Cryogenic cooling system with active heat exchanger

Abstract

The cryogenic cooling system comprises a dilution refrigerator, the dilution refrigerator having a mixing chamber (401), a fractionator (402), and a circulator for circulating the working fluid through the mixing chamber (401) and the fractionator (402). At least one active heat exchanger (408, 801, 802, 901) has a first end and a second end and is configured to use input energy to transfer heat from the first end to the second end during operation. The first end and the second end of the at least one active heat exchanger (408, 801, 802, 901) are thermally coupled at their respective positions along the circulator to transfer heat between the respective portions of the circulating working fluid.

Description

Technical Field

[0001] The present invention generally relates to the cooling of cryostats. In particular, the present invention relates to a structural solution and a cooling mechanism that can efficiently cool a cryostat and reasonably suppress the complexity of the structure.

Background Art

[0002] Initial cryostats were cooled with liquid refrigerants such as liquid nitrogen and liquid helium. Subsequently, mechanical cooling devices such as Stirling cryocoolers, Gifford-McMahon coolers, pulse tube refrigerators (PTRs), and Joule-Thomson coolers were introduced, realizing so-called refrigerant-free cooling. When an additional cooling system such as a dilution refrigerator that can operate only at temperatures below about 4 K is provided in the core part of the cryostat, the necessary precooling can be performed, for example, by a PTR. In a typical case, a PTR is used to consist of two cooling stages. The first stage achieves a temperature of about 40 K to 70 K, and the second stage precools to the 3 K to 4 K level required for the fractionator of the dilution refrigerator.

[0003] FIG. 1 is a simplified schematic diagram of a cryostat equipped with a dilution refrigerator and a mechanical precooler. The outermost structure of the cryostat is a vacuum enclosure 101 shown by a dashed line in FIG. 1. The uppermost flange 102 is the lid of the vacuum enclosure. The room temperature stage 103 of the mechanical precooler is attached thereto. The first stage 104 of the mechanical precooler is attached to the first flange 105, and the second stage 106 of the mechanical precooler is attached to the second flange 107. The first flange and the second flange may be called, for example, the 50 K flange and the 4 K flange, reflecting the operating temperature.

[0004] Further down, there are several more flanges, including the fractionator flange 108, to which the fractionator 109 of the dilution refrigerator is attached. In Figure 1, the mixing chamber 110 of the dilution refrigerator is attached to the base temperature flange 111. There may be one or more intermediate flanges between the fractionator flange 108 and the base temperature flange 111, but these are not shown in Figure 1 for clarity. Reference numeral 112 indicates the target region of the payload to be frozen. The payload, often called a sample, needs to be securely attached to the base temperature flange 111 to ensure the best possible thermal conductivity.

[0005] The cylindrical radiation shield, although not shown in Figure 1 for clarity, is typically mounted in a nested configuration to the flange. Aligned openings may be provided in the flange, together with the upper cover 113, to provide a so-called line-of-sight port to the target region 112. If rapid sample exchange is required, a sample changer may be provided to deliver the sample to the target region 112 through the aligned openings without warming or opening the entire cryostat. The sample changer may be a top-loading type as applied in Figure 1, or a bottom-loading type.

[0006] Figure 2 schematically shows the main components of the dilution refrigerator. The coldest part is the mixing chamber 110, where the concentrated phase 202 and the diluted phase 203 of the He3-He4 mixture are separated by a phase boundary 201. The circulation of He3 is maintained by a pump (not shown). The path of the He3 inflow stream is provided with flow impedances 204 and 205 and a heat exchanger for cooling the helium flowing toward the mixing chamber 110. Figure 2 shows a first heat exchanger 206 in the fractionator 109 where the helium mixture and He3 vapor cool the inflow stream, and a second heat exchanger 207 where the outflowing He3 cools the inflow stream. A heater 208 is provided to ensure an appropriate amount of He3 evaporation in the fractionator 109.

[0007] Figure 3 shows the heat flow between the parts shown in Figure 2. In the bottom layer, heat flows from the payload 301 to the mixing chamber 110 due to an endothermic reaction in which He3 moves across the phase boundary into the dilution phase. In the second heat exchanger 207 layer, heat flows from the warmer He3 inflow stream to the cooler outflow stream. In the fractionator 109 layer, heat flows to the fractionator from both the inflow stream 304 and the heater 208. The inflow streams are indicated by different reference numerals 304 to highlight the slightly different temperatures and interactions, even though they are the same He3 inflow streams as in the intermediate layer.

