A cavity processing method for the conductive cooling of a radio frequency superconducting accelerating cavity

Abstract

The application discloses a cavity processing method for RF superconducting accelerating cavity conduction cooling, and the steps of the method comprise the following steps: 1) depositing a niobium tri-tin film on the inner surface of a superconducting cavity, and vertically testing the obtained niobium tri-tin film cavity in liquid helium; 2) after cleaning and vacuumizing the niobium tri-tin film cavity, filling the inside with a protective gas; 3) performing sand blasting treatment on the outer surface of the superconducting cavity to facilitate copper powder deposition; 4) adopting a cold spraying process to coat copper on the outer surface of the superconducting cavity after sand blasting treatment, so as to form a cold sprayed copper layer; 5) performing mechanical polishing on the equatorial position of the superconducting cavity and the positions close to the ellipsoid part on both sides of the beam tube; 6) sleeving an oxygen-free copper hoop ring matched with the equatorial position of the superconducting cavity into the equator of the superconducting cavity, and fixing an oxygen-free copper hoop matched with the beam tube of the superconducting cavity on both beam tubes of the superconducting cavity; and 7) performing cold sprayed copper treatment on the oxygen-free copper hoop ring and the positions on both sides of the oxygen-free copper hoop of the superconducting cavity again, so as to form a secondary cold sprayed copper layer.

Description

Technical Field

[0001] This invention belongs to the field of radio frequency superconducting accelerators and relates to a cavity processing method for conductive cooling of radio frequency superconducting accelerator cavities. Background Technology

[0002] Radio frequency (RF) superconducting accelerator cavities are microwave resonant cavities with specially shaped structures made of superconducting materials. In their superconducting state, these cavities exhibit very low microwave losses and can establish extremely strong microwave electric fields, making them suitable for accelerating charged particles such as electrons and heavy ions. Currently, ellipsoidal RF superconducting cavities for accelerating electrons are widely used in a range of accelerator fields, including high-energy physics, nuclear physics, and free-electron lasers. Current RF superconducting cavities are made of niobium and require operation in superfluid helium at 2K temperatures. This often necessitates large, complex liquid helium cryogenic systems with demanding personnel requirements, hindering their application in industrial and defense sectors. In recent years, internationally proposed conduction-cooled acceleration technology based on niobium-tin tri-conducting RF superconducting thin-film cavities and miniature cryogenic refrigerators promises to miniaturize superconducting electron gun accelerators, enabling their application in industrial, medical, environmental, and even defense sectors.

[0003] Due to the limited cooling capacity of commonly used commercial cryogenic refrigerators, for superconducting cavities, lower temperatures result in lower surface losses and higher quality factors. Therefore, the key to conductive cooling acceleration technology is to perform certain processing on the outer surface of the superconducting cavity and utilize a cooling structure with excellent thermal conductivity to connect the superconducting cavity and the refrigerator. This allows the heat generated by radio frequency losses within the superconducting cavity to be dissipated in a timely manner and maintained at the lowest possible temperature for stable operation. To cover more application scenarios, the conductive cooling structure must be able to ensure stable operation of the niobium-tritin cavity at an acceleration gradient of 10 MV / m. Furthermore, because the superconducting cavity has a very high quality factor and a very narrow resonant bandwidth, even minor mechanical vibrations can cause it to detune. Therefore, the cooling structure must not only have excellent thermal conductivity but also isolate mechanical vibrations from the refrigerator.

