ENHANCED MULTI-STAGE DILUTION REFRIGERATOR WITH OPTIMIZED THERMAL EXCHANGE

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S. C. § 119(e) of U.S. Provisional Patent Application No. 63/714480 filed on Oct. 31, 2024, and titled “ENHANCED MULTI-STAGE DILUTION REFRIGERATOR WITH OPTIMIZED THERMAL EXCHANGE,” the contents of which are incorporated by reference herein in their entirety.

BACKGROUND

Dilution refrigerators are cryogenic devices that rely on the heat of mixing of the ³He and ⁴He isotopes to provide cooling to temperatures between approximately 2 mK and 1 K.

SUMMARY

Some embodiments are directed to a dilution refrigerator including: a still; a mixing chamber; a first continuous heat exchanger and a second continuous heat exchanger disposed between the mixing chamber and the still; and an intermediate cold plate disposed between the first continuous heat exchanger and the second continuous heat exchanger, wherein an outgoing flow path of helium from the mixing chamber to the still is thermalized to the intermediate cold plate.

In some embodiments, the intermediate cold plate is configured to, during operation of the dilution refrigerator, have a temperature that is between temperatures of the still and the mixing chamber.

In some embodiments, the still is thermally coupled to a still cold plate, the mixing chamber is thermally coupled to a mixing chamber cold plate, the first continuous heat exchanger is disposed between the still cold plate and the intermediate cold plate, and the second continuous heat exchanger is disposed between the intermediate cold plate and the mixing chamber cold plate.

In some embodiments, the dilution refrigerator further includes at least one discrete heat exchanger disposed between the intermediate cold plate and the mixing chamber cold plate.

In some embodiments, the at least one discrete heat exchanger is disposed between the second continuous heat exchanger and the mixing chamber cold plate.

In some embodiments, the first continuous heat exchanger and/or the second continuous heat exchanger include one of a counterflow heat exchanger, a cross-counterflow heat exchanger, and/or coflow heat exchanger.

In some embodiments, the at least one discrete heat exchanger includes a sintered metal heat exchanger and/or a plastic heat exchanger.

In some embodiments, the intermediate cold plate includes a first intermediate cold plate, the dilution refrigerator further includes a second intermediate cold plate and a third intermediate cold plate, the second intermediate cold plate and the third intermediate cold plate are disposed between the first intermediate cold plate and the mixing chamber, and the outgoing flow path of helium from the mixing chamber to the still is thermalized to two or more of the first, second, and/or third intermediate cold plates.

In some embodiments, during operation of the dilution refrigerator: the first intermediate cold plate has a temperature of approximately 200 mK, the second intermediate cold plate has a temperature of approximately 100 mK, and the third intermediate cold plate has a temperature of approximately 50 mK.

In some embodiments, the still is thermally coupled to a still cold plate, the mixing chamber is thermally coupled to a mixing chamber cold plate, the first continuous heat exchanger is disposed between the still cold plate and the first intermediate cold plate, and the second continuous heat exchanger is disposed between the first intermediate cold plate and the second intermediate cold plate.

In some embodiments, the dilution refrigerator includes at least one discrete heat exchanger disposed between the first intermediate cold plate and the mixing chamber.

In some embodiments, the at least one discrete heat exchanger includes a sintered metal heat exchanger and/or a plastic heat exchanger.

In some embodiments, the at least one discrete heat exchanger includes a first discrete heat exchanger, a second discrete heat exchanger, and a third discrete heat exchanger.

In some embodiments, the first discrete heat exchanger is disposed between the first intermediate cold plate and the second intermediate cold plate.

In some embodiments, the second discrete heat exchanger is disposed between the second intermediate cold plate and the third intermediate cold plate.

In some embodiments, the third discrete heat exchanger is disposed between the third intermediate cold plate and the mixing chamber.

