A Defect Detection Device for Superconducting Cavities Based on Laser Interferometry and Visualization of Superfluid Helium Heat Transfer Temperature Field

Abstract

This invention discloses a superconducting cavity defect detection device based on laser interferometry visualization of the superfluid helium heat transfer temperature field. A laser generator produces a femtosecond laser beam, which is magnified by a beam expander and then passes through an electric shutter. The beam is then split into two beams by a first semi-transparent mirror: an object beam and a reference beam. The object beam passes through the first mirror, glides across the surface of the superconducting cavity to be detected, and is tangent to the detection point before reaching a second semi-transparent mirror. The reference beam passes through a second mirror and also reaches the second semi-transparent mirror. The two beams converge and interfere at the second semi-transparent mirror. The interference image is captured by a high-speed camera after passing through an imaging lens. The high-speed camera captures interference images generated during steady-state and transient measurements and sends them to a computer. The computer analyzes the interference images to determine whether there are defects at the detection point on the superconducting cavity surface. This invention utilizes laser interferometry to image minute temperature disturbances in a superfluid helium cell, thereby achieving high-precision superconducting cavity defect detection.

Description

Technical Field

[0001] This invention belongs to the field of non-destructive testing technology for superconducting cavities, and in particular relates to a superconducting cavity defect detection device based on visualization of the temperature field of superfluid helium heat transfer by laser interferometry. Background Technology

[0002] Superconducting radio frequency resonant cavities cooled by superfluid helium are core components of modern particle accelerators and other large-scale scientific projects. The highest acceleration field of a superconducting cavity is often limited by micron-level surface defects or contaminants, such as tiny surface protrusions, depressions, solder spatter, dust, etc., which can cause quenching. Detecting and removing these surface defects can significantly improve acceleration performance.

[0003] Traditional detection methods, including array thermometry and second acoustic wave methods, each have their limitations. Fixed array thermometry requires thousands of thermometers, consuming a lot of manpower and resources, while rotating scanning detection has very low sensitivity. The accuracy of second acoustic wave methods can only reach the centimeter level, and there is a problem of "too fast" sound speed during data processing. Therefore, there is an urgent need to explore a high-precision superconducting cavity defect detection method based on new principles.

[0004] When the superconducting cavity operates stably under a high-frequency electromagnetic field, surface defects generate Joule heating, raising the temperature of the surrounding cavity material. This heat is conducted into the superfluid helium pool and then dissipated to the outside through its unique internal convection heat transfer mechanism. The internal convection phenomenon of superfluid helium can be described by the Landau two-fluid model: superfluid helium consists of two components, a superfluid and a normal fluid. When an external heat source is present, the viscous and entropy-laden normal fluid moves away from the heat source and carries away heat, while the inviscid and entropy-free superfluid moves in the opposite direction. This heat transfer mechanism is extremely efficient, allowing the cavity material around the defects to remain in a superconducting state below the critical temperature. When the relative velocity between the two fluids exceeds a critical value, quantum vortices are triggered in the superfluid. These quantum vortices impede heat transfer and convert it into the fluid's internal energy, thus forming a local thermal boundary layer that diffuses outward from the heating point.

[0005] When the acceleration gradient of the superconducting cavity is further increased until the temperature around the defect point exceeds the critical temperature of the cavity material, a portion of the cavity material will lose its superconductivity and generate more heat. The heated region rapidly diffuses outward, causing a quenching accident in the entire superconducting cavity. During the quenching process, a large amount of heat will be released into the superfluid helium pool around the defect point. This transient heat flow can propagate outward in the form of waves. Unlike the conventional first sound wave characterized by pressure oscillations, this nonlinear wave characterized by oscillations in the content of normal / superfluid components and total entropy is called the second sound wave. The second sound wave generates a large number of quantum vortices during propagation and is attenuated by them. At the same time, in the variable cross-section structure, rarefaction waves with decreasing temperature will also be generated at the tail to maintain energy conservation. Finally, when the heat flow is large, noisy / noiseless boiling heat transfer may also occur around the defect point.

