Cryogenic cooling system with active heat exchanger

Abstract

The cryogenic cooling system comprises a circuit (302, 501, 601) configured to transport a working fluid to or from a first heat exchanger area (303), the circuit having a thermal coupling to a working area (301). The circuit (inlet part (304, 502, 602)) precedes the first heat exchanger area in the direction of flow of the working fluid through the circuit. The circuit outlet part (305, 503, 603) follows the first heat exchanger area in the direction of flow of the working fluid through the circuit. One or more active heat exchangers (401, 504, 505, 506) are coupled between the inlet part and the outlet part and configured to transfer heat from the inlet part to the outlet part during operation using input energy.

Description

[Technical Field]

[0001] The present invention relates generally to the cooling of cryostats, and more particularly to structural solutions and cooling mechanisms that allow for efficient cooling of cryostats while keeping structural complexity reasonably low. [Background technology]

[0002] Early cryostats were cooled with a liquid refrigerant bath, such as liquid nitrogen or liquid helium. Later, mechanical cooling devices, such as Stirling refrigerators, Gifford-McMahon refrigerators, pulse tube refrigerators (PTRs), and Joule-Thomson refrigerators, were introduced, realizing so-called cryogen-free cooling. When the core of a cryostat is equipped with an additional cooling system, such as a dilution refrigerator, which can only operate at temperatures below approximately 4 K, the necessary pre-cooling can be achieved, for example, by a PTR. Typically, a PTR is used to configure two cooling stages: the first stage achieves temperatures of approximately 40 K to 70 K, and the second stage pre-cools the dilution refrigerator's still to the required 3 K to 4 K level.

[0003] A dilution refrigerator is a closed-circuit system in which the isotopic composition of the working fluid changes between the different stages of the circuit. The two isotopes involved are 3He and 4He. In the mixing chamber, a 3He-rich phase floats above a 3He-poor phase. By constant pumping, some of the 3He moves across the phase boundary to the 3He-poor phase, and the heat required for this dilution provides useful cooling power at the base temperature. A so-called dilution channel leads from the mixing chamber to the still, from which the 3He vaporizes and circulates back through compression and pre-cooling before finally returning to the mixing chamber.

[0004] There are also cryogenic cooling systems in which the working fluid circulates in a closed circuit without any change in its isotopic composition. An example of such a cooling system is the so-called siphon probe, which can be used to insert a sample into a cryogenically cooled environment. A cold working fluid, such as liquid helium, circulates through an inlet capillary tube in the probe and reaches the end where the sample holder is attached. The cold working fluid absorbs heat from the sample holder, and the heated (and at least partially evaporated) working fluid is then collected through a return tube in the probe. Corresponding fluid circulation devices can also be used in cryostats that are not equipped with a sample exchange mechanism, but the circulating working fluid cools only the working area or some other stationary part of the cryostat.

[0005] The essential performance merits of any cryogenic cooling system include the lowest base temperature achievable and the cooling capacity available at each temperature level. In a closed circuit based cryogenic cooling system, achieving the best performance at the working region or lowest temperature level requires reducing the enthalpy of the working fluid as much as possible before it reaches the portion of the circuit where it begins to absorb heat from the components to be cooled in the working region of the circuit. Summary of the Invention [Problem to be solved by the invention]

[0006] It is an object of the present invention to provide a cryogenic cooling system and method that provides a reasonable balance between achievable base temperature and cooling capacity. Another object is to combine effective cooling with a reasonable increase in structural complexity. [Means for solving the problem]

[0007] These and further advantageous objects are achieved by utilizing one or more active heat exchangers in combination with a circuit for circulating a working fluid for cooling.

[0008] According to a first aspect, there is provided a cryogenic cooling system for cooling a working area, the cryogenic cooling system comprising: a circuit configured to transport a working fluid to and from a first heat exchanger area, the circuit having a thermal coupling to the working area; an inbound part of the circuit preceding the first heat exchanger area in a flow direction of the working fluid through the circuit; and an outbound part of the circuit following the first heat exchanger area in a flow direction of the working fluid through the circuit; the cryogenic cooling system comprising one or more active heat exchangers coupled between the inbound part and the outbound part and configured to transfer heat from the inbound part to the outbound part during operation using input energy.

