A method for preparing a copper-niobium-trinickel composite superconducting cavity

Abstract

The application discloses a preparation method of a copper-niobium-niobium tri-tin composite superconducting cavity, which comprises the following steps: copper-niobium composite plate manufacturing, using the copper-niobium composite plate to manufacture a half cavity and a beam tube, using the half cavity and the beam tube to manufacture a copper-niobium composite superconducting cavity, pretreatment of the copper-niobium composite superconducting cavity, plating niobium tri-tin film on the superconducting cavity by using a CVD medium-temperature plating method, wherein plating material is Nb and SnCl2, evaporation temperature is 400-500 DEG C, and deposition temperature is 700-750 DEG C, cold chain ring brazing of the superconducting cavity, buffer electrochemical polishing of the niobium tri-tin film superconducting cavity and packaging. The niobium tri-tin film reduces the surface resistance of the composite superconducting cavity, improves the superconducting critical temperature and the critical magnetic field, and realizes the replacement of a liquid helium refrigerator with a low-temperature refrigerator; and the cold chain conduction ring brazing realizes good heat conduction connection between the low-temperature refrigerator and the superconducting cavity.

Description

Technical Field

[0001] This invention belongs to the technical field of metal composite processing and acid-washed metal materials, specifically a method for preparing a copper-niobium-niobium-tin composite superconducting cavity. Background Technology

[0002] Particle accelerators are vacuum experimental devices that use electromagnetic fields to accelerate charged particles. Their applications include advanced light sources, particle colliders, industrial accelerators, and medical accelerators. Particle accelerators not only provide crucial research tools in materials science, nuclear science, medicine, and chemistry, but also play a unique role in various sectors of the national economy, such as isotope production, radiation therapy, and industrial irradiation. Radio frequency (RF) superconducting technology is an advanced technology in modern particle accelerators. By coupling external RF power through antennas or waveguides, an electromagnetic field is excited within a metallic superconducting cavity. Charged particles enter the electromagnetic field at a suitable phase and interact with it, thus increasing their energy. Compared to room-temperature accelerators, RF superconducting accelerators use superconductors as the accelerating cavity material, which reduces RF losses and operating costs, and improves energy utilization. The superconducting cavity material is generally pure niobium. In large superconducting accelerators, the superconducting cavity is cooled by immersion in liquid helium to maintain temperature stability. To provide sufficient liquid helium, a complex and expensive cryogenic system is required for the superconducting module. This cryogenic system is bulky, costly, and requires long-term maintenance. This has greatly limited the application of superconducting accelerators in other fields.

[0003] The main methods for niobium-tin plating in the cavity are vapor deposition (temperature greater than 1100℃, exceeding the melting point of oxygen-free copper at 1083℃) and bronze method / electrodeposition method (Sn-Cu-Nb ternary reaction, temperature less than 700℃, the film contains impurities such as Cu and Cu3Sn).

[0004] Meanwhile, the overheating magnetic field of Nb3Sn at lower temperatures is 420 mT, far exceeding the 240 mT of pure Nb. This means that, theoretically, under the same cavity type, Nb3Sn cavities can withstand higher peak magnetic fields and provide a higher acceleration gradient. Taking a 1.3 GHz Tesla cavity as an example, the theoretical gradient of Nb3Sn is close to 100 MV / m (compared to 57 MV / m for pure Nb cavities). Therefore, Nb3Sn cavities have good development potential. Especially at low temperatures, the thermal conductivity of Nb3Sn is low, about three orders of magnitude lower than that of pure Nb; the thermal conductivity of copper is much greater than that of pure niobium and Nb3Sn.

[0005] Combining the characteristics of Nb3Sn, pure niobium, and copper, a copper-niobium composite plate is fabricated using an explosive process. A copper-niobium composite superconducting cavity is then fabricated using die stamping, machining, and electron beam welding. A niobium-tin composite thin film is deposited inside the cavity using chemical vapor deposition. Finally, the composite cavity is connected to a cold chain ring by brazing, and a heating cable is used to connect it to a cryogenic industrial refrigerator, thus completing the cooling of the superconducting cavity. Therefore, the copper, niobium, and niobium-tin composite cold chain superconducting cavity improves the superconducting critical temperature, acceleration gradient, superheated magnetic field, and thermal conductivity. The industrial refrigerator, replacing the liquid helium system, significantly reduces the accelerator's construction and operating costs, making it an ideal solution for the miniaturization and civilian application of superconducting accelerators. Summary of the Invention

[0006] To address the problems existing in the background art, this invention provides a method for preparing a copper-niobium-niobium-tin composite superconducting cavity, the technical solution of which includes:

[0007] Step 1: Fabrication of copper-niobium composite plate;

[0008] Step 2: Fabricate the semi-cavity and bundle tube using copper-niobium composite plates;

[0009] Step 3: Fabricate a copper-niobium composite superconducting cavity using a half-cavity and a bundle tube;

[0010] Step 4: Pretreatment of the copper-niobium composite superconducting cavity;

[0011] Step 5: Deposit a niobium-tin thin film onto the superconducting cavity using a medium-temperature CVD deposition method;

[0012] Step 6: Cold chain brazing of the superconducting cavity;

[0013] Step 7: Perform buffered electrochemical polishing and packaging on the niobium-tin thin film superconducting cavity;

[0014] The coating materials in step 5 are Nb and SnCl2, with an evaporation temperature of 400℃-500℃ and a deposition temperature of 700℃-750℃.

[0015] Step 1 includes:

[0016] Step 1.1: The copper-niobium composite plate is manufactured by parallel explosive welding of 2-4mm oxygen-free copper plate and 2-4mm high-purity niobium plate; wherein, the residual resistivity ratio of the high-purity niobium plate is ≥300.

