Preparation method of copper-niobium-niobium-tin composite superconducting cavity

Abstract

The invention discloses a preparation method of a copper-niobium-niobium-tin composite superconducting cavity. Comprising the following steps: manufacturing a copper-niobium composite board; manufacturing a half cavity and a beam tube by using a copper-niobium composite plate; manufacturing a copper-niobium composite superconducting cavity by using the half cavity and the beam tube; pretreating the copper-niobium composite superconducting cavity; the method comprises the following steps: plating a niobium-tin thin film on a superconducting cavity by adopting a CVD (Chemical Vapor Deposition) medium-temperature film plating method, wherein film plating materials are Nb and SnCl2, the evaporation temperature is 400-500 DEG C, and the deposition temperature is 700-750 DEG C; brazing a cold chain ring of the superconducting cavity; and carrying out buffer electrochemical polishing and packaging on the Nb3Sn thin film superconducting cavity. According to the Ni3Sn thin film, the surface resistance of the composite superconducting cavity is reduced, the superconducting critical temperature and the critical magnetic field are improved, and a low-temperature refrigerator is used for replacing liquid helium for refrigeration; the cold chain conduction ring braze welding realizes good heat conduction connection between the low-temperature refrigerator and the superconducting cavity.

Description

Technical Field

[0001] The invention belongs to the technical field of metal combination processing and pickling metal materials, and specifically relates to a method for preparing a copper-niobium-niobium-tin composite superconducting cavity. Background Art

[0002] Particle accelerators are vacuum experimental devices that use electromagnetic fields to accelerate charged particles. Their application areas include advanced light sources, particle colliders, industrial accelerators, and medical accelerators. Particle accelerators not only provide important research methods in the fields of materials science, nuclear science, medicine, and chemistry, but also play a unique role in various national economic fields such as isotope production, radiation medicine, and industrial irradiation. Radio frequency superconducting technology is an advanced technology of modern particle accelerators. It couples external radio frequency power through antennas or waveguides to excite electromagnetic fields in metal superconductor cavities. Charged particles enter the electromagnetic field at a suitable phase and interact with it, which increases the energy of the particles. Compared with room temperature accelerators, radio frequency superconducting accelerators use superconductors as acceleration cavity materials, which can reduce the radio frequency loss and operating costs of the acceleration cavity and improve energy utilization. Superconducting cavity materials generally use pure niobium. In large superconducting accelerators, superconducting cavities are cooled by immersion in liquid helium to maintain the temperature of the superconducting cavity stable. In order to provide sufficient liquid helium, a complete set of complex and expensive liquid helium cryogenic systems are required for superconducting modules. The cryogenic system is large in size, expensive, and requires long-term investment for maintenance. This leads to significant limitations on the application of superconducting accelerators in other fields.

[0003] The main methods for cavity niobium-tin coating include evaporation (temperature greater than 1100°C, exceeding the melting point of oxygen-free copper 1083°C) and bronze method / electrodeposition method (Sn-Cu-Nb ternary reaction, temperature less than 700°C, the film contains impurities such as Cu and Cu3Sn).

[0004] At the same time, the overheat magnetic field of Nb3Sn at a lower temperature is 420mT, far exceeding the 240mT of pure Nb, which means that under the same cavity conditions, the Nb3Sn cavity can theoretically withstand a higher peak magnetic field and provide a higher acceleration gradient. Taking the 1.3GHz TESLA cavity as an example, the theoretical gradient of Nb3Sn is nearly 100MV / m (57MV / m for pure Nb cavity), so the Nb3Sn cavity has 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, the copper-niobium explosion is used to make the composite plate; the copper-niobium composite superconducting cavity is made by die stamping, processing and electron beam welding; the niobium-tin film is plated inside the cavity by chemical vapor deposition; finally, the composite cavity is connected to the cold chain ring by brazing, and connected to the low-temperature industrial refrigerator by a conductive belt to complete the cooling of the superconducting cavity. Therefore, the copper, niobium and niobium-tin composite cold chain conduction superconducting cavity improves the superconducting critical temperature, acceleration gradient, overheat magnetic field and thermal conductivity. The industrial refrigerator replaces the liquid helium system and greatly reduces the accelerator cost and operating cost. It is an ideal solution for the miniaturization and civilian use of superconducting accelerators. Summary of the invention

[0006] In view of the problems existing in the background technology, the present invention provides a method for preparing a copper-niobium-niobium-tin composite superconducting cavity, and the technical solution includes:

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

[0008] Step 2: Use copper-niobium composite plates to make half cavities and bundle tubes;

[0009] Step 3, using the half cavity and the beam tube to make a copper-niobium composite superconducting cavity;

[0010] Step 4, pretreatment of the copper-niobium composite superconducting cavity;

[0011] Step 5, using a CVD medium-temperature coating method to coat the superconducting cavity with a niobium-tin thin film;

[0012] Step 6, brazing of superconducting cavity cold chain ring;

[0013] Step 7, performing buffer electrochemical polishing and packaging on the niobium-tin thin film superconducting cavity;

[0014] The coating materials in step 5 are Nb and SnCl2, the evaporation temperature is 400°C-500°C, and the deposition temperature is 700°C-750°C.

