Superconducting cavity Nb3Sn coating method

Abstract

The invention discloses a superconducting cavity Nb3Sn coating method. Comprising the steps of 1, niobium cavity pretreatment, 2, niobium cavity and tin source crucible installation, 3, superconducting cavity niobium-tin coating film preparation and 4, coating film post-treatment. In the step 3, after several times of heating, the tin source area is heated to 1300 DEG C, and the coating area is heated to 1200 DEG C; cooling the tin source area and the coating area to 1100 DEG C; the tin source area is cooled to 1000 DEG C, and the film coating area is kept at 1100 DEG C; the tin source area and the coating area are cooled to 231 DEG C, and the tin source temperature is always 100 DEG C or above lower than the coating temperature; and gradually and naturally cooling the tin source area and the coating area from 231 DEG C to 50 DEG C. According to the invention, by adjusting the vapor pressure of the coating film, a small amount of residual tin molecules in the superconducting cavity are gradually diffused to the tin source area and are condensed and attached to the crucible in the tin source area, so that the problem that residual tin in the superconducting cavity affects the balance of the coating film during cooling is solved.

Description

Technical Field

[0001] The invention belongs to the technical field of plating materials with metal materials, in particular to a niobium-tin film plating method for a 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] As a key component of radio frequency superconducting accelerators, radio frequency superconducting cavities have the advantages of high quality factor, continuous wave operation capability, large beam aperture, etc., and are widely used in high-current particle accelerators. With the development of niobium (Nb) material smelting technology, the improvement of superconducting cavity processing level, and the research of post-processing processes such as nitrogen doping and baking, the performance of traditional pure niobium cavities has approached its material limit. Compared with Nb, niobium tin (Nb3Sn) materials have high superconducting transition temperature, high overheat magnetic field, and low surface resistance. The overheat magnetic field of Nb3Sn at a lower temperature is 420 mT, which is far higher than the 240 mT of pure Nb, which means that under the same cavity conditions, the Nb3Sn cavity can theoretically withstand a higher peak magnetic field and can provide a higher acceleration gradient. Taking the 1.3 GHz TESLA cavity as an example, the theoretical acceleration gradient of Nb3Sn is nearly 100 MV / m (pure Nb cavity is 57 MV / m), so the Nb3Sn superconducting cavity has good development potential.

[0004] Nb3Sn thin film superconducting cavity has the ability to operate at a high quality factor at 4.2 K, which can realize the application of liquid helium-free conduction cooling thermostat. The tin vapor diffusion method is currently the most effective method for preparing Nb3Sn thin film cavity; however, the tin vapor diffusion method currently used still has the problem of uneven diffusion. Summary of the invention

[0005] In view of the problems existing in the background technology, the present invention provides a method for coating a superconducting cavity with niobium-tin films, and the technical solution includes: Step 1: Superconducting cavity pretreatment, Step 2: Installation of superconducting cavity and tin source crucible. Step 3: Preparation of superconducting cavity niobium-tin coating. Step 4, post-plating treatment; The step 3 comprises: Step 3.1: Add Sn and SnCl into the tin source crucibles at both ends of the superconducting cavity. 2 ; Place the superconducting cavity into the evaporation furnace and evacuate it; Step 3.2, the temperature of the tin source area and the coating area is raised to 200°C and kept warm; Step 3.3, the temperature of the tin source area is raised to 550°C and the temperature of the coating area is raised to 500°C, and the temperatures are kept constant; Step 3.4, the temperature of the tin source area is raised to 1300°C and the temperature of the coating area is raised to 1200°C, and the temperatures are kept constant; Step 3.5, the temperature of the tin source area and the coating area is reduced to 1100°C and kept warm; Step 3.6, the temperature of the tin source area is lowered to 1000°C, and the coating area is kept at 1100°C and kept warm; Step 3.7, the temperature of the tin source area and the coating area is reduced to 231°C, and the tin source temperature is always more than 100°C lower than the coating temperature; Step 3.8, the tin source area and the coating area are gradually cooled naturally from 231°C to 50°C; Step 3.9: Break the vacuum in the evaporation furnace and take out the coated superconducting cavity.