[0017] Step 1.2: After the copper-niobium composite plate is explosively welded, it is cut into round and square plates according to the material size specifications for the superconducting cavity.

[0018] Step 1.3, Polishing of copper-niobium composite plate: The thickness of the polished composite plate is 3-4mm, of which the niobium layer is 1-1.5mm thick and the copper layer is 2-2.5mm thick.

[0019] The fabrication of the semi-cavity includes:

[0020] Cutting copper-niobium semi-cavity plates;

[0021] Copper-niobium half-cavity stamping;

[0022] Machining of copper-niobium half-cavities.

[0023] The machining sequence of the copper-niobium half-cavity is as follows:

[0024] Fix the stamped half cavity to the tooling and tighten the M12 bolts;

[0025] Clamp the equatorial end plate on the lathe three-jaw chuck, remove the IRIS end plate, and complete the machining of the IRIS end. Leave a 0.5mm allowance on the IRIS end face and remove 8-10mm of copper layer near the weld bead.

[0026] Install the IRIS end plate and tighten it with bolts. Turn the tooling around and clamp the IRIS end plate on the three-jaw chuck of the machine tool. Remove the equatorial end plate and complete the machining of the equatorial end. Leave a 2mm allowance on the equatorial end face. Thin the inside and outside of the equatorial end and remove 10-12mm of copper layer near the weld bead.

[0027] Remove all tooling and take out the half-cavity part.

[0028] Step 2, which involves fabricating the semi-cavity and bundle tube using a copper-niobium composite plate, includes:

[0029] Step A: Cutting copper-niobium bundle tubes;

[0030] Step B: Roll the copper-niobium bundle tube into a circle;

[0031] Step C, Copper-Niobium Bundle Processing: Milling removes the copper layer near the sealing weld bead of the bundle tube, removing 8-10mm on one side;

[0032] Step D: Perform ultrasonic cleaning on the copper-niobium bundle tube;

[0033] Step E: Perform chemical polishing on the copper-niobium bundle tube;

[0034] Step F: Perform ultrasonic treatment on the copper-niobium bundle tube ultrapure water;

[0035] Step G, Copper-Niobium Bundle Tube Sealing: After the bundle tube is removed from the cleanroom, it must be sealed within 8 hours. The sealing process utilizes electron beam back-forming welding technology.

[0036] Step H, Post-weld processing of copper-niobium bundle tubes: Remove 8-10mm of the outer copper layer from the IRIS welding end, leaving a 0.5mm allowance on the end face; remove the copper layer from the flange end;

[0037] The ultrasonic cleaning process for the copper-niobium bundle tube is as follows: ① Wipe the inner and outer surfaces of the bundle tube with a degreasing agent and anhydrous ethanol; ② Place the bundle tube in an ultrasonic cleaner for degreasing and ultrasonication at a temperature of 40℃ for 60 minutes and an ultrasonic frequency of 40kHz; the cleaning agent is Micro90, and the ratio of cleaning agent to pure water is 1:40; after ultrasonication, rinse with pure water until no foam is generated; ③ Place the bundle tube in pure water for ultrasonication for 60 minutes at a water temperature of 40℃ and an ultrasonic frequency of 40kHz; after ultrasonication, rinse with pure water and air dry.

[0038] The chemical polishing process of the copper-niobium bundle tube is as follows: the acid used for polishing is a mixed acid solution with a mass concentration of 40% hydrofluoric acid, 68% nitric acid and 85% phosphoric acid, and a volume ratio of 1:1:2; the pickling temperature is below 20 degrees Celsius; the niobium layer and weld beads inside the bundle tube are pickled by wiping, and the pickling time is 10 minutes; after pickling, it is rinsed with pure water, soaked in pure water and transferred to a clean room.

[0039] The process of ultrasonication of the copper-niobium bundle tube with ultrapure water is as follows: the bundle tube is placed in ultrapure water and ultrasonicated for 60 minutes at a temperature of 40℃ and an ultrasonic frequency of 40kHz; after ultrasonication, the bundle tube is rinsed with flowing ultrapure water for ≥15 minutes until the resistivity of the rinsed water is ≥18.2MΩ·cm; the bundle tube is then placed in a Class 100 clean room to air dry for 3-5 hours.

[0040] Step 3 includes:

[0041] Step 3.1, Copper-Niobium Cavity Welding Process;

[0042] Step 3.2, Flange sealing surface machining: After the bundle tube is welded to the niobium-titanium flange, the flange end face is machined on a lathe to remove the allowance and ensure that the roughness of the flange sealing surface is less than 0.8.

[0043] Step 3.3, Semi-cavity assembly frequency control: After the bundle tube is welded to the IRIS end face of the semi-cavity assembly, the room temperature frequency of the semi-cavity assembly is measured using a vector network analyzer. The bundle tube flange is clamped on a lathe, and the equatorial end face is machined. Based on the frequency results, the number of machining operations and the amount of material removed are determined.

[0044] Step 3.4, RF surface polishing: After the semi-cavity component is processed to the target frequency at room temperature, the RF surface defects are polished. The polishing sandpaper is diamond sandpaper, and the grit of the sandpaper is increased from low to high. After polishing, the degreasing ultrasonic treatment is performed for 30 minutes. The RF surface defects are checked again until there are no defects before the whole cavity equator welding can be performed.

[0045] Step 3.1 further includes:

[0046] Step 3.1.1: First, perform ultrasonic cleaning, chemical polishing, and ultrapure water ultrasonic cleaning on the copper-niobium bundle tube. Then, mount the bundle tube flat with the niobium-titanium flange and perform electron beam welding on the end face of the bundle tube and the niobium-titanium flange.