[0009] According to one embodiment, the cryogenic refrigeration system is configured to maintain a constant isotopic composition of the working fluid throughout the circuit, which has the advantage at least that relatively simple and robust refrigeration systems may also benefit from the advantageous features of the present invention.

[0010] According to one embodiment, the cryogenic cooling system is configured to control flow characteristics of the working fluid such that a specific heat capacity of the working fluid in a portion of the outflow part to which one of the one or more active heat exchangers transfers heat is greater than a specific heat capacity of the working fluid in a portion of the inflow part to which the one active heat exchanger transfers said heat, which has the advantage at least that the active heat exchangers can be operated with a relatively high efficiency.

[0011] According to one embodiment, the at least one active heat exchanger comprises at least one of a solid-state microrefrigerator based on a normal metal-insulator-superconductor tunnel junction (hereinafter referred to as NIS refrigerator), a solid-state microrefrigerator based on a superconductor-insulator-normal metal-insulator-superconductor tunnel junction (hereinafter referred to as SINIS refrigerator), and a solid-state microrefrigerator based on a semiconductor-superconductor tunnel junction (hereinafter referred to as Sm-S refrigerator), which has the advantage of being able to use at least the sophisticated properties and relatively high efficiency at cryogenic temperatures of these types of refrigerators.

[0012] According to one embodiment, the at least one active heat exchanger includes at least one NIS or SINIS refrigerator, comprising, in this order from a first end of the NIS or SINIS refrigerator to a second end of the NIS or SINIS refrigerator, a first bias contact electrode layer, a conductive or semiconductive substrate layer, a normal metal-insulator-superconductor tunnel junction or a superconductor-insulator-normal metal-insulator-superconductor tunnel junction layer, a superconductor layer, and a second bias contact electrode layer, which has at least the advantage of allowing for systematic and efficient coupling of the refrigerator to a circuit.

[0013] According to one embodiment, the superconductor layer comprises a constriction for restricting the propagation of phonons from the superconductor layer to a layer of the normal metal-insulator-superconductor tunnel junction or superconductor-insulator-normal metal-insulator-superconductor tunnel junction, which has the advantage at least that the efficiency of the refrigerator can be improved.

[0014] According to one embodiment, the at least one active heat exchanger includes a solid-state micro-refrigerator 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, which has the advantage of at least utilizing the elegant properties and relatively high efficiency at very low temperatures of these types of refrigerators.

[0015] According to one embodiment, the at least one active heat exchanger includes an electric calorific cooler, which has the advantage of making use of at least the sophisticated characteristics and relatively high efficiency at very low temperatures of these types of refrigerators.

[0016] According to one embodiment, the cryogenic cooling system comprises a set of active heat exchangers. Each active heat exchanger of the set may then be coupled between the inflow part and the outflow part and configured to transfer heat from the inflow part to the outflow part during operation using input energy. At least two active heat exchangers of the set may be NIS refrigerators, and may have superconductor portions of their normal metal-insulator-superconductor tunnel junctions made of superconductors with different transition temperatures. The at least two NIS refrigerators may then be arranged along the circuit in order of increasing distance from the first heat exchanger region, which order is also in order of increasing transition temperature. This has the advantage of at least being able to optimally utilize the best performance of re-refrigerators based on different superconductor materials.

[0017] According to one embodiment, the cryogenic cooling system comprises a further active heat exchanger coupled between a first portion of the outflow part and a second portion of the outflow part, the second portion being further along the outflow part from the first heat exchanger area than the first portion. The further active heat exchanger may then be configured to use input energy to transfer heat from the first portion to the second portion during operation. This has the advantage of at least allowing the coldest portion of the system to reach an even lower temperature.

