Helium liquefaction system based on two-stage pulse tube refrigerator

Abstract

The invention provides a helium liquefaction system based on a two-stage pulse tube refrigerator, and aims to solve the problems of low heat transfer efficiency and unreliable mechanical support in the prior art. The system comprises a pulse tube refrigerator, a first-stage cold disc, a second-stage cold disc, a first-stage cold screen, a second-stage cold screen, supporting devices connected between a vacuum cavity and the first-stage cold disc and between the first-stage cold disc and the second-stage cold disc, and the first-stage cold screen and the second-stage cold screen are adjustable supporting devices composed of supporting rods and springs. Efficient and controllable direct contact heat conduction between the cold disc and the cold head is achieved, and stretching or compression of the cold head is avoided; meanwhile, a fully-wrapped secondary nested radiation shielding system is arranged, a secondary cold shield is directly and thermally anchored with a secondary cold head, a split top cover design is adopted, core components of a secondary system are completely wrapped, and dual radiation protection is formed. Through the synergistic effect of efficient heat conduction and comprehensive protection, heat leakage of the system is remarkably reduced, the heat transfer temperature difference between all components of the secondary system is reduced, and the cold energy utilization rate and the helium liquefaction rate are increased.

Description

Technical Field

[0001] This invention relates to the field of cryogenic heat transfer technology, specifically to a helium liquefaction system based on a two-stage pulse tube refrigerator. Background Technology

[0002] Pulse tube refrigerators, as a highly efficient cryogenic refrigeration device with no moving parts, are widely used in cryogenic research. Two-stage pulse tube refrigerators, by connecting high and low temperature cold heads in series, significantly expand the cooling temperature range, enabling them to reach the liquid helium temperature range, while balancing cooling efficiency and system complexity. This has become an important choice for achieving helium liquefaction in laboratories. In the process of obtaining cooling capacity in a two-stage pulse tube refrigerator, direct contact between the cold head and the cold plate is undoubtedly the most efficient way to transfer heat. However, when the weight of the component being cooled is too large and requires an additional system support structure, the rigid support method of the system cannot accurately control the force exerted by the cold plate on the cold head. Improper installation can cause the cold head to be squeezed or stretched by the cold plate. These additional forces acting on the cold head, once they exceed a certain threshold, will deteriorate the pulse tube cooling performance, preventing each stage of the cold head from reaching the expected minimum temperature and cooling capacity.

[0003] To circumvent the aforementioned problems, existing technologies for obtaining cooling capacity in two-stage pulse tube refrigerators primarily rely on flexible connections between the cold plate and the cold head using copper braided ropes. This additional flexible connection helps prevent performance degradation. However, the flexible connection increases contact thermal resistance and its own thermal conductivity, reducing heat transfer efficiency between the cold head and the cold plate, and increasing the temperature difference between the cold head and the liquid pool. Currently, the rated operating temperature of the cold head is 4.2K, which is close to the helium liquefaction temperature. This increased temperature difference between the cold head and the liquid pool leads to a slow liquefaction rate, or even prevents liquefaction altogether.

[0004] In addition, the common way to install a cold shield is to fix it to the lower edge of the cold plate, thereby reducing the radiative heat loss of the lower surface of the cold plate and the area surrounded by the cold shield. However, this layout ignores the radiative protection of the upper surface of the cold plate and the cold head itself, which constitutes a parasitic heat load that cannot be ignored. Furthermore, there is a temperature difference between the secondary cold head and the secondary cold plate due to radiative heat leakage, which further reduces the effective cooling power of the system. Summary of the Invention

