High-order mode coupler based on double suppression structure of transmission line model and design method

Abstract

The application discloses a high-order mode coupler based on a double-inhibition structure of a transmission line model and a design method, and belongs to the accelerator physics and superconducting high-frequency technical field.The method comprises the following steps: determining a superconducting cavity base mode frequency, a high-order mode frequency band and an external quality factor Qe requirement; establishing an equivalent transmission line model comprising two parallel resonance base mode inhibition structures; optimizing a transmission curve through a normalized scattering matrix theory; converting the optimized transmission line model into a three-dimensional structure and further optimizing; modeling and calculating the external quality factor Qe in combination with the superconducting cavity; performing secondary electron multiplication effect simulation analysis; performing thermal load analysis by using an iterative process; and finally completing mechanical design.The application adopts the double-inhibition structure, greatly improves the base mode inhibition bandwidth, realizes deep inhibition of the base mode without pre-tuning, significantly reduces the superconducting module integration cost, improves the assembly efficiency and reduces the operation risk.

Description

Technical Field

[0001] This invention belongs to the fields of accelerator physics and superconducting high-frequency technology, and particularly relates to a high-order mode coupler and its design method based on a transmission line model dual suppression structure. 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 motion direction; these are called higher-order modes. These higher-order modes can cause multi-beam instability, increase emittance and beam energy dissipation, and also introduce unnecessary cryogenic losses. Using higher-order mode couplers to absorb the energy of these higher-order modes within the resonant cavity is the most important means of solving the higher-order mode problem. Higher-order mode couplers are one of the core components of superconducting high-frequency systems in accelerators, especially for high-energy, high-brightness accelerators, where they directly determine the quality of the electron beam.

[0003] Coaxial couplers are compact and suitable for low-frequency and mid-to-high-order mode power extraction. Designing coaxial high-order mode couplers presents two main challenges: maximizing the coupling of dangerous high-order modes within a wide bandwidth and effectively suppressing the fundamental mode. Coaxial high-order mode couplers have been successfully applied in the 1980s in TRISTAN, HERA, and LEP. LEP-type high-order mode couplers have been successfully applied in SOLEIL, LHC, SLS's Super-3HC cryostat, and ELETTRA. TESLA high-order mode couplers are optimized designs based on HERA couplers. Because the high-order mode power is very small (1% duty cycle), the high-order mode coupler is designed to be placed outside the liquid helium bath. If TESLA high-order mode couplers are used for continuous wave operation, the extraction probe and the coupler's internal conductors experience severe overheating. Therefore, the coupler needs to be redesigned for continuous wave operation to reduce overheating. The damping effect of a coaxial high-order mode coupler depends primarily on the RF design of the structure. A drawback of the coupler is the tuning of the fundamental mode's RF band-stop filter, which requires extreme care during actual fundamental mode tuning. In any coaxial high-order mode coupler design, the portion located in the high magnetic field region must be made of superconducting material and adequately cooled to prevent quenching. After the coupler is mounted on a superconducting cavity, it is essential to ensure that the band-stop filter's capacitor can be finely adjusted to effectively suppress the fundamental mode. This invention proposes an improvement for the design of high-order mode couplers for superconducting cavities.

[0004] Coaxial high-order mode couplers are typically mounted on a superconducting cavity bundle tube and consist of a coaxial resonant cavity, a coupling antenna, and a load circuit. The high-order mode coupler extracts the field energy of the high-order modes from within the bundle tube via the coupling antenna, and then dissipates it to the absorbing load through a conduction circuit. The coupling of the coaxial high-order mode coupler to the electromagnetic field can be simplified into two forms, one of which is as follows: Figure 1 As shown in the left figure, the top of the inner conductor is open-circuited, forming a probe, or as... Figure 1 As shown in the right figure, the top of the inner conductor is short-circuited, forming a loop. The probe or loop is equivalent to a resistor R.