[0004] Currently, there are two main conductive cooling processes designed for superconducting cavities internationally. One is the method designed by Fermilab in the United States, which involves electron beam welding of niobium rings on both sides of the equator of the niobium cavity and using 5N aluminum to connect the superconducting cavity and the cryostat. This method greatly reduces the contact thermal resistance between the cooling structure and the superconducting cavity through electron beam welding. However, since the thermal conductivity of niobium at 4K is only 50-80 W / (m·K), when the cavity RF loss is high, it will be difficult to keep the cavity surface temperature uniform, which can easily lead to hot spots and thermal quench failure. Therefore, it is suitable for niobium-tin thin film cavities with high quality factors (quality factors higher than the tenth power when the acceleration gradient is 10MV / m). Another process developed by Thomas Jefferson National Laboratory in the United States involves electroplating a thick copper layer with high residual resistivity (RRR>300) on the outer surface of the niobium cavity, and then fixing a toolable copper hoop onto the superconducting cavity by electroplating. This method also effectively solves the problem of high contact thermal resistance between the cooling structure and the superconducting cavity, and the thermal conductivity of electroplated copper is very high, which significantly improves the stability and uniformity of the cavity. However, this method is too time-consuming. To obtain a copper layer of sufficient thickness, the electroplating process for a 1-cell cavity takes more than 3 months, and there is a risk that the electroplating solution will enter the superconducting cavity and contaminate the film. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention aims to provide a method for processing radio frequency (RF) superconducting cavities based on a niobium-copper-aluminum composite heat transfer structure. The surface loss of a superconducting cavity is related to its surface resistance and tangential magnetic field. Due to the high magnetic field strength near the equator of the cavity ellipsoid, the loss is mainly concentrated in this region. This invention solves the problems of poor cavity thermal conductivity and excessive contact thermal resistance faced by RF superconducting cavities when used for conductive cooling by cold-spraying copper onto the outer surface of the RF superconducting cavity and tightly assembling copper clamps; then using copper or aluminum flexible connections to connect the cold head of a small cryogenic refrigerator to the superconducting cavity. This invention provides a stable operating temperature for the superconducting cavity and reduces the impact of external vibrations, significantly improving the thermal stability and thermal uniformity of conductively cooled superconducting cavities, thereby enhancing their RF performance. Specifically:

[0006] 1) Applying cold spraying technology to copper plating on the outer surface of a superconducting cavity can quickly obtain a copper layer several millimeters thick, significantly shortening the copper plating time. Furthermore, it can greatly reduce the contact thermal resistance while achieving a good bond between the copper layer and the superconducting cavity, thus improving the thermal conductivity of the conductive cooling structure.

[0007] Because copper and niobium have very different physical properties and there is no miscible phase between them, the copper layer obtained by conventional deposition methods such as electroplating has low bonding strength with the niobium substrate, and the coating is prone to peeling off. Cold spraying can obtain a copper coating with high bonding strength to the niobium substrate, and within a certain range, the higher the powder gas pressure and temperature, the higher the bonding strength. To ensure that the copper coating does not peel off after multiple cooling and reheating cycles from room temperature to liquid helium temperature, high-pressure cold spraying technology is required. The material selection and process parameter control for high-pressure cold spraying are as follows:

[0008] 1. The selected copper powder is oxygen-free copper powder with a purity higher than 99.95% and a particle size of 10-50μm;

[0009] 2. The powder feeding gas is helium or nitrogen. Helium is preferred because it makes it easier to accelerate the copper powder to above the critical velocity, thus improving the bonding strength;

[0010] 3. The powder delivery air pressure is 3-7 MPa, and the flow rate is 120-150 SLM;

[0011] 4. The temperature of the spray gun chamber is 500-700℃;

[0012] 5. Spraying distance is 20-50mm;

[0013] 6. The thickness of the copper coating is 2-5mm.

[0014] 7. Copper powder softens but does not melt at temperatures between 180-220℃.

[0015] 8. During the spraying process, monitor the substrate temperature to ensure it does not exceed 60℃. The rotatable cavity can be used to increase the linear velocity for heat dissipation and prevent oxidation of the copper layer.

[0016] 2) The oxygen-free copper hoop is fixed by cold spraying, which achieves a good bond between the copper hoop and the copper layer, and can also greatly reduce the contact thermal resistance and increase the heat conduction area.

[0017] 3) The use of high-purity aluminum sheet metal in a C-shape reduces the impact of refrigerator vibration on the superconducting cavity. The niobium-copper-aluminum composite conductive cooling structure can achieve high gradient stable operation of the low quality factor niobium three-tin thin film cavity in CW mode.