[0015] The step 1 comprises:

[0016] Step 1.1, the copper-niobium composite plate is manufactured by parallel explosion welding of a 2-4 mm oxygen-free copper plate and a 2-4 mm 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 plates and square plates according to the size specifications of the superconducting cavity manufacturing material;

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

[0019] The half-cavity production includes:

[0020] Cutting of copper-niobium half-cavity plates;

[0021] Copper-niobium half-cavity stamping;

[0022] Copper-niobium half-cavity machining.

[0023] The order of the copper-niobium half-cavity processing is:

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

[0025] Clamp the equatorial end plate on the three-jaw plate of the lathe, remove the IRIS end plate, complete the processing of the IRIS end, reserve 0.5mm margin on the IRIS end surface, and remove 8-10mm of copper layer near the weld;

[0026] Install the IRIS end plate and tighten it with bolts, turn the tooling around, clamp the IRIS end plate on the three jaws of the machine tool, remove the equatorial end plate, complete the processing of the equatorial end, reserve a 2mm margin on the equatorial end face, thin the inside and outside of the equatorial end, and remove 10-12mm of the copper layer near the weld;

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

[0028] The step 2 of using the copper-niobium composite plate to make the half cavity and the bundle tube comprises:

[0029] Step A, cutting copper-niobium bundle tube;

[0030] Step B, rolling the copper-niobium bundle tube into a circle;

[0031] Step C, copper-niobium bundle tube processing: milling to remove the copper layer near the bundle tube sealing weld, removing 8-10 mm on one side;

[0032] Step D, performing ultrasonic cleaning on the copper-niobium beam tube;

[0033] Step E, chemically polishing the copper-niobium bundle tube;

[0034] Step F, performing ultrasound on the copper-niobium beam tube ultrapure water;

[0035] Step G, sealing welding of copper-niobium beam tube: After the beam tube is taken out of the clean room, the sealing welding of the beam tube is completed within 8 hours. The sealing welding adopts electron beam back forming welding technology;

[0036] Step H, post-weld processing of copper-niobium bundle tube: remove 8-10mm of outer copper layer at the IRIS welding end, and reserve 0.5 margin at the end face; remove the copper layer at the flange end;

[0037] The ultrasonic cleaning process of 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 put the bundle tube into an ultrasonic cleaning machine for oil removal, the temperature is 40°C, the ultrasonic wave is for 60 minutes, and the ultrasonic frequency is 40khz; the cleaning agent is Micro90, and the ratio of the cleaning agent to pure water is 1:40; after the ultrasonic wave is finished, rinse with pure water until no foam is generated; ③ put the bundle tube into pure water for ultrasonic wave for 60 minutes, the water temperature is 40°C, the ultrasonic frequency is 40khz, and after the ultrasonic wave is finished, rinse with pure water and dry;

[0038] The chemical polishing process of the copper-niobium beam tube is as follows: the acid solution 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 lower than 20 degrees Celsius; the niobium layer and the weld inside the beam tube are pickled by scrubbing, and the pickling time is 10 minutes; after pickling, rinse with pure water, and transport to a clean room by soaking in pure water;

[0039] The process of the copper-niobium bundle tube ultrapure water ultrasound is as follows: the bundle tube is placed in ultrapure water ultrasound for 60 minutes, the temperature is 40° C., and the ultrasound frequency is 40 khz; after the ultrasound is finished, the bundle tube is rinsed with flowing ultrapure water for ≥15 minutes until the water resistivity after rinsing is ≥18.2 MΩ·cm; the bundle tube is placed in a Class 100 clean room to 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 processing: after the bundle tube is welded to the niobium-titanium flange, the flange end face is processed by a lathe to remove the reserved amount to ensure that the flange sealing surface roughness is less than 0.8;

[0043] Step 3.3, half-cavity assembly frequency control: After the beam tube is welded to the half-cavity IRIS end face, use a vector network analyzer to measure the half-cavity assembly room temperature frequency. Use a lathe to clamp the beam tube flange, process the equatorial end face, and determine the number of processing times and processing removal amount based on the frequency results;

[0044] Step 3.4, RF surface polishing: After the half-cavity component is processed to the target frequency at room temperature, the RF surface defects are polished. The grinding sandpaper is diamond sandpaper, and the sandpaper mesh is from low to high. After polishing, perform oil removal ultrasound for 30 minutes, and check the RF surface defects again. Only when there are no defects can the whole cavity equatorial welding be performed;

[0045] The step 3.1 further comprises:

[0046] Step 3.1.1, firstly ultrasonically clean the copper-niobium bundle tube, chemically polish the copper-niobium bundle tube, and perform ultrapure water ultrasonic treatment on the copper-niobium bundle tube, align the bundle tube with the niobium-titanium flange, and perform electron beam welding on the end faces of the bundle tube and the niobium-titanium flange.