[0005] To address the aforementioned deficiencies in the prior art, this invention provides a helium liquefaction system based on a two-stage pulse tube refrigerator. While ensuring that no harmful mechanical stress is generated on the refrigerator's cold head, it achieves efficient direct heat conduction between the cold head and the cold plate. At the same time, it constructs a comprehensive radiation protection system to minimize parasitic heat load, thereby significantly improving the helium liquefaction efficiency and the overall thermodynamic performance of the system.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A helium liquefaction system based on a two-stage pulse tube refrigerator includes: A helium liquefaction system based on a two-stage pulse tube refrigerator, characterized in that it comprises: Vacuum cavity; A pulse tube refrigerator, comprising a primary cold head and a secondary cold head located inside a vacuum chamber; The primary cold plate is in direct thermal contact with the primary cold head. The secondary cold plate is in direct thermal contact with the secondary cold head. A helium liquefaction circuit includes a helium inlet pipe, a primary precooling heat exchanger, a secondary precooling heat exchanger, and a liquid pool connected in sequence. The primary precooling heat exchanger is fixedly connected below the primary cold plate, and the secondary precooling heat exchanger and the liquid pool are both fixedly connected below the secondary cold plate. The primary cold shield is fixedly connected below the primary cold plate, and the primary precooling heat exchanger is located inside the primary cold shield; The secondary cold shield is located inside the primary cold shield. The secondary cold shield is fixedly connected to the upper surface of the secondary cold head and completely encloses the secondary cold head, the secondary cold plate, the secondary precooling heat exchanger, and the liquid pool. The support device includes a primary support device connected between the vacuum chamber and the primary cold plate, and a secondary support device connected between the primary cold plate and the secondary cold plate. Both the primary and secondary support devices are used to provide adjustable support force to compensate for the weight of the supported components and ensure direct thermal contact between the cold plate and the corresponding cold head.

[0007] Furthermore, the primary support device consists of a primary support rod and a primary spring. The upper end of the primary support rod is fixedly connected to the upper wall of the vacuum chamber, and the lower end passes through the primary cold plate and extends downwards by a certain length, with a primary spring sleeved on its outer side. The lower end of the primary spring is provided with a primary adjusting nut that is threadedly connected to the primary support rod. The secondary support device consists of a secondary support rod and a secondary spring. The upper end of the secondary support rod is fixedly connected to the primary cold plate, and the lower end passes downwards through the secondary cold plate, with a secondary spring sleeved on its outer side. The lower end of the secondary spring is provided with a secondary adjusting nut that is threadedly connected to the secondary support rod.

[0008] Furthermore, each of the primary and secondary support devices is provided with no fewer than three sets, and the multiple sets of primary and secondary support devices are evenly distributed along the circumferential direction of their respective cold plate edges.

[0009] Furthermore, the secondary cold screen is composed of a secondary cold screen cylinder and a multi-part split-type cold screen top cover. The upper end of the secondary cold screen cylinder is fixedly connected to multiple sets of support rod side ears corresponding to the secondary support device. The support rod side ears are provided with a first U-shaped groove for the secondary support device to pass through. The split-type cold screen top cover is detachably connected to the upper end of the support rod side ears and fits against the upper surface of the secondary cold head. It is also provided with a second U-shaped groove for the secondary support device to pass through and is distributed in a manner corresponding to the position of the first U-shaped groove.

[0010] Furthermore, the secondary support device is also equipped with a clamping assembly for pressing the split-type cold screen top cover tightly against the side lug of the support rod.

[0011] Furthermore, the split-type cold screen top cover is also provided with an inlet pipe pre-reserved hole for the helium gas inlet pipe to pass through.

[0012] Furthermore, a secondary cold head side lug is fixedly connected to the upper end of the secondary cold screen cylinder. The secondary cold head side lug is located between two adjacent support rod side lugs, and the secondary cold head side lug is pressed onto the upper surface of the secondary cold head by a fastening device.

[0013] Furthermore, the fastening device is a G-type clamp, and the clamping part of the G-type clamp is located between the adjacent split-type cold screen top cover and the secondary cold head side ear.