In some embodiments, the intermediate cold plate includes a continuous heat exchange system. In some embodiments, the continuous heat exchange system includes a plurality of attenuating wires thermally coupled to a plurality of discrete points along a length of, a substantial length of, at least half of a length of, or an entire length of a flow path of a dilute ³He/⁴He mixture and/or to a heat exchanger disposed between the still and the mixing chamber.

In some embodiments, wires of the plurality of attenuating wires are disposed so that, during operation of the dilution refrigerator, the wires are in direct contact with fluid flowing along the flow path of the dilute ³He/⁴He mixture.

In some embodiments, an incoming flow path of a concentrated ³He/⁴He mixture between the still and the mixing chamber is not thermally coupled to the intermediate cold plate.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings:

FIG. 1 is a schematic diagram of an example of a closed-cycle dilution refrigerator, in accordance with some embodiments of the technology described herein.

FIGS. 2A and 2B are views of an illustrative implementation of a dilution refrigerator, in accordance with some embodiments of the technology described herein.

FIG. 3 is a schematic diagram of another example of a closed-cycle dilution refrigerator having multiple intermediate cold plates, in accordance with some embodiments of the technology described herein.

DETAILED DESCRIPTION

In a closed-cycle dilution refrigerator, cooling power is generated at two principal phase boundaries: (i) the liquid-vapor interface in the still, where evaporation of ³He removes heat, and (ii) the liquid-liquid interface in the mixing chamber, where ³He crossing from the concentrated phase to the dilute phase absorbs the enthalpy of mixing. Active cooling of the lower thermal stages (e.g., the experimental stage) therefore occurs mainly at the still and mixing chamber cooling plates. However, the cooling power at these lower thermal stages is limited by, as one example, the temperature-dependent behavior of heat exchangers (e.g., for the mixing chamber cold plate).

For temperatures above approximately 100 mK, conventional heat exchangers (e.g., tube-in-tube) can be used effectively. Below this temperature, however, Kapitza resistance limits the effectiveness of conventional heat exchangers, such that specialized heat exchangers (e.g., comprising high surface area materials) are better suited to facilitate effective heat exchange. In such designs, an intermediate cold plate (ICP) is sometimes placed just above the specialized heat exchangers or otherwise coupled between the mixing chamber and the still. However, the placement defines the utility of the ICP because heating from the ICP (e.g., of the specialized heat exchanger or other components between the mixing chamber and the still) will have cascading impacts on the cooling power of the dilution refrigerator, thereby increasing the system's base temperature.

The inventors have recognized and appreciated that conventional dilution refrigerator systems significantly underutilize the potential cooling power that can be provided by the enthalpy deficit between the incoming ³He/⁴He mixture, comprising a high concentration of ³He, and the outgoing ³He/⁴He mixture, comprising a small concentration of ³He (a “dilute” ³He/⁴He mixture). Additionally, while the effectiveness of a dilution refrigerator to cool experiments or devices is conventionally understood to be limited by the 3 K thermal stage—where high-electron-mobility transistor (HEMT) amplifiers are typically thermalized—or the mixing chamber plate—where the experiment, quantum processing unit (QPU), or sample resides—the inventors have further recognized and appreciated that the heat associated with thermalization and the attenuation of signals between 700 mK and 10 mK is often a greater bottleneck in dilution refrigerator performance.

Accordingly, the inventors have developed systems and methods to improve the cooling utility and operation of dilution refrigerators. The techniques include extracting heat from the outgoing, dilute helium stream at one or more locations between the still and the mixing chamber cold plates. By including this additional thermalization, the effectiveness of a given dilution refrigerator with otherwise identical pumps, compressors, and helium volumes can be dramatically increased.

Following below are more detailed descriptions of various concepts related to, and embodiments of, techniques for improved thermal exchange in dilution refrigerators. It should be appreciated that various aspects described herein may be implemented in any of numerous ways. Examples of specific implementations are provided herein for illustrative purposes only. In addition, the various aspects described in the embodiments below may be used alone or in any combination and are not limited to the combinations explicitly described herein.