[0006] The step 1 comprises: Step 1.1, wipe the inner and outer surfaces of the superconducting cavity with degreasing agent and anhydrous ethanol to remove oil stains, etc.; rinse with pure water until there is no foam; Step 1.2, ultrasonic degreasing of superconducting cavity: the cleaning agent is Micro90, the ratio of which to pure water is 1:40, the temperature is 40°C, the ultrasonic process is performed for 60 minutes, and the ultrasonic frequency is 40 khz; after the ultrasonic process is completed, rinse with pure water until there is no foam; Step 1.3, pure water ultrasound: temperature 40°C, ultrasound for 60 minutes, and dry in a clean room for 3-5 hours after ultrasound; Step 1.4, chemical polishing of superconducting cavity: 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°C and the flow rate is 17 liters / minute. After pickling, rinse with pure water and transfer to the clean room by soaking in pure water. Step 1.5, ultrapure water ultrasound: the superconducting cavity is placed in ultrapure water ultrasound for 60 minutes, the temperature is 40°C, and the ultrasound frequency is 40 kHz; Step 1.6, high-pressure water flushing: The superconducting cavity is fixed on the high-pressure water flushing equipment and flushed for 3 strokes; the flushing water pressure in each stroke is ≥10MPa, the flushing water resistivity is ≥18.2MΩ•cm, the superconducting cavity rotation speed is 20 turns / minute, and the nozzle movement speed is 20mm / minute; after the high-pressure water flushing is completed, the superconducting cavity is placed in a Class 100 clean room to dry for 3-5 hours; Step 1.7, 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; after annealing, naturally cool it to ≤50℃ and take it out.

[0007] The step 2 comprises: firstly connecting the superconducting cavity with the tin source crucibles on both sides; then horizontally placing the superconducting cavity on the product support of the evaporation furnace; and finally wrapping the interface of the superconducting cavity except the beam tube flange with niobium foil.

[0008] The step 4 comprises: Step 4.1, Ultrapure water ultrasound: After the superconducting cavity coating is completed, ultrapure water is carried out in a clean room: the coating cavity is placed in ultrapure water ultrasound for 60 minutes, the temperature is 40°C, and the ultrasound frequency is 40khz; Step 4.2, high-pressure water flushing and drying: In a Class 100 clean room, the superconducting cavity is fixed on the high-pressure water flushing equipment and flushed for 3 strokes; in each stroke, the flushing water pressure is ≥10MPa, the flushing water resistivity is ≥18.2MΩ•cm, the superconducting cavity rotation speed is 20 turns / minute, and the nozzle movement speed is 20mm / minute; after the high-pressure water flushing is completed, the superconducting cavity is placed in the Class 100 clean room to dry; Step 4.3, clean packaging: complete clean packaging in the clean room and install the vacuum valve; Step 4.4, vacuuming: In the clean room, first slowly pump low vacuum, use a mechanical pump to pump low vacuum, and adjust the pumping speed to less than 50 bar•mL / s based on the opening of the needle valve, pumping from normal pressure to below 1 Pa; after the slow pumping is completed, turn on the molecular pump and pump to 10 -3 Pa; Step 4.5: Leak detection: Use a helium mass spectrometer to detect leaks, with a leak rate of ≤1×10 -11 mbar•l / s, the leak test is passed; then, continue to evacuate the vacuum to better than 10 -4Pa, close the vacuum valve and seal the suction port with a blind flange.

[0009] After step 4: Step 5, low temperature test: install the superconducting cavity on the hanger in the vertical measurement system and enter the vertical shaft with the hanger; the interior of the vertical shaft has normal temperature and low temperature magnetic shielding, and the ambient magnetic field near the superconducting cavity inside the vertical shaft is less than 5 mG; a slow cooling rate of 0.1 K / min or less is used.

[0010] In step 3.1, 30 g of Sn and SnCl were added to each tin source crucible. 2 is 15g.

[0011] In the step 3.2, the temperature of the tin source area and the coating area is kept at 200°C for 36 to 48 hours, and the furnace temperature is 220°C ± 20°C; In the step 3.3, the tin source area is kept at 550°C and the coating area is kept at 500°C for 5 hours, and the furnace temperature is 550°C±30°C; In step 3.4, the tin source area is kept at 1300° C. and the coating area is kept at 1200° C. for 3 hours, and the furnace temperature is 1350° C.±50° C.; In step 3.8, the furnace temperature is gradually cooled to 50°C.

[0012] In the step 3.5, the tin source area and the coating area are kept at 1100° C. for 40 minutes, and the furnace temperature is 1200° C.±50° C.

