A high-order mode coupler with double-inhibiting structure and a manufacturing method thereof

Abstract

The application discloses a high-order mode coupler with double-inhibiting structure and a manufacturing method thereof. The high-order mode coupler comprises an extraction component and a coupling component; the coupling component is provided with a coupling hook for deeply coupling electromagnetic field transmitted by a superconducting cavity and inputting the electromagnetic field to the extraction component; the extraction component is used for extracting the input electromagnetic field to a normal temperature zone; a first coupling capacitor is formed between the lower end of the coupling hook and the tube wall of the superconducting cavity tube for determining the frequency point of the base mode inhibition; a second coupling capacitor is formed between a T-shaped probe and the tube wall of the extraction component for adjusting the bandwidth of the base mode inhibition frequency point; a third coupling capacitor is formed between the bottom end of the inner conductor of the extraction component and the top end of the coupling hook for adjusting the transmission efficiency of electromagnetic field energy; the first coupling capacitor is a first inhibiting structure, and the LC resonant circuit composed of the T-shaped probe and the second coupling capacitor is a second inhibiting structure. The application solves the base mode tuning problem and the heating problem of the high-order mode coupler.

Description

Technical Field

[0001] This invention belongs to the field of superconducting high-frequency technology and relates to a high-order mode coupler with a dual suppression structure and its manufacturing method. Background Technology

[0002] As a charged beam enters the accelerating cavity for acceleration, it excites other resonant modes in the cavity besides the acceleration mode along the beam's direction of motion; these are called higher-order modes. These higher-order modes dissipate some of the beam's energy and establish electromagnetic fields. Higher-order modes can cause beam instability and additional cavity wall losses. Therefore, specialized devices for extracting or absorbing the energy of higher-order modes within the cavity are needed—i.e., higher-order mode suppressors. Coaxial higher-order mode suppressors (generally called higher-order mode couplers) are used in superconducting cavities, typically mounted on the superconducting cavity beam tube, and consist of a coaxial resonant cavity, a coupling antenna, and a load circuit. The higher-order mode coupler extracts the field energy of the higher-order modes from within the beam tube through the coupling antenna, and then dissipates it on the absorbing load via a conduction circuit. The advantages of this type of higher-order mode coupler are its compact structure and complex design; it extracts higher-order modes without affecting the operating mode (generally the fundamental mode), thus requiring a fundamental mode suppression structure.

[0003] Coaxial high-order mode suppressors were proposed in the 1980s and were primarily used in the HERA accelerator at DESY and the LEP accelerator at CERN, and have been widely applied in many accelerators. TESLA-type high-order mode suppressors evolved from HERA and are currently mainly used in the 1.3 GHz 9-cell superconducting cavity at Fermilab and the 1.5 GHz superconducting cavity of the 12 GeV CEBAF upgrade project. Their structure is as follows: Figure 1 (1) shows that the LEP type high-order mode suppressor was later applied to SOLEIL, LHC and SLS and ELETTRA's Super-3HC thermostat.

[0004] The LEP-type high-order mode coupler operates at a fundamental mode frequency of 352MHz, and its structure is as follows: Figure 1 As shown in (2), the high-order mode coupler structure includes a coupling hook to couple the electromagnetic fields of the modes, and the fundamental mode suppression structure is realized by a series LC resonant circuit composed of hooks. A liquid helium pipe is designed inside the inner conductor to cool the inner conductor, making the structure complex.

[0005] The common feature of the widely used TESLA 1.3GHz and LEP 352MHz high-order mode couplers is that they all have only one suppression structure for fundamental mode suppression. Their principle is to use a series LC resonant circuit to form the fundamental mode suppression mechanism, such as... Figure 1As shown in (2), the disadvantage of this suppression structure is its relatively small suppression bandwidth for the fundamental mode, typically around 10MHz. These types of higher-order mode couplers, after being installed on a superconducting cavity, generally operate at a low temperature of 2K-4.2K. Considering factors such as processing errors and thermal shrinkage, the suppression frequency of the fundamental mode by the higher-order mode coupler will change. Furthermore, the higher-order mode coupler is generally located inside a cryogenic thermostat; once the thermostat is assembled, the higher-order mode coupler cannot be operated. Therefore, after the higher-order mode coupler is installed, it needs to be pre-tuned at room temperature. This requires accurate estimation of various factors affecting the frequency, such as processing errors and thermal shrinkage. Pre-tuning generally requires specially designed pre-tuning fixtures to cause mechanical deformation of the higher-order mode coupler, thereby achieving the purpose of frequency tuning. For example... Figure 1 (1) The 1.3GHz high-order mode coupler shown uses a tuning fixture to change the fundamental mode suppression structure capacitor at the top to achieve the purpose of tuning frequency. The tuning range is limited, and the thickness of the top end cap of the high-order mode coupler is about 2mm. The tuning force must be adjusted properly during tuning to avoid excessive deformation of the end cap and breakage. Figure 1 (2) The 352MHz high-order mode coupler changes the frequency by applying force to the crossbar of the helium inlet, which causes the internal hook to deform. The pre-tuning of the fundamental mode complicates the whole process. If the pre-tuning is not correct at room temperature, the frequency will not be at the operating frequency when working at low temperature, resulting in fundamental mode leakage. In severe cases, it can lead to the entire module losing quench or even vacuum leakage.

[0006] In addition, such as Figure 1 (1) The high-order mode coupler shown does not provide additional cooling for the inner and outer conductors. When in use, the heating anchor is generally heated at the extraction port, and the cooling capacity is limited. Therefore, it is not suitable for continuous wave and high-current accelerators. Figure 1 (2) The 352MHz high-order mode coupler shown uses internal cooling, which is complex to manufacture and will cause insufficient cooling when there is helium inside. Summary of the Invention

[0007] To address the problems existing in the prior art, the present invention aims to provide a high-order mode coupler with a dual-suppression structure and its manufacturing method. The high-order mode coupler of the present invention features a compact design, solves the fundamental mode tuning problem and the heat generation problem of high-order mode couplers, and is suitable for high-current, continuous-wave accelerators. Currently, there are no precedents for its development domestically or internationally. This invention solves the manufacturing difficulties of high-order mode couplers and successfully achieved the manufacturing and testing of the first high-order mode coupler suitable for continuous-wave, high-current 650MHz superconducting cavity. Furthermore, its design method can be used to develop high-order mode couplers for superconducting cavities at any frequency.

