A cryostat for conduction-cooled superconducting cavity testing and its cooling method

Abstract

The present invention discloses a cryostat for conduction-cooled superconducting cavity testing and its cooling method. The cryostat of the present invention includes a vacuum container, and a support rod is connected to the top cover of the vacuum container; a magnetic shield is provided inside the vacuum container, and a cold screen is provided inside the magnetic shield; a superconducting cavity support plate is provided inside the cold screen; the magnetic shield, the cold screen and the superconducting cavity support plate are respectively connected and fixed to the support rod; a pulse tube refrigerator is provided on the vacuum container, and its second-stage cold head is connected to the superconducting cavity through a flexible heat conduction material connection unit; a liquid nitrogen input pipe and a liquid nitrogen output pipe are provided on the vacuum container for inputting liquid nitrogen to cool the cold screen. The cooling and rewarming times of the cryostat of the present invention are greatly shortened, greatly improving the test efficiency; at the same time, the volume and cost are also reduced, and it can also be used as a cryogenic experimental platform for other cryogenic measurement experiments.

Description

Technical Field

[0001] The present invention is a cryostat for the research and development test of conduction-cooled superconducting cavities, which is used for the research and development test of conduction-cooled superconducting cavities and the research and development of conduction-cooling processes. Background Art

[0002] As a core component in superconducting accelerators, the performance of superconducting cavities directly determines the energy and quality of the beam. Currently, superconducting cavities are mostly used in particle experimental accelerators, and all of them need to be equipped with expensive and complex liquid helium cryogenic systems to remove the electromagnetic losses on the inner surface of the superconducting cavities. Superconducting cavities are generally cooled by the liquid helium immersion method, and the operating temperature is between 2 - 4.5K. The liquid helium cryogenic system needs to provide a stable liquid helium cryogenic environment for the superconducting cavity to ensure the stability of the temperature of the superconducting cavity and the pressure of the helium pool. The large-scale liquid helium cryogenic system is a key factor restricting the miniaturization and industrial application of accelerators.

[0003] With the continuous progress of superconducting cavity technology, the electromagnetic losses during the operation of superconducting cavities have gradually decreased. A scheme using a small refrigerator to cool superconducting cavities has been proposed, and various accelerator laboratories around the world have studied this scheme. Cooling superconducting cavities with a small refrigerator has outstanding characteristics such as a small cryostat volume, a compact structure, a low construction cost, and easy maintenance compared with the traditional liquid helium immersion scheme. Therefore, using a small refrigerator to cool superconducting cavities can greatly promote the application of superconducting accelerators in the industrial field.

[0004] The design of the conduction-cooling structure and related processes are the research focuses in the current cooling scheme. It is necessary to conduct experimental research on the designed scheme to accumulate sufficient data, and on this basis, optimize the design of the conduction-cooling structure. The traditional vertical test of superconducting cavities uses a liquid helium dewar, which needs to be equipped with a complete cryogenic system to provide sufficient liquid helium. Each test requires a long cooling time and a long experimental period, and a large amount of cold energy will also be wasted. The research and development test of superconducting cavities using conduction cooling requires designing a new experimental platform to meet its special experimental needs. Summary of the Invention

[0005] The present invention designs a cryostat for the test of conduction-cooled superconducting cavities, which is a cryostat using a pulse tube refrigerator as a cold source. The cryostat includes components such as a vacuum vessel, a cold screen, a magnetic shield, a support rod, a superconducting cavity support plate, and a pulse tube refrigerator. This cryostat can be used for the research and development test of conduction-cooled superconducting cavities and can also test the performance of the conduction-cooling structure. When designing, comprehensive consideration is given to reducing the cooling time of the equipment to improve the test efficiency, and at the same time, the design scheme also needs to be convenient for testing various cavity types and different conduction-cooling structures.

[0006] The technical solution of the present invention is as follows:

[0007] A cryostat for conduction-cooled superconducting cavity testing, characterized in that it includes a vacuum container, a support rod 4 is arranged in the vacuum container, and the support rod 4 is connected to the top cover of the vacuum container; a magnetic shield is arranged in the vacuum container, and a cold screen is arranged inside the magnetic shield; a superconducting cavity support plate 6 is arranged inside the cold screen for connecting and fixing the superconducting cavity to be tested.

[0008] The magnetic shield, the cold screen and the superconducting cavity support plate 6 are respectively connected and fixed to the support rod 4.