[0008] Dilution refrigerators inherently have relatively low cooling capacity. For example, attempting to increase cooling capacity by increasing the circulation rate of He3 usually fails to reach the base temperature as low as when the flow rate is moderate. As cryogenic cooling trends involve larger payloads, and consequently, the demands on cooling capacity are also increasing, developments that can improve cooling capacity without sacrificing achievable base temperature levels would be welcome. [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The objective is to present a cryostat and a cooling method for a cryostat that solves the problem of large thermal loads in an advantageous and technically simple manner. Another objective is to ensure that this solution is scalable to larger cryostats. A further objective is to solve the problem of increasing thermal loads without sacrificing operational reliability. A further objective is to combine effective cooling with a moderate increase in structural complexity. [Means for solving the problem]

[0010] These and further advantageous objectives are achieved by utilizing an active heat exchanger combined with a helium flow that provides a working fluid for cooling.

[0011] According to a first embodiment, a cryogenic cooling system is provided which includes a dilution refrigerator. The components of the dilution refrigerator are a mixing chamber, a still, and a circulation arrangement for circulating a working fluid through the mixing chamber and the still. The cryogenic cooling system includes at least one active heat exchanger having a first end and a second end, configured to use input energy to transfer heat from the first end to the second end during operation. The first end and the second end of the at least one active heat exchanger are thermally coupled at their respective positions along the circulation arrangement to transfer heat between the respective portions of the circulating working fluid.

[0012] According to one embodiment, among the at least one active heat exchanger, • Solid-state microrefrigerator (hereinafter referred to as NIS refrigerator) based on a normal-conducting metal-insulator-superconductor tunnel junction. • A solid-state micro-refrigerator based on a superconductor-insulator-normal-conducting metal-insulator-superconductor tunnel junction (hereinafter referred to as a SINIS refrigerator), • Solid-state micro-refrigeration based on semiconductor-superconductor tunnel junctions (hereinafter referred to as Sm-S refrigerators) It includes at least one of the following.

[0013] This has the advantage of allowing the use of active heat exchanger technology that possesses properties known to be suitable for this type of application.

[0014] According to one embodiment, in the at least one active heat exchanger, from the first end toward the second end, • A first contact electrode layer for biasing, • A conductive or semiconductive substrate layer, • Layers of a normal-conducting metal-insulator-superconductor tunnel junction or a superconductor-insulator-normal-conducting metal-insulator-superconductor tunnel junction, • Superconducting layer, • A second contact electrode layer for biasing, This includes at least one NIS or SINIS refrigerator equipped with the following in this order.

[0015] This has the advantage that, at the very least, efficient thermal coupling to the working fluid flow can be achieved, coupled with sufficient heat transfer in the desired direction through an active heat exchanger.

[0016] According to one embodiment, the superconducting layer includes a constriction for restricting the propagation of phonons from the superconducting layer to the layer of the normal-conducting metal-insulator-superconductor tunnel junction or the superconductor-insulator-normal-conducting metal-insulator-superconductor tunnel junction. This has the advantage of reducing the leakage of thermal energy to the rear, at least in the active heat exchanger.

[0017] According to one embodiment, the at least one active heat exchanger includes a solid microrefrigerator based on the use of electrons in a standard transistor structure as a gas equivalent refrigerant that is alternately expanded and compressed in a Carnot cycle to induce cooling. This has the advantage of adding further versatility to the implementation of the active heat exchanger.

[0018] According to one embodiment, the at least one active heat exchanger includes an electric heat cooler. This has the advantage of adding further versatility to the implementation of at least the active heat exchanger.

[0019] According to one embodiment, the circulation device comprises a first channel for introducing an inflow stream of working fluid into the mixing chamber and a second channel for leading out an outflow stream of working fluid from the mixing chamber. The first end of the first active heat exchanger is thermally coupled to the first channel, and the second end of the first active heat exchanger is thermally coupled to the second channel. This has the advantage of very effectively directly cooling the flow of working fluid into the mixing chamber.

[0020] According to one embodiment, the cryogenic cooling system comprises a set of active heat exchangers, including the first active heat exchanger. Each active heat exchanger in the set may then have its first end thermally coupled to the first channel and its second end thermally coupled to the second channel. At least two of the active heat exchangers in the set may be NIS refrigerators and may have superconducting portions of their normal-conducting metal-insulator-superconductor tunnel junctions made of superconductors with different transition temperatures. The at least two NIS refrigerators may be arranged in a sequence in which the distance along the first channel increases from the point where the working fluid exits the first channel into the mixing chamber during operation, and the sequence is also the sequence in which the transition temperatures increase. This has the advantage that the operation of the active heat exchangers in the set can be optimized to match the temperature gradually decreasing toward the coldest part of the dilution refrigerator.