[0008] The technical solution of this invention is as follows: A high-order mode coupler with a dual-suppression structure, characterized in that it includes an extraction component and a coupling component; The coupling component is equipped with a coupling hook, which is used to penetrate into the superconducting cavity bundle tube to couple the electromagnetic field transmitted from the superconducting cavity and input it to the extraction component; The extraction component is used to extract the input electromagnetic field to a room temperature region outside the superconducting cavity; A first coupling capacitor is formed between the lower end of the coupling hook and the wall of the superconducting cavity tube, which is used to determine the frequency point of fundamental mode suppression; a second coupling capacitor is formed between the T-shaped probe in the extraction assembly and the wall of the extraction assembly, which is used to adjust the bandwidth of the fundamental mode suppression frequency point; a third coupling capacitor is formed between the bottom end of the inner conductor of the extraction assembly and the top end of the coupling hook, which is used to adjust the transmission efficiency of electromagnetic field energy. The first coupling capacitor is the first suppression structure, and the LC resonant circuit formed by the T-shaped probe and the second coupling capacitor is the second suppression structure.

[0009] Preferably, the extraction assembly has an inner conductor inside its tube wall, on which the T-shaped probe is mounted. The bottom end of the inner conductor has a chamfered extraction assembly coupling capacitor for a smooth transition to the microwave surface. One end of the tube wall is connected to the outer conductor of the extraction assembly via a flange, and the other end of the tube wall is sealed to the coupling assembly via a flange. A ceramic ring is located inside the outer conductor; the outer surface of the ceramic ring is brazed to the outer conductor, and the inner surface of the ceramic ring is brazed to the inner conductor. The top end of the inner conductor is connected to the inner conductor of the tapered transition section. The top end of the outer conductor is connected to the outer conductor of the tapered transition section. The top ends of the inner and outer conductors of the tapered transition section are connected to an extraction port using a standard N-type connector.

[0010] Preferably, the coupling assembly includes a coupling hook, a coupling capacitor at the coupling assembly end, an inner conductor support rod, a niobium outer cylinder, a liquid helium tank outer cylinder, a niobium-titanium flange, a titanium tube, and a titanium flange; the niobium outer cylinder is disposed inside the liquid helium tank outer cylinder, and the two niobium-titanium flanges are respectively connected to the niobium outer cylinder and the liquid helium tank outer cylinder, forming a hollow interlayer between the niobium outer cylinder and the liquid helium tank outer cylinder; the coupling hook is disposed inside the niobium outer cylinder, and the coupling capacitor at the coupling assembly end is disposed at the top of the coupling hook; one end of the inner conductor support rod is connected to the niobium outer cylinder, and the other end is connected to the coupling hook; a titanium tube is connected to each side of the liquid helium tank outer cylinder for injecting or discharging liquid helium into or out of the hollow interlayer; the extraction component end coupling capacitor and the coupling assembly end coupling capacitor form the third coupling capacitor.

[0011] Preferably, the extraction component end coupling capacitor is integrally formed with the inner conductor and welded to the tapered transition section inner conductor to form a complete inner conductor.

[0012] Preferably, the parallelism error of the upper and lower surfaces of the ceramic ring is ±0.1 mm, and the concentricity error of the inner and outer surfaces of the ceramic ring is ±0.1 mm.

[0013] Preferably, the maximum depth to which the coupling hook penetrates the superconducting cavity bundle tube is 1 / 4 of the inner diameter of the superconducting cavity bundle tube.

[0014] Preferably, the T-shaped probe includes a crossbar and a fan-shaped structure; one end of the crossbar is connected to the inner conductor, and the other end is connected to the inner center of the fan-shaped structure; the fan-shaped structure is parallel to the tube wall of the extraction component, forming the second coupling capacitor.

[0015] A method for manufacturing a high-order mode coupler with the aforementioned dual-suppression structure includes the following steps: Preparation of extraction and coupling components; The oxygen-free copper parts in the prepared extraction assembly are first coarsely ground, and then polished a second time using polishing paste and a canvas wheel to achieve the required surface finish. Then, the stainless steel parts are ultrasonically cleaned, and the oxygen-free copper parts are ultrasonically cleaned and then chemically polished using mixed acid and a gloss treatment solution. Finally, the treated extraction assembly is placed in a container filled with anhydrous ethanol for ultrasonic cleaning. The prepared coupling components were rinsed, buffered chemically polished, and annealed.

[0016] Preferably, the method for performing buffered chemical polishing on the prepared coupling component is as follows: the prepared coupling component is subjected to buffered chemical heavy polishing and buffered chemical light polishing; the acid solution ratio for buffered chemical heavy polishing is 1:1:2, the acid solution temperature is below 15℃, the acid solution flow rate is <18L / min, and the heavy polishing time is about 80min; the acid solution ratio for buffered chemical light polishing is 1:1:2, the acid solution temperature is below 15℃, the acid solution flow rate is <18L / min, and the light polishing time is about 30min. After the process, pure water is poured in and the system is vented several times until the pH is >6. Finally, the thickness of the BCP light polishing is less than 30μm, and the thickness of the two BCP polishing processes is controlled within 100μm.

[0017] Preferably, the parallelism error between the axis of the coupling hook and the reference plane is within ±0.1 mm; during welding and machining, the flange hole of the niobium-titanium flange of the coupling assembly and the center of the coupling hook are on the same center line, with an angle error of ±0.5°; the reference plane is a plane perpendicular to the top surface of the coupling hook that passes through the center of the threaded holes at both ends of the niobium-titanium flange and the center of the top surface of the coupling hook.