[0047] Step 3.1.2, firstly perform ultrasonic cleaning of the copper-niobium beam tube, chemical polishing of the copper-niobium beam tube, and ultrapure water ultrasonic treatment of the copper-niobium beam tube, and then level the beam tube with the end face of the half-cavity IRIS, fix it with a tool, and perform electron beam back forming welding from the inner layer of the beam tube and the end face of the half-cavity IRIS to ensure the quality of the inner layer weld;

[0048] Step 3.1.3, first perform ultrasonic cleaning of the copper-niobium bundle tube, chemical polishing of the copper-niobium bundle tube and ultrapure water ultrasonic treatment of the copper-niobium bundle tube, flatten the equatorial end faces of the two half-cavities, fix them with tooling, perform symmetrical electron beam spot welding from the outside of the equatorial end of the half-cavity, then remove the tooling and perform electron beam back forming welding from the outside to ensure the quality of the weld on the inner side of the equator.

[0049] The step 4 comprises:

[0050] Step 4.1, ultrasonic cleaning of superconducting cavity: ① wipe the inner and outer surfaces of the superconducting cavity with degreasing agent and anhydrous ethanol; ② put the superconducting cavity into an ultrasonic cleaning machine for oil removal and ultrasonic cleaning, the temperature is 40℃, the ultrasonic cleaning is 60 minutes, and the ultrasonic frequency is 40khz; the cleaning agent is Micro90, and the ratio of cleaning agent to pure water is 1:40; after the ultrasonic cleaning is completed, rinse with pure water until no foam is generated; ③ put the superconducting cavity into pure water for ultrasonic cleaning for 60 minutes, the water temperature is 40℃, the ultrasonic frequency is 40khz, and after the ultrasonic cleaning is completed, rinse with pure water and dry;

[0051] Step 4.2, chemical polishing of the superconducting cavity: the superconducting cavity is fixed on the BCP whole cavity cleaning equipment, and the acid used 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; forward pickling for 70 minutes, and reverse pickling for 90 minutes; the RF surface removal amount is 150μm, the acid temperature is ≤20℃, and the flow rate is 15-18L / min; then rinse with pure water until the pH value 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 rotates at a speed of about 20 revolutions / minute, and the nozzle moves at a speed of about 20mm / minute; then the superconducting cavity is placed in a Class 100 clean room to 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, raise the temperature to 750-800℃, and keep it at this temperature for 3-5 hours; cool it naturally to ≤50℃ and then take 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 and a volume ratio of 1:9 is used; polishing for 30 minutes to remove 20-25 μm; the purpose of further improving the surface roughness of the superconducting cavity is achieved;

[0055] Step 4.6, performing desulfurization twice;

[0056] Step 4.7: Rinse the superconducting cavity with high-pressure water according to step 4.3, and then dry it in a clean room.

[0057] The step 6 comprises: brazing the oxygen-free copper cold chain ring on the outer side of the superconducting cavity weld: using soft soldering, the solder is a tin-silver alloy with a silver content of 72%, a high vacuum environment, and a brazing temperature of <450°C; during the brazing process, the superconducting cavity flange is wrapped with niobium foil

[0058] In step 7, the niobium tin thin film superconducting cavity is subjected to buffered electrochemical polishing to remove residual Sn on the surface of the film and improve the roughness of the niobium tin thin film, comprising: 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 desulfurization is performed twice, and then ultrapure water is rinsed and dried in a clean room.

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

[0060] The beneficial effects of the present invention are:

[0061] 1. A new process for making copper-niobium cavities was used: first, the copper-niobium composite superconducting cavity was made through mold stamping, machine tool processing, clean ultrasound, post-processing and electron beam welding; then, the copper-niobium composite superconducting cavity was pretreated and niobium-tin film was plated to complete the copper, niobium and niobium-tin composite superconducting cavity; finally, the cold chain conduction cooling ring was made through brazing of oxygen-free copper and the composite superconducting cavity.