[0021] According to one embodiment, the circulating device includes a second channel for directing the outflow stream of working fluid from the mixing chamber to the fractionator. The first end of the second active heat exchanger may then be thermally coupled to a first part of the second channel. The second end of the second active heat exchanger may also be thermally coupled to a second part of the second channel, the second part being located further along the second channel than the first part from the position where the second channel draws working fluid from the mixing chamber during operation. This has the advantage that it can indirectly promote the cooling of the working fluid flowing into the mixing chamber by lowering the temperature of the portion of the working fluid that absorbs heat from the inflow stream of working fluid.

[0022] According to one embodiment, the cryogenic cooling system comprises one or more first thermal couplers that are coupled to each portion of the working fluid circulating at the first end of each of the active heat exchangers. This has the advantage of being able to enhance the thermal coupling between the working fluid and the active heat exchangers.

[0023] According to one embodiment, the cryogenic cooling system includes one or more second thermal couplers that couple to respective portions of the working fluid circulating at the second end of each of the active heat exchangers. This has the advantage of at least enhancing the thermal coupling between the working fluid and the active heat exchanger.

[0024] According to one embodiment, at least one of the thermal couplers includes a fixed amount of sintered heat conductor material within a space that forms part of each of the channels. This has the advantage of at least being able to use well-proven techniques well-known for the implementation of thermal couplers.

[0025] According to one embodiment, at least one of the thermal couplers includes a structured internal surface made of a heat conductor material on one or more walls of a space that forms part of each of the channels, the structured internal surface being formed by an additive manufacturing process and comprising a plurality of extended heat conduction paths in the form of regular-shaped portions of the heat conductor material that extend through most of the structured thickness of the internal surface. This has the advantage of at least being able to utilize an effective thermal coupling structure that cannot be achieved with other manufacturing techniques.

Brief Description of the Drawings

[0026] The accompanying drawings, which are incorporated to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

[0027] [Figure 1] Shows a cryostat with refrigerant-free cooling according to known techniques. [Figure 2] Shows a known dilution refrigerator. [Figure 3] Shows the heat flow in the dilution refrigerator of FIG. 2. [Figure 4] Shows a dilution refrigerator according to one embodiment. [Figure 5] Shows a dilution refrigerator according to one embodiment. [Figure 6] Shows a dilution refrigerator according to one embodiment. [Figure 7] A dilution refrigerator according to one embodiment is shown. [Figure 8] A dilution refrigerator according to one embodiment is shown. [Figure 9] A dilution refrigerator according to one embodiment is shown. [Figure 10] This shows a cryogenic cooling system according to one embodiment. [Modes for carrying out the invention]

[0028] The following description refers to the accompanying drawings, which form part of this disclosure and illustrate specific embodiments in which this disclosure may be arranged. It should be understood that other embodiments may be utilized and structural or logical modifications may be implemented without departing from the scope of this disclosure. Therefore, the following detailed description should not be constrained, as the scope of this disclosure is defined by the accompanying claims.

[0029] For example, it should be understood that disclosures relating to a described method may also apply to a corresponding device or system configured to perform the method, and vice versa. For example, if a particular method step is described, the corresponding device may include a unit for performing the described method step, even if such a unit is not explicitly described or illustrated in the figures. On the other hand, for example, if a particular device is described based on a functional unit, the corresponding method may include steps for performing the described function, even if such steps are not explicitly described or illustrated in the figures. Furthermore, it should be understood that features of the various exemplary embodiments described herein may be combined with each other unless otherwise specified.

[0030] Figure 4 shows a dilution refrigerator that constitutes part of a cryogenic cooling system. The components of the dilution refrigerator are a mixing chamber 401, a fractionator 402, and a circulator that circulates the working fluid through the mixing chamber 401 and the fractionator 402. A fractionator heater 403 is also shown in Figure 4. In the arrangement shown in Figure 4, the circulator includes, among other things, a first channel 404 for introducing the working fluid inflow stream 405 into the mixing chamber 401 and a second channel 406 for leading the working fluid outflow stream 407 out of the mixing chamber 401. All of these components may be similar to the corresponding components of a conventional dilution refrigerator shown in Figure 2 above.

[0031] In contrast to Figure 2, the cryogenic cooling system in Figure 4 includes an active heat exchanger 408 having a first end (left side in Figure 4) and a second end (right side in Figure 4). The fact that the heat exchanger 408 is an active heat exchanger means that it is configured to use input energy to transfer heat from the first end to the second end during operation.