[0018] According to one embodiment, the cryogenic cooling system comprises one or more first thermal couplers coupling a respective end of each of the active heat exchangers to a respective portion of the working fluid in each of the parts of the circuit, which has the advantage that at least an optimal thermal coupling between the working fluid and the active heat exchanger can be ensured.

[0019] According to one embodiment, the cryogenic cooling system comprises one or more flow impedances along the inlet part, which has the advantage that at least the phase characteristics and the enthalpy of the working fluid flowing through the inlet part can be influenced. [Brief explanation of the drawings]

[0020] The accompanying drawings, which are included 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.

[0021] [Figure 1] Shows the refrigeration cycle [Figure 2] A refrigeration cycle augmented with active heat transfer is shown. [Figure 3] 1 shows a cryogenic cooling system. [Figure 4] 1 shows a cryogenic cooling system. [Figure 5] 1 shows a cryogenic cooling system. [Figure 6] 1 shows a cryogenic cooling system. DETAILED DESCRIPTION OF THE INVENTION

[0022] In the following description, reference is made to the accompanying drawings which form a part hereof, and which show, by way of illustration, specific embodiments in which the present disclosure may be arranged. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present disclosure. Therefore, the following detailed description is not to be taken in a limiting sense, as the scope of the present disclosure is defined by the appended claims.

[0023] For example, it should be understood that disclosure related to a described method may also apply to a corresponding device or system configured to perform the method, and vice versa. For example, where particular method steps are described, a corresponding device may include units for performing the described method steps, even if such units are not explicitly described or shown in the figures. On the other hand, for example, where a particular apparatus is described in terms of functional units, a corresponding method may include steps for performing the described functions, even if such steps are not explicitly described or shown in the figures. Furthermore, it should be understood that features of various exemplary aspects described herein may be combined with each other, unless otherwise specified.

[0024] FIG. 1 illustrates some thermodynamic aspects of a cryogenic cooling system for cooling a working region. A circuit is configured to transport a working fluid to and from the working region, which is represented in FIG. 1 by the bottom of the drawing. The inlet portion of the circuit is on the left side of FIG. 1, and the flow direction of the working fluid through the circuit is downward, toward the working region. The outlet portion of the circuit is on the right side of FIG. 1, and the flow direction of the working fluid through the circuit is upward, away from the working region.

[0025] The working area contains some type of load to be cooled. Heat from the load causes a working fluid to evaporate 101 at or near the working area. The area where heat from the load is absorbed by the working fluid may be referred to as the (first) heat exchanger area. The escaping vapor carries the heat extracted from the load as part of its enthalpy. In another appropriate part of the system, the vapor is compressed 102 and then condensed 103 to a liquid, which in this part of the cycle releases heat to the surroundings. In liquid form, the working fluid expands 104 and is circulated, causing partial evaporation and auto-cooling. The cooled liquid-vapor mixture is circulated back towards the working area, where a new cycle begins.

[0026] Essentially, a cycle such as that of Figure 1 cools a working area (and a load therein), and the greater the increase in enthalpy of the working fluid as it passes through a (first) heat exchanger area at or near the working area, the more efficiently it can be cooled. Figure 2 illustrates a principle that can be followed to make cooling of the working area more efficient. According to the principle illustrated in Figure 2, one or more active heat exchangers can be coupled between the inlet and outlet parts of the circuit, creating an effect shown in Figure 2 as active heat transfer 201.

[0027] While passive heat exchangers rely solely on the second law of thermodynamics, essentially forcing two bodies at different temperatures to reach thermal equilibrium, active heat exchangers are devices that use input energy to transfer heat from a first end to a second end during operation. In this definition, input energy refers to any form of energy other than that carried as part of the enthalpy of the substance being cooled at the first end. In electrically operated active heat exchangers, the input energy is electrical energy flowing through an electrical circuit of which the active heat exchanger is a part, receiving operating power from an external voltage or current source. Examples of the types of active heat exchangers that can be used, such as those shown in Figure 4, are described in more detail later in this text.