[0047] Step 3.1.2: First, perform ultrasonic cleaning, chemical polishing and ultrapure water ultrasonic cleaning of the copper niobium bundle tube. Then, flatten the bundle tube and fix it with tooling. Perform back-forming welding of the inner electron beam from the inner layer of the bundle tube and the inner layer of the IRIS end face to ensure the quality of the inner layer weld.

[0048] Step 3.1.3: First, perform ultrasonic cleaning, chemical polishing, and ultrapure water ultrasonic cleaning of the copper-niobium bundle tube. Then, flatten the equatorial end faces of the two semi-cavities and fix them with tooling. First, perform symmetrical electron beam spot welding from the outside of the equatorial end of the semi-cavities. Then, remove the tooling and perform electron beam back forming welding from the outside to ensure the quality of the weld bead on the inside of the equatorial side.

[0049] Step 4 includes:

[0050] Step 4.1, Ultrasonic Cleaning of the Superconducting Cavity: ① Wipe the inner and outer surfaces of the superconducting cavity with a degreasing agent and anhydrous ethanol; ② Place the superconducting cavity in an ultrasonic cleaner for degreasing and ultrasonication at a temperature of 40℃ for 60 minutes and a frequency of 40kHz; the cleaning agent is Micro90, and the ratio of cleaning agent to pure water is 1:40; after ultrasonication, rinse with pure water until no foam is produced; ③ Place the superconducting cavity in pure water for ultrasonication for 60 minutes at a water temperature of 40℃ and a frequency of 40kHz; after ultrasonication, rinse with pure water and air dry.

[0051] Step 4.2: Chemical polishing of the superconducting cavity: The superconducting cavity is fixed on the BCP cavity cleaning equipment. The acid used is a mixed acid solution with a mass concentration of 40% hydrofluoric acid, 68% nitric acid, and 85% phosphoric acid, in a volume ratio of 1:1:2. The cavity is acid-washed in the forward direction for 70 minutes, then in the reverse direction for 90 minutes. The removal rate on the radio frequency surface is 150 μm, the acid temperature is ≤20℃, and the flow rate is 15-18 L / min. The cavity is then rinsed with pure water until the pH reaches 7.

[0052] Step 4.3: High-pressure water flushing of the superconducting cavity: The superconducting cavity is fixed on the HPR equipment and flushed for 3 strokes. The flushing water pressure is ≥10MPa, the flushing water resistivity is ≥18.2MΩ·cm, the superconducting cavity rotation speed is about 20 revolutions / minute, and the nozzle movement speed is about 20mm / minute. Then place the superconducting cavity in a Class 100 clean room to air dry for 3-5 hours.

[0053] Step 4.4, superconducting cavity annealing: Place the superconducting cavity horizontally on the annealing furnace platform, with a vacuum degree better than 1E-2Pa, heat to 750-800℃, hold for 3-5 hours; let it cool naturally to ≤50℃ before taking it out.

[0054] Step 4.5, Electrochemical polishing of the superconducting cavity: The superconducting cavity is fixed on the EP device, and a mixed acid solution with a mass concentration of 40% hydrofluoric acid and 98% sulfuric acid in a volume ratio of 1:9 is used; polishing is performed for 30 minutes to remove 20-25 μm; this aims to further improve the surface roughness of the superconducting cavity.

[0055] Step 4.6: Perform desulfurization twice;

[0056] Step 4.7: After rinsing the superconducting cavity with high-pressure water as in step 4.3, air dry it in a clean room.

[0057] Step 6 includes: brazing an oxygen-free copper cold chain ring on the outer side of the superconducting cavity weld bead: using soft brazing, the brazing material is a tin-silver alloy with a silver content of 72%, in a high vacuum environment, and at a brazing temperature <450℃; during the brazing process, the superconducting cavity flange is wrapped with niobium foil.

[0058] Step 7 involves buffered electrochemical polishing of the niobium-tin thin film superconducting cavity to remove residual Sn on the film surface and improve the roughness of the niobium-tin thin film. This includes fixing the niobium-tin thin film superconducting cavity on an EP device, using a mixed acid solution with a mass concentration of 40% hydrofluoric acid, 98% sulfuric acid, and lactic acid, wherein the volume ratio of hydrofluoric acid, sulfuric acid, and lactic acid is 4:5:11, and the polishing rate is less than 0.1 μm / min; then, two desulfurization processes are performed, followed by rinsing with ultrapure water and drying in a clean room.

[0059] After step 7, proceed to step 8, the low-temperature test.

[0060] The beneficial effects of this invention are as follows:

[0061] 1. A new process for fabricating copper-niobium cavities was used: First, the copper-niobium composite superconducting cavity was fabricated through mold stamping, machine tool processing, clean ultrasonication, post-processing, and electron beam welding; then, the copper-niobium and niobium-tin composite superconducting cavities were fabricated through copper-niobium composite superconducting cavity pretreatment and niobium-tin plating; finally, the cold chain conduction cooling ring was fabricated by brazing oxygen-free copper to the composite superconducting cavity.

[0062] 2. Nb3Sn thin films were deposited on the superconducting cavity using CVD and magnetron sputtering methods to fabricate a copper-niobium and niobium-tin composite superconducting cavity. Oxygen-free copper solved the problem of poor thermal conductivity of niobium and increased the mechanical strength of the superconducting cavity. The niobium-tin thin film reduced the surface resistivity of the composite superconducting cavity, increased the superconducting critical temperature and critical magnetic field, and enabled the use of a cryogenic refrigerator to replace liquid helium refrigeration. The cold chain conduction ring brazing ensured a good thermal connection between the cryogenic refrigerator and the superconducting cavity. High-purity niobium enabled the reaction of tin on the niobium surface to form niobium-tin, and the high-purity niobium reduced the risk of quench loss and superconducting cavity collapse caused by surface defects in the composite superconducting cavity.