[0014] The technical solution provided by this invention has the following advantages compared with the prior art: 1. The vacuum chamber top cover is connected to the primary cold plate, and the secondary cold plate is connected to the primary cold plate by a support device consisting of a support rod and a spring. The spring compression can be adjusted by adjusting the nut to precisely set the support force and balance it with the load weight. This ensures stable, large-area direct thermal contact between the cold head and the cold plate. The force exerted by the load on the cold head is controlled by adjusting the spring compression, thus solving the problem in the existing technology where the direct hard connection between the cold head and the cold plate affects the performance of the pulse tube refrigerator due to the inability to control the force on the cold head. 2. The secondary cold shield is tightly attached to the upper surface of the secondary cold head to completely enclose the secondary cold head, secondary cold plate, secondary precooling heat exchanger, and liquid pool. Together with the primary cold shield, it encloses the lower surface of the primary cold plate and all components of the secondary heat transfer system. The radiative heat transfer path is split from the inner surface of the primary cold shield to the secondary components to the inner surface of the primary cold shield to the outer surface of the secondary cold shield and the inner surface of the secondary cold shield to the secondary components. This avoids direct radiative heat exchange between the secondary cold head, secondary cold plate, and secondary precooling heat exchanger and the primary cold shield, forming a secondary radiation protection, enabling the components of the secondary heat transfer system to reach lower temperatures and smaller temperature differences. 3. The secondary cold shield adopts a design of cylindrical body + support rod side lugs + secondary cold head side lugs + split cold shield top cover. The split cold shield top cover and support rod side lugs are correspondingly distributed and each is provided with a U-shaped groove for the support rod to pass through. Traditional integral cold shields cannot be installed. During the assembly of this device, after the secondary support rod and secondary cold plate are assembled in place, tilting the secondary cold shield cylindrical body can place the secondary cold plate inside the secondary cold shield cylindrical body. The secondary cold shield and the upper surface of the secondary cold head can be tightly connected by the secondary cold head side lugs and fastening device. The first fastening nut is used to support and fix it at the support rod side lugs. Finally, the split cold shield top cover is installed and pressed into the upper surface of the support rod side lugs by the second fastening nut to achieve comprehensive protection of the secondary cold head, secondary cold plate and its installation components by the secondary cold shield. 4. The efficient heat conduction structure maximizes the cooling performance of the refrigerator, while comprehensive radiation protection minimizes parasitic heat "leaking" into the system from the outside. The combination of the two promotes each other and forms a virtuous cycle, enabling the system to reach a lower limit temperature. The lower limit temperature and smaller temperature difference further improve the phase change efficiency of helium liquefaction. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0016] Figure 1 This is a schematic diagram of the structure of the present invention; Figure 2 This is a schematic diagram of the connection structure between the secondary cold shield cylinder and the secondary cold head; Figure 3 This is a schematic diagram of a G-type clamp structure; Figure 4 This is a schematic diagram of the connection structure between the secondary cold shield and the secondary cold head; in: 1-Vacuum chamber; 2-Pulse refrigerator; 3-First-stage cold head; 4-Second-stage cold head; 5-First-stage cold plate; 6-Second-stage cold plate; 7-First-stage cold shield; 8-Secondary cooling screen, 801-Secondary cooling screen body, 802-Secondary cooling head side lug, 803-Support rod side lug, 804-First U-shaped groove, 805-Split-type cooling screen top cover, 806-Second U-shaped groove, 807-Air inlet pipe reserved hole; 9-First-stage support device, 901-First-stage support rod, 902-First-stage spring, 903-First-stage adjusting nut; 10-Secondary support device, 1001-Secondary support rod, 1002-Secondary spring, 1003-Secondary adjusting nut, 1004-First fastening nut, 1005-Secondary fastening nut; 11-Helium gas inlet pipe; 12-First-stage precooling heat exchanger; 13-Second-stage precooling heat exchanger; 14-Liquid pool; 15-Vacuum angle valve; 16-G-type clamp, 1601-clamp body, 1602-clamping bolt, 1603-rubber washer, 1604-handle. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0018] like Figures 1-4 As shown, this invention provides a helium liquefaction system based on a two-stage pulse tube refrigerator. The core function of this system is to liquefy helium using the cooling capacity generated by the two-stage pulse tube refrigerator at the two-stage cold heads. The system includes a vacuum chamber 1, a pulse tube refrigerator 2, a first-stage cold plate 5, a second-stage cold plate 6, a first-stage cold screen 7, a second-stage cold screen 8, and a helium liquefaction circuit. A vacuum valve 15 is installed on the vacuum chamber 1, which is connected to a vacuum pump to perform a vacuuming operation inside the vacuum chamber 1. The vacuum level inside the vacuum chamber 1 is lower than 10. -5 Pa, the effect of air convection on the system heat exchange is negligible, and the main heat loss of the system is radiation heat loss; the primary cold head 3 and the secondary cold head 4 of the pulse tube refrigerator 2 are located inside the vacuum chamber 1 and are connected to the upper end faces of the primary cold plate 5 and the secondary cold plate 6 respectively. The primary cold plate 5 is rigidly connected to the upper side wall of the vacuum chamber 1 through the primary support device 9, and the primary cold plate 5 and the secondary cold plate 6 are rigidly connected through the secondary support device 10. The helium liquefaction circuit includes a helium inlet pipe 11 and a primary precooling heat exchanger 12 connected in sequence. The secondary precooling heat exchanger 13 and the liquid pool 14 are fixed to the lower end faces of the primary cold plate 5 and the secondary cold plate 6, respectively. The liquid pool 14 is fixed to the lower end face of the secondary cold plate 6. Furthermore, the upper inner wall of the secondary cold screen 8 is fixed to the upper end face of the secondary cold head 4, and the secondary cold head 4, the secondary cold plate 6, the secondary precooling heat exchanger 13 and the liquid pool 14 are completely sealed and wrapped. The primary cold screen 7 is fixedly connected to the lower part of the primary cold plate 5 and completely wraps the secondary cold screen 8 and the primary precooling heat exchanger 12.