[0018] The technical solution of this invention is as follows:

[0019] A cavity processing method for conductive cooling of a radio frequency superconducting accelerating cavity, comprising the following steps:

[0020] 1) A niobium-tin thin film was deposited on the inner surface of the superconducting cavity, and the resulting niobium-tin thin film cavity was subjected to vertical testing in liquid helium;

[0021] 2) After cleaning and evacuating the cavity of the niobium-tin thin film that has passed the test, fill it with gas to protect the niobium-tin thin film.

[0022] 3) The outer surface of the superconducting cavity is sandblasted to facilitate copper powder deposition;

[0023] 4) A cold spraying process is used to coat copper on the outer surface of the sandblasted superconducting cavity to form a cold sprayed copper layer;

[0024] 5) Perform mechanical polishing on the equatorial position of the superconducting cavity and on both sides of the bundle tube near the ellipsoid after the coating is completed;

[0025] 6) Fit an oxygen-free copper hoop that matches the equatorial position of the superconducting cavity into and fix it to the equatorial position of the superconducting cavity. Fix an oxygen-free copper hoop that matches the superconducting cavity bundle tube to each of the two bundle tubes of the superconducting cavity. The oxygen-free copper hoop includes two half rings.

[0026] 7) Apply cold-sprayed copper to the oxygen-free copper hoop on the superconducting cavity and on both sides of the oxygen-free copper hoop position to form a secondary cold-sprayed copper layer.

[0027] Furthermore, when applying copper coating to the outer surface of the superconducting cavity using a cold spraying process, the selected copper powder is oxygen-free copper powder with a purity higher than 99.95% and a particle size of 10–50 μm; the powder feeding gas is helium or nitrogen, with a powder feeding gas pressure of 3–7 MPa and a flow rate of 120–150 SLM; the spray gun chamber temperature is 500–700 °C, the spraying distance is 20–50 mm, and the substrate temperature of the superconducting cavity is monitored during the spraying process to ensure it does not exceed 60 °C.

[0028] Furthermore, the secondary cold-sprayed copper layer 6 has a near-triangular cross-section, which is used to increase the heat conduction area between the oxygen-free copper hoop, the oxygen-free copper hoop and the superconducting cavity, and to reinforce the oxygen-free copper hoop and the oxygen-free copper hoop.

[0029] Furthermore, the thickness of the sprayed copper layer is 1-5mm.

[0030] Furthermore, a niobium-tin thin film was deposited on the inner surface of the superconducting cavity 1 using the tin vapor method.

[0031] Furthermore, the superconducting cavity processed in step 7) is ultrasonically cleaned to remove loosely bonded copper powder.

[0032] Furthermore, the superconducting cavity treated in step 7) is evacuated and then sealed; subsequently, it undergoes vacuum annealing in a high-temperature vacuum furnace at a temperature of 300-600℃ and a vacuum level better than 1×10⁻⁶. -2 Pa, annealing time 0.5-1h.

[0033] A superconducting cavity, characterized in that it comprises a cavity processed by the above method.

[0034] A superconducting cavity with a cooling structure is characterized in that the superconducting cavity treated by the above method is connected to the cold head of a refrigerator through a cavity equator connector 7, wherein the cavity equator connector 7 is bent into a C-shape; one end of the cavity equator connector 7 is connected to the secondary cold head of the refrigerator, and the other end is connected to a high-purity aluminum ring at the equator of the superconducting cavity; one end of the cavity equator bundle tube connector 8 is connected to the oxygen-free copper hoop 5, and the other end is connected to the high-purity aluminum ring at the equator of the superconducting cavity.

[0035] Furthermore, the material of the cavity equator connector 7 is a high-purity aluminum plate; the cavity equator bundle tube connector 8 is a high-purity aluminum plate sheet metal part; the mating surfaces of the oxygen-free copper hoop 3 and the high-purity aluminum ring 4, the mating surfaces of the oxygen-free copper hoop 5 and the cavity equator bundle tube connector 8, and the mating surfaces of the cavity equator connector 7 and the cavity equator bundle tube connector 8 are all padded with indium foil with a thickness of 0.1mm, and low-temperature vacuum thermal grease is applied between the secondary cold head of the refrigerator and the cavity equator connector 7.