[0063] 3. The copper-niobium superconducting cavity plate in this scheme is obtained by copper-niobium explosive welding. The copper-niobium plate is 3-4mm thick, the niobium layer is 1-1.5mm thick, and the copper layer is 2-2.5mm thick. Explosive welding of copper-niobium plates results in higher bonding strength in a low-temperature environment, and 3-4mm copper-niobium plate is the ideal thickness for semi-cavity forming.

[0064] 4. During the fabrication of the copper-niobium cavity, the niobium layer needs to be cleaned and the copper layer protected before welding to prevent contamination of the niobium layer by the copper layer during electron beam welding. The treatment method described in this article can effectively clean and prevent niobium layer contamination.

[0065] 5. The niobium-tin triple coating of the superconducting cavity is prepared by medium-temperature chemical vapor deposition (CVD). CVD produces more uniform coatings on irregular curved surfaces, and the coating parameters are easier to control.

[0066] 6. The cold chain brazing uses tin-silver alloy (Sn-Ag) solder, with a temperature <450℃. The thermal conductivity of tin-silver alloy is close to that of oxygen-free copper, ensuring the uniformity of low-temperature conduction in the superconducting cavity. The brazing temperature is lower than the coating temperature, reducing the impact of brazing temperature on the coating layer. Low-temperature brazing also reduces thermal deformation, minimizing its impact on the superconducting cavity frequency and field flatness. Attached Figure Description

[0067] Figure 1 This is a schematic flowchart illustrating an embodiment of the preparation method of a copper-niobium-niobium-tin composite superconducting cavity according to the present invention.

[0068] Figure 2 This is a schematic cross-sectional view of the parallel explosive welding method in an embodiment of the present invention;

[0069] Figure 3 This is a schematic diagram of the whole cavity welding process and the position of the brazed cold chain ring in an embodiment of the present invention;

[0070] Figure 4 This is a schematic diagram of the structure of the superconducting cavity after brazing the cold chain ring in an embodiment of the present invention. Detailed Implementation

[0071] The present invention will be further described in detail below with reference to the accompanying drawings.

[0072] like Figure 1 The embodiment of the present invention shown includes:

[0073] Step 1: Fabrication of copper-niobium composite plate;

[0074] Step 2: Fabricate the semi-cavity and bundle tube using copper-niobium composite plates;

[0075] Step 3: Fabricate a copper-niobium composite superconducting cavity using a half-cavity and a bundle tube;

[0076] Step 4: Pretreatment of the copper-niobium composite superconducting cavity:

[0077] Step 5: Deposit a niobium-tin thin film onto the superconducting cavity using a medium-temperature CVD deposition method;

[0078] Step 6: Cold chain brazing of the superconducting cavity;

[0079] Step 7: Perform buffered electrochemical polishing and packaging on the niobium-tin thin film superconducting cavity.

[0080] Step 1, the fabrication of the copper-niobium composite plate, includes:

[0081] Step 1.1: The copper-niobium composite plate is made of 2-4mm oxygen-free copper plate and 2-4mm high-purity niobium plate using the parallel explosive welding method (e.g. Figure 1 The fabrication is completed as shown; among which, the copper is made of oxygen-free copper plate, the niobium is made of high-purity niobium plate, and the RRR (residual resistivity ratio) is ≥300.

[0082] Step 1.2: After the copper-niobium composite plate is explosively welded, it is cut into round and square plates according to the material size specifications for the superconducting cavity.

[0083] Step 1.3, Polishing the copper-niobium composite plate: The thickness of the polished composite plate is 3-4mm, with the niobium layer being 1-1.5mm thick and the copper layer being 2-2.5mm thick. If the niobium layer is too thin, it is prone to cracking during cavity molding; if the niobium layer is too thick, it will affect low-temperature heat conduction; the copper layer cannot be too thick, otherwise the cavity cannot be molded in one go, and multiple molding processes will increase the damage to the niobium layer.

[0084] Step 2 involves fabricating the half-cavity and bundle tube using a copper-niobium composite plate, including:

[0085] Step 2.1, semi-cavity construction, includes:

[0086] Step 2.1.1, Copper-Niobium Semi-Cavity Plate Blanking: Based on the semi-cavity surface development and mold design calculations, determine the blanking dimensions of the copper-niobium plate. For example, the blanking dimensions of the 1.3GHz superconducting cavity semi-cavity are... δ represents the thickness of the composite board, which is typically 3-4 mm.

[0087] Step 2.1.2, Copper-niobium semi-cavity stamping: The stamping die includes a die, a punch and a pressure plate. The stamping equipment is a 200T four-column hydraulic press with a stamping pressure of 120T and a holding time of 3 minutes. The lubricant is a 1:1 mixture of vegetable oil and salad oil.

[0088] Step 2.1.3, Machining of the Copper-Niobium Half-Cavity: The half-cavity is fixed using a machining fixture, which includes an equatorial end plate, a punch, a die, and an IRIS end plate. The machining sequence is as follows: ① Fix the stamped half-cavity to the fixture and tighten the M12 bolts. ② Clamp the equatorial end plate on the lathe's three-jaw chuck, remove the IRIS end plate, and complete the machining of the IRIS end. Leave a 0.5mm allowance on the IRIS end face and remove 8-10mm of copper layer near the weld. ③ Install the IRIS end plate and tighten it with bolts. Turn the fixture around, clamp the IRIS end plate on the machine tool's three-jaw chuck, and remove the equatorial end plate. Complete the machining of the equatorial end, leaving a 2mm allowance on the equatorial end face. Thin the equatorial end both inside and out, and remove 10-12mm of copper layer near the weld. ④ Remove all fixtures and take out the half-cavity part.