[0009] A vacuum pumping port 11 is arranged on the vacuum container for connecting to a vacuum pump.

[0010] A pulse tube refrigerator 10 is arranged on the vacuum container. The first-stage cold head 8 of the pulse tube refrigerator 10 passes through the top cover of the magnetic shield and is connected to the top cover of the cold screen; the second-stage cold head at the lower end of the first-stage cold head 8 passes through the top cover of the cold screen and is connected to the superconducting cavity through a flexible heat conduction material connection unit 7 for transmitting the cold quantity generated by the pulse tube refrigerator 10 to the superconducting cavity.

[0011] A liquid nitrogen input pipe 19 and a liquid nitrogen output pipe 20 are arranged on the vacuum container. One end of the liquid nitrogen input pipe 19 is connected to the liquid nitrogen inlet on the vacuum container and the other end is connected to the liquid nitrogen inlet of the magnetic shield for inputting liquid nitrogen into the space between the magnetic shield and the cold screen to cool the cold screen; one end of the liquid nitrogen output pipe 20 is connected to the liquid nitrogen outlet of the vacuum container and the other end is connected to the liquid nitrogen outlet of the magnetic shield for discharging the liquid nitrogen between the magnetic shield and the cold screen.

[0012] Further, the vacuum container is composed of a vacuum cylinder body 1 and a vacuum container top cover 15, and a vacuum pumping port is arranged on the vacuum container top cover 15 for connecting to a vacuum pump.

[0013] Further, the vacuum cylinder body 1 is connected to the vacuum container top cover by bolts; the material of the vacuum container is stainless steel 316L.

[0014] Further, the magnetic shield is composed of a magnetic shield barrel 3 and a magnetic shield top cover 16. The magnetic shield top cover 16 is fixed to the vacuum container top cover 15 through the support rod 4, and the upper side wall of the magnetic shield barrel 3 is connected to the vacuum cylinder body 1 through a magnetic shield fixing fastener 9.

[0015] Further, the top cover 17 of the cold screen is fixed to the vacuum container top cover 15 through the support rod 4, and the cylinder body 2 of the cold screen is connected to the top cover 17 by bolts.

[0016] Further, four support rods 4 are evenly distributed inside the vacuum container; the 4K heat leakage Q of each support rod 4 is not higher than 0.1W, and the weight is not lower than 200kg; wherein, according to the heat leakage the structural dimensions of the support rod 4 are determined, A is the cross-sectional area, λ is the thermal conductivity, and T is the temperature.

[0017] Further, a sensor interface 13 and a heater channel 18 are also provided on the vacuum container; the sensor interface 13 is used to lead out the signal wires of the sensors installed on the superconducting cavity to be measured, and the heater channel 18 is used to lead out the wires of the heaters installed on the superconducting cavity to be measured.

[0018] A superconducting cavity cooling method based on the cryostat is characterized in that a two-stage cooling method is adopted. In the first stage, the liquid nitrogen inlet valve is opened to cool the cold screen. When the cold screen drops to the liquid nitrogen temperature range, the liquid nitrogen inlet valve is closed; in the second stage, the pulse tube refrigerator 10 is started, and the first-stage cold head 8 of the pulse tube refrigerator 10 is used to continue cooling the cold screen until the temperature of the cold screen reaches a stable value.

[0019] Further, the liquid nitrogen outlet of the vacuum container is connected to a vaporizer for vaporizing the exported liquid nitrogen and discharging it into the outdoor atmosphere.

[0020] The advantages of the present invention are as follows:

[0021] The design of this cryostat takes into account comprehensive factors such as the test efficiency of the conduction-cooled superconducting cavity, the space requirements of different cavity types, and the signal transmission channel interfaces for different cryogenic experiments, and can meet the cryogenic test of the conduction-cooled superconducting cavity and the process research of the conduction-cooled structure. Compared with the traditional vertical test dewar for superconducting cavity testing, the cooling and rewarming time of this cryostat has been greatly shortened, greatly improving the test efficiency. At the same time, the volume and cost are also reduced. It can also be used as a cryogenic experimental platform for other cryogenic measurement experiments. Description of the Drawings

[0022] Figure 1 It is a structure diagram of the cryostat.

[0023] Figure 2 It is a structure diagram of the top cover of the vacuum container.

