigerator is easier and the size of the dilution refrigerator is prevented from increasing.

[0036] (3) The helium 4 heated by the circulation pump 41 is cooled by the helium 3 pot 31. Therefore, the above embodiment is suitable for application to a configuration in which heat is used to separate the helium 4 or to circulate the helium 4.

[0037] (4) The control unit 20 monitors the temperature of the circulation pump 41, etc., so that the circulation of the helium 4 is stabilized, thereby stabilizing the output temperature of the mixer 10. The above-described embodiments may be modified and implemented as follows.

[0038] The 4He circulation unit 40 may further include a second reservoir (1K pot) that stores 4He and cools the 4He to 1K. The second reservoir cools the 4He introduced from the circulation pump 41 and introduces the 4He into the dilute phase liquid mixture SD1 in the mixer 10.

[0039] The control unit 20 may monitor the flow rate of the dilute phase mixture SD5 introduced from the separator 50 to the circulation pump 41, or may monitor the flow rate of helium 4 in the tube 14. The control unit 20 controls the output of the heater 41H so that the monitored value remains constant.

[0040] The heat exchange section 60 is not limited to a combination of the cold plate 61, the first heat exchange tube 16, and the second heat exchange tube 62, but may be composed of only the cold plate 61 or only the first heat exchange tube 16.

[0041] SC1, SC5, SC6... dense phase liquid mixture SD1, SD5, SD6... dilute phase liquid mixture T11, T31, T61... output temperature 10... mixer 14... third circulation tube 16... first heat exchange tube 20... control unit 30... circulation unit 31... pot 32... inlet tube 33... outlet tube 34... vacuum pump 35... mechanical refrigerator 40... helium 4 circulation unit 41... circulation pump 42... storage unit 43... second circulation tube 50... separator 54... first circulation tube 60... heat exchange unit 61... cold plate 62... second heat exchange tube

Claims

1. A dilution refrigerator in which a mixed liquid is composed of 3He and 4He, comprising a mixer that forms a phase separation interface between a dilute phase mixed liquid and a dense phase mixed liquid inside, and that creates a cryogenic temperature by moving the phase separation interface in the 3He, comprising: a separator that is located below the mixer and separates the 3He from the mixed liquid; a heat exchange unit that is located between the mixer and the separator and creates a countercurrent flow between the dense phase mixed liquid floating up from the separator to the mixer and the dilute phase mixed liquid flowing down from the mixer to the separator, and performs heat exchange between the dense phase mixed liquid and the dilute phase mixed liquid by means of the countercurrent flow; a 4He circulation unit that has a filter that separates 4He from the mixed liquid, and uses thermomechanical effect to separate 4He from the dilute phase mixed liquid in the separator and return the 4He to the dilute phase mixed liquid in the mixer; and an auxiliary refrigerator that creates a low temperature by decompressing and exhausting the 3He, a dilution refrigerator, wherein the auxiliary refrigerator cools the 4He that the 4He circulation unit returns to the mixer; 2. The dilution refrigerator according to claim 1, wherein the 4He circulation unit includes a circulation pump, and the circulation pump draws 4He from the separator by using a thermomechanical effect caused by raising the temperature of the circulation pump higher than that of the separator.

3. The dilution refrigerator according to claim 2, wherein the auxiliary refrigerator comprises: a pot for storing liquid 3He; and an outlet tube for discharging the 3He from the pot; and the 4He circulation unit comprises: a storage unit cooled by the auxiliary refrigerator, which stores the 4He introduced from the circulation pump and introduces the 4He into the mixer; and a circulation tube connecting the circulation pump to the storage unit, which is positioned so as to exchange heat with the outlet tube.

4. A dilution refrigerator according to claim 2 or 3, further comprising a control unit that controls the operation of the circulation pump, wherein the control unit drives the circulation pump so that any one selected from the group consisting of the temperature of the circulation pump, the pressure inside the circulation pump, and the amount of 4He circulated by the circulation pump remains constant.

5. A dilution refrigeration method in which a mixed liquid is composed of 3He and 4He, a phase separation interface between a dilute phase mixed liquid and a dense phase mixed liquid is formed inside a mixer, and a cryogenic temperature is created by moving the phase separation interface in the 3He, comprising: separating the 3He from the mixed liquid in a separator located below the mixer; forming countercurrents in a heat exchange unit located between the mixer and the separator, between a dense phase mixed liquid rising from the separator toward the mixer and a dilute phase mixed liquid flowing down from the mixer to the separator, and performing heat exchange between the dense phase mixed liquid and the dilute phase mixed liquid by the countercurrents; separating the 4He from the dilute phase mixed liquid in the separator using a thermomechanical effect in a 4He circulating unit equipped with a filter that separates the 4He from the mixed liquid, and returning the 4He to the dilute phase mixed liquid in the mixer, and cooling the 4He returned to the mixer by the 4He circulating unit using an auxiliary refrigerator that creates a low temperature by decompressing and exhausting the 3He. A dilution refrigeration method characterized by the above.