[0062] 2. CVD and magnetron sputtering methods were used to deposit Nb3Sn thin films on the superconducting cavity to produce copper, niobium and niobium-tin composite superconducting cavities. 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 resistance of the composite superconducting cavity, increased the superconducting critical temperature and critical magnetic field, and realized the use of low-temperature refrigerators instead of liquid helium refrigeration. The cold chain conduction ring brazing achieved a good thermal conduction connection between the low-temperature refrigerator and the superconducting cavity. High-purity niobium achieved the reaction of tin on the niobium surface to form niobium-tin. High-purity niobium reduced the risk of quenching and superconducting cavity collapse caused by surface defects in the composite superconducting cavity.

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

[0064] 4. In the process of making the copper-niobium cavity, the niobium layer should be cleaned and the copper layer should be protected before welding to prevent the niobium layer from being contaminated by the copper layer during electron beam welding. According to the treatment method in this article, the niobium layer can be effectively cleaned and the niobium layer can be prevented from being contaminated.

[0065] 5. The superconducting cavity niobium-tin coating is prepared by medium-temperature chemical vapor deposition (CVD). The CVD method is more uniform in coating on anisotropic surfaces, and the coating parameters are easy to control.

[0066] 6. The cold chain ring brazing uses tin-silver alloy (Sn-Ag) solder, and the temperature is less than 450℃. The thermal conductivity of tin-silver alloy is close to that of oxygen-free copper, which can ensure the uniformity of low-temperature conduction of the superconducting cavity. The brazing temperature is lower than the coating temperature, which can reduce the effect of the brazing temperature on the coating layer. Low-temperature brazing can reduce the effect of brazing thermal deformation on the frequency and field flatness of the superconducting cavity. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Figure 1 The present invention is a schematic flow chart of an embodiment of a method for preparing a copper-niobium-niobium-tin composite superconducting cavity.

[0068] Figure 2 A schematic cross-sectional view of parallel explosion welding in an embodiment of the present invention;

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

[0070] Figure 4 Schematic diagram of the structure of the superconducting cavity after the cold chain ring is brazed in an embodiment of the present invention. DETAILED DESCRIPTION

[0071] The present invention is further described in detail below in conjunction with the accompanying drawings.

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

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

[0074] Step 2: Use copper-niobium composite plates to make half cavities and bundle tubes;

[0075] Step 3, using the half cavity and the beam tube to make a copper-niobium composite superconducting cavity;

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

[0077] Step 5, using a CVD medium-temperature coating method to coat the superconducting cavity with a niobium-tin thin film;

[0078] Step 6, brazing of superconducting cavity cold chain ring;

[0079] Step 7: Buffer electrochemical polishing and packaging of the niobium-tin thin film superconducting cavity.

[0080] Wherein, step 1, copper-niobium composite plate production includes:

[0081] Step 1.1: Copper-niobium composite plate: 2-4 mm oxygen-free copper plate and 2-4 mm high-purity niobium plate are welded by parallel explosion welding (such as Figure 1 As shown in the figure, 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 plates and square plates according to the size specifications of the superconducting cavity manufacturing material;

[0083] Step 1.3, polishing of copper-niobium composite plate: the thickness of the composite plate after polishing is 3-4mm, of which the thickness of the niobium layer is 1-1.5mm and the thickness of the copper layer is 2-2.5mm. If the niobium layer is too thin, it is easy to crack during cavity forming; if the niobium layer is too thick, it will affect low-temperature thermal conductivity; the copper layer cannot be too thick, otherwise the cavity cannot be formed in one time, and multiple forming will increase the damage of the niobium layer.

[0084] Step 2: Using copper-niobium composite plates to make half cavities and bundle tubes includes:

[0085] Step 2.1, half-cavity production, also includes:

[0086] Step 2.1.1, copper-niobium half-cavity plate blanking: According to the half-cavity surface development and mold design calculation, determine the copper-niobium plate blanking size. For example, the half-cavity blanking size of a 1.3GHz superconducting cavity is δ is the thickness of the composite sheet, generally 3-4 mm.

[0087] Step 2.1.2, copper-niobium half-cavity stamping: the stamping die includes a concave die, a punch and a pressing plate, the stamping equipment is a 200T four-column hydraulic press, the stamping pressure is 120T, the holding time is 3 minutes, and the lubricating fluid is a 1:1 mixture of vegetable oil and salad oil.