[0006] Therefore, there is an urgent need to design a new superconducting cavity detection technology to achieve high-precision superconducting cavity defect detection. Summary of the Invention

[0007] To address the issues of high cost or low accuracy in existing superconducting radio frequency resonant cavity defect detection methods, this invention provides a superconducting cavity defect detection device based on laser interferometry visualization of the superfluid helium heat transfer temperature field. This device utilizes laser interferometry to detect and image minute temperature disturbances in the superfluid helium pool, thereby achieving high-precision superconducting cavity defect detection.

[0008] A superconducting cavity defect detection device based on laser interferometry visualization of the superfluid helium heat transfer temperature field includes an optical path system. In this system, a laser generator produces a femtosecond laser beam, which is amplified by a beam expander and then passes through an electric shutter. The beam is then split into two beams by a first semi-transparent mirror: the transmitted beam is the object beam, and the reflected beam is the reference beam. The object beam passes through the first mirror, grazes the surface of the superconducting cavity to be detected, and is tangent to the detection point on the surface before reaching a second semi-transparent mirror. The reference beam also reaches the second semi-transparent mirror after passing through the second mirror. The two beams converge and interfere at the second semi-transparent mirror, and the interference image is captured by a high-speed camera after passing through an imaging lens.

[0009] A high-speed camera captures interferometric images generated during steady-state and transient measurements and sends them to a computer. The computer analyzes the interferometric images to determine whether there are defects at the probe points on the surface of the superconducting cavity.

[0010] Furthermore, the superconducting cavity detection device also includes a superconducting cavity system, which includes a liquid helium cavity and a vacuum cavity disposed outside the liquid helium cavity. The liquid helium cavity fixes the superconducting cavity to be detected inside the liquid helium cavity through a rotation mechanism. Optical windows for allowing object light to pass through are provided on the cavity bodies of the liquid helium cavity and the vacuum cavity.

[0011] The rotating mechanism is fixed to the outer wall of the liquid helium cavity. The rotating shaft of the rotating mechanism extends into the liquid helium cavity and is fixed to the central axis of the superconducting cavity, so as to realize 360-degree detection of the superconducting cavity.

[0012] Furthermore, the superconducting cavity detection device also includes a data acquisition and control system, which includes a pressure sensor, a temperature-level sensor, a controller, and a data acquisition unit.

[0013] Among them, the pressure sensor is used to measure the pressure in the liquid helium chamber, the temperature-level sensor is used to measure the temperature and level in the liquid helium chamber, and the data acquisition unit collects the data from the pressure sensor and the temperature-level sensor and sends it to the computer.

[0014] The controller is connected to a computer and is used to control the operation of the laser generator, electric shutter, and high-speed camera according to the computer's instructions.

[0015] Furthermore, the liquid helium chamber is connected to a vacuum pump via a pipeline. The vacuum pump controls the pressure inside the liquid helium chamber based on data from the pressure sensor and the temperature-level sensor collected by the data acquisition device, thereby controlling the temperature to achieve control over the quenching state of the superconducting chamber.

[0016] Furthermore, if there are defects at the detection points on the surface of the superconducting cavity, the point heat source at the defect point will induce the formation of a thermal boundary layer, local boiling, and outward diffusion of a second acoustic wave. Based on laser interferometry, the high-speed camera captures the interferometric image containing these features.

[0017] During steady-state measurement, the superconducting cavity operates under stable conditions slightly below the quench limit. The minute changes in refractive index caused by the thermal boundary layer will manifest as local deformation of the interference fringes. At this time, the superconducting cavity can be rapidly scanned across its entire surface by slowly rotating it and taking pictures simultaneously.

[0018] During transient measurements, the moment the superconducting cavity loses quench, the controller needs to send instructions to the laser generator, electric shutter, and high-speed camera to quickly acquire interferometric images. Transient heat transfer near the defect point generates a large-scale thermal boundary layer, local boiling, and outward diffusion of the second acoustic wave.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The device of this invention can use laser interferometry to detect minute temperature disturbances in a superfluid helium cell, obtain quantitative full-field temperature data through analysis of the interferometric images, and then analyze the temperature field to achieve high-precision superconducting cavity defect detection. Since interferometry is particularly suitable for detecting minute refractive index changes, and the defect location can be obtained by fitting two-dimensional full-field phase information, higher detection accuracy can be achieved than traditional methods. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the overall structure of a superconducting cavity defect detection device based on laser interferometry superfluid helium heat transfer temperature field visualization according to the present invention.