FIG. 1 is a schematic diagram of a dry, closed-cycle dilution refrigerator 100, in accordance with some embodiments described herein. In some embodiments, the dilution refrigerator 100 includes an outer vacuum chamber 106 at room temperature (e.g., approximately 300 K) and a number of thermal stages 108a-108f (e.g., thermalization plates or cold plates) held at decreasing temperature intervals (e.g., approximately 50 K, 9-10 K, 3 K, etc.) during operation of the dilution refrigerator 100. For example, the first thermal stage 108a (e.g., a primary cold plate) may be at approximately 50 K, the second thermal stage 108b (e.g., a regenerator cold plate) may be at approximately 9-10 K, the third thermal stage 108c (e.g., a cold foot cold plate) may be at approximately 2-3.5 K or 3-4 K, the fourth thermal stage 108d (e.g., a still cold plate) may be at approximately 500 mK, the fifth thermal stage 108e (e.g., an intermediate cold plate) may be at approximately 100 mK, and the sixth thermal stage 108f (e.g., the mixing chamber cold plate) may be at approximately 10 mK.

In some embodiments, one or more of the thermal stages 108a-108f may be coupled to radiation shielding configured to reduce thermal noise by blocking ambient radiation generated by warmer parts of the dilution refrigerator from reaching colder parts of the dilution refrigerator. As shown in the example of FIG. 1, the first thermal stage 108a, the third thermal stage 108c, and the fourth thermal stage 108d are respectively coupled to radiation shielding 109a, radiation shielding 109c, and radiation shielding 109d. It should be appreciated that fewer radiation shields or more radiation shields may be provided in dilution refrigerator 100, as aspects of the technology described herein are not limited in this respect.

In some embodiments, the fifth thermal stage 108e may be configured to provide additional thermalization of the outgoing helium flow path between the mixing chamber 122 and the still 114, as described herein. While a single fifth thermal stage 108e is shown in the example of FIG. 1, it should be appreciated that more than one intermediate cold plate may be present in the dilution refrigerator 100 between the mixing chamber 122 and the still 114. For example, there could be two intermediate cold plates, three intermediate cold plates (e.g., as depicted in the example of FIG. 3), four intermediate cold plates, or five intermediate cold plates. Alternatively, the fifth thermal stage 108e may not be present in dilution refrigerator 100, and instead the outgoing helium flow path may be continuously or semi-continuously thermalized to wires thermally coupled along the outgoing helium flow path (e.g., thermally coupled to the fluid flowing along the outgoing helium flow path and thermally coupled to a substantial length, more than half of the length, or the entire length of the outgoing helium flow path), as described herein.

In some embodiments, the dilution refrigerator 100 may include a pump system 102 that pressurizes a ³He/⁴He gas mixture (e.g., to a pressure in a range from 0.3 bar to 1.5 bar, at approximately 1 bar, at approximately 2 bar, in a range from 0.3 bar to 5 bar). The ³He/⁴He gas mixture may enter the outer vacuum chamber 106 through one or more inlets and thereafter may travel through the inner thermal stages 108a-108f through the condensing line 102a. After performing its cooling function, the ³He/⁴He mixture may return to the pump system 102 through a return.

In some embodiments, during operation of the dilution refrigerator, the ³He/⁴He mixture may be progressively cooled as it travels along the condensing line 102a from the first thermal stage 108a to the mixing chamber 122. At the first thermal stage, the helium may be initially cooled to approximately 50 K. After exiting the cooldown turbo charger device, the ³He/⁴He mixture may next be cooled by a cryocooler 104. A portion of the cryocooler 104 may be disposed partially outside of the outer vacuum chamber 106, in some embodiments. The cryocooler 104 may be vibrationally isolated from outer vacuum chamber 106 by a vibration isolation stage, which may comprise padding and/or any other suitable vibration isolation techniques.