[0018] This invention incorporates a dual-suppression structure design for fundamental mode suppression during the design phase, increasing the fundamental mode suppression bandwidth from approximately 10MHz in traditional single-suppression structures to approximately 100MHz. Through RF structure optimization design, mechanical performance analysis, and thermal load analysis, the final mechanical structure design was determined. The high-order mode coupler employing the dual-suppression structure eliminates fundamental mode tuning, significantly reducing module assembly complexity, saving time and manufacturing costs, and improving the stability of the module during belt-driven operation. To reduce low-temperature thermal load and enable the high-order mode coupler to meet high current intensity and continuous wave operation requirements, a liquid helium tank design is added to the high-order mode coupler structure.

[0019] This invention proposes a high-order mode coupler with a dual-suppression structure suitable for high-current, continuous-wave accelerators and its manufacturing method. The design phase of the dual-suppression high-order mode coupler utilizes multiphysics simulation software and finite element analysis software for simulation analysis, while the electromagnetic and mechanical design employs a bidirectional iterative optimization design method. The high-order mode coupler structural design is initially determined based on the transmission characteristic curve. Subsequently, the high-order mode coupler and the superconducting cavity are jointly modeled, and simulation calculations are performed to assess the suppression of the fundamental and high-order modes. If the suppression requirements are not met, the structural parameters of the high-order mode coupler need to be locally adjusted. After meeting the suppression requirements, secondary electron multiplication simulation and thermal analysis are conducted. A liquid helium tank is added to the outside of the high-order mode coupler coupling assembly to ensure sufficient cooling of the inner and outer conductors. The impact of processing errors on the suppression effect of the high-order mode coupler is evaluated, and the requirements for processing and installation errors of each part during actual processing and module assembly are determined accordingly. The factors that have a significant impact on the suppression results of the fundamental and higher-order modes are: the gap between the coupling capacitors between the extraction end assembly and the coupling assembly, the parallelism of the coupling capacitors, the parallelism between the first and second suppression structures and the niobium outer cylinder, and the angle of the coupling hook along the vertical direction. The errors of the above parameters should be carefully controlled during the processing and welding process.

[0020] The high-order mode coupler of this invention, with its dual-suppression structure, consists of two parts: an extraction component and a coupling component. The high-order mode coupler is mounted on the bundle tube of a superconducting cavity. During operation, its interior is in a vacuum state, while the exterior is in a thermally insulating vacuum. Therefore, after the component is soldered, leak testing is required, and the vacuum leak rate must be less than 1 × 10⁻⁶. -10 mbar*L / s.

[0021] To reduce the weight of the inner conductor and minimize its deformation during processing, installation, and cooling, the inner conductor of the high-order mode coupler extraction assembly employs a shell structure design. The ceramic ring and T-shaped probe assembly of the extraction assembly are key components for achieving the dual suppression structure and high-order mode transmission; therefore, processing errors must be strictly controlled during fabrication. The dimensions of the ceramic ring are critical, requiring assurance of parallelism and concentricity on both its upper and lower surfaces. The ceramic ring serves two purposes: firstly, it acts as an equivalent capacitor element, matching the inner conductor of the extraction assembly to form a high-pass circuit, allowing electromagnetic energy to pass through and reducing circuit reflection; secondly, it acts as a dielectric element allowing high-power transmission while also isolating the cavity vacuum, preventing vacuum leakage and contaminants from entering the superconducting cavity. To ensure effective fundamental mode suppression and high-order mode transmission, the processing errors of each component of the extraction assembly must be controlled within ±0.1mm, with concentricity and parallelism within ±0.1mm.

[0022] All welds in the high-order mode coupler coupling assembly are manufactured using electron beam welding. After welding, each weld on the entire component must be leak-tested, and the vacuum leak rate must be less than 1×10⁻⁶. -10 mbar*L / s. The leakage rate of the liquid helium tank interlayer between the niobium outer cylinder and the titanium cylinder is less than 1×10. -10 mbar*L / s. The coupling hook of the coupling assembly is another key component for realizing the dual suppression structure and high-order mode transmission. During welding, ensure the parallelism of the coupling hook's axis with the reference plane. Here, the reference plane refers to the plane perpendicular to the coupling hook's end face, passing through the center of the threaded holes at both ends of the niobium-titanium flange and the center of the coupling hook's end face (the top face of the coupling capacitor at the coupling assembly end). During welding and machining, ensure that the threaded holes of the niobium-titanium flange and the center of the hook are on the same center line, with an angular error of ±0.5°. During welding and machining, welding shrinkage allowance should be considered, and the final dimensional error should be within ±0.1mm.

[0023] As a crucial component of the superconducting cavity, the higher-order mode coupler (HMC) is assembled with it and operates in a low-temperature environment. Dust, fibers, metal shavings, and sharp surface protrusions adhering to the cavity surface become electron emission sources under a high electric field, causing localized electric field concentration and resulting in field emission, which in turn affects the overall performance of the superconducting cavity. To ensure the HMC surface maintains extremely high cleanliness and does not affect the quality factor of the superconducting cavity, appropriate post-processing should be performed on the extraction and coupling components after fabrication. Therefore, the following post-processing procedures have been specifically developed based on the materials and characteristics of different components of the HMC: 1) The extraction assembly consists of components made of two different materials: oxygen-free copper and stainless steel. To improve surface finish, remove surface particulate contaminants, and prevent surface particles, residual grease, and polishing agents from adhering to the assembly surface, the oxygen-free copper parts in the extraction assembly are first coarsely ground with 800-grit sandpaper to ensure a surface finish of 0.4. Then, a second polishing is performed using polishing paste and a canvas wheel to achieve the required surface finish. For stainless steel parts, ultrasonic degreasing is performed for 30 minutes (with degreasing solvent provided). For oxygen-free copper parts, ultrasonic cleaning (with degreasing solvent provided) is performed for 30 minutes to remove oil, followed by chemical polishing with mixed acid and a gloss treatment solution to fully achieve the required surface finish. If the weld seams on the inner surface are not smooth, manual grinding is required. After grinding, the entire assembly is placed in anhydrous ethanol and ultrasonically cleaned for 30 minutes. After final inspection and approval, protective fixtures should be installed and vacuum-sealed for protection, or the assembly should be placed in a drying cabinet to prevent oxidation of the oxygen-free copper.