[0022] Figure 2 The temperature changes of liquid helium at different locations away from the superconducting cavity under steady-state conditions and the corresponding interference images;

[0023] Figure 3 This is a transient image showing the temperature change of liquid helium at different positions relative to the superconducting cavity at a given moment, along with the corresponding interference pattern.

[0024] In the diagram: 1. Laser generator; 2. Beam expander; 3. Motorized shutter; 4. First semi-transparent mirror; 5. Beam; 6. First reflector; 7. Optical window; 8. Pressure sensor; 9. Superconducting cavity; 10. Vacuum pump; 11. Rotation mechanism; 12. Superconducting cavity power supply; 13. Temperature-level sensor; 14. Liquid helium cavity; 15. Vacuum cavity; 16. Second reflector; 17. Second semi-transparent mirror; 18. High-speed camera; 19. Controller; 20. Computer; 21. Data acquisition unit; 22. Defect point; 23. Superconducting cavity surface; 24. Thermal boundary layer; 25. Local boiling; 26. Second acoustic wave. Detailed Implementation

[0025] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the embodiments described below are intended to facilitate the understanding of the present invention and do not constitute any limitation thereof.

[0026] A superconducting cavity defect detection device based on laser interference superfluid helium heat transfer temperature field visualization is based on the following principle: a superconducting cavity 9 is placed in a superfluid helium pool with an optical window 7 and fixed by a rotating mechanism 11 that can rotate around the central axis. A laser beam is split into two beams through an optical path. One beam of object light passes over the cavity surface of the superconducting cavity 9 from the side and interferes with the other reference light. The resulting interference image is recorded by a high-speed camera 18. Analyzing the captured image can detect whether there are defects on the surface of the superconducting cavity.

[0027] like Figure 1 As shown, laser generator 1 is used to generate femtosecond lasers, which can produce lasers of different frequencies. Beam expander 2 is used to magnify the optical path, amplifying the thin-line laser generated by the laser generator into a laser with a certain width. Motorized shutter 3 is used to control the on / off state of the optical path, and can be commanded by controller 19. In transient measurements, it works in conjunction with other equipment to achieve the measurement purpose. First semi-transparent and semi-reflective mirror 4 is used to split the optical path in two, one beam as the object beam and the other as the reference beam.

[0028] The first reflecting mirror 6 and the second reflecting mirror 16 are used to change the direction of light travel, enabling the object beam and the reference beam to converge. The second semi-transparent mirror 17 is used for the convergence of the object beam and the reference beam and for the occurrence of interference. The high-speed camera 18 is used to capture interference images generated during steady-state and transient measurements.

[0029] The superconducting cavity 9 is the object to be inspected; the superconducting cavity power supply 12 is used to supply power to the superconducting cavity 9; the rotating mechanism 11 is used to rotate the superconducting cavity 9 during defect inspection to achieve 360-degree inspection of the superconducting cavity 9.

[0030] An optical window 7 is mounted on the bodies of the liquid helium chamber 14 and the vacuum chamber 15, allowing object light to pass over the surface of the superconducting cavity 9 at low temperatures. A vacuum pump 10 is used to evacuate the liquid helium chamber 14 to control the pressure within the chamber. The liquid helium chamber 14 contains liquid helium, providing a superconducting environment for the superconducting cavity 9, and controlling the internal pressure controls the liquid helium temperature. The vacuum chamber 15 provides a vacuum environment for the liquid helium chamber, reducing heat leakage; a liquid nitrogen chamber can be added between the vacuum chamber 15 and the liquid helium chamber 14 to further reduce heat leakage.

[0031] Pressure sensor 8 is used to collect the pressure inside the liquid helium chamber; temperature-level sensor 13 is used to collect temperature data and liquid helium level inside the chamber; data acquisition unit 21 is used to collect data from various sensors and transmit it to computer 20; computer 20 is used to display the collected data and issue commands; controller 19 is used to receive commands and issue commands to control high-speed camera, laser generator, electric shutter, etc.