[0019] Specifically, such as Figure 1As shown, the primary support device 9 consists of a primary support rod 901 and a primary spring 902. The primary support rod 901 is distributed vertically along its length, and both its upper and lower ends are provided with external threads. The upper end is threadedly connected to the upper inner wall of the vacuum chamber 1, and the lower end passes through the through hole on the primary cold plate 5 and extends downwards for a certain length, where it is threadedly connected to a primary adjusting nut 903. The primary spring 902 is sleeved on the outside of the primary support rod 901, and both ends of the primary spring 902 along its length are connected to the lower end face of the primary cold plate 5 and the primary adjusting nut 903, respectively. Similarly, the secondary support device 10... It consists of a secondary support rod 1001 and a secondary spring 1002. The secondary support rod 1001 is distributed vertically along its length. Both the upper and lower ends are provided with external threads. The upper end is threaded to the lower end face of the primary cold plate 5, and the lower end passes through the through hole of the secondary cold plate 6 and extends downward for a certain length. The lower end of the secondary support rod 1001 is threaded to a secondary adjusting nut 1003. The secondary spring 1002 is sleeved on the outside of the secondary support rod 1001, and the upper and lower ends of the secondary spring 1002 are respectively connected to the lower end face of the secondary cold plate 6 and the secondary adjusting nut 1003.

[0020] In this embodiment, three sets of primary support devices 9 and secondary support devices 10 are provided. The three sets of primary support devices 9 are evenly distributed along the circumference of the primary cold plate 5, and the primary support rod 901 of the primary support device 9 is connected to the outer edge of the primary cold plate 5. The three sets of secondary support devices 10 are evenly distributed along the circumference of the secondary cold plate 6, and the lower end of the secondary support rod 1001 of the secondary support device 10 is connected to the outer edge of the secondary cold plate 6.

[0021] It should be noted that the total elastic force provided by the three sets of primary springs 902 should be approximately equal to the total weight of the components they support. These components include the primary cold plate 5, primary cold shield 7, primary precooling heat exchanger 12, secondary cold plate 6, secondary cold shield 8, secondary precooling heat exchanger 13, secondary support device 10, and liquid pool 14. This ensures that the primary cold plate 5 does not exert significant tensile or compressive stress on the primary cold head 3. Similarly, the total elastic force provided by the three sets of secondary springs 1002 should be approximately equal to the total weight of the components they support. These components include the secondary cold plate 6, secondary cold shield 8, secondary precooling heat exchanger 12, secondary cold plate 6, secondary cold shield 8, secondary precooling heat exchanger 13, secondary support device 10, and liquid pool 14. Heat exchanger 13 and liquid pool 14; Before installation, the weight of each component and the spring coefficient should be determined in advance, the force on each spring should be calculated, and the required deformation of the spring should be calculated. The compression deformation of the first-stage spring 902 and the second-stage spring 1002 can be adjusted by the first-stage adjusting nut 903 and the second-stage adjusting nut 1003 respectively. By adjusting the spring compression, the magnitude of the load force on the cold head is controlled, thus solving the problem that the cold head is damaged due to the inability to control the force on the cold head when the cold head and cold plate are directly rigidly connected, which affects the performance of the pulse tube refrigerator 2.