[0013] In the step 3.6, the tin source area is kept at 1000°C and the coating area is kept at 1100°C for 20 minutes, and the furnace temperature is 1200°C±50°C.

[0014] In step 3.7, the temperature drops evenly and the process takes 3-4 hours.

[0015] The beneficial effects of the present invention are: 1. In the final stage of maintaining the coating area at 1100°C, an additional 20 minutes of operation was performed to keep the tin source area lower than the coating area. By adjusting the coating vapor pressure, the small amount of tin molecules remaining in the superconducting cavity gradually diffused to the tin source area, and condensed and attached to the crucible in the tin source area, thus solving the problem that the residual tin in the superconducting cavity would affect the coating balance when cooling down, and ensuring the performance of the superconducting cavity.

[0016] 2. Before the conventional temperature drop to 231°C, the tin source temperature was maintained at a level more than 100°C lower than the coating temperature (lasting about 3-4 hours); the formation of tin islands was eliminated, ensuring the performance of the superconducting cavity.

[0017] 3. Before superconducting cavity coating, clean the cavity and remove the surface contamination of the RF surface by oil removal ultrasound and pure water ultrasound; remove the contamination layer of the RF surface material of the superconducting cavity by chemical polishing. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The present invention is a schematic diagram of the process of an embodiment of the method for coating a superconducting cavity with niobium-tin.

[0019] Figure 2 This is a schematic diagram of the temperature curve structure for preparing the niobium-tin coating in an embodiment of the present invention. DETAILED DESCRIPTION

[0020] The present invention is further described in detail below in conjunction with the accompanying drawings.

[0021] like Figure 1 The embodiment of the present invention shown includes: Step 1: Niobium cavity pretreatment: Step 1.1. Wipe the inner and outer surfaces of the niobium cavity with degreasing agent and anhydrous ethanol to remove oil stains, etc.; rinse with pure water (resistivity ≥ 2MΩ•cm) until there is no foam.

[0022] Step 1.2, ultrasonic degreasing of superconducting cavity: the cleaning agent is Micro90, the ratio of which to pure water is 1:40, the temperature is 40°C, the ultrasonic process is performed for 60 minutes, and the ultrasonic frequency is 40 khz; after the ultrasonic process is completed, rinse with pure water until there is no foam.

[0023] Step 1.3: Pure water ultrasound: temperature 40°C, ultrasound for 60 minutes, and dry in a clean room for 3-5 hours after ultrasound.

[0024] Step 1.4, Niobium cavity chemical polishing (BCP): 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 below 20°C and the flow rate is 17 liters / minute. After pickling, rinse with pure water and transfer to the clean room by soaking in pure water.

[0025] Step 1.5, Ultrapure water ultrasound: Ultrapure water (resistivity 18.2 MΩ•cm) was placed in the niobium cavity and ultrasound was applied for 60 minutes at a temperature of 40°C and an ultrasound frequency of 40 kHz.

[0026] Step 1.6, High-pressure water flushing (HPR): 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.

[0027] Step 1.7, 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.

[0028] Step 2: Installation of niobium chamber and tin source crucible: First, connect the niobium cavity to the tin source crucibles on both sides; then place the niobium cavity horizontally on the product support of the evaporation furnace, adjust the position of the heater and the thermocouple; the interface of the superconducting cavity except the beam tube flange is wrapped with niobium foil. The crucibles on both sides need to be filled with 0.5‰-1‰ of Sn and SnCl 2 .

[0029] Step 3: Preparation of superconducting cavity niobium-tin coating: Figure 2 The process and temperature control shown include: Step 3.1 Add 30g Sn and 15g SnCl into the tin source crucibles at both ends of the superconducting cavity. 2 The superconducting cavity was placed into the evaporation furnace and the vacuum was pumped to 1×10 -6 Pa, it takes 3-4 hours.

[0030] Step 3.2 The temperature of the tin source area and the coating area is raised to 200℃, which takes about 0.5 hours, and the furnace temperature is about 220℃ (±20℃); keep at 200℃ for 36-48 hours. The purpose of low-temperature degassing is to remove water molecules and other gas molecules in the cavity, and to avoid the deformation of the cavity affecting the change of superconducting cavity parameters. During the coating process, the escape of material gas molecules will reduce the quality of the niobium tin film, and even cause film defects, resulting in non-superconductivity.