[0028] 2 shows how one or more active heat exchangers coupled between inlet and outlet parts of a circuit (referring to a circuit configured to carry a working fluid) are configured to use input energy to transfer heat from the inlet part to the outlet part during operation. This increases the enthalpy difference between the portion of the working fluid flowing towards the working region and the portion of the working fluid flowing away from the working region, so that the working region is cooled more efficiently than in FIG. 1.

[0029] 3 and 4 show two cryogenic cooling systems that actually demonstrate the main differences between FIGS. 1 and 2. Both cryogenic cooling systems exist to cool a working region 301. A circuit 302 is configured to transport a working fluid (e.g., nitrogen, helium, hydrogen, a mixture thereof, or some other cryogenic cooling fluid) between a first heat exchanger region 303, to which the circuit 302 has thermal coupling to the working region 301. The inlet part 304 of the circuit is on the left side in both systems and is located before the first heat exchanger region 303 in the direction of flow of the working fluid through the circuit 302. The outlet part 305 is on the right side in both systems and is located after the first heat exchanger region 303 in the direction of flow of the working fluid through the circuit 302. A flow impedance 306, constructed, for example, in the form of a throttle valve, may be located at some point in the inlet part 304 to cause expansion of the working fluid, indicated by reference numeral 104 in FIGS. 1 and 2. Also shown is a second heat exchanger area 307 in which a passive heat exchanger is provided between the inlet part 304 and the outlet part 305 of the circuit. If a passive heat exchanger is provided, the purpose of the passive heat exchanger is to transfer heat from the incoming working fluid to the outgoing working fluid, i.e. to cool the incoming working fluid before it enters the first heat exchanger area 303.

[0030] The cryogenic cooling system of Figure 4 comprises an active heat exchanger 401 coupled between the inlet part 304 and the outlet part 305 of the circuit. The active heat exchanger 401 is configured to transfer heat from the inlet part 304 to the outlet part 305 during operation using input energy (not shown separately in Figure 4).

[0031] Referring to the definition of the adsorption heat exchanger above, the first end of the cross-flow heat exchanger 401 is its left end and the second end is its right end in Figure 4. The second end is thermally coupled to the outgoing flow of working fluid so that the working fluid acts as a vehicle to carry away the heat extracted by the active heat exchanger from the incoming flow of working fluid.

[0032] One possible form of active heat exchanger 401 is a solid-state micro-refrigerator 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 active heat exchangers are known, for example, from U.S. Patent Application Publication No. 2022 / 0208644, the contents of which are incorporated herein by reference.

[0033] Another possible form of active heat exchanger 401 is an electrocaloric cooler. Such active heat exchangers are known, for example, from the scientific literature (Adriana Greco, Claudia Masselli, "Electrocaloric Cooling: A Review of the Thermodynamic Cycles, Materials, Models, and Devices," Magnetochemistry 2020, 6, 67; doi:10.3390 / magnetochemistry6040067), the contents of which are incorporated herein by reference.

[0034] Another possible form of the active heat exchanger 401 is a solid-state microrefrigerator based on a normal metal-insulator-superconductor tunnel junction, a superconductor-insulator-normal 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. Examples of NIS refrigerators are known, for example, from U.S. Pat. No. 6,581,387, the contents of which are incorporated herein by reference. Examples of Sm-S refrigerators are known, for example, from the scientific literature (Emma Mykkanen et al., "Thermionic junction devices utilizing phonon blocking," Sci. Adv. 2020; 6: eaax9191, April 10, 2020), the contents of which are incorporated herein by reference.

[0035] Assuming that the active heat exchanger 401 is an NIS refrigerator, it may have a layer structure as shown schematically in FIG. 4. A first contact electrode layer 402 made of gold, copper, or another material with good thermal and electrical conductivity is provided at a first end of the NIS refrigerator for biasing the NIS refrigerator. To the right of the first contact electrode layer 402 in FIG. 4 are a conductive or semiconductive substrate layer 403, which may be made of, for example, metal or crystalline silicon, and a normal metal-insulator-superconductor tunnel junction layer 404. In the case of a SINIS refrigerator, layer 404 is composed of a superconductor-insulator-normal metal-insulator-superconductor tunnel junction.