[0036] The advantages of this invention are as follows:

[0037] This invention enables rapid copper plating of the outer surface of a superconducting cavity, facilitating the quick acquisition of a complete conductive cooling structure. Furthermore, it exhibits excellent thermal uniformity and stability, allowing for stable operation of a low-quality-factor niobium-tritin thin-film cavity in continuous-wave mode with a high acceleration gradient. Additionally, its flexible installation makes it suitable for various types of conductive cooling test platforms.

[0038] This invention will improve the thermal stability and thermal uniformity of the cavity, making it suitable for use in niobium-tin thin film cavities with lower quality factors, while simplifying the process and significantly shortening the processing time, making it more convenient for practical applications. Attached Figure Description

[0039] Figure 1 This is a diagram of the surface treatment process for the outer surface of a superconducting cavity.

[0040] Figure 2 This is a schematic diagram of the cross-section of a superconducting cavity.

[0041] Figure 3 This is a schematic diagram of a superconducting cavity conduction cooling structure.

[0042] Figure 4 The surface morphology of a cold-sprayed copper layer on a niobium surface;

[0043] (a) 2000 times, (b) 10000 times.

[0044] Figure reference numerals: 1-Superconducting cavity, 2-Cold sprayed copper layer, 3-Oxygen-free copper hoop, 4-High-purity aluminum ring, 5-Oxygen-free copper hoop, 6-Secondary cold sprayed copper layer, 7-Cold head and cavity equator connection, 8-Cavity equator bundle tube connection. Detailed Implementation

[0045] The present invention will now be described in further detail with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0046] The surface loss of a superconducting cavity is related to its surface resistance and the tangential magnetic field. Since the magnetic field strength is high near the equator of the cavity ellipsoid, the loss is mainly concentrated in this region; therefore, the primary consideration is to remove heat from this area. Figure 1 This is a diagram of the superconducting cavity processing technology of the present invention. The specific process flow is as follows:

[0047] 1. A niobium-tin thin film was deposited on the inner surface of the superconducting cavity 1 using the tin vapor method to obtain the niobium-tin thin film cavity. Vertical testing was performed in liquid helium. Subsequent processing was carried out after the radio frequency performance met the application requirements.

[0048] 2. After the niobium-tin thin film cavity that meets the requirements is cleaned with high-pressure water and evacuated to a vacuum, it is filled with argon gas at about 1 atmosphere to protect the niobium-tin thin film.

[0049] 3. Sandblasting is performed on the outer surface of superconducting cavity 1 to remove surface oil and oxide layer, and at the same time increase surface roughness to facilitate copper powder deposition. After sandblasting, residual brown corundum and other substances on the surface must be cleaned.

[0050] 4. After sandblasting, a 1-5 mm thick copper layer is cold-sprayed onto the outer surface of the superconducting cavity. The spraying area is as follows: Figure 1 As shown;

[0051] 5. Mechanical polishing was performed on the equatorial position of the superconducting cavity and the positions near the ellipsoid on both sides of the bundle tube after spraying, and the average surface roughness reached 0.4μm;

[0052] 6. Pre-process a high-purity oxygen-free copper hoop 3 that matches the equatorial position of the superconducting cavity and an oxygen-free copper hoop 5 that matches the superconducting cavity bundle tube, and process the surface to an average roughness of 0.4μm;

[0053] 7. The processed oxygen-free copper hoop ring 3 is about 0.2 mm smaller in diameter than the copper layer sprayed on the outer surface of the polished superconducting cavity. After cooling the superconducting cavity with liquid nitrogen, the oxygen-free copper hoop ring 3 is fitted into the equator of the superconducting cavity. After reheating, a tight fit is achieved between the superconducting cavity and the oxygen-free copper hoop ring 3; or the copper ring is heated to about 100°C to expand it and then fitted into the equator of the superconducting cavity. After cooling and shrinking, a tight fit is achieved.