[0088] Step 2.1.3, copper-niobium half-cavity processing: The half-cavity is fixed with a processing tooling, which includes an equatorial end plate, a punch, a die, and an IRIS end plate. The processing sequence is as follows: ① Fix the stamped half-cavity to the tooling and tighten with M12 bolts. ② Clamp the equatorial end plate on the three-jaw plate of the lathe, remove the IRIS end plate, complete the processing of the IRIS end, reserve a 0.5mm margin on the IRIS end face, and remove 8-10mm of the copper layer near the weld. ③ Install the IRIS end plate and tighten it with bolts, turn the tooling around, clamp the IRIS end plate on the three-jaw plate of the machine tool, and remove the equatorial end plate; complete the processing of the equatorial end, reserve a 2mm margin on the equatorial end face, thin the inside and outside of the equatorial end, and remove 10-12mm of the copper layer near the weld. ④ Remove all tooling and take out the half-cavity parts.

[0089] Step 2.2, bundle tube production, also includes:

[0090] Step 2.2.1, cutting of copper-niobium beam tube: Size: Determine the size of the copper-niobium beam tube according to the midline method of the coil. For example, the cutting size of the 1.3GHz cavity beam tube is 254.15 (circumference) × 165 × δ, where δ is the thickness of the composite plate, which is generally 3-4mm.

[0091] Step 2.2.2, rolling the copper-niobium bundle tube: use a three-roller rolling machine to roll it, 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 bundle tube sealing weld, removing 8-10mm on one side.

[0093] Step 2.2.4, ultrasonic cleaning of copper-niobium bundle tube: ① Use degreasing agent and anhydrous ethanol to wipe the inner and outer surfaces of the bundle tube to remove oil stains, etc. ② Put the bundle tube into an ultrasonic cleaning machine for oil removal, the temperature is 40℃, the ultrasonic frequency is 40khz, the ultrasonic cleaning agent is Micro90, and the ratio of pure water (resistivity 2MΩ·cm) is 1:40; after the ultrasonic cleaning, rinse with pure water until no foam is generated. ③ Put the bundle tube into pure water for ultrasonic cleaning for 60 minutes, the water temperature is 40℃, the ultrasonic frequency is 40khz, and after the ultrasonic cleaning, rinse with pure water and dry.

[0094] Step 2.2.5, Chemical polishing (BCP) of copper-niobium beam tube: 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, with a volume ratio of 1:1:2; the pickling temperature is lower than 20 degrees Celsius. The niobium layer and weld inside the beam tube are pickled by scrubbing, and the pickling time is 10 minutes. Pay attention to protecting the copper layer from acid corrosion. After pickling, rinse with pure water and transfer to the clean room by soaking in pure water.

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

[0096] Step 2.2.7, copper-niobium beam tube sealing welding: After the beam tube is taken out of the clean room, the beam tube sealing welding is completed within 8 hours. The sealing welding adopts electron beam back forming welding technology, and the specific welding parameters are omitted.

[0097] Step 2.2.8, post-weld processing of the copper-niobium beam tube: remove 8-10mm of the outer copper layer at the IRIS welding end, and reserve 0.5mm of margin on the end face; the length of the copper layer removed at 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, and reserve 1mm of margin on the end face.

[0098] Step 3: Using the half cavity and the beam tube to make a copper-niobium composite superconducting cavity includes:

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

[0100] Step 3.1.1, firstly perform ultrasonic cleaning, chemical polishing and ultrapure water ultrasonic treatment on the copper-niobium bundle tube (the process is the same as step 2.2.4, step 2.2.5 and step 2.2.6 respectively), and then align the bundle tube with the niobium-titanium flange. Figure 3 Electron beam welding is performed between the beam tube and the end face of the niobium-titanium flange at position ①.

[0101] Step 3.1.2: First, ultrasonically clean the copper-niobium bundle tube, chemically polish the copper-niobium bundle tube, and ultrasonically treat the copper-niobium bundle tube with ultrapure water (the process is the same as step 2.2.4, step 2.2.5, and step 2.2.6, respectively). The bundle tube is flush with the end face of the half-cavity IRIS and fixed with a tool. Figure 3 The inner layer electron beam back forming welding is performed from the beam tube to the end face of the half-cavity IRIS at position ② to ensure the quality of the inner layer weld.

[0102] Step 3.1.3, firstly perform ultrasonic cleaning, chemical polishing and ultrapure water ultrasonic treatment on the copper-niobium bundle tube (the process is the same as step 2.2.4, step 2.2.5 and step 2.2.6 respectively), flatten the equatorial end faces of the two half cavities and fix them with tooling. Figure 3 At position ③, symmetrical electron beam spot welding is first performed from the outside of the half cavity equatorial end, and then the tooling is removed and the electron beam back-molding welding is performed from the outside to ensure the quality of the weld on the inside of the equator. (In step 3.1, the copper-niobium beam tube needs to be chemically polished again every 8 hours, and the process is the same as step 2.2.5).