[0036] Further to the right are the superconductor layer 405 and second contact electrode layer 406 for biasing. Like the first contact electrode layer 402, the second contact electrode layer 406 is made of gold, copper, or another material with good thermal and electrical conductivity. The bias voltage between the first contact electrode layer 402 and the second contact electrode layer 406 generates the electric field across them necessary for proper operation of the NIS refrigerator. In addition to this bias, the second contact electrode layer 406 also functions as a quasiparticle trap, and for this purpose it is advantageous to place it near the tunnel junction.

[0037] The operating principle of an NIS (or SINIS, or Sm-S) refrigerator is based on the tunneling effect. The bias voltage across the NIS refrigerator is set so that only the most energetic, i.e., hottest, electrons can tunnel through the thin insulating layer and reach the superconductor, lowering the average electron temperature of the normal metal. The transferred heat energy can leak back in the form of phonons, especially if the overall temperature is too high. However, by keeping the overall temperature low enough and using certain structural solutions, the unwanted back leakage of heat energy can be suppressed.

[0038] As an example of a specific structural solution, the superconductor layer 405 in FIG. 4 may include a constriction to restrict the propagation of phonons from the superconductor layer 405 to the layer 404 of the normal metal-insulator-superconductor tunnel junction (or superconductor-insulator-normal metal-insulator-superconductor tunnel junction). The constriction may be created by any known means, for example, by patterning the region near the tunnel junction and using an appropriate material with a varying acoustic impedance. The purpose is to block phonon-based heat transport by reflecting phonons. Reflection may also be enhanced by using metamaterials, which refer to layered materials created by atomic layer deposition, in which two or more materials alternate in layers of atomic-scale thickness. Photolithography methods can also be used for this type of patterning.

[0039] 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, the contents of which are incorporated herein by reference.

[0040] The efficiency of cooling that can be achieved with an NIS, SINIS, or Sm-S refrigerator depends, among other things, on the superconductor selected for the "S" side of the tunnel junction. It is known that the general temperature level at which an NIS, SINIS, or Sm-S refrigerator should operate should be below and appropriately related to the critical temperature of the selected superconductor. As a rough guideline, it can be advantageous to select the critical temperature of the superconductor at a level approximately twice the expected temperature of the warm end of the NIS, SINIS, or Sm-S refrigerator during operation.

[0041] In general, a drawback of NIS, SINIS, or Sm-S refrigerators has been the relatively small temperature differential that can be achieved between their ends. Attempting to increase the temperature differential results in an increasingly large proportion of the transferred heat leaking back through the joints via phonon conduction. Furthermore, as noted above, NIS, SINIS, or Sm-S refrigerators can operate efficiently over a relatively narrow temperature range, depending on the materials selected. To circumvent these drawbacks, attempts have been made to stack two or more NIS, SINIS, or Sm-S refrigerators on top of each other, optimizing the materials of the refrigerator closest to the cold end of the stack for the lowest temperature and the next refrigerator in the stack for a slightly higher temperature. However, it should be noted that the efficiency factor of NIS, SINIS, or Sm-S refrigerators can be rather low, on the order of 25%. In a stacked configuration, each refrigerator toward the warmer end of the stack must transfer not only the original thermal energy extracted from the object to be cooled at the cold end, but also the waste heat introduced by the refrigerator preceding it in the stack. Essentially, the efficiency factor of a series of NIS, SINIS, or Sm-S refrigerators is the product of the efficiency factors of all the individual refrigerators in the series. For this reason, it may be more advantageous to place only a single NIS, SINIS, or Sm-S refrigerator, or at most two in series, at the location indicated by reference numeral 401 in Figure 4. In other words, it is advantageous to use refrigerators of this type such that the thermal energy transferred across them passes through only a single layer, or at most two layers, of the bonded structure that constitutes the NIS, SINIS, or Sm-S refrigerator, respectively.