[0022] The first-stage cold shield 7 has a cylindrical structure without a top cover. The upper edge is fixedly connected to the outer edge of the lower end face of the first-stage cold plate 5 by bolts. The upper surface of the first-stage cold plate 5 and the first-stage cold shield 7 exchange heat with the vacuum chamber 1 through radiation. The cooling capacity comes from the first-stage cold head 3.

[0023] like Figures 1-4 As shown, the secondary cooling screen 8 is made of 1mm thick aluminum alloy plate to reduce its weight. It consists of a secondary cooling screen cylinder 801, secondary cooling head side ears 802, support rod side ears 803, and a split cooling screen top cover 805. The secondary cooling screen cylinder 801 is a cylinder structure without a top cover. Three sets of support rod side ears 803 are welded to its upper end face. The three sets of support rod side ears 803 are evenly distributed along the circumference of the secondary cooling screen cylinder 801. Each set of support rod side ears 803 is provided with a first U-shaped groove 804 opening towards the axis of the secondary cooling screen cylinder 801. The three sets of secondary support rods 1001 correspond to the positions of the first U-shaped grooves 804 on the three sets of support rod side ears 803. The secondary support rods 1001 located below the support rod side ears 803 are threaded with a first fastening nut 1004. The first fastening nut 1004 is tightened against the lower end face of the support rod side lug 803, which supports the support rod side lug 803, thereby enabling the secondary cold head side lug 802 to overlap the upper surface of the secondary cold head 4 while supporting the secondary cold screen 8. The secondary cold head side lug 802 is welded to the upper end face of the secondary cold screen cylinder 801 and is located between two adjacent sets of support rod side lugs 803. Since the secondary cold head side lug 802 is in contact with the upper surface of the secondary cold head 4 to act as a heat transfer bridge between the secondary cold head 4 and the secondary cold screen 8, in order to enhance the heat transfer effect, the secondary cold head side lug 802 adopts an irregular shape design to increase the contact area while avoiding obstacles on the upper surface of the secondary cold head 4 to the greatest extent. In this embodiment, two sets of G-type clamps 16 are used to clamp the secondary cold head side lug 802 to the secondary cold head 4. The G-type clamps 16 are as follows: Figure 3As shown, it consists of a clamping body 1601 and a clamping bolt 1602. The clamping bolt 1602 is threadedly connected to the clamping body 1601, and a rubber pad 1603 is fixedly connected to one end located inside the opening of the clamping body 1601. A handle 1604 is fixedly connected to the upper end. When the upper end face of the secondary cold head 4 contacts the lower end face of the secondary cold head side ear 802, the secondary cold head 4 and the secondary cold screen 8 can be clamped and fixed by two sets of G-type clamps 16. The split-type cold screen top cover 805 is provided with three sets. Each of the three sets of split-type cold screen top covers 805 has a set of second U-shaped grooves 806 on its outer edge. The second U-shaped grooves 806 on the three sets of split-type cold screen top covers 805 are respectively distributed in a corresponding position to the first U-shaped grooves 804 on the three sets of support rod side ears 803. In addition, a pressing component is also provided on the secondary support rod 1001. The clamping assembly is a second fastening nut 1005 threaded onto the secondary support rod 1001. The second fastening nut 1005 is located above the split-type cold screen top cover 805. When installing the split-type cold screen top cover 805, first align the second U-shaped groove 806 on the split-type cold screen top cover 805 with the secondary support rod 1001, so that the second U-shaped groove 1006 wraps around the secondary support rod 1001. Then move the split-type cold screen top cover 805 downward until it reaches the upper end face of the support rod side ear 803. Finally, tighten the second fastening nut 1005 downward to press the split-type cold screen top cover 805 onto the upper surface of the support rod side ear 803 and the secondary cold head 4. In addition, in this embodiment, a pre-drilled air inlet pipe hole 807 is provided between two adjacent split-type cold screen top covers 805, which is distributed vertically for the helium gas inlet pipe 11 to pass through.