[0024] Reference numerals: 1 - vacuum cylinder, 2 - 40 - 50K cold screen cylinder, 3 - magnetic shielding barrel, 4 - support rod, 5 - test superconducting cavity, 6 - superconducting cavity support plate, 7 - flexible heat conduction material connection unit, 8 - first - stage cold head, 9 - magnetic shielding fixing piece, 10 - pulse tube refrigerator, 11 - vacuum pumping port, 12 - support rod adjusting device, 13 - sensor interface, 14 - RF signal input / output opening, 15 - vacuum vessel top cover, 16 - magnetic shielding top cover, 17 - upper cold screen top cover, 18 - heater channel, 19 - liquid nitrogen input pipe, 20 - liquid nitrogen output pipe. Detailed implementation mode

[0025] The present invention will be further described in detail below with reference to the drawings. The examples given are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0026] The present invention designs a cryostat using a pulse tube refrigerator as a cold source. The cryostat includes components such as a vacuum vessel, a cold screen, a magnetic shield, a support rod, a superconducting cavity support plate, and a pulse tube refrigerator, and uses a two - stage cooling method of liquid nitrogen and a refrigerator for cooling.

[0027] The main outstanding features of the invention are as follows:

[0028] The interior of the cold screen is the main space for experimental testing. In this space, the shape of the support plate can be modified to meet the test tasks of different types of conduction - cooled superconducting cavities. Corresponding support plates are designed for superconducting cavities with different shapes and sizes for support. In addition, this container can also be used for research on conduction - cooling processes, including experiments such as measuring the thermal physical properties of materials and measuring the contact thermal resistance of different material connections. The specific structure and materials of the cryostat are introduced as follows:

[0029] Feature 1.

[0030] The vacuum vessel is composed of a vacuum cylinder 1 and a vacuum vessel top cover 15, which are connected by bolts. The vacuum vessel is made of stainless steel 316L. A vacuum pumping port is arranged on the vacuum vessel top cover 15 to connect to a vacuum pump. After the temperature drop is in place, the pressure of the vacuum vessel is required to be lower than 10 -6 Pa.

[0031] Feature 2.

[0032] The magnetic shielding is composed of two parts. The upper magnetic shielding top cover 16 is fixed to the vacuum vessel top cover 15 through the support rod 4 made of G10 material. The upper side wall of the magnetic shielding barrel 3 is connected to the vacuum cylinder body 1 through the magnetic shielding fixing part 9 made of G10 material. After using the magnetic shielding, it is required that the magnetic field near the superconducting cavity in the cryostat is less than 10 mG. To meet this requirement, enough space is reserved for the installation of the magnetic shielding during the cryostat design process to ensure that its structure does not deform or even short-circuit to reduce the shielding effect. In addition, the materials inside the magnetic shielding need to be demagnetized.

[0033] Feature 3.

[0034] The cold shield is mainly used to reduce the radiative heat at the room temperature end. The cold shield is also divided into two parts. The upper top cover 17 of the cold shield is fixed to the vacuum vessel top cover 15 through the support rod 4. The lower cylinder body 2 of the cold shield is connected to the upper top cover 17 by bolts. The first-stage cold head 8 of the cryocooler is connected to the upper top cover 17 of the cold shield, and the second-stage cold head at the lower end of the first-stage cold head 8 is connected to the superconducting cavity through the flexible heat conduction material connection unit 7 to reduce the vibration of the cold head and ensure good heat transfer. After the temperature reduction reaches the required level, the highest temperature of the cold shield is required to be lower than 52 K, and the temperature difference does not exceed 5 K. The outside of the cold shield will be wrapped with insulating materials to further reduce the heat leakage. In addition, the temperature uniformity of the cold shield is also very important. Therefore, AL with high thermal conductivity is selected to make the cold shield, so as to ensure that the radiative heat leakage of the cold shield is not higher than 0.2 W.

[0035] Feature 4.

[0036] The superconducting cavity support plate 6 is connected to the vacuum vessel top cover 17 through the support rod 4. For different experimental contents, the shape of the support plate can be modified. Since the support plate is directly connected to the superconducting cavity, sufficient mechanical strength and thermal insulation performance need to be ensured, and its weight should be minimized as much as possible.

[0037] Feature 5.

[0038] The second-stage cold head at the lower end of the first-stage cold head 8 of the pulse tube cryocooler 10 will be connected to the superconducting cavity or the cooling structure to be tested through the flexible heat conduction material. The second-stage cold head is the main path for cooling the 4 K temperature region. To improve the experimental efficiency, the cooling rate needs to be accelerated. Therefore, the cold mass in the 4 K temperature region should be optimized to the best value. According to the design requirements, the 4 K heat leakage of the four support rods is not higher than 0.1 W, and the weighed weight is not less than 200 kg. The temperature reduction of the whole device should be controlled to be completed within 12 hours; generally, heaters are arranged on the test device (such as the superconducting cavity to be tested) to adjust the cooling rate.