[0032] Passive heat exchangers rely solely on the second law of thermodynamics, where two objects at different temperatures essentially aim for thermal equilibrium, whereas active heat exchangers use input energy to transfer heat from one end to the other during operation. Examples of the types of active heat exchangers that can be used, as shown in Figure 4, will be described in more detail later in this text.

[0033] In Figure 4, the principle of thermally coupling the active heat exchanger 408 to the parts of the dilution refrigerator is important. The first and second ends of the active heat exchanger 408 are thermally coupled at their respective positions along the circulation device to transfer heat between each part of the circulating working fluid. This allows the working fluid to be used not only for its normal purpose as the working fluid of the dilution refrigerator, but also as a heat transfer fluid to transfer the heat transferred through the active heat exchanger to the second end, and ultimately to an even further distance.

[0034] In particular, in the configuration shown in Figure 4, the first end of the active heat exchanger 408 is thermally coupled to the first channel 404, and the second end of the active heat exchanger 408 is thermally coupled to the second channel 406. Thus, the active heat exchanger 408 is configured to transfer heat between the working fluid inlet stream 405 and the working fluid outlet stream 407. The temperature of the working fluid in the inlet stream 405 is approximately 100 mK, and the temperature of the working fluid in the outlet stream 407 is approximately 10 mK to 30 mK. Once heat is transferred from the inlet stream 405 to the outlet stream 407, the working fluid that ultimately flows into the mixing chamber 401 may be at a lower temperature. This allows the mixing chamber 401 to reach a lower base temperature during operation and / or allows the dilution refrigerator to absorb heat more efficiently from the payload on which it is installed for cooling. The fact that the working fluid outflow stream 407 becomes warmer than it would be without the active heat exchanger 408 is not a problem because, further upstream of the dilution refrigerator, the working fluid flowing out of the mixing chamber will be warm anyway.

[0035] In Figure 4, the active heat exchanger 408 is shown somewhere between the mixing chamber 401 and the distillation unit 402; however, it should be noted that this is for the sake of clarity in the diagram. More important is the thermal coupling between the ends of the active heat exchanger and the parts of the circulation system.

[0036] In the schematic diagram of Figure 4, the cryogenic cooling system includes a first thermal coupler 409 that connects the first end of the active heat exchanger 408 to the working fluid inflow stream 405. Similarly, the cryogenic cooling system includes a second thermal coupler 410 that connects the second end of the active heat exchanger 408 to the working fluid outflow stream 407. Examples of how such thermal couplers can be constructed will be described in more detail later in this paper.

[0037] Furthermore, the cryogenic cooling system in Figure 4 includes a passive heat exchanger 411 between the first channel 405 and the distillation unit 402. At this level, the working fluid flowing into the mixing chamber 401 is warmer than the working fluid in the distillation unit 402, so the purpose of the passive heat exchanger 411 is to transfer heat from the upper part of the working fluid inflow stream 405 to the main body of the working fluid in the distillation unit 402. Since the distillation unit 402 locally forms part of the working fluid circulation system, it can also be said that the passive heat exchanger 411 transfers heat between each part of the circulating working fluid.

[0038] Figure 5 shows a part of another cryogenic cooling system, similar in some respects to Figure 4, but with the positions and roles of the two heat exchangers reversed. The passive heat exchanger 501 is coupled between the first channel 404 and the second channel 406 in the dilution refrigerator section between the mixing chamber 401 and the fractionator 402. The active heat exchanger 502 is coupled between the top of the first channel 404 and the fractionator 402. Functionally, their purpose is the same as in Figure 4; that is, both transfer heat between their respective parts of the circulating working fluid, ultimately making the working fluid entering the mixing chamber 401 as cold as possible.

[0039] One possible form of the active heat exchanger 408 or 502 is a solid microrefrigerator based on the use of electrons in a standard transistor structure as a gas equivalent refrigerant that is alternately expanded and compressed in a Carnot cycle to induce cooling. Such an active heat exchanger is known, for example, in U.S. Patent Application Publication No. 2022 / 0208644, the contents of which are incorporated herein by reference.

[0040] Another possible form of the active heat exchanger 408 or 502 is an electrocaloric cooler. Such active heat exchangers are known, for example, from scientific literature ("Electrocaloric Cooling: A Review of the Thermodynamic Cycles, Materials, Models, and Devices" by Adriana Greco and Claudia Masselli, Magnetochemistry 2020, 6, 67; doi:10.3390 / magnetochemistry6040067), which is incorporated herein by reference.