[0024] The installation procedure for this device is as follows: S1. Fixed primary cold plate 5 and primary cold head 3: First, connect the upper end of the primary support rod 901 to the upper wall of the vacuum chamber 1 via a thread. Then, rotate the primary cold plate 5 so that the three sets of through holes on the outer edge of the primary cold plate 5 correspond to the lower ends of the three primary support rods 901. Move the primary cold plate 5 from bottom to top to the lower end face of the primary cold head 3. Next, put the primary spring 902 on the outer side of the lower end of the primary support rod 901. Finally, install the primary adjusting nut 903 at the lower end of the primary support rod 901. Rotate the primary adjusting nut 903 a certain number of turns so that the upper end face of the primary cold plate 5 initially contacts the lower end face of the primary cold head 3. The lower end face of the primary cold head 3 has threaded holes evenly distributed along the circumference. Through holes are pre-reserved at the corresponding positions of the primary cold plate 5. Tighten and fix the primary cold plate 5 and the primary cold head 3 with bolts.

[0025] S2. Fixed secondary cold plate 6 and secondary cold head 4: First, the upper end of the secondary support rod 1001 is threadedly connected to the lower end face of the primary cold plate 5. Then, the secondary cold plate 6 is rotated so that the three sets of through holes on the outer edge of the secondary cold plate 6 correspond to the lower ends of the three sets of secondary support rods 1001. The secondary cold plate 6 is then moved from bottom to top to the lower end face of the secondary cold head 4. Next, a secondary spring 1002 is fitted onto the outer side of the lower end of the secondary support rod 1001, and a secondary adjusting nut 1003 is installed on the secondary support rod 1001 located below the secondary spring 1002. By rotating the secondary adjusting nut 1003, the secondary cold plate 6 is initially brought into contact with the lower end face of the secondary cold head 4. The lower end face of the secondary cold head 4 has threaded holes evenly distributed along the circumferential direction. A through hole is pre-drilled at the corresponding position of the secondary cold plate 6, and the secondary cold plate 6 is tightened and fixed to the secondary cold head 4 with bolts. It should be noted that before installing the secondary support rod 1001, the first fastening nut 1004 and the second fastening nut 1005 need to be pre-installed on the secondary support rod 1001. The first fastening nut 1004 is located above the split-type cold screen top cover 805, and the second fastening nut is located between the secondary cold plate 6 and the side ear 803 of the support rod. The primary precooling heat exchanger 12 and the secondary precooling heat exchanger 13 are fixedly installed at the lower ends of the primary cold plate 5 and the secondary cold plate 6, respectively, and the liquid pool 14 is fixedly installed on the lower end face of the secondary cold plate 6.

[0026] S3. Install secondary cold screen 8: In this embodiment, the diameter of the secondary cold plate 6 is slightly smaller than the inner diameter of the secondary cold screen 8. During installation, firstly, align the first U-shaped groove 804 on the three sets of support rod side ears 803 with the three sets of secondary support rods 1001. Then, tilt the secondary cold screen cylinder 801 appropriately to enclose the secondary cold plate 6 inside the secondary cold screen cylinder 801. Ensure that the secondary support rods 1001 pass through the first U-shaped groove 804 from top to bottom. At this time, the lower end face of the secondary cold head side ear 802 is in contact with the upper end face of the secondary cold head 4, and the secondary cold head side ear 802 serves as the initial overlapping surface with the secondary cold head 4. Next, use two sets of G-clamps 16 to connect the secondary cold head side ear 802 with the secondary cold head 4. The secondary cold head 4 and the secondary cold plate 6 are clamped together. Next, the secondary support rod 1001 is fixed to the support rod side lug 803 using the first fastening nut 1004 located below the support rod side lug 803 to support the secondary cold screen cylinder 801. Simultaneously, according to the designed weight of the secondary cold plate 6, secondary cold screen 8, secondary precooling heat exchanger 13, and liquid pool 14, the secondary adjusting nut 1003 located below the secondary spring 1002 is used to press the secondary spring 1002 to a matching length to achieve the final fixation of the secondary support device 10. Finally, the three-part split cold screen top cover 80 is... 5. After the second U-shaped groove 806 on the split-type cold screen top cover 805 aligns with the first U-shaped groove 804 on the support rod side ear 803, the split-type cold screen top cover 805 is placed on the upper end of the support rod side ear 803. The split-type cold screen top cover 805 is then pressed against the support rod side ear 803 and the upper surface of the secondary cold head 4 by the second fastening nut 1005 located above it, thereby fixing the split-type cold screen top cover 805 and ensuring good contact between the split-type cold screen top cover 805, the support rod side ear 803, and the secondary cold head 4. To enhance the heat transfer effect between the split-type cold screen top cover 805 and other parts, it should be noted that after the above installation is completed, the gaps between the three split-type cold screen top covers 805, the gaps between the split-type cold screen top cover 805 and the secondary cold head side ears 802 and G-type clips 16, and the exposed part of the upper end face of the secondary cold head 4 are all sealed with aluminum foil tape; and the helium gas inlet pipe 11 passes through the upper wall of the vacuum chamber 1 and the primary cold plate 5 from top to bottom and is connected to the primary pre-cooling heat exchanger 12, and then passes through the helium gas inlet pipe reserved hole 807 on the secondary cold plate 6 and is connected to the secondary pre-cooling heat exchanger 13 and the liquid pool 14 in sequence.