[0032] Specifically, the entire device is divided into three parts: optical path system, superconducting cavity system, and data acquisition and control system.

[0033] In the optical system, laser generator 1 generates a femtosecond laser, which is magnified by beam expander 2 and then passes through motorized shutter 3. The light is then split into two beams by first semi-transparent mirror 4. The transmitted beam 5 is the object beam, and the reflected beam is the reference beam. The object beam passes through first reflector 6 and then grazes the surface of the superconducting cavity 9 to be tested, tangent to the detection point on the surface, and then reaches second semi-transparent mirror 17. The reference beam passes through second reflector 16 and also reaches second semi-transparent mirror 17. The two beams converge and interfere at second semi-transparent mirror 17. The interference image is captured by high-speed camera 18 after passing through an imaging lens.

[0034] The superconducting cavity system includes a liquid helium cavity 14 and a vacuum cavity 15 disposed outside the liquid helium cavity 14. The superconducting cavity 9 to be tested is fixed inside the liquid helium cavity 14 by a rotating mechanism 11. Optical windows 7 for allowing object light to pass through are provided on the cavities of the liquid helium cavity 14 and the vacuum cavity 15. The rotating mechanism 11 is fixed to the outer wall of the liquid helium cavity 14, and the rotation axis of the rotating mechanism 11 extends into the liquid helium cavity 14 and is fixed to the central axis of the superconducting cavity 9, so as to realize 360-degree detection of the superconducting cavity 9.

[0035] The data acquisition and control system includes a pressure sensor 8, a temperature-level sensor 13, a controller 19, and a data acquisition unit 21. The pressure sensor 8 measures the pressure inside the liquid helium chamber 14, the temperature-level sensor 13 measures the temperature and level inside the liquid helium chamber 14, and the data acquisition unit 21 collects data from the pressure sensor 8 and the temperature-level sensor 13 and sends it to the computer 20.

[0036] The controller 19 is connected to the computer 20 and is used to control the operation of the laser generator 1, the electric shutter 3 and the high-speed camera 18 according to the instructions of the computer 20.

[0037] The liquid helium chamber 14 is connected to the vacuum pump 10 through a pipeline. The vacuum pump 10 controls the pressure in the liquid helium chamber 14 based on the data collected by the pressure sensor 8 and the temperature-liquid level sensor 13 from the data acquisition device 13, and then controls the temperature to control the quench state of the superconducting chamber 9.

[0038] The high-speed camera 18 captures interference images generated during steady-state and transient measurements and sends them to the computer 20. The computer 20 analyzes the interference images to determine whether there are defects at the detection points on the surface of the superconducting cavity 9.

[0039] like Figure 2 As shown, this illustrates the temperature variations of liquid helium at different locations relative to the superconducting cavity under steady-state conditions, and the corresponding interference patterns. Under steady-state measurement conditions, the minute changes in refractive index caused by the thermal boundary layer 24 formed by steady-state heat transfer near the defect point 22 on the surface 23 of the superconducting cavity will manifest as localized deformation of the interference fringes.

[0040] like Figure 3 The image shows the temperature change of liquid helium at different positions relative to the superconducting cavity at a certain moment during transient conditions, and the corresponding interference images. During transient measurements, at the instant the superconducting cavity loses quench, the control system needs to send commands to the laser generator, shutter, and camera to quickly acquire interference images. The transient heat transfer near the defect point 22 will generate key features such as a large-scale thermal boundary layer 24, local boiling 25, and outwardly diffused second acoustic waves 26.

[0041] The embodiments described above provide a detailed explanation of the technical solutions and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A superconducting cavity defect detection device based on laser interferometry visualization of the superfluid helium heat transfer temperature field, characterized in that, The system includes an optical path system. In the optical path system, the laser generator (1) generates a femtosecond laser, which is amplified by the beam expander (2) and then passes through the electric shutter (3). The optical path is then split into two beams by the first semi-transparent mirror (4). The transmitted beam is the object beam, and the reflected beam is the reference beam. After passing through the first mirror (6), the object beam passes over the surface of the superconducting cavity (9) to be tested, is tangent to the detection point on the surface, and then reaches the second semi-transparent mirror (17). The reference beam also reaches the second semi-transparent mirror (17) after passing through the second mirror (16). The two beams of light converge and interfere at the second semi-transparent mirror (17). The interference image is captured by a high-speed camera (18) after passing through an imaging lens. The high-speed camera (18) captures the interference images generated during steady-state and transient measurements and sends them to the computer (20). The computer (20) analyzes the interference images to determine whether there are defects at the detection points on the surface of the superconducting cavity (9).