[0103] Step 3.2, flange sealing surface processing: After the bundle tube is welded to the niobium-titanium flange, the flange end face is processed by a lathe to remove the reserved amount to ensure that the flange sealing surface roughness is less than 0.8.

[0104] Step 3.3, half-cavity assembly frequency control: After the beam tube is welded to the half-cavity IRIS end face, use a vector network analyzer to measure the half-cavity assembly room temperature frequency. Use a lathe to clamp the beam tube flange and process the equatorial end face. According to the frequency results, determine the number of processing times and the amount of processing removal, which is generally 3-5 times. The coefficients of frequency and processing removal can be calculated using simulation software such as CST and comsol or determined based on manufacturing experience. The half-cavity assembly room temperature frequency deviation is ≤±0.2MHz.

[0105] Step 3.4, RF surface grinding: After the half-cavity assembly is processed to the target frequency at room temperature, the RF surface (half-cavity and beam tube inner surface) needs to be polished for defects. The grinding sandpaper is diamond sandpaper, and the sandpaper mesh is generally from low to high, 120 mesh → 320 mesh → 600 mesh → 1000 mesh. Grinding is generally performed 3-5 times to remove all weld bead protrusions, scratches, bumps, sand holes and contaminants on the RF surface. After the final grinding, perform oil removal ultrasound for 30 minutes, and check the RF surface defects again until there are no defects before the whole cavity equatorial welding can be performed.

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

[0107] Step 4.1, ultrasonic cleaning of superconducting cavity: The process of ultrasonic cleaning 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 the BCP whole cavity cleaning equipment, and the acid used is a mixed acid with a mass concentration of 40% hydrofluoric acid, 68% nitric acid and 85% phosphoric acid, and a volume ratio of 1:1:2; forward pickling for 70 minutes, and reverse pickling for 90 minutes. The difference in forward and reverse directions and time is to ensure uniform pickling removal; the RF surface removal is about 150μm, the acid temperature is ≤20℃, and the flow rate is 15-18L / minute. After BCP, rinse with pure water until the pH value reaches 7; thereby achieving the purpose of removing the surface contamination layer inside the superconducting cavity and improving the surface roughness.

[0109] Step 4.3, high pressure water flushing (HPR) of 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 rotates at a speed of about 20 revolutions / minute, and the nozzle moves at a speed of about 20mm / minute. After the HPR is completed, the superconducting cavity is placed in a Class 100 clean room to 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 better than 1E-2Pa, and heat it to 750-800℃ for 3-5 hours. After annealing, cool it naturally to ≤50℃ and take it out.

[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 and a volume ratio of 1:9 is used; polishing for 30 minutes to remove 20-25 μm; the purpose of further improving the surface roughness of the superconducting cavity is achieved;

[0112] Step 4.6, performing desulfurization twice;

[0113] Step 4.7: Perform high pressure water flushing (HPR) of the superconducting cavity according to step 4.3, and then dry it in a clean room;

[0114] Step 5: Use CVD medium-temperature coating method to coat the superconducting cavity with a niobium-tin thin film: the coating materials are Nb and SnCl2, the evaporation temperature is 400°C-500°C, and the deposition temperature is 700°C-750°C;

[0115] Step 6: Superconducting cavity cold chain ring brazing: Figure 3 All positions ④ (including two beam tube cold chain rings and one equatorial cold chain ring) are brazed with oxygen-free copper cold chain rings on the outside of the superconducting cavity weld: soft soldering is used, the solder is tin-silver alloy (BAg-6, silver content is 72%, high silver content has better thermal conductivity), high vacuum environment, brazing temperature <450℃. The brazing temperature is low-temperature niobium tin coating temperature to prevent film damage. After completion, Figure 4 As shown; during the brazing process, the superconducting cavity flange is wrapped with niobium foil to prevent the entry of volatile gases that may be generated by brazing.

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

[0117] Step 7.1, Buffered electrochemical polishing (BEP) of the niobium-tin film superconducting cavity: The main purpose is to remove the residual Sn on the film surface and improve the roughness of the niobium-tin film. The niobium-tin film superconducting cavity is fixed on the EP device, using a mass concentration of 40% hydrofluoric acid, 98% sulfuric acid and lactic acid, with a volume ratio of 4:5:11. The polishing rate is less than 0.1μm / min, and the polishing rate is mainly determined by the content of hydrofluoric acid. After the BEP is completed, two desulfurizations are required, followed by ultrapure water rinsing and clean room drying.

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

[0119] After step 7, step 8, low temperature testing, can be performed.