[0041] Another possible form of the active heat exchanger 408 or 502 is a solid micro-refrigerator based on a normal-conducting metal-insulator-superconductor tunnel junction, a superconductor-insulator-normal-conducting metal-insulator-superconductor tunnel junction, or a semiconductor-superconductor tunnel junction. Such active heat exchangers are often referred to as NIS refrigerators, SINIS refrigerators, or Sm-S refrigerators, respectively. An example of an NIS refrigerator is known, for example, by U.S. Patent No. 6,581,387, the contents of which are incorporated herein by reference. An example of an Sm-S refrigerator is known, for example, by scientific literature ("Thermionic junction devices utilizing phonon blocking" by Emma Mykkanen et al., Sci. Adv. 2020; 6: eaax9191, April 10, 2020), the contents of which are incorporated herein by reference.

[0042] Figure 6 shows a part of a cryogenic cooling system that uses an NIS refrigerator, SINIS refrigerator, or Sm-S refrigerator as an active heat exchanger, similar to the active heat exchanger 408 shown in Figure 4. For simplicity, the active heat exchanger will be referred to as the NIS refrigerator below.

[0043] The vessel shown in Figure 6 functions as the mixing chamber of a dilution refrigerator. On the left side, a first channel 404 is provided for introducing the working fluid inflow stream into the mixing chamber. On the right side, a second channel 406 is provided for leading the working fluid outflow stream from the mixing chamber. At the top of the vessel, a sintered body 601 is schematically shown as housing a passive heat exchanger between the working fluid inflow and outflow streams.

[0044] The NIS refrigerator 408 in Figure 6 has a layered structure. A first contact electrode layer 602, made of gold, copper, or another material with excellent thermal and electrical conductivity, is provided at the first end of the NIS refrigerator to bias it. To the right of the first contact electrode layer 602 in Figure 6 are a conductive or semiconducting substrate layer 603, made of, for example, metal or crystalline silicon, and a layer 604 of a normal-conducting metal-insulator-superconductor tunnel junction. In the case of a SINIS refrigerator, layer 604 consists of a superconductor-insulator-normal-conducting metal-insulator-superconductor tunnel junction.

[0045] Further to the right are a superconducting layer 605 for biasing and a second contact electrode layer 606. Similar to the first contact electrode layer 602, the second contact electrode layer 606 is made of gold, copper, or another material with excellent thermal and electrical conductivity. The bias voltage between the first and second contact electrode layers 602 and 606 generates the electric field across them necessary for the proper operation of the NIS refrigerator 408. In addition to this bias, the second contact electrode layer 606 also functions as a quasiparticle trap, and for this purpose, it is advantageous to place it near the tunnel junction.

[0046] The operating principle of NIS (or SINIS, or Sm-S) refrigerators is based on the tunneling effect. The bias voltage across the NIS refrigerator is set so that only the highest-energy, i.e., the hottest electrons can tunnel through the thin insulating layer to reach the superconductor, thereby lowering the mean electron temperature of the normal-conducting metal. The transferred thermal energy can leak in the opposite direction in the form of phonons, especially if the overall temperature is too high. However, by keeping the overall temperature sufficiently low and using specific structural solutions, the undesirable reverse leakage of thermal energy can be suppressed.

[0047] As an example of a specific structural solution, the superconducting layer 605 in Figure 6 may have a constriction to restrict phonon propagation from the superconducting layer 605 to the layer 604 of the normal-conducting metal-insulator-superconductor tunnel junction (or superconductor-insulator-normal-conducting metal-insulator-superconductor tunnel junction). The constriction may be fabricated by any known means, for example, by patterning a region near the tunnel junction and using a suitable material that changes the acoustic impedance. This is intended to block phonon-based heat transport by reflecting phonons. The reflection may be enhanced by using metamaterials, which mean layered materials fabricated by atomic layer deposition in which two or more materials alternate in layers of atomic-scale thickness. Photolithography is also available for the aforementioned type of patterning.

[0048] Structural solutions in NIS-, SINIS-, or Sm-S refrigerators may also include one or more solutions disclosed in U.S. Patent Application Publication No. 2022 / 0272869, which are incorporated herein by reference.

[0049] The cooling efficiency that can be achieved using NIS, SINIS, or Sm-S refrigerators depends, in particular, on the superconductor selected for the "S" side of the tunnel junction. It is known that the typical temperature level at which NIS, SINIS, or Sm-S refrigerators should operate should be below and appropriately related to the critical temperature of the selected superconductor. In a cryogenic cooling system like the one in Figure 6, the typical temperature range is expected to be below 100 mK, because this is the temperature that the working fluid inflow stream can achieve in the upper (passive) heat exchanger 601. As a rough guideline, it may be advantageous to select the critical temperature of the superconductor at a level approximately twice the assumed temperature of the warmer end of the operating NIS, SINIS, or Sm-S refrigerator.