[0089] Step 2.2, Bundle fabrication, includes:

[0090] Step 2.2.1, Copper-Niobium Bundle Tube Cutting: Dimensions: Calculate the dimensions of the copper-niobium bundle tube according to the roll center line method. For example, the cutting dimensions of the 1.3GHz cavity bundle tube are 254.15 (circumference) × 165 × δ, where δ is the thickness of the composite plate, which is generally 3-4mm.

[0091] Step 2.2.2, Copper-niobium bundle tube rolling: The copper layer is rolled into a round shape using a three-roll rolling machine, with the copper layer on the outside of the bundle tube.

[0092] Step 2.2.3, Copper-niobium bundle tube processing: Milling to remove the copper layer near the sealing weld of the bundle tube, removing 8-10mm on one side.

[0093] Step 2.2.4, Ultrasonic Cleaning of Copper-Niobium Bundle Tubes: ① Wipe the inner and outer surfaces of the bundle tubes with a degreasing agent and anhydrous ethanol to remove oil and other contaminants. ② Place the bundle tubes in an ultrasonic cleaner for degreasing and ultrasonication at 40℃ for 60 minutes at a frequency of 40kHz. The cleaning agent is Micro90, mixed with pure water (resistivity 2MΩ·cm) at a ratio of 1:40. After ultrasonication, rinse with pure water until no foam is produced. ③ Place the bundle tubes in pure water for ultrasonication for 60 minutes at 40℃ and a frequency of 40kHz. After ultrasonication, rinse with pure water and air dry.

[0094] Step 2.2.5, Chemical Polishing (BCP) of Copper-Niobium Bundle Tubes: The polishing acid solution used is a mixture of 40% hydrofluoric acid, 68% nitric acid, and 85% phosphoric acid in a volume ratio of 1:1:2; the pickling temperature is below 20 degrees Celsius. The niobium layer and weld seams inside the bundle tube are pickled using a wiping method for 10 minutes, taking care to protect the copper layer from acid corrosion. After pickling, rinse with pure water, soak in pure water, and transfer to a clean room.

[0095] Step 2.2.6, Ultrasonic sonication of copper-niobium bundle tube: Place the bundle tube in ultrapure water (resistivity 18.2 MΩ·cm) and sonicate for 60 minutes at 40℃ and 40 kHz. After sonication, rinse the bundle tube with flowing ultrapure water for ≥15 minutes until the resistivity of the rinsed water is ≥18.2 MΩ·cm. Place the bundle tube in a Class 100 clean room to air dry for 3-5 hours.

[0096] Step 2.2.7, Copper-Niobium Bundle Tube Sealing: After the bundle tube is removed from the cleanroom, it must be sealed within 8 hours. The sealing process uses electron beam back-forming welding technology; specific welding parameters are omitted.

[0097] Step 2.2.8, Post-weld processing of copper-niobium bundle tube: Remove 8-10mm of the outer copper layer from the IRIS welding end, leaving a 0.5mm allowance on the end face; the length of the copper layer removed from the flange end is determined according to the thickness of the flange, such as removing 18.5mm of copper layer from the 1.3GHz cavity, leaving a 1mm allowance on the end face.

[0098] Step 3: Fabrication of a copper-niobium composite superconducting cavity using a half-cavity and a bundle tube includes:

[0099] Step 3.1, the copper-niobium cavity welding process, includes:

[0100] Step 3.1.1: First, perform ultrasonic cleaning, chemical polishing, and ultrapure water ultrasonic cleaning of the copper-niobium bundle tube (the processes are the same as steps 2.2.4, 2.2.5, and 2.2.6 respectively). The bundle tube is then mounted flush with the niobium-titanium flange. Figure 3 Electron beam welding was performed between the bundle tube and the end face of the niobium-titanium flange at position ①.

[0101] Step 3.1.2: First, perform ultrasonic cleaning, chemical polishing, and ultrapure water ultrasonic cleaning of the copper-niobium bundle tube (the processes are the same as steps 2.2.4, 2.2.5, and 2.2.6, respectively). The bundle tube is then flush with the end face of the semi-cavity IRIS and fixed with tooling. Figure 3 At position ②, the inner electron beam is formed and welded from the bundle tube to the end face of the semi-cavity IRIS to ensure the quality of the inner layer weld bead.

[0102] Step 3.1.3: First, perform ultrasonic cleaning, chemical polishing, and ultrapure water ultrasonic cleaning of the copper-niobium bundle tube (the processes are the same as steps 2.2.4, 2.2.5, and 2.2.6, respectively). Flatten the equatorial end faces of the two hemi-cavities and fix them with fixtures. Figure 3 At position ③, symmetrical electron beam spot welding is first performed from the outer side of the equatorial end of the semi-cavity, and then the tooling is removed and electron beam back forming welding is performed from the outer side to ensure the quality of the weld bead on the inner side of the equatorial cavity. (In step 3.1, the copper niobium beam tube chemical polishing must be redone every 8 hours, and the process is the same as in step 2.2.5).

[0103] Step 3.2, Flange sealing surface machining: After the bundle tube is welded to the niobium-titanium flange, the flange end face is machined on a lathe to remove the allowance and ensure that the roughness of the flange sealing surface is less than 0.8.