[0054] 8. The inner diameter of the processed oxygen-free copper hoop 5 is consistent with that of the polished cavity bundle tube. Then, the oxygen-free copper hoop 5 is tightly fixed to the two bundle tubes of the superconducting cavity using a clamp.

[0055] 9. Apply cold-sprayed copper to both sides of the mating points between the superconducting cavity 1 and the oxygen-free copper hoop 3 and 5, forming a secondary cold-sprayed copper layer 6. The cross-sectional shape of the secondary cold-sprayed copper layer 6 is nearly triangular, ensuring the connection between the oxygen-free copper hoop 3 and 5 and the superconducting cavity 1 while increasing the heat conduction area. During the secondary spraying, the polished copper rings and hoops must be protected. After spraying, remove the clamps from the copper hoops. Figure 2 As shown;

[0056] An optional processing step is added between steps 9 and 10: After evacuating the superconducting cavity treated in steps 1-9, seal it. Perform vacuum annealing in a high-temperature vacuum furnace at a temperature of 300-600℃, with a vacuum level better than 1×10⁻⁶. -2 Pa, annealing time 0.5-1h. Annealing increases the residual resistivity (RRR) of the copper coating, improves its thermal conductivity, and eliminates internal stress in the copper coating.

[0057] 10. Perform ultrasonic cleaning on the treated superconducting cavity to remove loosely bonded copper powder.

[0058] 11. After the superconducting cavity undergoes the aforementioned processing, it is connected to the cryo-cold head of the refrigerator using a cold head-cavity equator connector 7. This connector 7 is made of high-purity aluminum plate. Specifically, the connector 7 is bent into a C-shape to obtain a C-shaped high-purity aluminum plate. One end is bolted to the secondary cold head of the cryo-cold head, and the other end is connected to the high-purity aluminum ring at the equator of the superconducting cavity. The C-shape significantly reduces vibration from the cryo-cold head. Then, a cavity equator bundle tube connector 8 is used to connect the C-shaped high-purity aluminum plate to the copper clamp 5 on the bundle tube, making the cooling of the superconducting cavity more uniform and enhancing its thermal stability. The cavity equator bundle tube connector 8 is a high-purity aluminum sheet metal part. Figure 3 At the points where copper and aluminum, and aluminum and aluminum are fixed together by bolts, a 0.1mm thick indium shim must be placed on the contact surface to reduce contact thermal resistance. The contact surface between the cold head and the aluminum plate must be coated with Apiezon N low-temperature vacuum thermal grease. The screws used are 316L non-magnetic screws, and the nuts are silicon bronze / brass nuts. Non-magnetic washers must be used to increase the tightening force. The bolt preload must exceed 10kN to ensure a contact thermal resistance below 1×10⁻⁶ kN. -4 (K·m 2 ) / W. Figure 3 The diagram shows the conductive cooling structure when using four refrigerators. The position and number of C-shaped high-purity aluminum plates can be adjusted according to the arrangement of the refrigerators on the superconducting cavity conductive cooling test platform.

[0059] This invention is the first to apply cold spraying technology to prepare a relatively thick copper layer on the outer surface of a superconducting cavity. The high-pressure cold spraying process can provide a good bond between the copper layer and the niobium cavity, with a bonding strength exceeding 50 MPa. Figure 4The surface morphology of the cold-sprayed copper layer on the niobium surface shows that the copper powder underwent significant plastic deformation. Simultaneously, the cold spraying greatly reduced the contact thermal resistance between the niobium cavity and the conductive cooling structure. After cooling from room temperature to 4K and then reheating a niobium-tin thin film cavity with a 2-3 mm thick copper layer applied by cold spraying, no peeling or cracking of the coating occurred.

[0060] Although specific embodiments of the invention have been disclosed for illustrative purposes to aid in understanding and implementing the invention, those skilled in the art will understand that various substitutions, variations, and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the invention should not be limited to the content disclosed in the preferred embodiments, and the scope of protection claimed by the invention is defined by the claims.