[0031] Step 3.3 The temperature of the tin source area is raised to 550℃ and the temperature of the coating area is raised to 500℃. The furnace temperature is about 550℃ (±30℃) and it takes about 0.5 hours. The temperature of the tin source area is higher than that of the coating area in order to create a vapor pressure difference between the two, which is beneficial to SnCl 2 The molecules continue to diffuse into the cavity. The coating area is kept warm for 5 hours to fully nucleate on the surface of the superconducting cavity coating. JLab proposes the following possible processes: 3SnCl 2 + 2 NbO → 2 NbOCl 3 + 3 Sn x SnCl 2 + 2 Nb → 2 NbCl x + xSn ( x = 3.45) Step 3.4 The temperature of the tin source area is raised to 1300℃ and the temperature of the coating area is raised to 1200℃. The furnace temperature is about 1350℃ (±50℃). It takes about 1.5 hours. The temperature of the tin source area is higher than that of the coating area in order to generate a vapor pressure difference between the two, which is conducive to the continuous diffusion of Sn molecules into the cavity. The coating area is kept warm for 3 hours in order to fully form a Nb3Sn film on the surface of the superconducting cavity coating.

[0032] Step 3.5 The temperature of the tin source area and the coating area is reduced to 1100℃, the furnace temperature is about 1200℃ (±50℃), and the temperature is kept for 40 minutes; the purpose of cooling the tin source area is to reduce the evaporation of tin molecules, the pressure difference of tin molecules in the tin source area and the coating area is consistent, and the uniformity of tin molecules gradually becomes the same.

[0033] Step 3.6 The temperature of the tin source area is cooled to 1000℃, the coating area is maintained at 1100℃, and the furnace temperature is about 1200℃ (±50℃), and it is kept warm for 20 minutes; at this time, the temperature of the tin source is lower than the temperature of the coating area, and the small amount of tin molecules remaining in the superconducting cavity gradually diffuses to the tin source area, and condenses in the tin source area and adheres to the crucible.

[0034] Step 3.7 The temperature of the tin source area and the coating area is continuously and evenly cooled to below 231°C. The cooling time is about 3-4 hours. During the cooling process, the tin source temperature is always more than 100°C lower than the coating temperature. Because tin exceeds the melting point of 231.8°C, tin vapor will be formed. The higher the temperature, the more tin vapor there is. In the cooling range of 1000°C to 231°C, the temperature of the coating area is higher than that of the tin source area. The tin molecules in the coating area diffuse to the tin source area and condense in the tin source area. The temperature difference of 100°C can prevent the tin vapor molecules from being deposited on the coating surface to form tin islands, which will affect the performance of the superconducting cavity.

[0035] Step 3.8 The temperature of the tin source area and the coating area gradually decreases from 231°C to 50°C, and the furnace body temperature also gradually decreases to 50°C. The temperature decrease takes about 5-6 hours.

[0036] Step 3.9: Break the vacuum in the evaporation furnace, take out the coated superconducting cavity, and check the status of the niobium-tin coating.

[0037] Step 4: Post-coating treatment, including: Step 4.1 Ultrapure water ultrasound: After the superconducting cavity coating is completed, ultrapure water is carried out in a clean room: ultrapure water (resistivity 18.2MΩ•cm) is placed in the coating cavity and ultrasound is performed for 60 minutes at a temperature of 40°C and an ultrasonic frequency of 40khz.

[0038] Step 4.2 High-pressure water flushing (HPR) and drying: In a Class 100 clean room, the superconducting cavity is fixed on the HPR equipment and flushed for 3 strokes; in each stroke, the flushing water pressure is ≥10MPa, the flushing water resistivity is ≥18.2MΩ•cm, the superconducting cavity rotates at about 20 turns / min, and the nozzle moves at about 20mm / min; after the HPR, the superconducting cavity is placed in the Class 100 clean room to dry for 3-5 hours.

[0039] Step 4.3 Clean packaging: Complete clean packaging in the clean room, including the installation of the feed extraction antenna and the three-way pipe. The antenna, screws, nuts, flanges and other parts used need to be ultrasonically degreased, cleaned with pure water ultrasonically, and dried with filtered nitrogen in advance, and finally install the vacuum valve. During the entire packaging process, it is necessary to ensure that the particle counter reading meets the standard, and try not to introduce dust and particles into the superconducting cavity during the operation.