[0104] Step 3.3, Half-cavity assembly frequency control: After welding the bundle tube to the IRIS end face of the half-cavity assembly, the room temperature frequency of the half-cavity assembly is measured using a vector network analyzer. The bundle tube flange is clamped on a lathe, and the equatorial end face is machined. Based on the frequency results, the number of machining operations and the amount of material removed are determined, generally 3-5 times. The coefficients for frequency and material removal can be calculated using simulation software such as CST or COMSOL, or determined based on manufacturing experience. The room temperature frequency deviation of the half-cavity assembly should be ≤ ±0.2MHz.

[0105] Step 3.4, RF Surface Grinding: After the half-cavity assembly is processed to the target frequency at room temperature, defects on the RF surface (half-cavity and inner surface of the tube bundle) need to be ground. Diamond sandpaper is used, with the grit generally increasing from low to high: 120 grit → 320 grit → 600 grit → 1000 grit. Grinding is usually performed 3-5 times to remove all weld protrusions, scratches, dents, pinholes, and contaminants from the RF surface. After final grinding, degreasing ultrasonication is performed for 30 minutes, and the RF surface defects are checked again until there are no defects before full-cavity equator welding can be performed.

[0106] Step 4: Pretreatment of the copper-niobium composite superconducting cavity:

[0107] Step 4.1, Ultrasonic cleaning of the superconducting cavity: The ultrasonic cleaning process is the same as step 2.2.4.

[0108] Step 4.2: Chemical Polishing (BCP) of the Superconducting Cavity: The superconducting cavity is fixed on a BCP cavity cleaning device. The acid used is a mixed acid solution with a mass concentration of 40% hydrofluoric acid, 68% nitric acid, and 85% phosphoric acid, in a volume ratio of 1:1:2. The solution is acid-washed in the forward direction for 70 minutes, then in the reverse direction for 90 minutes. The difference in direction and time is to ensure uniform acid removal. The removal amount on the radio frequency surface is approximately 150 μm. The acid temperature is ≤20℃, and the flow rate is 15-18 L / min. After BCP, the cavity is rinsed with pure water until the pH reaches 7. This achieves the purpose of removing the contaminant layer on the inner surface of the superconducting cavity and improving surface roughness.

[0109] Step 4.3, High-Pressure Water Flushing (HPR) of the Superconducting Chamber: The superconducting chamber is fixed on the HPR equipment and flushed for 3 strokes. The flushing water pressure is ≥10MPa, the flushing water resistivity is ≥18.2MΩ·cm, the superconducting chamber rotates at approximately 20 revolutions / minute, and the nozzle moves at approximately 20mm / minute. After HPR, the superconducting chamber is placed in a Class 100 cleanroom to air dry for 3-5 hours.

[0110] Step 4.4, Superconducting cavity annealing: Place the superconducting cavity horizontally on the annealing furnace platform, with a vacuum level better than 1E-2Pa, heat to 750-800℃, and hold for 3-5 hours. After annealing, allow it to cool naturally to ≤50℃ before removing it.

[0111] Step 4.5, Electrochemical Polishing (EP) of the Superconducting Cavity: The superconducting cavity is fixed on the EP device, and a mixed acid solution with a mass concentration of 40% hydrofluoric acid and 98% sulfuric acid in a volume ratio of 1:9 is used; polishing is performed for 30 minutes to remove 20-25 μm of material; this aims to further improve the surface roughness of the superconducting cavity.

[0112] Step 4.6: Perform desulfurization twice;

[0113] Step 4.7: After performing high-pressure water rinsing (HPR) of the superconducting cavity as in Step 4.3, air dry it in a clean room;

[0114] Step 5: Deposit a niobium-tin thin film on the superconducting cavity using the CVD medium-temperature deposition method: The deposition materials are Nb and SnCl2, the evaporation temperature is 400℃-500℃, and the deposition temperature is 700℃-750℃.

[0115] Step 6, Brazing of the cold chain ring in the superconducting cavity: (as shown in the image) Figure 3 All positions ④ (including two bundle tube cold chain rings and one equatorial cold chain ring) are brazed with oxygen-free copper cold chain rings on the outer side of the superconducting cavity weld bead: soft brazing is used, with tin-silver alloy (BAg-6, silver content 72%, higher silver content improves thermal conductivity) as the solder. The brazing environment is high vacuum, and the brazing temperature is <450℃. The brazing temperature is the same as the low-temperature niobium triple tin plating temperature to prevent film damage. After completion, as shown... Figure 4 As shown; during the brazing process, the superconducting cavity flange is wrapped with niobium foil to prevent volatile gases that may be generated during brazing from entering.

[0116] Step 7: Perform buffered electrochemical polishing and packaging on the niobium-tin thin film superconducting cavity, including:

[0117] Step 7.1, Buffered Electrochemical Polishing (BEP) of Niobium-Tin Thin Film Superconducting Cavity: The main purpose is to remove residual Sn from the film surface and improve the roughness of the Niobium-Tin thin film. The Niobium-Tin thin film superconducting cavity is fixed on the EP device, using a mixture of 40% hydrofluoric acid, 98% sulfuric acid, and lactic acid in a volume ratio of 4:5:11. The polishing rate is less than 0.1 μm / min, and the polishing rate is mainly determined by the hydrofluoric acid content. After BEP, two desulfurization processes are required, followed by rinsing with ultrapure water and drying in a clean room.

[0118] Step 7.2: Clean packaging of the copper-niobium composite superconducting cavity.

[0119] After step 7, step 8, the low-temperature test, can be performed.