Claims

1. A method for preparing a copper-niobium-niobium-tin composite superconducting cavity, characterized in that: include: Step 1: Preparation of copper-niobium composite plate; Step 2: Use copper-niobium composite plates to make half cavities and bundle tubes; Step 3, using the half cavity and the beam tube to make a copper-niobium composite superconducting cavity; Step 4, pretreatment of the copper-niobium composite superconducting cavity; Step 5, using a CVD medium-temperature coating method to coat the superconducting cavity with a niobium-tin thin film; Step 6, brazing of superconducting cavity cold chain ring; Step 7, electrochemical polishing and packaging of the niobium-tin thin film superconducting cavity buffer; The coating materials in step 5 are Nb and SnCl2, the evaporation temperature is 400°C-500°C, and the deposition temperature is 700°C-750°C.

2. The method for preparing a copper-niobium-niobium-tin composite superconducting cavity according to claim 1, characterized in that: The step 1 comprises: Step 1.1, the copper-niobium composite plate is manufactured by parallel explosion welding of a 2-4 mm oxygen-free copper plate and a 2-4 mm 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 plates and square plates according to the size specifications of the superconducting cavity manufacturing material; Step 1.3, polishing of the copper-niobium composite plate: the thickness of the composite plate after polishing is 3-4 mm, of which the thickness of the niobium layer is 1-1.5 mm and the thickness of the copper layer is 2-2.5 mm.

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

4. The method for preparing a copper-niobium-niobium-tin composite superconducting cavity according to claim 3, characterized in that: The order of the copper-niobium half-cavity processing is: Fix the stamped half cavity to the tooling and tighten with M12 bolts; Clamp the equatorial end plate on the three-jaw plate of the lathe, remove the IRIS end plate, complete the processing of the IRIS end, reserve 0.5mm margin on the IRIS end surface, and remove 8-10mm of copper layer near the weld; Install the IRIS end plate and tighten it with bolts, turn the tooling around, clamp the IRIS end plate on the three jaws of the machine tool, remove the equatorial end plate, complete the processing of the equatorial end, reserve a 2mm margin on the equatorial end face, thin the inside and outside of the equatorial end, and remove 10-12mm of the copper layer near the weld; Remove all tooling and take out the half-cavity parts.

5. The method for preparing a copper-niobium-niobium-tin composite superconducting cavity according to claim 1, characterized in that: The step 2 of using the copper-niobium composite plate to make the half cavity and the bundle tube comprises: Step A, cutting copper-niobium bundle tube; Step B, rolling the copper-niobium bundle tube into a circle; Step C, copper-niobium bundle tube processing: milling to remove the copper layer near the bundle tube sealing weld, removing 8-10 mm on one side; Step D, performing ultrasonic cleaning on the copper-niobium beam tube; Step E, chemically polishing the copper-niobium bundle tube; Step F, performing ultrasound on the copper-niobium beam tube ultrapure water; Step G, sealing welding of copper-niobium beam tube: After the beam tube is taken out of the clean room, the sealing welding of the beam tube is completed within 8 hours. The sealing welding adopts electron beam back forming welding technology; Step H, post-weld processing of copper-niobium bundle tube: remove 8-10mm of outer copper layer at the IRIS welding end, and reserve 0.5 margin at the end face; remove the copper layer at the flange end; The ultrasonic cleaning process of 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 put the bundle tube into an ultrasonic cleaning machine for oil removal, the temperature is 40°C, the ultrasonic wave is for 60 minutes, and the ultrasonic frequency is 40khz; the cleaning agent is Micro90, and the ratio of the cleaning agent to pure water is 1:40; after the ultrasonic wave is finished, rinse with pure water until no foam is generated; ③ put the bundle tube into pure water for ultrasonic wave for 60 minutes, the water temperature is 40°C, the ultrasonic frequency is 40khz, and after the ultrasonic wave is finished, rinse with pure water and dry; The chemical polishing process of the copper-niobium beam tube is as follows: the acid solution 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 lower than 20 degrees Celsius; the niobium layer and the weld inside the beam tube are pickled by scrubbing, and the pickling time is 10 minutes; after pickling, rinse with pure water, and transport to a clean room by soaking in pure water; The process of the copper-niobium bundle tube ultrapure water ultrasound is as follows: the bundle tube is placed in ultrapure water ultrasound for 60 minutes, the temperature is 40° C., and the ultrasound frequency is 40 khz; after the ultrasound is finished, the bundle tube is rinsed with flowing ultrapure water for ≥15 minutes until the water resistivity after rinsing is ≥18.2 MΩ·cm; the bundle tube is placed in a Class 100 clean room to dry for 3-5 hours.