Claims

1. A cavity processing method for conductive cooling of a radio frequency superconducting accelerating cavity, comprising the following steps: 1) A niobium-tin thin film was deposited on the inner surface of the superconducting cavity, and the resulting niobium-tin thin film cavity was subjected to vertical testing in liquid helium; 2) After cleaning and evacuating the cavity of the niobium-tin thin film that has passed the test, fill it with gas to protect the niobium-tin thin film. 3) The outer surface of the superconducting cavity is sandblasted to facilitate copper powder deposition; 4) A cold spraying process is used to coat copper on the outer surface of the sandblasted superconducting cavity to form a cold sprayed copper layer; 5) Perform mechanical polishing on the equatorial position of the superconducting cavity and on both sides of the bundle tube near the ellipsoid after the coating is completed; 6) Fit an oxygen-free copper hoop that matches the equatorial position of the superconducting cavity into and fix it to the equatorial position of the superconducting cavity. Fix an oxygen-free copper hoop that matches the superconducting cavity bundle tube to each of the two bundle tubes of the superconducting cavity. The oxygen-free copper hoop includes two half rings. 7) Apply cold-sprayed copper to the oxygen-free copper hoop on the superconducting cavity and on both sides of the oxygen-free copper hoop position to form a secondary cold-sprayed copper layer.

2. The method according to claim 1, characterized in that, When applying copper coating to the outer surface of a superconducting cavity using a cold spraying process, the selected copper powder is oxygen-free copper powder with a purity higher than 99.95% and a particle size of 10–50 μm; the powder feeding gas is helium or nitrogen, with a pressure of 3–7 MPa and a flow rate of 120–150 SLM; the spray gun chamber temperature is 500–700℃, the spraying distance is 20–50 mm, and the substrate temperature of the superconducting cavity is monitored to be no higher than 60℃ during the spraying process.

3. The method according to claim 1, characterized in that, The secondary cold-sprayed copper layer has a near-triangular cross-section, which is used to increase the heat conduction area between the oxygen-free copper hoop and the superconducting cavity, as well as to reinforce the oxygen-free copper hoop and the oxygen-free copper hoop.

4. The method according to claim 1, 2, or 3, characterized in that, The thickness of the copper coating is 1-5mm.

5. The method according to claim 1, 2, or 3, characterized in that, Niobium-tin thin films were deposited on the inner surface of a superconducting cavity using the tin vapor method.

6. The method according to claim 1, 2, or 3, characterized in that, The superconducting cavity processed in step 7) is ultrasonically cleaned to remove loosely bonded copper powder.

7. The method according to claim 1, 2, or 3, characterized in that, After evacuating and sealing the superconducting cavity treated in step 7), vacuum annealing is then performed in a high-temperature vacuum furnace at a temperature of 300-600℃ and a vacuum level better than 1×10⁻⁶. -2 Pa, annealing time 0.5-1h.

8. A superconducting cavity, characterized in that, This includes cavities processed using the method described in claim 1.

9. A superconducting cavity with a cooling structure, characterized in that, The superconducting cavity processed by the method described in claim 1 is connected to the cold head of the refrigerator through the cavity equator connector (7), which is bent into a C-shape. One end of the cavity equator connector (7) is connected to the secondary cold head of the refrigerator, and the other end is connected to the high-purity aluminum ring at the equator of the superconducting cavity. One end of the cavity equator bundle tube connector (8) is connected to the oxygen-free copper hoop, and the other end is connected to the high-purity aluminum ring at the equator of the superconducting cavity.

10. The superconducting cavity according to claim 9, characterized in that, The cavity equator connector (7) is made of high-purity aluminum plate; the cavity equator bundle tube connector (8) is a high-purity aluminum plate sheet metal part; the mating surfaces of the oxygen-free copper hoop ring and the high-purity aluminum ring, the mating surfaces of the oxygen-free copper hoop ring and the cavity equator bundle tube connector (8), and the mating surfaces of the cavity equator connector (7) and the cavity equator bundle tube connector (8) are all padded with indium foil with a thickness of 0.1mm; low-temperature vacuum heat-conducting grease is applied between the secondary cold head of the refrigerator and the cavity equator connector (7).

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