In some embodiments, the ³He/⁴He mixture may be cooled by the cryocooler 104. The condensing line 102a may be wound around two portions of the cryocooler 104 to effect heat exchange between the ³He/⁴He mixture in the condensing line 102a and the cryocooler 104. In a first step, the ³He/⁴He mixture may be cooled to approximately 10 K by the cryocooler 104 (e.g., thermalizing to the second thermal stage 108b). In a second step, the ³He/⁴He mixture may be cooled to approximately 3-4 K by the cryocooler 104 (e.g., thermalizing to the third thermal stage 108c).

In some embodiments, after being cooled by the cryocooler 104, the ³He/⁴He mixture may pass through the third thermal stage 108c. The third thermal stage 108c may be thermally coupled but mechanically decoupled from cryocooler 104, in some embodiments, in order to provide vibration isolation to the later thermal stages 108d-108f. As a non-limiting example, in some embodiments, third thermal stage 108c may be mechanically decoupled from the cryocooler by a copper braid, heat strap, or other hanging component configured to maintain thermal coupling between the third thermal stage 108c and the cryocooler 104.

In some embodiments, after passing through the third thermal stage 108c, the incoming ³He/⁴He mixture may pass through a heat exchanger 111 coupled to a pumping line forming a portion of the outgoing dilute ³He/⁴He flow path 126. The heat exchanger 111 may be a counter-flow heat exchanger configured to use the enthalpy deficit of the cold vapor being pumped from the still bath to pre-cool the incoming fluid ³He/⁴He traveling from the third thermal stage 108c to the fourth thermal stage 108d.

In some embodiments, after passing through the heat exchanger 111, the incoming ³He/4He mixture may enter a primary impedance stage 112. The primary impedance stage 112 may be a Joule-Thomson expander configured to reduce the temperature and/or pressure of the ³He/⁴He mixture. For example, in some embodiments, the ³He/⁴He mixture may be at approximately 3-5 K before entering the primary impedance stage 112 and may be at approximately 1 K after exiting the primary impedance stage 112.

In some embodiments, after exiting the primary impedance stage 112, the ³He/⁴He mixture then flows into the still 114. The still 114 may contain a different mixture of fluid ³He/⁴He that cools the incoming ³He/⁴He mixture as it passes through the portion of the condensing line 102a and a still heat exchanger 115 disposed in the still 114. In some embodiments, the ³He/⁴He mixture in the condensing line may be cooled to approximately 400-900 mK by the still 114. In some embodiments, the ³He/⁴He mixture may be cooled to approximately 700 mK by the still 114.

In some embodiments, after exiting the still 114, the incoming ³He/⁴He mixture may flow through the fourth thermal stage 108d and into a secondary impedance stage 116. The secondary impedance stage 116 may be configured to ensure that only fluid ³He/⁴He proceeds further downstream in the dilution refrigerator 100 and that gas cavitation in the still 114 does not occur (e.g., by maintaining a threshold pressure in the still 114). The secondary impedance stage 116 may therefore reduce downstream cooling loads due to a latent heat of gaseous ³He/4He.

In some embodiments, after exiting the secondary impedance stage 116, the ³He/⁴He mixture may then flow into a first heat exchanger 118. The first heat exchanger 118 may be a continuous heat exchanger. For example, the first heat exchanger 118 may be a counterflow (e.g., a tube-tube heat exchanger), a cross-counterflow, and/or coflow heat exchanger. At the exit of the first heat exchanger 118, the ³He/⁴He mixture in the condensing line 102a may be cooled to a temperature in a range from approximately 50 to 200 mK, or to approximately 120 mK.

In some embodiments, after exiting the first heat exchanger 118, the incoming ³He/4He mixture passes through the fifth thermal stage 108e and enters continuous heat exchanger 119. The continuous heat exchanger 119 may be a counterflow (e.g., a tube-tube heat exchanger), a cross-counterflow, and/or coflow heat exchanger. The continuous heat exchanger 119 is disposed below the fifth thermal stage 108e. The fifth thermal stage 108e may be an intermediate cold plate (ICP) configured to be cooled to a temperature of approximately 50-200 mK. While continuous heat exchangers are typically more efficient than discrete heat exchangers, they become less efficient below a temperature of approximately 100 mK. However, adding continuous heat exchanger 119 below the fifth thermal stage 108e may enable the fifth thermal stage 108e to operate at a higher temperature and with more cooling power during steady state operation of the dilution refrigerator 100.