Claims

1. A method for preparing a copper-niobium-niobium-tin composite superconducting cavity, characterized in that, include: Step 1: Fabrication of copper-niobium composite plate; Step 2: Fabricate the semi-cavity and bundle tube using copper-niobium composite plates; Step 3: Fabricate a copper-niobium composite superconducting cavity using a half-cavity and a bundle tube; Step 4: Pretreatment of the copper-niobium composite superconducting cavity; Step 5: Deposit a niobium-tin thin film onto the superconducting cavity using a medium-temperature CVD deposition method; Step 6: Cold chain brazing of the superconducting cavity; Step 7: Electrochemical polishing and packaging of niobium-tin thin film superconducting cavity buffer; The coating materials in step 5 are Nb and SnCl2, with an evaporation temperature of 400℃-500℃ and a deposition temperature of 700℃-750℃. Step 3 includes: Step 3.1, Copper-Niobium Cavity Welding Process; Step 3.2, Flange sealing surface machining: After the bundle tube is welded to the niobium-titanium flange, the flange end face is machined on a lathe to remove the allowance and ensure that the roughness of the flange sealing surface is less than 0.

8. Step 3.3, Frequency control of the semi-cavity assembly: After the bundle tube is welded to the end face of the semi-cavity IRIS, the room temperature frequency of the semi-cavity assembly is measured using a vector network analyzer; the bundle tube flange is clamped on a lathe, and the equatorial end face is machined. Based on the frequency results, the number of machining operations and the amount of material removed are determined. Step 3.4, RF surface polishing: After the semi-cavity component is processed to the target frequency at room temperature, the RF surface defects are polished. The polishing sandpaper is diamond sandpaper, and the grit of the sandpaper is increased from low to high. After polishing, the component is subjected to degreasing ultrasonication for 30 minutes. The RF surface defects are checked again until there are no defects before the whole cavity equator welding is performed. Step 3.1 further includes: Step 3.1.1: First, perform ultrasonic cleaning, chemical polishing and ultrapure water ultrasonic cleaning of the copper-niobium bundle tube. Then, install the bundle tube flat with the niobium-titanium flange and perform electron beam welding on the end face of the bundle tube and the niobium-titanium flange. Step 3.1.2: First, perform ultrasonic cleaning, chemical polishing and ultrapure water ultrasonic cleaning of the copper niobium bundle tube. Then, flatten the bundle tube and fix it with tooling. Perform back-forming welding of the inner layer electron beam from the end face of the bundle tube and the end face of the half-cavity IRIS to ensure the quality of the inner layer weld. Step 3.1.3: First, perform ultrasonic cleaning, chemical polishing and ultrapure water ultrasonic cleaning of the copper-niobium bundle tube. Then, flatten the equatorial end faces of the two half-cavities and fix them with tooling. First, perform symmetrical electron beam spot welding from the outside of the equatorial end of the half-cavities. Then, remove the tooling and perform back-forming welding from the outside of the electron beam to ensure the quality of the weld bead on the inside of the equatorial side. Step 4 includes: Step 4.1, Ultrasonic Cleaning of the Superconducting Cavity: ① Wipe the inner and outer surfaces of the superconducting cavity with a degreasing agent and anhydrous ethanol; ② Place the superconducting cavity in an ultrasonic cleaner for degreasing and ultrasonication at a temperature of 40℃ for 60 minutes and a frequency of 40kHz; the cleaning agent is Micro90, and the ratio of cleaning agent to pure water is 1:40; after ultrasonication, rinse with pure water until no foam is produced; ③ Place the superconducting cavity in pure water for ultrasonication for 60 minutes at a water temperature of 40℃ and a frequency of 40kHz; after ultrasonication, rinse with pure water and air dry. Step 4.2: Chemical polishing of the superconducting cavity: The superconducting cavity is fixed on the BCP cavity cleaning equipment. The acid used is a mixed acid solution with a mass concentration of 40% hydrofluoric acid, 68% nitric acid, and 85% phosphoric acid, in a volume ratio of 1:1:

2. The cavity is acid-washed in the forward direction for 70 minutes, then in the reverse direction for 90 minutes. The removal rate on the radio frequency surface is 150 μm, the acid temperature is ≤20℃, and the flow rate is 15-18 L / min. The cavity is then rinsed with pure water until the pH reaches 7. Step 4.3, High-pressure water rinsing of the superconducting cavity: The superconducting cavity is fixed on the HPR equipment and rinsed for 3 strokes. The rinsing water pressure is ≥10MPa, the rinsing water resistivity is ≥18.2MΩ•cm, the rotation speed of the superconducting cavity is 20 revolutions / minute, and the nozzle movement speed is 20mm / minute. Then place the superconducting cavity in a Class 100 clean room to air dry for 3-5 hours. Step 4.4, superconducting cavity annealing: Place the superconducting cavity horizontally on the annealing furnace platform, with a vacuum degree better than 1E-2Pa, heat to 750-800℃, hold for 3-5 hours; allow to cool naturally to ≤50℃ before removing. Step 4.5, Electrochemical polishing of the superconducting cavity: The superconducting cavity is fixed on the EP device, and a mixed acid solution with a mass concentration of 40% hydrofluoric acid and 98% sulfuric acid in a volume ratio of 1:9 is used; polishing is performed for 30 minutes to remove 20-25 μm; this aims to further improve the surface roughness of the superconducting cavity. Step 4.6: Perform desulfurization twice; Step 4.7: After rinsing the superconducting cavity with high-pressure water as in Step 4.3, air dry it in a clean room. Step 6 includes: brazing an oxygen-free copper cold chain ring on the outside of the superconducting cavity weld bead: using soft brazing, the brazing material is a tin-silver alloy with a silver content of 72%, in a high vacuum environment, and at a brazing temperature of <450℃; during the brazing process, the superconducting cavity flange is wrapped with niobium foil.