[0042] Thermal couplers 407 are provided to thermally couple the first and second ends of the active heat exchanger 401 to the portions of the working fluid circulating through the inlet part 304 and outlet part 305, respectively. Sintered thermal conductor materials are frequently used in cryogenic cooling systems as a means of providing an effective thermal coupling between the working fluid and a piece of solid material. Accordingly, at least one of the thermal couplers 407 may again include a quantity of sintered thermal conductor material within the space forming the respective part of the circuit. The use of sintered thermal conductor material relies 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. At the same time, the thermal conduction paths through the sintered material allow heat to flow between the quantity of sintered material and the solid surface to which it is attached.

[0043] Additionally or alternatively, at least one of the thermal couplers 407 may comprise a structured internal surface made of a heat conductor on one or more walls of the space forming part of the inlet or outlet part, respectively. The structured internal surface may be formed by an additive manufacturing process and may comprise a plurality of extended heat conduction paths in the form of regularly shaped portions of the heat conductor material extending through most of the structured thickness of the internal surface. The objective is the same as for sintered materials: to provide both a large overall heat transfer surface for the working fluid and simultaneously provide heat conduction paths to the underlying solid surface. This type of structured internal surface is known, for example, from EP 3910276, the contents of which are incorporated herein by reference.

[0044] The cryogenic cooling system represented by the schematic diagram of Figure 4 may be configured to maintain a constant isotopic composition of the working fluid throughout the circuit. This distinguishes it from a system in which the inlet flow of working fluid consists of essentially pure 3He pumped into the mixing chamber of the dilution refrigerator, and the outlet flow of working fluid is part of the respective circuit between the mixing chamber and the still of the dilution refrigerator (or a line leading further from the still toward the room temperature portion of the circuit). In a simple embodiment, the circuit of Figure 4 is a closed circuit, where the working fluid simply flows without any intermediate changes that would significantly affect its isotopic composition.

[0045] Nevertheless, it is advantageous if the cryogenic cooling system is configured to control the flow characteristics of the working fluid so that the specific heat capacity of the working fluid in the portion of the outlet part 305 (the portion to which the active heat exchanger 401 transfers heat) is greater than the specific heat capacity of the working fluid in the portion of the inlet part 304 to which the active heat exchanger transfers heat. This can be achieved, for example, so that the working fluid flowing in the outlet part 305 is gaseous to a greater relative extent and at a lower pressure than in the inlet part 304. The relative proportions of liquid and gas phases in the inlet part 304 may be controlled by adjusting the speed at which the working fluid is pumped therethrough and by selecting the location and dimensions of one or more flow impedances 306 along the inlet part 304. Even a single flow impedance in the inlet part of the circuit is not required, provided that the flow conditions of the working fluid can be appropriately adjusted. The relative degree to which the working fluid is gaseous in the outlet part 305 is affected by the relationship between the circulation speed of the working fluid and the heat load from the working area 301, in addition to the factors listed above.

[0046] Figure 5 illustrates principles that may be used to increase the efficiency of actively transferring heat from an inlet part to an outlet part of a circuit 501. In the embodiment of Figure 5, a cryogenic cooling system comprises a set of active heat exchangers 504, 505, and 506. Each active heat exchanger in the set is coupled between an inlet part 502 and an outlet part 503 of the circuit 501. Being an active heat exchanger, each active heat exchanger in the set is configured to use input energy (other than energy entering with the working fluid) to transfer heat from the inlet part 502 to the outlet part 503 during operation.

[0047] At least two of the active heat exchangers 504, 505 and 506 are NIS, SINIS or Sm-S refrigerators, the superconductor parts of the tunnel junctions being made of superconductors with different transition temperatures, and such at least NIS, SINIS or Sm-S refrigerators are arranged along the circuit 501 in order of increasing distance from the (first) heat exchanger region, which is therefore also the order of increasing transition temperatures.