In some embodiments, after exiting the continuous heat exchanger 119, the ³He/⁴He mixture enters multiple discrete heat exchangers 120. The discrete heat exchangers 120 may be formed of high surface area materials, in some embodiments. For example, the high surface area materials may include sintered metal particles, sintered nanoparticles and/or sintered nanowires, in some embodiments. In some embodiments, the sintered metal may comprise sintered silver, although it should be appreciated that other metals with suitable thermal conductivities may be used to form the discrete heat exchangers 120. Alternatively or additionally, the discrete heat exchangers 120 may be formed of plastic having a phonon structure matching or approximately matching that of helium (e.g., a semi-crystalline plastic, including but not limited to polyether ether ketone (PEEK), polycarbonate, high-density polyethylene (HDP), and/or other vacuum-compatible plastics). The discrete heat exchangers 120 may be configured to further cool the ³He/⁴He mixture to a temperature below approximately 30 mK.

In some embodiments, after the ³He/⁴He mixture exits the last of discrete heat exchangers 120, it may pass through the sixth thermal stage 108f and enter the mixing chamber 122. Alternatively, in some embodiments, the last of the discrete heat exchangers 120 may be positioned at or below the sixth thermal stage 108f such that the incoming concentrated ³He/⁴He mixture may directly pass to the mixing chamber 122. In the mixing chamber 122, ³He atoms may be pumped from a concentrated phase into a dilute phase (e.g., mixed with ⁴He). This mixing causes the ³He to be cooled as it passes through the phase transition between the concentrated phase to the dilute phase, and this endothermic phase transition provides the final cooling power of the dilution refrigerator 100.

In some embodiments, an experimental volume (e.g., a sample stage or plate) may be thermally coupled to the mixing chamber 122 and configured to support a sample, an experimental device, and/or quantum device (e.g., a quantum processing unit (QPU)). Because the experimental volume is thermally coupled to the mixing chamber 122, the sample, experimental device, and/or quantum device may be held at or near the mixing chamber temperature.

In some embodiments, the experimental volume may be accessed by the user when the dilution refrigerator 100 is not in operation through an opening in the vacuum chamber 106 and door 125. The door 125 may be, in some embodiments, a removable panel (e.g., secured with mechanical fasteners) or may be a hinged door that a user may open using a clamped handle. Additional doors or removable panels (not depicted) may be provided in the radiation shielding 109a, 109c, and 109d, in some embodiments.

As illustrated in the example of FIG. 1, certain components of the dilution refrigerator 100 may be thermally coupled to a thermal stage and disposed on one side (e.g., above or below) the thermal stage. For example, the still 114 is shown as being disposed on an upper surface of the fourth thermal stage 108d. It should be appreciated that in some embodiments, such components may be disposed on either side of the associated thermal stage, as aspects of the technology described herein are not limited in this respect. For example, in some embodiments the still 114 may be disposed on a lower surface of the fourth thermal stage 108d.

In some embodiments, after entering the dilute phase, a dilute ³He/⁴He mixture may be pumped out of the mixing chamber 122 and back towards the still 114 along the outgoing dilute ³He/⁴He flow path 126. From the still 114, a ³He/⁴He mixture may exit the outer vacuum chamber 106 through a return which passes the outgoing ³He/⁴He mixture back to the pump system 102, which re-pressurizes the ³He/⁴He mixture to be used as the incoming ³He/⁴He flowing through condensing line 102a.