[0027] S4. Install Level 1 Cold Screen 7: Before installing the primary cold shield 7, the length of the primary spring 902 needs to be adjusted according to the total weight of the primary cold plate 5, primary cold shield 7, primary precooling heat exchanger 12, secondary cold plate 6, secondary cold shield 8, secondary precooling heat exchanger 13, secondary support device 10, and liquid pool 14. This ensures the reliability of the connection between the primary cold head 3 and the primary cold plate 5 and avoids damage to the primary cold head 3 that could affect the transfer of cooling capacity. After adjusting the length of the primary spring 902, the primary cold shield 7 is completely wrapped around the secondary cold shield 8 and then connected to the lower end face of the primary cold plate 5 with bolts.

[0028] The cold energy transfer path in the technical solution of this invention is as follows: The cold head is the component that directly generates cold energy in the two-stage pulse tube refrigerator 2. The two-stage pulse tube refrigerator can achieve staged cooling. The first-stage cold head 3 and the second-stage cold head 4 are the direct cold sources in the system. The lowest no-load temperature of the first-stage cold head 3 is close to 30K, and the lowest no-load temperature of the second-stage cold head 4 can reach about 3K. The cold energy of the first-stage cold head 3 and the second-stage cold head 4 is transferred to the first-stage cold plate 5 and the second-stage cold plate 6, respectively. Helium in the helium liquefaction system flows in from the gas storage through the helium inlet pipe 11, flows through the first-stage precooling heat exchanger 12 and the second-stage precooling heat exchanger 13, and then enters the liquid pool 14 for liquefaction. The first-stage precooling heat exchanger 12 is installed on the first-stage cold plate 5, which obtains cold energy from the first-stage cold plate 5 and performs a first precooling on the incoming helium. The second-stage precooling heat exchanger 13 is installed on the second-stage cold plate 14, which performs a second precooling on the helium that has already been precooled once. After two precoolings, the helium liquefaction time can be significantly shortened.

[0029] In this system, the primary cold plate 5 and the primary cold head 3, and the secondary cold plate 6 and the secondary cold head 4 are supported and connected by support rods and springs. The force between the cold plate and the cold head can be adjusted by adjusting the spring extension and contraction according to the load-bearing components, so as to avoid the cold plate stretching or squeezing the cold head and affecting the heat transfer effect. The primary cold screen 7 is installed below the primary cold plate 5 and completely encloses the secondary cold screen 8. The primary cold screen 7 exchanges heat with the vacuum chamber 1 through radiation, and the cooling energy comes from the primary cold head 3. The secondary cold screen 8 is tightly attached to the upper surface of the secondary cold head 4, so that the secondary cold plate 6, the secondary cold head 4, the secondary precooling heat exchanger 13 and the liquid pool 14 are all included in the radiation protection range of the secondary cold screen 8, avoiding direct radiation heat exchange between them and the primary cold screen 7, forming a secondary radiation protection, effectively reducing the radiative heat leakage of the secondary cold plate 6, and improving the temperature uniformity of the secondary cold head 4, the secondary cold plate 6, the liquid pool 14 and other devices, so that they can achieve lower temperatures and smaller temperature differences.