[0048] The configuration shown in Figure 5 overcomes the narrow optimal temperature range, relatively small temperature difference across the tunnel junction, and relatively low efficiency problems discussed above with reference to stack configurations attempted with NIS, SINIS, or Sn-S refrigerators. In a configuration like Figure 5, the temperature of the working fluid in both the inlet part 502 and the outlet part 503 increases with increasing distance from the working region 301 (or, more precisely, from the heat exchanger region 303). Assuming that the gradient of this temperature increase with increasing distance is approximately equal in both sections, the temperature difference across each individual active heat exchanger 504, 505, and 506 can be kept small enough for efficient operation. Furthermore, because heat needs to be transferred through only a single NIS, SINIS, or Sn-S refrigerator (i.e., a single layer junction of the structure) at each level, the efficiency factor can be kept relatively high in both cases.

[0049] 6 illustrates a further possible use of an active heat exchanger along a circuit 601, where the circuit 601 is configured to transport a working fluid to and from a heat exchanger region 303 having a thermal coupling to the working region 301. The inlet and outlet parts of the circuit are indicated by reference numerals 602 and 603, respectively. In the embodiment of FIG. 6, the cryogenic cooling system comprises a further active heat exchanger 604 coupled between a first portion of the outlet part 603 and a second portion of the outlet part 603, where the second portion is further along the outlet part 603 from the heat exchanger region 303 than the first portion. As the definition indicates, the further active heat exchanger 604 is configured to transfer heat from the first portion to the second portion during operation using input energy.

[0050] The idea in using an additional active heat exchanger as in Figure 6 is to actively transfer some heat further along the outflow part, which should reduce the temperature of the working fluid in the outflow part closer to the working area. This can then improve the likelihood that the active heat exchanger closest to the working area (reference number 401 in Figure 6) can reduce the enthalpy of the working fluid in the inflow part of the circuit as low as possible, which can in turn help to cool the working area 301 to as low a temperature as possible.

[0051] The enlarged portion at the top of FIG. 6 illustrates a possible modification to the main principle shown at the bottom. Instead of having separate thermal couplers 407 and 605, respectively, which are at substantially the same temperature, the second end of the lower active heat exchanger 401 and the first end of the upper active heat exchanger 604 can have a common thermal coupler 606 along the outlet part 603 of the circuit 601, thermally coupling both the second end of the lower active heat exchanger 401 and the first end of the upper active heat exchanger 604 to the working fluid flowing in the outlet part 603. In addition to or instead of the common thermal coupler 606, other means may be used to ensure good heat transfer between the components indicated by reference numerals 407 and 605 in the main part of the drawing. Examples of such other means include, but are not limited to, heat transfer liquids on the walls of this part of the circuit, heat transfer tubes, and solid heat conductors. Even the working fluid flowing in the outlet part of the circuit may constitute some kind of heat conductor to provide a heat-conducting connection between parts 407 and 605.

[0052] It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of ​​the invention can be implemented in various ways, therefore the 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. A cryogenic cooling system for cooling a working area (301), comprising: a circuit (302, 501, 601) configured to convey a working fluid to or from a first heat exchanger area (303), the circuit (302, 501, 601) having a thermal connection to said working area (301); an inlet part (304, 502, 602) of the circuit (302, 501, 601) before the first heat exchanger area (303) in the flow direction of the working fluid through the circuit (302, 501, 601); an outlet part (305, 503, 603) of the circuit (302, 501, 601) after the first heat exchanger area (303) in the direction of flow of the working fluid through the circuit (302, 501, 601); one or more active heat exchangers (401, 504, 505, 506) coupled between the inflow part (304, 502, 602) and the outflow part (305, 503, 603) and configured to transfer heat from the inflow part (304, 502, 602) to the outflow part (305, 503, 603) during operation using input energy; A cryogenic cooling system comprising:

2. The cryogenic cooling system of claim 1 , wherein the cryogenic cooling system is configured to maintain a constant isotopic composition of the working fluid throughout the circuit (302, 501, 601).