FIGS. 2A and 2B are views of an illustrative implementation of a dilution refrigerator 200, in accordance with some embodiments of the technology described herein. The examples of FIGS. 2A and 2B illustrate components of the lower portions of the dilution refrigerator (e.g., below the third thermal stage 108c) and may be used to implement the lower portions of dilution refrigerator 100 of the example of FIG. 1, in some embodiments. For example, FIGS. 2A and 2B show examples of the heat exchanger 111, the primary impedance stage 112, the still 114, the secondary impedance stage 116, the first heat exchanger 118, the fifth thermal stage 108e, the continuous heat exchanger 119, discrete heat exchangers 120, and the sixth thermal stage 108f.

In some embodiments, the dilution refrigerator 200 may further include a detachable plate 208e removably coupled to the fifth thermal stage 108e. The detachable plate may be configured to provide thermalization of the outgoing ³He/⁴He flow path 126 to the fifth thermal stage 108e, as described herein. As shown in the examples of FIGS. 2A and 2B, the detachable plate 208e may be removably coupled using mechanical fasteners (e.g., bolts and/or screws) to the fifth thermal stage 108e. In some embodiments, the detachable plate 208e may include integrated heat exchangers. In some embodiments, the integrated heat exchangers may be channels formed in the detachable plate 208e. The channels may be configured to have a high surface area. In some embodiments, the channels may be formed by machining, welding, and/or by additive fabrication techniques (e.g., three-dimensional printing techniques).

In some embodiments, the channels may further include or, alternatively, be replaced by a large surface area material attached to or disposed within the channels to form the integrated heat exchanger. For example, the integrated heat exchanger may have a lattice structure. The lattice structure may have a periodicity in a range from approximately 400 μm to approximately 1000 μm. For example, the lattice structure may have a periodicity of approximately 600 μm. In some embodiments, the lattice structure may be fabricated using additive fabrication techniques (e.g., three-dimensional printing techniques). The lattice structure may be fabricated to have a rough surface texture to increase the surface area of the material in contact with the helium mixture passing through the integrated heat exchanger, thereby improving heat exchange. In some embodiments, the lattice structure may be formed of a metal. As non-limiting examples, the lattice structure may be formed of copper, silver, aluminum, and/or other metals having suitable thermal properties.

In some embodiments, and as shown in FIG. 2B, the dilution refrigerator may include bypass lines 250 to allow the incoming concentrated ³He/⁴He mixture to bypass making thermal contact with the fifth thermal stage 108e. While not explicitly illustrated, it should be understood that the condensing line 102a may include additional bypass lines configured to allow the incoming ³He/4He mixture to bypass making thermal contact with any other of the thermal stages (e.g., any one or more of the first thermal stage 108a, the second thermal stage 108b, the third thermal stage 108c, the fourth thermal stage 108d, and/or the sixth thermal stage 108f), as aspects of the technology described herein are not limited in this respect.

FIG. 3 is a schematic diagram of another example of a closed-cycle dilution refrigerator 300 having multiple intermediate cold plates, in accordance with some embodiments of the technology described herein. As shown in FIG. 3, the dilution refrigerator 300 includes many of the same components as the illustrative dilution refrigerator 100 described in connection with the example of FIG. 1 herein. However, in some embodiments, the dilution refrigerator 300 includes multiple intermediate cold plates, rather than a single intermediate cold plate (e.g., fifth thermal stage 108e of dilution refrigerator 100).

In some embodiments, the dilution refrigerator 300 may include three intermediate cold plates: a first intermediate cold plate 308e-1, a second intermediate cold plate 308e-2, and a third intermediate cold plate 308e-3. The intermediate cold plates 308e-1, 308e-2, and/or 308e-3 may be configured to provide additional thermalization of the outgoing helium flow path between the mixing chamber 122 and the still 114, as described herein. The intermediate cold plates 308e-1, 308e-2, and/or 308e-3 may be disposed between portions of the discrete heat exchangers 120 (e.g., between one or more individual heat exchangers of the discrete heat exchangers 120).