[0024] 2) The inner conductor and outer cylinder of the coupling assembly are made of high-purity niobium. After the coupling assembly is welded, the inner surface needs to be inspected. The inner surface must be clean, smooth, and flat. The weld seam on the inner surface should be ground until there is no obvious unevenness. After ultrasonic cleaning and degreasing, buffer chemical polishing is performed. The acid solution ratio for the second polishing is 1:1:2, the acid temperature is below 15℃, and the acid flow rate is <18L / min. Both sides are acid-washed. During the acid washing, the inner surface is inspected and ground as needed. The thickness of the second polishing should be less than 80μm. After the second polishing, high-pressure water rinsing is performed. After rinsing and drying, high-temperature annealing is performed at 700 degrees Celsius for 3 hours and then held for 3 hours. After annealing, ultrasonic degreasing and cleaning are performed. After drying, buffer chemical polishing is performed. The thickness of the second polishing is less than 30μm. The thickness of the two BCP polishing processes should be controlled within 100μm. Pickling is primarily for smoothing and polishing the surface of the inner conductor of the coupling assembly, removing surface defect layers and oxide layers to prepare for subsequent superconductivity. High-pressure water rinsing thoroughly cleans the interior of the coupler assembly, removing residues and microparticles left from the pickling process, preventing field emission and protecting superconducting performance. After all post-processing of the coupling assembly is completed, the coupling assembly should be stored in a cleanroom environment to prevent surface impact and contamination.

[0025] The advantages of this invention are as follows: 1) A dual-suppression structure for fundamental mode suppression is incorporated into the design stage of the high-order mode coupler. Multiphysics and finite element analysis simulation software are used to iteratively analyze the RF structure, mechanical performance, and thermal load, ultimately determining the mechanical structure design. This invention increases the fundamental mode suppression bandwidth of the high-order mode coupler from approximately 10MHz in traditional single-suppression structures to approximately 100MHz. The dual-suppression structure eliminates fundamental mode tuning, significantly reducing module assembly complexity, saving time and manufacturing costs, and improving the stability of the module's belt bundle during operation.

[0026] 2) In order to reduce the low-temperature heat load and enable the high-order mode coupler to meet the requirements of high current intensity and continuous wave operation, a liquid helium tank design was added to the structure of the high-order mode coupler, which simplifies the processing of the inner conductor.

[0027] 3) The high-order module coupler adopts a detachable design, consisting of two main components sealed with an aluminum ring. This detachable design reduces manufacturing difficulty. If one component malfunctions, it is easier to repair. If it cannot be repaired and a spare part needs to be replaced, only that component needs to be replaced, effectively reducing module integration costs. Attached Figure Description

[0028] Figure 1 It is a traditional high-order mode coupler; (1) Cross-sectional view of TESLA 1.3GHz superconducting cavity high-order mode coupler, (2) Cross-sectional view of LEP 352MHz high-order mode coupler.

[0029] Figure 2 Flowchart for designing a high-order mode coupler.

[0030] Figure 3 This is a model diagram of a 650MHz high-order mode coupler.

[0031] Figure 4 This is a cross-sectional view of a 650MHz high-order mode coupler model.

[0032] Figure 5 This is a cross-sectional view of the extracted component model for a 650MHz high-order mode coupler.

[0033] Figure 6 This is a cross-sectional view of the coupling component model of a 650MHz high-order mode coupler.

[0034] Figure 7 To extract the component process diagram.

[0035] Figure 8 This is a process diagram for ceramic brazing components.

[0036] Figure 9 This is a welding assembly diagram of a T-type probe and a stainless steel flange.

[0037] Figure 10 This is a process diagram of the coupled component.

[0038] Figure 11 This is a flowchart of the post-processing for component extraction.

[0039] Figure 12 This is a flow chart of the post-processing technology for coupled components. Detailed Implementation

[0040] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0041] One embodiment of the present invention provides a high-order mode coupler with a dual-suppression structure suitable for high-current, continuous-wave accelerators and its manufacturing method. This coupler can be used in large electron-positron colliders to deeply suppress the fundamental and higher-order modes in a superconducting cavity, ensuring beam stability. The fundamental mode frequency of the superconducting cavity is 650MHz. The following description uses a 650MHz high-order mode coupler as an example to illustrate the dual-suppression structure and its manufacturing method. The design and manufacturing method can be applied to similar high-order mode couplers.

[0042] 1) Design flow of high-order mode couplers The design flowchart of a high-order mode coupler is as follows: Figure 2 As shown, the electromagnetic and mechanical design of a high-order mode coupler is a two-way iterative optimization design process.

[0043] After the initial design of the high-order mode coupler structure was determined, the structure was adjusted based on the transmission characteristic curve, and the suppression of the fundamental and higher-order modes was analyzed to meet the requirements for suppressing both modes. Figure 3 As shown, the higher-order mode coupler is mounted on the bundle tube of the superconducting cavity. The coupling hook extends deep into the bundle tube to couple the electromagnetic field transmitted from the superconducting cavity. The coupled energy is then extracted through the power extraction port. It is important to note that the depth of the coupling hook into the bundle tube affects the suppression strength of the fundamental and higher-order modes. Therefore, before optimization, the depth of the coupler hook into the bundle tube should be determined, with the maximum depth limited to 1 / 4 of the inner diameter of the bundle tube. Suppression of the fundamental mode is achieved by adjusting... Figure 4 The length of the first suppression structure (i.e., the coupling hook) and its distance from the tube wall are shown. For the 650MHz high-order mode coupler of this invention, the first suppression structure is a fundamental mode suppression structure, with a length ranging from 10cm to 17cm and a distance from the tube wall ranging from 2mm to 8mm. The second suppression structure is an LC resonant circuit composed of a T-shaped probe and the gap between the T-shaped probe and the tube wall. This is determined by the length of the crossbar of the T-shaped probe and the size of the fan-shaped portion. For the 650MHz high-order mode coupler of this invention, the length of the crossbar of the T-shaped probe in the second suppression structure ranges from 3cm to 8cm, and the fan-shaped portion has a height of 4cm, a thickness of 4cm, and a fan angle of 120°. Suppression of higher-order modes is achieved by adjusting... Figure 5The inner conductor dimensions shown are (for the 650MHz high-order mode coupler of this invention, the initial radius of the inner conductor ranges from 8mm to 10mm). Figure 6 The dimensions of the hooks and crossbars, as well as the distance between the coupling capacitors at the coupling component end and the coupling capacitors at the extraction probe end, and the dimensions of their respective end faces, are adjusted.