[0050] A first thermal coupler 409 is provided to thermally couple the first end of the active heat exchanger 408 to the portion of the working fluid circulating through the first channel 404. Similarly, a second thermal coupler 410 is provided to thermally couple the second end of the active heat exchanger 408 to the portion of the working fluid circulating through the second channel 406.

[0051] In cryogenic cooling systems, a certain amount of sintered thermal conductive material is frequently used as a means of providing an effective thermal bond between the working fluid and the solid material. Therefore, here again, at least one of the first thermal coupler 409 and the second thermal coupler 410 may contain a certain amount of sintered thermal conductive material in the space forming part of its respective channel. The use of sintered thermal conductive material is based on the fact that, in its sintered form, the material provides a relatively large overall contact surface through which heat can flow between the working fluid and the solid material. Simultaneously, a heat conduction path through the sintered material allows heat to flow between a certain amount of sintered material and the solid surface to which it is attached.

[0052] As an addition or alternative, at least one of the first thermal coupler 409 and the second thermal coupler 410 may have a structured internal surface made of a thermal conductive material on one or more walls of the space forming part of each channel. The structured internal surface may be formed by an additive manufacturing process and may have a plurality of extending heat conduction paths in the form of regularly shaped portions of the thermal conductive material that extend through most of the structured thickness of the internal surface. The objective is the same as in the case of sintered materials, namely, to provide a large overall heat transfer surface to the working fluid and to simultaneously provide heat conduction paths to the solid surface below. Such structured internal surfaces are known, for example, in European Patent Application Publication No. 3910276, which is incorporated herein by reference.

[0053] Figure 7 shows a part of a cryogenic cooling system using an NIS refrigerator or SINIS refrigerator as an active heat exchanger, similar to the active heat exchanger 408 shown in Figures 4 and 6. For simplicity, the active heat exchanger will be referred to as an NIS refrigerator below.

[0054] In addition to the embodiment shown in Figure 6, the cryogenic cooling system of Figure 7 includes a passive heat exchanger 501 that provides another thermal coupling between the first channel 404 and the second channel 406. In the embodiment of Figure 7, the passive heat exchanger 501 is located further away from the point where the working fluid exits the first channel 404 into the mixing chamber during operation than the active heat exchanger 408 (measured along the first channel 404). Also, the passive heat exchanger 501 is located further away from the point where the second channel 406 draws the working fluid out of the mixing chamber during operation (measured along the second channel 406). In other words, the passive heat exchanger 501 is located slightly warmer than the active heat exchanger 408 with respect to the temperature of the working fluid flowing through the first channel 404 and the second channel 406. As an addition or alternative, a passive heat exchanger may be located between the first channel 404 and the second channel 406 below the active heat exchanger 408, that is, close to the position where the working fluid exits the first channel 404 into the mixing chamber during operation, and close to the position where the second channel 406 draws the working fluid out of the mixing chamber.

[0055] Figure 8 shows a portion of a cryogenic cooling system in which a set of NIS refrigerators and / or SINIS refrigerators 408, 801, and 802 are used as active heat exchangers, similar to the single active heat exchanger 408 shown in Figures 4 and 6.

[0056] Each of the active heat exchangers 408, 801, and 802 has its first end thermally coupled to the first channel 404 and its second end thermally coupled to the second channel 406. At least two of the active heat exchangers in set 408, 801, and 802 are NIS refrigerators or SINIS refrigerators.

[0057] Since the active heat exchangers 408, 801, and 802 operate at different typical temperature levels, with the lowest operating at the lowest typical temperature, it may be advantageous to construct the superconducting portions of their NIS or SINIS tunnel junctions with superconductors having different transition temperatures. In particular, since at least two such NIS or SINIS refrigerators are arranged in a sequence where the distance along the first channel 404 increases from the point where the working fluid exits the first channel 404 into the mixing chamber during operation, it may also be advantageous to arrange them in a sequence where their transition temperatures increase.

[0058] Figure 9 shows a part of a cryogenic cooling system that uses an NIS refrigerator or SINIS refrigerator as an active heat exchanger, similar to the active heat exchanger 408 shown in Figures 4 and 6. Furthermore, the cryogenic cooling system in Figure 9 includes a second NIS refrigerator or SINIS refrigerator 901.