3. 3. The cryogenic cooling system of claim 1, wherein the cryogenic cooling system is configured to control the flow characteristics of the working fluid so that the specific heat capacity of the working fluid in the portion of the outlet part (305, 503, 603) to which one active heat exchanger of the one or more active heat exchangers transfers heat is greater than the specific heat capacity of the working fluid in the portion of the inlet part (304, 502, 602) to which the one active heat exchanger transfers the heat.

4. Among said at least one active heat exchanger (401, 504, 505, 506): - Solid-state microrefrigerators based on normal metal-insulator-superconductor tunnel junctions (hereafter referred to as NIS refrigerators); - A solid-state microrefrigerator based on a superconductor-insulator-normal metal-insulator-superconductor tunnel junction (hereinafter referred to as a SINIS refrigerator), - Solid-state microrefrigerators based on semiconductor-superconductor tunnel junctions (hereafter referred to as Sm-S refrigerators) The cryogenic cooling system of claim 1 or 2, comprising at least one of:

5. The at least one active heat exchanger (401, 504, 505, 506) includes at least one NIS or SINIS refrigerator, the NIS or SINIS refrigerator having, from a first end of the NIS or SINIS refrigerator to a second end of the NIS or SINIS refrigerator: a first contact electrode layer (402) for biasing; a conductive or semi-conductive substrate layer (403); a layer (404) of a normal metal-insulator-superconductor tunnel junction or a superconductor-insulator-normal metal-insulator-superconductor tunnel junction; a superconductor layer (405); a second contact electrode layer (406) for biasing; 4. The cryogenic cooling system of claim 3, comprising, in this order:

6. 5. The cryogenic cooling system of claim 4, wherein the superconductor layer (405) comprises a constriction for restricting the propagation of phonons from the superconductor layer (405) to the layer (404) of the normal metal-insulator-superconductor tunnel junction or superconductor-insulator-normal metal-insulator-superconductor tunnel junction.

7. 7. The cryogenic cooling system of any one of claims 1 to 6, wherein among said at least one active heat exchanger (401, 504, 505, 506) is included a solid-state micro-refrigerator 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.

8. The cryogenic cooling system of any one of claims 1 to 7, wherein the at least one active heat exchanger (401, 504, 505, 506) comprises an electric calorific cooler.

9. the cryogenic cooling system comprises a set of active heat exchangers (504, 505, 506); each active heat exchanger (504, 505, 506) of the set is coupled between the inflow part (502) and the outflow part (503) and is configured to transfer heat from the inflow part (502) to the outflow part (503) during operation using input energy; at least two active heat exchangers (504, 505, 506) of said set are NIS refrigerators, with the superconductor parts of their normal metal-insulator-superconductor tunnel junctions made of superconductors with different transition temperatures; The cryogenic cooling system according to any one of claims 1 to 8, wherein the at least two NIS refrigerators are arranged along the circuit (501) in order of increasing distance from the first heat exchanger area (303), said order also being in order of increasing transition temperatures.

10. the cryogenic cooling system comprises a further active heat exchanger (604) coupled between a first portion of the outflow part (603) and a second portion of the outflow part (603), the second portion being further along the outflow part (603) from the first heat exchanger area (303) than the first portion; The cryogenic cooling system of any one of claims 1 to 9, wherein the further active heat exchanger (604) is configured to use input energy to transfer heat from the first portion to the second portion during operation.

11. 11. The cryogenic cooling system of claim 1, comprising one or more first thermal couplers (407) coupling respective ends of each of the active heat exchangers (401, 504, 505, 506, 604) to respective portions of working fluid in each of the parts of the circuit.

12. The cryogenic cooling system of any one of claims 1 to 11, comprising one or more flow impedances (306) along the inflow part (304, 502, 602).

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