[0039] The design of the support rod needs to meet the heat transfer and strength requirements, and the material is G10. The heat leakage is mainly due to heat conduction, and the heat leakage amount is:

[0040]

[0041] Among them, Q is the heat leakage, with the unit of W, A is the cross-sectional area, with the unit of m 2 , λ is the thermal conductivity, with the unit of W / (m K), and T is the temperature, with the unit of K.

[0042] The normal stress is:

[0043]

[0044] Among them, σ is the normal stress, in MPa, W is the tensile or compressive load, in N, and [σ] is the allowable tensile stress of the material. The designed structure should meet the allowable requirements of the normal stress.

[0045] In order to reduce the heat leakage, the cross-sectional area of the material should be as small as possible. The cross-sectional area is mainly limited by the allowable tensile stress. Therefore, it is necessary to reduce the mass of the cold mass as much as possible, which can not only ensure the reduction of the heat leakage of the support rod and meet the strength requirements of the support rod, but also reduce the cooling time.

[0046] Feature 6

[0047] In order to reduce the cooling time and improve the test efficiency, a two-stage cooling method is adopted for the cold screen, and a liquid nitrogen circuit is designed on the cold screen for precooling. In the first stage, the liquid nitrogen inlet valve is opened to reduce the cold screen to the liquid nitrogen temperature range. When the cold screen reaches the liquid nitrogen temperature range, the liquid nitrogen inlet valve is closed. In the second stage, the pulse tube cryocooler is started, and the first-stage cold head of the pulse tube cryocooler is used to continue cooling the cold screen until the temperature of the cold screen reaches a stable value. In order to reduce the heat leakage of the liquid nitrogen inlet and outlet pipes, bellows are arranged at the positions of the liquid nitrogen pipe inlet and outlet, close to the vacuum top cover. The bellows extend the heat conduction path to reduce the heat leakage at the room temperature end of the pipe; the bellows are part of the pipe, which plays the role of compensating for thermal contraction and reducing heat leakage. The liquid nitrogen required for cooling the cold screen is provided by a moving liquid nitrogen dewar. The liquid nitrogen outlet is connected to a vaporizer and discharged to the outdoor atmosphere. This design can greatly reduce the cooling time and improve the test efficiency of the superconducting cavity.

[0048] Feature 7

[0049] This cryostat can also be used for the measurement of material physical property parameters, contact thermal resistance, etc. that require a low-temperature vacuum environment. On the vacuum container top cover of the cryostat, sensor interfaces 13 and heater channels 18 are reserved. When performing physical property measurements, the material to be measured and related clamping devices, heaters, sensors, etc. can be fixedly placed on the support plate. The low-temperature end required for measuring material physical property parameters and contact thermal resistance is provided by the second-stage cold head of the pulse tube cryocooler, and the hot end is controlled by the heater. The cables of the temperature sensors, the power supply of the heaters, the signal cables, etc. all pass through the vacuum container top cover. An RF signal input / output opening 14 is also reserved for feeding microwave power and signal transmission through the RF signal input / output opening 14 during the test of the superconducting cavity.

[0050] Before the test starts, when installing the superconducting cavity or related equipment, use a crane to lift the top cover 15 of the vacuum vessel, separate the connection between the top cover of the cold shield and the lower part, and then a new experimental device can be replaced on the support plate. This includes the replacement of measuring instruments such as sensors and heaters. After the installation is completed, first connect the cold shield to ensure that the connection between the top cover of the cold shield and the lower part is tight. Then wrap the outside of the cold shield with thermal insulation materials. During the process of wrapping the thermal insulation materials, the environment should be kept dry and dust-free. After the wrapping is completed, the top cover of the vacuum vessel can be lifted into the vacuum vessel for installation. After the installation is completed, leak detection should be carried out.

[0051] When measuring the thermophysical properties of materials and the contact thermal resistance, the thermostat can provide a stable vacuum environment and relevant signal channels for sensors and heaters. The refrigerator can provide different temperatures and cooling capacities to obtain experimental data in a relatively wide temperature range.