[0059] The circulation system of the cryogenic cooling system in Figure 9 includes a second channel 406 for leading the outflow stream of the working fluid from the mixing chamber to a fractionator (not shown in Figure 9). The second active heat exchanger 901 is thermally coupled between two positions along the second channel 406, rather than between the first and second channels. In other words, the first end of the second active heat exchanger 901 is thermally coupled to the first part of the second channel 406, and the second end of the second active heat exchanger 901 is thermally coupled to the second part of the second channel 406. The second part is further along the second channel 406 than the first part from the position where the second channel 406 draws the working fluid from the mixing chamber during operation. It is therefore important to note that in the graphical representation used in Figure 9, the layers of the NIS refrigerator shown as an example of the second active heat exchanger 901 are in a mirror image order compared to the layers of the first active heat exchanger 408.

[0060] In the embodiment of Figure 9, the first active heat exchanger 408 is not essential, but it may help keep the base temperature of the mixing chamber as low as possible. One or more similarly coupled passive heat exchangers can be used as an alternative to or addition to the first active heat exchanger 408. Alternatively, in the embodiment of Figure 8, a set of thermally coupled active heat exchangers can be provided between the first channel and the second channel.

[0061] The purpose of using the second active heat exchanger 901 in Figure 9 is to transfer the heat coupled to the working fluid outflow stream to further points along the second channel 408 at a speed that would be impossible by simply waiting for the working fluid to flow through it. As a result of the first end of the second active heat exchanger 901 (right end in Figure 9) becoming cold, the second end (right end in Figure 9) of the first active heat exchanger 408 (or any other type of heat exchanger that may be present) also becomes cold. Assuming that the first active heat exchanger 408 (or any other type of heat exchanger that may be present) can establish a constant maximum temperature difference ΔT between its ends, eventually the first end of the first active heat exchanger 408 (left end in Figure 9) also becomes cold, which also helps to obtain and maintain the lowest possible base temperature of the mixing chamber during operation.

[0062] In the graphical representation used in Figure 9, the second channel 406 is curved downward toward the second end (left end) of the second active heat exchanger 901. In reality, the working fluid flows through the second channel 406 under the influence of gravity, at least partially, due to the temperature gradient within the channel, so it is sometimes advantageous to avoid making the second channel meander vertically. It is relatively easy to design the actual path of the second channel in a way that avoids such meandering while maintaining proper thermal coupling to each part of the second channel. For example, each part of the second channel may be oriented horizontally.

[0063] In the main drawing of Figure 9, the thermal couplers 410 and 902 are shown as separate entities, even though they are adjacent to the longitudinal direction of the second channel 406. A partially enlarged view shows a slightly modified embodiment in which there is an integrated thermal coupler 903 that forms thermal bonds both between the second end of the first active heat exchanger 408 and the second channel 406, and between the first end of the second active heat exchanger 901 and the second channel 406. In this way, the additional cooling effect provided by the second active heat exchanger 901 makes it possible to cool the second end of the first active heat exchanger 408 more efficiently.

[0064] Figure 10 is a block diagram of a cryogenic cooling system. The main block shown in Figure 10 includes a mechanical refrigerator 1001, a dilution refrigerator 1002, a thermal switch 1003, a valve 1004, a sensor 1005, and a control system 1006. The mechanical refrigerator block comprises at least a compressor 1011, a sensor 1012, and a pump 1013. As an alternative to (or in addition to) the mechanical refrigerator, refrigeration based on a liquid cryogenic substance, such as liquid helium added and / or circulated from an external source, may be used. The dilution refrigerator block 1002 comprises subsystems such as at least a helium circulator 1021, an active heat exchanger 1022, and a sensor 1023. The active heat exchanger subsystem comprises one or more active heat exchangers arranged and operating as described above with reference to any of Figures 4 to 9. The control system 1006, among other things, is supplied to the vacuum chamber of the cryogenic cooling system via appropriate cables and supplies bias voltages, etc., to the active heat exchangers. The control system 1006 adjusts its operation in response to sensor signals received from various sensors, including the sensors shown in blocks 1012, 1023, and 1005 in Figure 10.

[0065] Those skilled in the art will see that, with advances in technology, the basic idea of ​​the present invention can be implemented in various ways. Therefore, the present invention and its embodiments are not limited to the examples described above, but rather may be modified within the scope of the claims.

Claims

1. • Dilution refrigerator and, - The dilution refrigerator includes a mixing chamber (401), a fractionator (402), and a circulation device that circulates the working fluid through the mixing chamber (401) and the fractionator (402), A cryogenic cooling system comprising, The cryogenic cooling system comprises at least one active heat exchanger (408, 801, 802, 901) having a first end and a second end, configured to use input energy to transfer heat from the first end to the second end during operation, - The first end and the second end of the at least one active heat exchanger (408, 801, 802, 901) are thermally coupled to their respective positions along the circulation device, and heat is transferred between the respective portions of the circulating working fluid. A cryogenic cooling system characterized by the following features.