While three additional cold plates 308e-1, 308e-2, and 308e-3 are shown in the example of FIG. 3, it should be appreciated that fewer than or more than three additional cold plates may be present in the dilution refrigerator 100 between the mixing chamber 122 and the still 114 in some embodiments. Alternatively, the intermediate cold plates 308e-1, 308e-2, and/or 308e-3 may not be present in dilution refrigerator 300, and instead the outgoing helium flow path may be continuously or semi-continuously thermalized by wires thermally coupled along the outgoing helium flow path (e.g., thermally coupled to the fluid flowing along the outgoing helium flow path and thermally coupled to a substantial length, more than half of the length, or the entire length of the outgoing helium flow path).

It should further be appreciated that the intermediate cold plates may be positioned at alternative locations than those shown in the example of FIG. 3, as aspects of the disclosure are not limited in these respects. For example, the continuous heat exchanger 119 may comprise two or more continuous heat exchangers, and additional intermediate cold plates may be positioned between each of the two or more continuous heat exchangers, in some embodiments. As another example, the intermediate cold plates may be positioned only between the continuous heat exchanger 119 and the discrete heat exchangers 120 such that intermediate cold plates are not positioned between heat exchangers of the discrete heat exchangers 120. In some embodiments, another intermediate cold plate may be positioned between the first heat exchanger 118 and the continuous heat exchanger 119. In some embodiments, during operation of the dilution refrigerator 300 the first intermediate cold plate 308e-1 may have a temperature of approximately 500 mK, the second intermediate cold plate 308e-2 may have a temperature of approximately 100-250 mK, and the third intermediate cold plate 108e-3 may have a temperature of approximately 50-100 mK. In some embodiments, the intermediate cold plates 308e-1, 308e-2, and/or 308e-3 may provide additional cooling power for the dilution refrigerator 300 by being thermalized to discrete points along the outgoing dilute ³He/4He flow path 126. In contrast, the incoming concentrated ³He/⁴He mixture may flow from the fourth thermal stage 108d to the sixth thermal stage 108f by passing through one or more of the heat exchangers (e.g., heat exchangers 118, 119, and/or 120) while bypassing (e.g., by not being thermalized to) the intermediate cold plates 308e-1, 308e-2, and/or 308e-3, as described in more detail with reference to FIG. 2B herein.

In some embodiments, rather than including the intermediate cold plates 308e-1, 308e-2, and/or 308e-3, wires (not depicted) may be continuously or semi-continuously thermalized by the outgoing dilute ³He/⁴He flow path 126. The attenuating wires may be coupled along a length (e.g., along a substantial length, at least half of the length, or the entire length) of the outgoing dilute ³He/⁴He flow path 126 and/or to the various heat exchangers 118, 119, and/or 120 between the still 114 and the mixing chamber 122. For example, the attenuating wires may be thermally coupled to an exterior surface of tubing defining the outgoing dilute ³He/⁴He flow path 126. Alternatively or additionally, the attenuating wires may extend into the outgoing dilute ³He/⁴He flow path 126 and/or the various heat exchangers 118, 119, and/or 120 to be in direct contact with the dilute ³He/⁴He along the outgoing dilute ³He/⁴He flow path 126.

In some embodiments, rather than including one or more of the various heat exchangers 118, 119, and/or 120 between the still 114 and the mixing chamber 122, the intermediate cold plates 308e-1, 308e-2, and/or 308e-2 may include or be configured to act as additional integrated heat exchangers. The additional integrated heat exchangers may be configured to remove heat from the incoming ³He/⁴He fluid as it travels towards the mixing chamber 122.

All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms.

The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with “and/or” should be construed in the same fashion, i.e., “one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to “A and/or B,” when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively.

The terms “approximately” and “about” may be used to mean within ±20% of a target value in some embodiments, within ±10% of a target value in some embodiments, within ±5% of a target value in some embodiments, within ±2% of a target value in some embodiments. The terms “approximately”and “about”may include the target value.