[0052] Although specific embodiments of the present invention are disclosed for illustrative purposes, the purpose is to help understand the content of the present invention and implement it accordingly. Those skilled in the art can understand that various substitutions, changes, and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the content disclosed in the best embodiments, and the scope of protection required by the present invention shall be defined by the scope defined in the claims.

Claims

1. A cryostat for conduction-cooled superconducting cavity testing, characterized in that, It includes a vacuum container, inside which there is a support rod (4) connected to the top cover of the vacuum container; inside the vacuum container, there is a magnetic shield, and inside the magnetic shield, there is a cold shield; inside the cold shield, there is a superconducting cavity support plate (6) for connecting and fixing the superconducting cavity to be tested. The magnetic shield, the cold shield, and the superconducting cavity support plate (6) are respectively connected and fixed to the support rod (4). The vacuum container is provided with a vacuum pumping port (11) for connecting to a vacuum pump. The vacuum container is provided with a pulse tube refrigerator (10). The first-stage cold head (8) of the pulse tube refrigerator (10) passes through the top cover of the magnetic shield and is connected to the top cover of the cold shield; the second-stage cold head at the lower end of the first-stage cold head (8) passes through the top cover of the cold shield and is connected to the superconducting cavity through a flexible heat conduction material connection unit (7) for transmitting the cold generated by the pulse tube refrigerator (10) to the superconducting cavity. The vacuum container is provided with a liquid nitrogen input pipe (19) and a liquid nitrogen output pipe (20). One end of the liquid nitrogen input pipe (19) is connected to the liquid nitrogen inlet on the vacuum container, and the other end is connected to the liquid nitrogen inlet of the magnetic shield, for inputting liquid nitrogen into the space between the magnetic shield and the cold shield to cool the cold shield. One end of the liquid nitrogen output pipe (20) is connected to the liquid nitrogen outlet of the vacuum container, and the other end is connected to the liquid nitrogen outlet of the magnetic shield, for discharging the liquid nitrogen between the magnetic shield and the cold shield.

2. The cryostat according to claim 1, characterized in that, The vacuum container is composed of a vacuum cylinder body (1) and a vacuum container top cover (15). The vacuum container top cover (15) is provided with a vacuum pumping port for connecting to a vacuum pump.

3. The cryostat according to claim 2, characterized in that, The vacuum cylinder body (1) is connected to the vacuum container top cover by bolts; the material of the vacuum container is stainless steel 316L.

4. The cryostat according to claim 2, characterized in that, The magnetic shield is composed of a magnetic shield barrel (3) and a magnetic shield top cover (16). The magnetic shield top cover (16) is fixed to the vacuum container top cover (15) through the support rod (4), and the upper side wall of the magnetic shield barrel (3) is connected to the vacuum cylinder body (1) through a magnetic shield fixing fastener (9).

5. The cryostat according to claim 2, characterized in that, The top cover (17) of the cold shield is fixed to the vacuum container top cover (15) through the support rod (4), and the cylinder body (2) of the cold shield is connected to the top cover (17) by bolts.

6. The cryostat according to any one of claims 1 to 5, characterized in that, There are four support rods (4) evenly distributed inside the vacuum container; the 4K heat leakage Q of each support rod (4) is not higher than 0.1 W and the weight is not lower than 200 kg; wherein, according to the heat leakage the structural dimensions of the support rod (4) are determined, A is the cross-sectional area, λ is the thermal conductivity, and T is the temperature.

7. The cryostat according to any one of claims 1 to 5, characterized in that, The vacuum container is also provided with a sensor interface (13) and a heater channel (18); the sensor interface (13) is used to lead out the signal wires of the sensors installed on the superconducting cavity to be tested, and the heater channel (18) is used to lead out the wires of the heaters installed on the superconducting cavity to be tested.

8. A superconducting cavity cooling method based on the cryostat described in claim 1, characterized in that, Adopt a two-stage cooling method. In the first stage, open the liquid nitrogen inlet valve to cool the cold shield. When the cold shield reaches the liquid nitrogen temperature range, close the liquid nitrogen inlet valve; in the second stage, start the pulse tube refrigerator (10), and use the first-stage cold head (8) of the pulse tube refrigerator (10) to continue cooling the cold shield until the temperature of the cold shield reaches a stable value.

9. The method according to claim 8, wherein The liquid nitrogen outlet of the vacuum container is connected to a vaporizer for vaporizing the exported liquid nitrogen and discharging it to the outdoor atmosphere.

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