2. Among the at least one active heat exchanger (408, 801, 802, 901), - Solid-state micro-refrigerator based on a normal-conducting metal-insulator-superconductor tunnel junction (hereinafter referred to as NIS refrigerator), • A solid-state micro-refrigerator based on a superconductor-insulator-normal-conducting metal-insulator-superconductor tunnel junction (hereinafter referred to as the SINIS refrigerator), • Solid-state micro-refrigerator based on semiconductor-superconductor tunnel junction (hereinafter referred to as Sm-S refrigerator) The cryogenic cooling system according to claim 1, comprising at least one of the following.

3. In the at least one active heat exchanger (408, 801, 802, 901), from the first end toward the second end, - A first contact electrode layer (602) for biasing, - A conductive or semiconductive substrate layer (603), - A layer (604) of a normal-conducting metal-insulator-superconductor tunnel junction or a superconductor-insulator-normal-conducting metal-insulator-superconductor tunnel junction, - Superconducting layer (605), - Second contact electrode layer (606) for bias The cryogenic cooling system according to claim 2, comprising at least one NIS refrigerator or SINIS refrigerator (408) having the elements in this order.

4. The cryogenic cooling system according to claim 3, wherein the superconductor layer (605) includes a constriction for restricting the propagation of phonons from the superconductor layer (605) to the layer (604) of the normal-conducting metal-insulator-superconductor tunnel junction or the superconductor-insulator-normal-conducting metal-insulator-superconductor tunnel junction.

5. The cryogenic cooling system according to any one of claims 1 to 4, wherein at least one of the active heat exchangers (408, 801, 802, 901) includes a solid microrefrigerator based on the use of electrons in a standard transistor structure as a gas equivalent refrigerant that is alternately expanded and compressed in a Carnot cycle to induce cooling.

6. The cryogenic cooling system according to any one of claims 1 to 5, wherein at least one of the active heat exchangers (408, 801, 802, 901) includes an electric heat cooler.

7. The circulation device comprises a first channel (404) for introducing an inflow stream (405) of working fluid into the mixing chamber (401), and a second channel (406) for leading out an outflow stream (407) of working fluid from the mixing chamber (401). - The first end of the first active heat exchanger (408) is thermally coupled to the first channel (404). The cryogenic cooling system according to any one of claims 1 to 6, wherein the second end of the first active heat exchanger (408) is thermally coupled to the second channel (406).

8. The cryogenic cooling system comprises a set of active heat exchangers (408, 801, 802), which includes the first active heat exchanger (408). Each active heat exchanger (408, 801, 802) in the set has its first end thermally coupled to the first channel (404) and its second end thermally coupled to the second channel (406). - At least two of the sets (408, 801, 802) are NIS refrigerators and have superconducting portions of their normal-conducting metal-insulator-superconductor tunnel junctions made of superconductors with different transition temperatures. The cryogenic cooling system according to claim 7, wherein the at least two NIS refrigerators are arranged in a sequence such that the distance along the first channel (404) increases from the point where the working fluid exits the first channel (404) into the mixing chamber during operation, and the sequence is also the sequence in which the transition temperature increases.

9. The circulation device includes a second channel (406) for guiding the outflow stream (407) of the working fluid from the mixing chamber (401) to the fractionator (402), - The first end of the second active heat exchanger (901) is thermally coupled to the first part of the second channel (406). The cryogenic cooling system according to any one of claims 1 to 8, wherein the second end of the second active heat exchanger (901) is thermally coupled to a second part of the second channel (406), and the second part is located further along the second channel (406) than the first part, from the position where the second channel (406) draws working fluid from the mixing chamber during operation.

10. A cryogenic cooling system according to any one of claims 1 to 9, comprising one or more first heat couplers (409) that are coupled to the respective portions of the working fluid circulating at the first ends of each of the active heat exchangers (408).

11. A cryogenic cooling system according to any one of claims 1 to 10, comprising one or more second heat couplers (410) coupled to each portion of the working fluid circulating at the second end of each of the active heat exchangers (408).

12. The cryogenic cooling system according to claim 10 or 11, wherein at least one of the thermal couplers (409, 410) contains a certain amount of sintered thermal conductive material in the space forming a portion of each of the channels.

13. The cryogenic cooling system according to claim 10 or 11, wherein at least one of the thermal couplers (409, 410) comprises a structured internal surface made of a thermal conductive material on one or more walls of a space forming part of each of the channels, the structured internal surface comprising a plurality of extended thermal conduction paths in the form of regularly shaped portions of the thermal conductive material that are formed by an additive manufacturing process and extend through most of the structured thickness of the internal surface.

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