[0030] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions will not cause the essence of the corresponding technical solutions to deviate from the protection scope of the technical solutions of the embodiments of the present invention.

Claims

1. A helium liquefaction system based on a two-stage pulse tube refrigerator, characterized in that, include: Vacuum cavity; A pulse tube refrigerator, comprising a primary cold head and a secondary cold head located inside a vacuum chamber; The primary cold plate is in direct thermal contact with the primary cold head. The secondary cold plate is in direct thermal contact with the secondary cold head. A helium liquefaction circuit includes a helium inlet pipe, a primary precooling heat exchanger, a secondary precooling heat exchanger, and a liquid pool connected in sequence. The primary precooling heat exchanger is fixedly connected below the primary cold plate, and the secondary precooling heat exchanger and the liquid pool are both fixedly connected below the secondary cold plate. The primary cold shield is fixedly connected below the primary cold plate, and the primary precooling heat exchanger is located inside the primary cold shield; The secondary cold shield is located inside the primary cold shield. The secondary cold shield is fixedly connected to the upper surface of the secondary cold head and completely encloses the secondary cold head, the secondary cold plate, the secondary precooling heat exchanger, and the liquid pool. The support device includes a primary support device connected between the vacuum chamber and the primary cold plate, and a secondary support device connected between the primary cold plate and the secondary cold plate. Both the primary and secondary support devices are used to provide adjustable support force to compensate for the weight of the supported components, so as to ensure direct thermal contact between the cold plate and the corresponding cold head while avoiding stretching or compression of the cold head.

2. The helium liquefaction system based on a two-stage pulse tube refrigerator according to claim 1, characterized in that, The primary support device consists of a primary support rod and a primary spring. The upper end of the primary support rod is fixedly connected to the upper wall of the vacuum chamber, and the lower end passes through the primary cold plate and extends downwards by a certain length, with a primary spring sleeved on its outer side. The lower end of the primary spring is provided with a primary adjusting nut that is threadedly connected to the primary support rod. The secondary support device consists of a secondary support rod and a secondary spring. The upper end of the secondary support rod is fixedly connected to the primary cold plate, and the lower end passes downwards through the secondary cold plate, with a secondary spring sleeved on its outer side. The lower end of the secondary spring is provided with a secondary adjusting nut that is threadedly connected to the secondary support rod.

3. The helium liquefaction system based on a two-stage pulse tube refrigerator according to claim 2, characterized in that, The primary support device and the secondary support device are each provided with no less than three sets, and the multiple sets of primary support devices and secondary support devices are evenly distributed along the circumferential direction of their respective cold plate edges.

4. The helium liquefaction system based on a two-stage pulse tube refrigerator according to claim 1, characterized in that, The secondary cold screen consists of a secondary cold screen cylinder and a multi-part split-type cold screen top cover. The upper end of the secondary cold screen cylinder is fixedly connected to multiple sets of support rod side ears corresponding to the secondary support device. The support rod side ears are provided with a first U-shaped groove for the secondary support device to pass through. The split-type cold screen top cover is detachably connected to the upper end of the support rod side ears and fits against the upper surface of the secondary cold head. It is also provided with a second U-shaped groove for the secondary support device to pass through and is distributed in a manner corresponding to the position of the first U-shaped groove.

5. The helium liquefaction system based on a two-stage pulse tube refrigerator according to claim 4, characterized in that, The secondary support device is also equipped with a clamping component for pressing the split-type cold screen top cover onto the upper surface of the support rod side ear.

6. The helium liquefaction system based on a two-stage pulse tube refrigerator according to claim 4, characterized in that, The split-type cold screen top cover is also provided with a reserved hole for the helium gas inlet pipe to pass through.

7. The helium liquefaction system based on a two-stage pulse tube refrigerator according to claim 4, characterized in that, The upper end of the secondary cold shield cylinder is also fixedly connected to a secondary cold head side ear. The secondary cold head side ear is located between two adjacent support rod side ears, and the secondary cold head side ear is pressed onto the upper surface of the secondary cold head by a fastening device.

8. The helium liquefaction system based on a two-stage pulse tube refrigerator according to claim 7, characterized in that, The fastening device is a G-type clamp, and the clamping part of the G-type clamp is located between the adjacent split-type cold screen top cover and the secondary cold head side ear.

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