[0044] After the transmission characteristics of the high-order mode coupler satisfy the suppression conditions for both the fundamental and high-order modes, the secondary electron multiplication effect analysis is carried out. Figure 3 The high-order mode coupler shown is most prone to secondary electron multiplication at the first fundamental mode suppression structure. This can be addressed through optimization. Figure 4 The distance between the gaps in the first coupling capacitor shown is designed to prevent secondary electron multiplication. The dynamic heat loss of the high-order mode coupler under the working field strength of the superconducting cavity is simulated, and the static heat leakage is also simulated to evaluate the temperature rise of each part of the high-order mode coupler. Based on the calculation results, the cooling method for the high-order mode coupler is determined. For this invention, a liquid helium tank is added to the outside of the high-order mode coupler coupling assembly. The design of the liquid helium tank's inlet and outlet needs to be determined by comprehensively considering the cryogenic thermostat design and the installation space.

[0045] Before mechanical design, it is necessary to evaluate the impact of machining errors on the suppression effect of high-order mode couplers. This determines the requirements for machining and installation errors of each part during actual machining and module assembly. The parts that have a significant impact on the transmission results are: such as... Figure 4 As shown, the parallelism and distance of the gaps of the third coupling capacitor, the first coupling capacitor, the second coupling capacitor, and the angle of the coupling hook along the vertical direction are specified. The gaps of the first and second coupling capacitors determine the fundamental mode suppression frequency and bandwidth, while the gap of the third coupling capacitor determines the transmission efficiency of electromagnetic field energy in the higher-order mode frequency band. The flange face seal between the extraction end assembly and the coupling assembly uses an aluminum ring seal design, as does the seal between the coupling assembly and the superconducting cavity bundle tube. Appropriate sealing grooves should be provided in the flange design.

[0046] The higher-order mode coupler is installed in the superconducting cavity, which operates under a high vacuum and cryogenic environment. Therefore, the mechanical design must consider the stress and deformation under vacuum and cooling conditions. The thickness of the niobium outer cylinder and helium tank are determined after considering mechanical stress. After the higher-order mode coupler is fabricated and welded, its inner surface needs to be polished. This inner surface is a microwave surface, and any defects will ultimately affect the overall performance of the superconducting cavity. After polishing, a leak test is performed. If the leak test is successful, post-processing is carried out. Next, the higher-order mode coupler needs to be tested for its transmission characteristics at room temperature and then installed in the superconducting cavity to test its suppression effect on the fundamental and higher-order modes. If the requirements are met, it is then cleanly assembled with the superconducting cavity for cryogenic vertical testing. If the vertical test is successful, the superconducting cavity, the higher-order mode coupler, and the cryogenic thermostat are assembled. After assembly, a horizontal test is performed. If the test results do not meet the design requirements, the structure of the higher-order mode coupler needs to be optimized again.

[0047] 2) Design of high-order mode coupler and liquid helium tank structure Higher-order mode coupler models such as Figure 3 As shown, Figure 4 A cross-sectional view is provided. The main components consist of two parts: the first part is... Figure 3 The assembly between the stainless steel flange and the power extraction port is called the extraction assembly; the second part is... Figure 3 The assembly consisting of the niobium-titanium flanges and the coupling hooks shown is called the coupling assembly. The extraction assembly and the coupler assembly are connected using M8 bolts through 12 8mm diameter threaded holes on the stainless steel flange of the extraction assembly and the niobium-titanium flange at the top of the coupling assembly. Here, the coupling assembly is assembled with the niobium-titanium flange on the superconducting cavity via the lower niobium-titanium flange. The coupler hooks penetrate into the superconducting cavity bundle tube, coupling the electromagnetic field energy within the superconducting cavity. This electromagnetic field energy is released through the coupling capacitance between the two components. Figure 4 The gap of the third coupling capacitor shown transmits electromagnetic field energy to the extraction component. Finally, the extraction component extracts the power to the room temperature region outside the superconducting cavity through the power extraction port, and finally absorbs it on the load in the room temperature region.

[0048] The detailed structure of the extraction component of the high-order mode coupler is as follows: Figure 5As shown in the figure, the 650MHz high-order mode coupler extraction assembly consists of an inner conductor, an extraction assembly-end coupling capacitor, a T-type probe, a ceramic ring, a stainless steel flange, an outer conductor, a tapered transition section outer conductor, and an inner conductor. The inner conductor is made of oxygen-free copper with an outer diameter of 16mm. The extraction assembly-end coupling capacitor is a cylindrical oxygen-free copper piece with an outer diameter of 25.14mm and a height of 5mm. The edges of the coupling capacitor are chamfered to ensure a smooth transition on the microwave surface. The stainless steel flange is made of 316L stainless steel with an inner diameter of 80mm, and the ceramic ring is made of alumina 95 ceramic with an inner diameter of 22.7mm and an outer diameter of 36.82mm. Both the outer conductor and the tapered transition section outer conductor are made of 316L stainless steel, while the inner conductor of the tapered transition section is made of oxygen-free copper. The extraction port uses a standard N-type connector. Its inner conductor is laser-welded to the tapered transition section of the inner conductor of the extraction assembly, and its outer conductor is laser-welded to the tapered transition section of the outer conductor of the extraction assembly. The extraction components are all made of non-superconducting materials. The outer conductor of the extraction components and the outer conductor of the tapered transition section are both solid thin-walled structures. The inner conductor of the extraction components is hollow. The detailed structure is described in the manufacturing process and welding section.