[0040] Step 4.4 Vacuuming: In the clean room, slowly pump low vacuum. Use a mechanical pump to pump low vacuum. Adjust the pumping speed to less than 50 bar•mL / s in combination with the opening of the needle valve. Pump from normal pressure to below 1 Pa. The reason for slowly pumping low vacuum is to avoid turbulence in the vacuum pipe, which will introduce dust and particles into the superconducting cavity and affect the test results. After the slow pumping is completed, turn on the molecular pump and pump to 10 -3 Pa, you can use a helium mass spectrometer leak detector to detect leaks.

[0041] Step 4.5 Leakage detection: Leakage rate ≤ 1×10 -11 mbar•l / s, the leak test is passed; then, continue to evacuate the vacuum to better than 10 -4 Pa, close the vacuum valve and seal the suction port with a blind flange.

[0042] Step 5, low temperature test: The superconducting cavity is installed on the hanger, connected to the coaxial line, fixed with low temperature thermometer, flux gate and other components, and enters the shaft with the hanger. The interior of the shaft has normal temperature and low temperature magnetic shielding, and the environmental magnetic field near the superconducting cavity inside the shaft is less than 5 mG. In this embodiment, the superconducting cavity vertical test system of Peking University is specifically used, including signal generator, power amplifier, directional coupler, filter, oscilloscope and other components; a slow cooling rate of 0.1 K / min or less is used. Reducing the cooling rate can reduce the flux capture caused by the temperature difference thermoelectric effect.

[0043] The niobium-tin film superconducting cavity produced by the present invention is uniformly coated, free of impurities, and has a small amount of impurity elements; the bonding strength between niobium and niobium-tin is high. According to low temperature testing, superconducting transition is achieved in the niobium-tin temperature range.

Claims

1. A method for coating a superconducting cavity with niobium-tin films, characterized in that: include: Step 1: Superconducting cavity pretreatment, Step 2: Installation of superconducting cavity and tin source crucible. Step 3: Preparation of superconducting cavity niobium-tin coating. Step 4, post-plating treatment; The step 3 comprises: Step 3.1, add Sn and SnCl2 into the tin source crucibles at both ends of the superconducting cavity; install the superconducting cavity into the evaporation furnace and evacuate it; Step 3.2, the temperature of the tin source area and the coating area is raised to 200°C and kept warm; Step 3.3, the temperature of the tin source area is raised to 550°C and the temperature of the coating area is raised to 500°C, and the temperatures are kept constant; Step 3.4, the temperature of the tin source area is raised to 1300°C and the temperature of the coating area is raised to 1200°C, and the temperatures are kept constant; Step 3.5, the temperature of the tin source area and the coating area is reduced to 1100°C and kept warm; Step 3.6, the temperature of the tin source area is lowered to 1000°C, and the coating area is kept at 1100°C and kept warm; Step 3.7, the temperature of the tin source area and the coating area is reduced to 231°C, and the tin source temperature is always more than 100°C lower than the coating temperature; Step 3.8, the tin source area and the coating area are gradually cooled naturally from 231°C to 50°C; Step 3.9: Break the vacuum in the evaporation furnace and take out the coated superconducting cavity.

2. A superconducting cavity niobium-tin coating method according to claim 1, characterized in that: The step 1 comprises: Step 1.1: Wipe the inner and outer surfaces of the superconducting cavity with degreasing agent and anhydrous ethanol to remove oil stains; rinse with pure water until there is no foam; Step 1.2, ultrasonic degreasing of superconducting cavity: the cleaning agent is Micro90, the ratio of which to pure water is 1:40, the temperature is 40°C, the ultrasonic process is performed for 60 minutes, and the ultrasonic frequency is 40 khz; after the ultrasonic process is completed, rinse with pure water until there is no foam; Step 1.3, pure water ultrasound: temperature 40°C, ultrasound for 60 minutes, and dry in a clean room for 3-5 hours after ultrasound; Step 1.4, chemical polishing of superconducting cavity: 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°C and the flow rate is 17 liters / minute. After pickling, rinse with pure water and transfer to the clean room by soaking in pure water. Step 1.5, ultrapure water ultrasound: the superconducting cavity is placed in ultrapure water ultrasound for 60 minutes, the temperature is 40°C, and the ultrasound frequency is 40 kHz; Step 1.6, high-pressure water flushing: The superconducting cavity is fixed on the high-pressure water flushing equipment and flushed for 3 strokes; the flushing water pressure in each stroke is ≥10MPa, the flushing water resistivity is ≥18.2MΩ•cm, the superconducting cavity rotation speed is 20 turns / minute, and the nozzle movement speed is 20mm / minute; after the high-pressure water flushing is completed, the superconducting cavity is placed in a Class 100 clean room to dry for 3-5 hours; Step 1.7, 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; after annealing, naturally cool it to ≤50℃ and take it out.