6. The method for preparing a copper-niobium-niobium-tin composite superconducting cavity according to claim 5, characterized in that: Step 3 includes: Step 3.1, copper-niobium cavity welding process; Step 3.2, flange sealing surface processing: after the bundle tube is welded to the niobium-titanium flange, the flange end face is processed by a lathe to remove the reserved amount to ensure that the flange sealing surface roughness is less than 0.8; Step 3.3, half-cavity assembly frequency control: After the beam tube is welded to the half-cavity IRIS end face, use a vector network analyzer to measure the half-cavity assembly room temperature frequency. Use a lathe to clamp the beam tube flange, process the equatorial end face, and determine the number of processing times and processing removal amount based on the frequency results; Step 3.4, RF surface polishing: After the half-cavity component is processed to the target frequency at room temperature, the RF surface defects are polished. The grinding sandpaper is diamond sandpaper, and the sandpaper mesh is from low to high. After polishing, perform oil removal ultrasound for 30 minutes, and check the RF surface defects again. Only when there are no defects can the whole cavity equatorial welding be performed; The step 3.1 further comprises: Step 3.1.1, firstly ultrasonically clean the copper-niobium bundle tube, chemically polish the copper-niobium bundle tube, and perform ultrapure water ultrasonic treatment on the copper-niobium bundle tube, align the bundle tube with the niobium-titanium flange, and perform electron beam welding on the end faces of the bundle tube and the niobium-titanium flange. Step 3.1.2, firstly perform ultrasonic cleaning of the copper-niobium beam tube, chemical polishing of the copper-niobium beam tube, and ultrapure water ultrasonic treatment of the copper-niobium beam tube, and then level the beam tube with the end face of the half-cavity IRIS, fix it with a tool, and perform electron beam back forming welding from the inner layer of the beam 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 of the copper-niobium bundle tube, chemical polishing of the copper-niobium bundle tube and ultrapure water ultrasonic treatment of the copper-niobium bundle tube, flatten the equatorial end faces of the two half-cavities, fix them with tooling, perform symmetrical electron beam spot welding from the outside of the equatorial end of the half-cavity, then remove the tooling and perform electron beam back forming welding from the outside to ensure the quality of the weld on the inner side of the equator.

7. The method for preparing a copper-niobium-niobium-tin composite superconducting cavity according to claim 1, characterized in that: The step 4 comprises: Step 4.1, ultrasonic cleaning of superconducting cavity: ① wipe the inner and outer surfaces of the superconducting cavity with degreasing agent and anhydrous ethanol; ② put the superconducting cavity into ultrasonic cleaning machine for oil removal, the temperature is 40℃, the ultrasonic frequency is 40khz, the cleaning agent is Micro90, the ratio of cleaning agent to pure water is 1:40; after the ultrasonic cleaning, rinse with pure water until no foam is generated; ③ put the superconducting cavity into pure water for ultrasonic cleaning for 60 minutes, the water temperature is 40℃, the ultrasonic frequency is 40khz, after the ultrasonic cleaning, rinse with pure water and dry; Step 4.2, chemical polishing of the superconducting cavity: the superconducting cavity is fixed on the BCP whole cavity cleaning equipment, and the acid used 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; forward pickling for 70 minutes, and reverse pickling for 90 minutes; the RF surface removal amount is 150μm, the acid temperature is ≤20℃, and the flow rate is 15-18L / min; then rinse with pure water until the pH value reaches 7; 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 rotates at a speed of about 20 revolutions / minute, and the nozzle moves at a speed of about 20mm / minute; then the superconducting cavity is placed in a Class 100 clean room to 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, raise the temperature to 750-800℃, and keep it at this temperature for 3-5 hours; cool it naturally to ≤50℃ and then take it out. 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 and a volume ratio of 1:9 is used; polishing for 30 minutes to remove 20-25 μm; the purpose of further improving the surface roughness of the superconducting cavity is achieved; Step 4.6, performing desulfurization twice; Step 4.7: Rinse the superconducting cavity with high-pressure water according to step 4.3, and then dry it in a clean room.

8. The method for preparing a copper-niobium-niobium-tin composite superconducting cavity according to claim 1, characterized in that: The step 6 comprises: brazing an oxygen-free copper cold chain ring on the outside of the superconducting cavity weld: using a soft soldering method, the solder is a tin-silver alloy with a silver content of 72%, a high vacuum environment, and a brazing temperature of <450° C.; during the brazing process, the superconducting cavity flange is wrapped with niobium foil.

9. The method for preparing a copper-niobium-niobium-tin composite superconducting cavity according to claim 1, characterized in that: In step 7, the niobium tin film superconducting cavity is subjected to buffered electrochemical polishing to remove residual Sn on the film surface and improve the roughness of the niobium tin film, comprising: fixing the niobium tin film superconducting cavity on the 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 desulfurization is performed twice; and then ultrapure water is rinsed and dried in a clean room; After step 7, proceed to step 8, low temperature test.

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