[0049] Figure 5 The connection relationships between the units are as follows: The coupling capacitor at the extraction component end is integrally formed with the inner conductor of the extraction component, and is welded to the tapered transition section of the inner conductor of the extraction component to form a complete inner conductor. The T-shaped probe is welded to the inner conductor of the extraction component through its crossbar end. The inner surface of the ceramic ring is connected to the inner conductor by brazing, and the outer surface of the ceramic ring is connected to the outer conductor by brazing. The welding process of the ceramic ring is described in [link to details]. Figure 8 .

[0050] The detailed structural cross-section of the coupling component of the high-order mode coupler is shown below. Figure 6 As shown, the main body consists of a coupling hook, a coupling capacitor at the coupling component end, an inner conductor support rod, a niobium outer cylinder, a liquid helium tank outer cylinder, niobium-titanium flanges, titanium tubes, and titanium flanges. The coupling hook, coupling capacitor at the coupling component end, inner conductor support rod, and niobium outer cylinder are all made of high-purity niobium (RRR300), while the liquid helium tank outer cylinder and connecting flanges are made of titanium (TA2). The large flanges welded to both ends of the niobium outer cylinder are also made of niobium-titanium. The outer diameter of the coupling hook and support rod is 16mm, and the height from the top of the coupling capacitor to the bottom of the hook is 146mm. The inner diameter of the niobium outer cylinder is 80mm, the wall thickness is 3mm, and the height of the niobium cylinder is 102.66mm. The inner diameter of the liquid helium tank outer cylinder is 94mm, the wall thickness is 2mm, the inner diameter of the helium inlet pipe is 6mm, and the inner diameter of the helium outlet pipe is 21.4mm. The length of the inlet and outlet pipes is determined by the installation space, ensuring that the upper and lower niobium-titanium flanges of the niobium tube can be installed normally. Except for the liquid helium tank outer cylinder and its connecting flanges, the entire coupling component is made of superconducting materials.

[0051] Figure 6The connection relationships between the units shown are as follows: the coupling capacitor and coupling hook at the coupling component end are formed into the complete inner conductor of the component by electron beam welding. The top of the inner conductor support rod is connected to the niobium outer cylinder by electron beam welding. The two niobium titanium flanges are connected to the niobium outer cylinder by electron beam welding, and are also connected to the liquid helium tank outer cylinder by electron beam welding. A hollow sandwich with a distance of 4mm is formed between the niobium outer cylinder and the liquid helium tank outer cylinder. One end of the titanium tubes on the left and right sides is connected to the titanium flange by electron beam welding, and the other end is connected to the liquid helium tank outer cylinder by electron beam welding.

[0052] 3) Manufacturing process and welding of high-order mode couplers The high-order mode coupler mainly consists of two parts: the extraction component and the coupling component. Due to the differences in materials of each component, the welding process adopted is different.

[0053] (1) Manufacturing and welding of extraction components Extract the process diagram of the component as follows Figure 7 As shown, the entire assembly consists of seven parts. To reduce the weight of the inner conductor and minimize its deformation during processing, installation, and cooling, the inner conductor of the extraction assembly, the lower half of the tapered transition section of the inner conductor in the ceramic brazing assembly, and the support crossbar of the T-shaped probe are all machined into solid shell structures. The inner diameter of this shell structure is 10mm. Simultaneously, the entire assembly is installed within the module's thermally insulated vacuum. Therefore, a small hole with a diameter of 2mm is pre-drilled in the tapered transition section of the outer conductor of the extraction assembly for vacuum extraction. A similar hole of the same size is also pre-drilled in the tapered transition section of the inner conductor of the ceramic brazing assembly, ensuring that the space between the tapered transition sections of the inner and outer conductors of the extraction assembly is also in a vacuum state within the module.

[0054] Figure 7 The inner conductor of the extraction component shown is made of oxygen-free copper and is brazed to the T-type probe. The fan-shaped part of the T-type probe component is also brazed to the supporting crossbar.

[0055] Figure 7 The ceramic brazing assembly shown in the diagram is as follows. Figure 8 As shown, the ceramic brazing assembly consists of three parts: a tapered transition section of the inner conductor of the extraction assembly, machined from oxygen-free copper; an oxygen-free copper outer conductor ring; and a ceramic ring. The ceramic ring needs to be metallized before brazing. The ceramic ring has an inner diameter of 22.7 mm, an outer diameter of 36.82 mm, and a thickness of 4.4 mm. It is important to note that the dimensions of the ceramic ring are critical; the flatness of both the upper and lower surfaces of the ceramic ring and the concentricity of the ceramic ring must be ensured. The outer diameter of the tapered transition section of the inner conductor and the inner diameter of the oxygen-free copper outer conductor ring must be matched with the ceramic.

[0056] Figure 7The outer conductor of the extraction component and the tapered transition section of the outer conductor are fixed by argon arc welding. Figure 8 The ceramic brazed assembly is brazed with the outer conductor of the extraction assembly, the outer conductor of the extraction assembly is brazed with the stainless steel flange, and the tapered transition section of the inner conductor of the ceramic brazed assembly is brazed with the inner conductor of the extraction assembly.

[0057] Figure 7 The welding angle between the T-type probe and the stainless steel flange shown is required, as follows: Figure 9 As shown, the center of the T-type probe's sector (circled in the diagram) is parallel to the flange's etched line, which is parallel to the center line of the stainless steel flange's flange hole. During welding, attention should be paid to the parallelism between the upper edge of the T-type probe's sector and the stainless steel flange surface.

[0058] Figure 7 The stainless steel flange shown has a sealing groove with a depth of 1mm and a width of 7mm. Care should be taken to protect the sealing surface during processing.

[0059] Pay attention to the following during welding Figure 7 The diagram shows the concentricity of the inner conductor of the component with the stainless steel flange, and the parallelism of the bottom plane of the inner conductor of the component with the surface of the stainless steel flange.

[0060] Figure 7 The machining error of each component of the extracted assembly is required to be controlled within ±0.1mm, the concentricity error is ±0.1mm, and the parallelism error is ±0.1mm.

[0061] Figure 7 After the extraction component is welded as shown, each weld seam of the entire component must be leak-tested, and the vacuum leak rate must be less than 1×10. -10 mbar*L / s.