3. A superconducting cavity niobium-tin coating method according to claim 1, characterized in that: The step 2 comprises: firstly connecting the superconducting cavity with the tin source crucibles on both sides; then horizontally placing the superconducting cavity on the product support of the evaporation furnace; and finally wrapping the interface of the superconducting cavity except the beam tube flange with niobium foil.

4. A superconducting cavity niobium-tin coating method according to claim 1, characterized in that: The step 4 comprises: Step 4.1, Ultrapure water ultrasound: After the superconducting cavity coating is completed, ultrapure water is carried out in a clean room: the coating cavity is placed in ultrapure water ultrasound for 60 minutes, the temperature is 40°C, and the ultrasound frequency is 40khz; Step 4.2, high-pressure water flushing and drying: In a Class 100 clean room, the superconducting cavity is fixed on the high-pressure water flushing equipment and flushed for 3 strokes; in each stroke, the flushing water pressure is ≥10MPa, the flushing water resistivity is ≥18.2MΩ•cm, the superconducting cavity rotation speed is 20 turns / minute, and the nozzle movement speed is 20mm / minute; after the high-pressure water flushing is completed, the superconducting cavity is placed in the Class 100 clean room to dry; Step 4.3, clean packaging: complete clean packaging in the clean room and install the vacuum valve; Step 4.4, vacuuming: In the clean room, first slowly pump low vacuum, use a mechanical pump to pump low vacuum, and adjust the pumping speed to less than 50 bar•mL / s based on the opening of the needle valve, pumping from normal pressure to below 1 Pa; after the slow pumping is completed, turn on the molecular pump and pump to 10 -3 Pa; then use a helium mass spectrometer leak detector to detect leaks; Step 4.5, leak detection: leak rate ≤ 1×10 -11 mbar•l / s, the leak test is passed; then, continue to evacuate the vacuum to better than 10 - 4 Pa, close the vacuum valve and seal the suction port with a blind flange.

5. The method for coating a superconducting cavity niobium-tin film according to claim 1, characterized in that: After step 4: Step 5, low temperature test: install the superconducting cavity on the hanger in the vertical measurement system and enter the vertical shaft with the hanger; the interior of the vertical shaft has normal temperature and low temperature magnetic shielding, and the ambient magnetic field near the superconducting cavity inside the vertical shaft is less than 5 mG; a slow cooling rate of 0.1 K / min or less is used.

6. A superconducting cavity niobium-tin coating method according to claim 1, characterized in that: In step 3.1, 30 g of Sn and 15 g of SnCl2 are added to each tin source crucible.

7. A superconducting cavity niobium-tin coating method according to claim 1, characterized in that: In the step 3.2, the temperature of the tin source area and the coating area is kept at 200°C for 36 to 48 hours, and the furnace temperature is 220°C ± 20°C; In the step 3.3, the tin source area is kept at 550°C and the coating area is kept at 500°C for 5 hours, and the furnace temperature is 550°C±30°C; In step 3.4, the tin source area is kept at 1300° C. and the coating area is kept at 1200° C. for 3 hours, and the furnace temperature is 1350° C.±50° C.; In step 3.8, the furnace temperature is gradually cooled to 50°C.

8. The method for coating a superconducting cavity niobium-tin film according to claim 1, characterized in that: In the step 3.5, the tin source area and the coating area are kept at 1100° C. for 40 minutes, and the furnace temperature is 1200° C.±50° C.

9. A superconducting cavity niobium-tin coating method according to claim 1, characterized in that: In the step 3.6, the tin source area is kept at 1000°C and the coating area is kept at 1100°C for 20 minutes, and the furnace temperature is 1200°C±50°C.

10. The method for coating a superconducting cavity niobium-tin film according to claim 1, characterized in that: In step 3.7, the temperature drops evenly and the process takes 3-4 hours.

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