2. The method for preparing a copper-niobium-niobium-tin composite superconducting cavity according to claim 1, characterized in that, Step 1 includes: Step 1.1: The copper-niobium composite plate is manufactured by parallel explosive welding of 2-4mm oxygen-free copper plate and 2-4mm high-purity niobium plate; wherein, the residual resistivity ratio of the high-purity niobium plate is ≥300. Step 1.2: After the copper-niobium composite plate is explosively welded, it is cut into round and square plates according to the material size specifications for the superconducting cavity. Step 1.3, Polishing of copper-niobium composite plate: The thickness of the polished composite plate is 3-4mm, of which the niobium layer is 1-1.5mm thick and the copper layer is 2-2.5mm thick.

3. The method for preparing a copper-niobium-niobium-tin composite superconducting cavity according to claim 1, characterized in that, The fabrication of the semi-cavity includes: Cutting copper-niobium semi-cavity plates; Copper-niobium half-cavity stamping; Machining of copper-niobium half-cavities.

4. The method for preparing a copper-niobium-niobium-tin composite superconducting cavity according to claim 3, characterized in that, The machining sequence of the copper-niobium half-cavity is as follows: Fix the stamped half cavity to the tooling and tighten the M12 bolts; Clamp the equatorial end plate on the lathe three-jaw chuck, remove the IRIS end plate, and complete the machining of the IRIS end. Leave a 0.5mm allowance on the IRIS end face and remove 8-10mm of copper layer near the weld bead. Install the IRIS end plate and tighten it with bolts. Turn the tooling around and clamp the IRIS end plate on the three-jaw chuck of the machine tool. Remove the equatorial end plate and complete the machining of the equatorial end. Leave a 2mm allowance on the equatorial end face. Thin the inside and outside of the equatorial end and remove 10-12mm of copper layer near the weld bead. Remove all tooling and take out the half-cavity part.

5. The method for preparing a copper-niobium-niobium-tin composite superconducting cavity according to claim 1, characterized in that, Step 2, which involves fabricating the semi-cavity and bundle tube using a copper-niobium composite plate, includes: Step A: Cutting copper-niobium bundle tubes; Step B: Roll the copper-niobium bundle tube into a circle; Step C, Copper-Niobium Bundle Processing: Milling removes the copper layer near the sealing weld bead of the bundle tube, removing 8-10mm on one side; Step D: Perform ultrasonic cleaning on the copper-niobium bundle tube; Step E: Perform chemical polishing on the copper-niobium bundle tube; Step F: Perform ultrasonic treatment on the copper-niobium bundle tube ultrapure water; Step G, Copper-Niobium Bundle Tube Sealing: After the bundle tube is removed from the cleanroom, the bundle tube sealing is completed within 8 hours; the sealing is performed using electron beam back forming welding technology. Step H, Post-weld processing of copper-niobium bundle tubes: Remove 8-10mm of the outer copper layer from the IRIS welding end, leaving a 0.5mm allowance on the end face; remove the copper layer from the flange end; The ultrasonic cleaning process for the copper-niobium bundle tube is as follows: ① Wipe the inner and outer surfaces of the bundle tube with a degreasing agent and anhydrous ethanol; ② Place the bundle tube in an ultrasonic cleaner for degreasing and ultrasonication at a temperature of 40℃ for 60 minutes and an ultrasonic frequency of 40kHz; the cleaning agent is Micro90, and the ratio of cleaning agent to pure water is 1:40; after ultrasonication, rinse with pure water until no foam is generated; ③ Place the bundle tube in pure water for ultrasonication for 60 minutes at a water temperature of 40℃ and an ultrasonic frequency of 40kHz; after ultrasonication, rinse with pure water and air dry. The chemical polishing process of the copper-niobium bundle tube is as follows: the acid used for polishing is a mixed acid solution with a mass concentration of 40% hydrofluoric acid, 68% nitric acid and 85% phosphoric acid, and a volume ratio of 1:1:2; the pickling temperature is below 20 degrees Celsius; the niobium layer and weld bead inside the bundle tube are pickled by wiping, and the pickling time is 10 minutes; after pickling, it is rinsed with pure water, soaked in pure water and transferred to a clean room. The process of ultrasonication of the copper-niobium bundle tube with ultrapure water is as follows: the bundle tube is placed in ultrapure water and ultrasonicated for 60 minutes at a temperature of 40℃ and an ultrasonic frequency of 40kHz; after ultrasonication, the bundle tube is rinsed with flowing ultrapure water for ≥15 minutes until the resistivity of the rinsed water is ≥18.2MΩ•cm; the bundle tube is then placed in a Class 100 clean room to air dry for 3-5 hours.

6. The method for preparing a copper-niobium-niobium-tin composite superconducting cavity according to claim 1, characterized in that, Step 7 involves buffered electrochemical polishing of the niobium-tin thin film superconducting cavity to remove residual Sn on the film surface and improve the roughness of the niobium-tin thin film. This includes: fixing the niobium-tin thin film superconducting cavity on an EP device; using a mixed acid solution with a mass concentration of 40% hydrofluoric acid, 98% sulfuric acid, and lactic acid, wherein the volume ratio of hydrofluoric acid, sulfuric acid, and lactic acid is 4:5:11; and a polishing rate of less than 0.1 μm / min; followed by two desulfurization processes; and then rinsing with ultrapure water and drying in a clean room. After step 7, proceed to step 8, the low-temperature test.

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