[0062] Figure 7 The inner surface of the extracted component should be free from bumps and scratches. It should be protected during processing and transportation to ensure the smoothness of the oxygen-free copper inner conductor surface.

[0063] (2) Manufacturing and welding of coupling components All welds of the coupling components were made using electron beam welding. The welding process and sequence are as follows.

[0064] S1: As Figure 10 The coupling hook shown is formed by CNC milling, and the coupling capacitor at the end of the coupling component is electron beam welded. The outer diameter of the coupling hook is 16 mm, the outer diameter of the coupling capacitor at the end of the coupling component is 25.14 mm, and the edges are chamfered with a radius of R1 mm.

[0065] S2: After the coupling hook and coupling capacitor at the coupling component end are welded, weld them to the niobium outer cylinder. The niobium outer cylinder has an inner diameter of 80mm, an outer diameter of 86mm, and a height of 102.66mm. Both ends of the niobium outer cylinder should have stop joints for welding to the top and bottom niobium-titanium flanges. The coupling hook, coupling capacitor at the coupling component end, and niobium outer cylinder are all made of high-purity niobium material (RRR300). S3: The components after S2 welding are electron beam welded to the bottom niobium-titanium flange.

[0066] S4: The titanium tube (liquid helium / helium gas outlet tube) is electron beam welded to the adjacent titanium flange, and the material is titanium.

[0067] S5: The titanium tube (liquid helium inlet tube) is electron beam welded to the adjacent titanium flange, and the material is titanium.

[0068] S6: The S4 component is electron beam welded to the outer cylinder of the liquid helium tank, which is made of titanium.

[0069] S7: Electron beam welding of the S5 component to the outer cylinder of the liquid helium tank.

[0070] S8: S7 assembly is electron beam welded to the top niobium-titanium flange.

[0071] S9: Electron beam welding of S3 and S8 components completed. Figure 10 The coupling components shown are welded as a whole.

[0072] Figure 10 After the coupling assembly shown is welded, each weld seam of the entire component must be leak-tested, and the vacuum leak rate must be less than 1×10⁻⁶. -10 mbar*L / s. The leakage rate of the interlayer between the niobium outer cylinder and the liquid helium tank outer cylinder is less than 1×10. -10 mbar*L / s.

[0073] During welding, ensure the parallelism error between the coupling hook and the reference surface is within ±0.1mm. During welding and machining, ensure the flange hole of the niobium-titanium flange and the center of the coupling hook are on the same center line, with an angular error of ±0.5°. During welding and machining, welding shrinkage allowance should be considered, and the final dimensional error should be within ±0.1mm.

[0074] 4) Post-processing (1) Post-processing of extracted components The flowchart of post-processing after component extraction is as follows: Figure 11 As shown.

[0075] After the parts are machined, for oxygen-free copper parts, after precision turning, to ensure a surface finish of 0.4, rough grinding is first performed using 800-grit sandpaper, followed by secondary polishing with polishing paste and a canvas wheel to achieve the required surface finish. The cleaning process for the resulting parts is divided into two parts: for stainless steel parts, ultrasonic degreasing for 30 minutes (with a degreasing solvent) is sufficient; for oxygen-free copper parts, ultrasonic cleaning (with a degreasing solvent) for 30 minutes is performed first, followed by chemical polishing with mixed acid and a gloss treatment solution to fully achieve the required surface finish.

[0076] For T-type probe assemblies, use a file and sandpaper to grind the weld seams to prevent protrusions. After processing, place the assembly in a container filled with anhydrous ethanol, then immerse the entire assembly in ultrasonic cleaning for 30 minutes to remove powder.

[0077] After all welding and post-processing are completed, dimensional inspection is carried out. The dimensional error is required to be within ±0.1mm. After passing the inspection, protective fixtures are installed and vacuum protection is applied, or the copper is placed in a drying cabinet for oxygen-free oxidation.

[0078] (2) Post-processing of coupled components After the coupling components are welded, they need to undergo internal surface inspection, buffered chemical polishing (BCP), and annealing. The specific process flow is as follows: Figure 12 As shown.

[0079] After the coupling components are welded, a visual and surface inspection is performed first. The inner and outer surfaces must be clean, smooth, and flat, and the weld seams on the inner surface must be ground until there are no obvious unevennesses.

[0080] After meeting the above requirements, rinse the outer surface; ultrasonically degrease with 50℃ solution for 30 minutes; rinse with pure water and air dry for 5 hours or more.

[0081] Before BCP repolishing, weighing is required. Both BCP repolishing and BCP light polishing require a total surface removal of 100μm.

[0082] BCP repolishing requires an acid solution ratio of 1:1:2, consisting of hydrofluoric acid (HF), nitric acid (HNO3), and phosphoric acid (H3PO4). The acid solution temperature should be below 15°C, the flow rate <18L / min, and the repolishing time approximately 80 minutes. Both sides should be acid-washed (with internal surface inspection and polishing as needed), with each side accounting for about half of the total time. After filling with pure water, the system should be emptied several times until the pH >6. The final BCP repolishing thickness should be less than 80μm. After BCP repolishing, HPR (high-pressure water rinsing) should be performed, requiring three cycles. After completion, the system should be placed in a cleanroom to air dry for 10 hours or more.

[0083] Then, high-temperature annealing is performed, requiring a vacuum degree better than 1.0 × 10⁻⁶. -3 Heat to 700 degrees Celsius over 3 hours, maintain the temperature for 3 hours, and then allow to cool naturally to 80 degrees Celsius before removing. After removal, perform ultrasonic cleaning by ultrasonically cleaning with a degreasing solution at 50 degrees Celsius for 30 minutes, followed by rinsing with pure water.

[0084] After the above process is completed, BCP light polishing is performed with an acid solution ratio of 1:1:2. The acid solutions are hydrofluoric acid (HF), nitric acid (HNO3), and phosphoric acid (H3PO4). The acid solution temperature is below 15℃, the acid solution flow rate is <18L / min, and the light polishing time is about 30min. After the process is completed, pure water is poured in and the system is emptied several times until the pH is >6. The final BCP light polishing thickness is less than 30μm, and the thickness of the two BCP polishing processes is controlled within 100μm.