2. The superconducting cavity defect detection device based on laser interferometry superfluid helium heat transfer temperature field visualization according to claim 1, characterized in that, The superconducting cavity defect detection device further includes a superconducting cavity system, which includes a liquid helium cavity (14) and a vacuum cavity (15) disposed outside the liquid helium cavity (14). The liquid helium cavity (14) fixes the superconducting cavity (9) to be detected inside the liquid helium cavity (14) by a rotating mechanism (11). The liquid helium cavity (14) and the vacuum cavity (15) are provided with optical windows (7) for light to pass through.

3. The superconducting cavity defect detection device based on laser interferometry superfluid helium heat transfer temperature field visualization according to claim 2, characterized in that, The rotating mechanism (11) is fixed on the outer wall of the liquid helium cavity (14). The rotating shaft of the rotating mechanism (11) extends into the liquid helium cavity (14) and is fixed to the central axis of the superconducting cavity (9) to realize 360-degree detection of the superconducting cavity (9).

4. The superconducting cavity defect detection device based on laser interferometry superfluid helium heat transfer temperature field visualization according to claim 2, characterized in that, The superconducting cavity defect detection device also includes a data acquisition and control system, which includes a pressure sensor (8), a temperature-level sensor (13), a controller (19), and a data acquisition unit (21). Among them, the pressure sensor (8) is used to measure the pressure in the liquid helium chamber (14), the temperature-liquid level sensor (13) is used to measure the temperature and liquid level in the liquid helium chamber (14), and the data acquisition unit (21) collects the data from the pressure sensor (8) and the temperature-liquid level sensor (13) and sends it to the computer (20). The controller (19) is connected to the computer (20) and is used to control the operation of the laser generator (1), the electric shutter (3) and the high-speed camera (18) according to the instructions of the computer (20).

5. The superconducting cavity defect detection device based on laser interferometry superfluid helium heat transfer temperature field visualization according to claim 4, characterized in that, The liquid helium chamber (14) is connected to the vacuum pump (10) through a pipeline. The vacuum pump (10) controls the pressure in the liquid helium chamber (14) based on the data collected by the pressure sensor (8) and the temperature-liquid level sensor (13) collected by the data acquisition unit (21), and then controls the temperature to achieve control of the superconducting cavity (9) in the quench state.

6. The superconducting cavity defect detection device based on laser interferometry superfluid helium heat transfer temperature field visualization according to claim 1, characterized in that, If there are defects at the detection points on the surface of the superconducting cavity (9), the point heat source at the defect point will cause the formation of a thermal boundary layer, local boiling and outward diffusion of a second sound wave. Based on the laser interferometry, the high-speed camera (18) captures the interferometric image containing these features.

7. The superconducting cavity defect detection device based on laser interferometry superfluid helium heat transfer temperature field visualization according to claim 6, characterized in that, During steady-state measurement, the superconducting cavity (9) operates under a stable condition slightly below the quench limit. The small change in refractive index caused by the thermal boundary layer will manifest as local deformation of the interference fringes. At this time, the superconducting cavity (9) can be rapidly scanned across its entire surface by slowly rotating it and taking pictures simultaneously.

8. The superconducting cavity defect detection device based on laser interferometry superfluid helium heat transfer temperature field visualization according to claim 6, characterized in that, During transient measurement, the moment the superconducting cavity (9) loses quench, the controller (19) needs to send instructions to the laser generator (1), electric shutter (3) and high-speed camera (18) to quickly acquire interference images. The transient heat transfer near the defect point generates a large-scale thermal boundary layer, local boiling, and outward diffusion of the second sound wave.

Images