[0085] After light tossing, perform HPR (High-Performance Reduction), air dry, and then weigh.

[0086] After all the above steps are completed, the post-processing of the coupling component is finished. After post-processing, the coupling component should be stored in a clean room environment to prevent surface bumps and contamination.

[0087] The above are preferred embodiments of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A high-order mode coupler with a dual-suppression structure, characterized in that, Includes extraction components and coupling components; The coupling component is equipped with a coupling hook, which is used to penetrate into the superconducting cavity bundle tube to couple the electromagnetic field transmitted from the superconducting cavity and input it to the extraction component; The extraction component is used to extract the input electromagnetic field to a room temperature region outside the superconducting cavity; A first coupling capacitor is formed between the lower end of the coupling hook and the wall of the superconducting cavity tube, which is used to determine the frequency point of fundamental mode suppression; a second coupling capacitor is formed between the T-shaped probe in the extraction assembly and the wall of the extraction assembly, which is used to adjust the bandwidth of the fundamental mode suppression frequency point; a third coupling capacitor is formed between the bottom end of the inner conductor of the extraction assembly and the top end of the coupling hook, which is used to adjust the transmission efficiency of electromagnetic field energy. The first coupling capacitor is the first suppression structure, and the LC resonant circuit formed by the T-shaped probe and the second coupling capacitor is the second suppression structure.

2. The high-order mode coupler according to claim 1, characterized in that, The extraction assembly has an inner conductor inside its tube wall, on which the T-shaped probe is mounted. The bottom end of the inner conductor has a chamfered extraction assembly coupling capacitor for a smooth microwave transition. One end of the tube wall is connected to the outer conductor of the extraction assembly via a flange, and the other end is sealed to the coupling assembly via a flange. A ceramic ring is located inside the outer conductor; the outer surface of the ceramic ring is brazed to the outer conductor, and the inner surface of the ceramic ring is brazed to the inner conductor. The top end of the inner conductor is connected to the inner conductor of the tapered transition section. The top end of the outer conductor is connected to the outer conductor of the tapered transition section. The top ends of the inner and outer conductors of the tapered transition section are connected to an extraction port using a standard N-type connector.

3. The high-order mode coupler according to claim 2, characterized in that, The coupling assembly includes a coupling hook, a coupling capacitor at the coupling assembly end, an inner conductor support rod, a niobium outer cylinder, a liquid helium tank outer cylinder, a niobium-titanium flange, a titanium tube, and a titanium flange. The niobium outer cylinder is housed inside the liquid helium tank outer cylinder. The two niobium-titanium flanges are respectively connected to the niobium outer cylinder and the liquid helium tank outer cylinder, forming a hollow interlayer between the niobium outer cylinder and the liquid helium tank outer cylinder. The coupling hook is located inside the niobium outer cylinder, and the coupling capacitor at the coupling assembly end is located at the top of the coupling hook. One end of the inner conductor support rod is connected to the niobium outer cylinder, and the other end is connected to the coupling hook. A titanium tube is connected to each side of the liquid helium tank outer cylinder for injecting or discharging liquid helium into or out of the hollow interlayer. The coupling capacitor at the extraction assembly end and the coupling capacitor at the coupling assembly end form the third coupling capacitor.

4. The high-order mode coupler according to claim 3, characterized in that, The extraction component end coupling capacitor is integrally formed with the inner conductor and welded to the tapered transition section inner conductor to form a complete inner conductor.

5. The high-order mode coupler according to claim 2, characterized in that, The parallelism error of the upper and lower surfaces of the ceramic ring is ±0.1 mm, and the concentricity error of the inner and outer surfaces of the ceramic ring is ±0.1 mm.

6. The high-order mode coupler according to claim 1, 2, or 3, characterized in that, The maximum depth to which the coupling hook penetrates the superconducting cavity bundle tube is 1 / 4 of the inner diameter of the superconducting cavity bundle tube.

7. The high-order mode coupler according to claim 1, 2, or 3, characterized in that, The T-shaped probe includes a crossbar and a fan-shaped structure; one end of the crossbar is connected to the inner conductor, and the other end is connected to the inner center of the fan-shaped structure; the fan-shaped structure is parallel to the tube wall of the extraction component, forming the second coupling capacitor.

8. A method for manufacturing a high-order mode coupler with a dual suppression structure as described in claim 1, comprising the following steps: Preparation of extraction and coupling components; The oxygen-free copper parts in the prepared extraction assembly are first coarsely ground, and then polished a second time using polishing paste and a canvas wheel to achieve the required surface finish. Then, the stainless steel parts are ultrasonically cleaned, and the oxygen-free copper parts are ultrasonically cleaned and then chemically polished using mixed acid and a gloss treatment solution. Finally, the treated extraction assembly is placed in a container filled with anhydrous ethanol for ultrasonic cleaning. The prepared coupling components were rinsed, buffered chemically polished, and annealed.

9. The method according to claim 8, characterized in that, The method for buffered chemical polishing of the prepared coupling components is as follows: the prepared coupling components are subjected to buffered chemical heavy polishing and buffered chemical light polishing; the acid ratio for buffered chemical heavy polishing is 1:1:2, the acid temperature is below 15℃, the acid flow rate is <18L / min, and the heavy polishing time is about 80min; the acid ratio for buffered chemical light polishing is 1:1:2, the acid temperature is below 15℃, the acid flow rate is <18L / min, and the light polishing time is about 30min. After the process, pure water is poured in and the system is vented several times until the pH is >6. The final BCP light polishing thickness is less than 30μm, and the thickness of the two BCP polishing processes is controlled within 100μm.

10. The method according to claim 8, characterized in that, The parallelism error between the axis of the coupling hook and the reference plane is within ±0.1 mm; during welding and machining, the flange hole of the niobium-titanium flange of the coupling assembly and the center of the coupling hook are on the same center line, with an angle error of ±0.5°; the reference plane is a plane perpendicular to the top surface of the coupling hook that passes through the center of the threaded holes at both ends of the niobium-titanium flange and the center of the top surface of the coupling hook.

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