[0005] The fundamental mode is used to accelerate charged particles. Therefore, the coupler structure designed based on transmission line theory must use a filter structure to suppress the fundamental mode. That is to say, the fundamental mode is a useful mode and cannot be coupled out while coupling higher-order modes. The addition of the filter structure makes the design more complicated, but the filter structure can be integrated into the reactance structure. Figure 2 The equivalent circuit of the traditional single-suppression structure is given. If the inductor L = When connected in parallel with a capacitor C, it will reach the fundamental mode frequency. A band-stop filter is formed at this point. In the high-frequency range, the filter reactance becomes capacitive to compensate for... The loop reactance at the point. For this circuit, if the higher-order mode frequency to be compensated is... Then L and C are:

[0006] ;

[0007] Practical high-order mode suppressors need to suppress multiple high-order modes. In addition to determining which high-order modes are dangerous and may cause beam instability, it is also necessary to determine the impedance of these high-order modes. The impedance of each high-order mode is determined by R / Q and Q at that mode frequency.

[0008] In the prior art, the closest technical solution to the present invention is the high-order mode coupler design of LEPII, the design method of which is described below.

[0009] The LEPII superconducting cavity has a fundamental mode frequency of 352 MHz. The modes requiring suppression by the high-order mode coupler are mainly concentrated at three frequencies: 480 MHz, 650 MHz, and 1.1 GHz. To prevent fundamental mode energy extraction, a fundamental mode filtering section is incorporated into the design. The LEPII high-order mode coupler achieves this function through the inductance of the hook structure and the capacitance formed between its open end and ground. These components together constitute a series notch filter tuned to 352 MHz. Fundamental mode tuning is achieved by adjusting the liquid helium connecting tube using an external mechanical device. By selecting the correct hook parameters, such as the equivalent inductance of the loop, the equivalent length of the line at 650 MHz being approximately half the wavelength, the inductance of the crossbar, and the capacitance at the output port, the high-order mode coupler exhibits resonant characteristics at these three frequency points, ultimately maximizing coupling. The transmission characteristic curve of the LEPII high-order mode coupler is shown below. Figure 3 As shown.

[0010] In existing technologies, high-order mode couplers with such a single-suppression structure typically have a bandwidth of several MHz for fundamental mode suppression, such as... Figure 3As shown, the fundamental mode suppression bandwidth is relatively small, and it is affected by processing errors and temperature drops, placing high demands on the fabrication and pre-tuning of the high-order mode coupler. Before formal operation, the fundamental mode needs to be tuned at room temperature. When the cavity and coupler are at low temperatures, only a negligible amount of fundamental mode power (approximately 10 watts) is extracted. If the tuning at room temperature is not done properly, the fundamental mode suppression frequency will deviate too much after cooling, causing fundamental mode leakage, and in severe cases, leading to superconducting cavity quench failure. Secondly, if electron discharge occurs (such as multi-stage hysteresis discharge), the notch filter may become detuned for a short time, resulting in a significant increase in peak power, which in severe cases can cause the ceramic window of the high-order mode coupler to crack, leading to vacuum leakage.

[0011] Furthermore, tuning the fundamental mode at room temperature requires the design of corresponding tuning fixtures, which increases the complexity of the system. For accelerator projects with multiple superconducting cavities, this undoubtedly increases the module integration time and cost. Summary of the Invention

[0012] To address the aforementioned technical problems, this invention provides a high-order mode coupler and its design method based on a transmission line model with a dual-suppression structure. This method is applicable to all superconducting cavities using coaxial high-order mode couplers, particularly large accelerator projects employing high beam current designs. High-order mode coupler design involves solving multiple physical quantity problems related to high frequency, thermal analysis, mechanical design, and cavity-beam interaction. The three-dimensional model design is determined through equivalent circuit analysis, transmission optimization, equivalent three-dimensional structure design, and finite element analysis. Due to their compact structure, coaxial high-order mode couplers are generally used in multi-cavity cryogenic reactor designs, where the challenge lies in fundamental mode tuning and broadband suppression of higher-order modes. This invention, starting from transmission line theory, optimizes the transmission line model to deeply suppress the fundamental mode and achieve high-pass transmission of higher-order modes. The optimized transmission line model is transformed into an equivalent three-dimensional structure, and a systematic high-order mode coupler design method is formed through electromagnetic optimization design of the three-dimensional structure. Based on the above design method, this invention also proposes a high-order mode coupler design scheme with a dual suppression structure, which greatly improves the bandwidth of fundamental mode suppression. In actual use, no tuning of the fundamental mode is required, which greatly reduces the difficulty and complexity of the design, greatly improves the assembly efficiency of superconducting modules at room temperature, and saves time and costs. At the same time, during actual beam carrying operation of the accelerator, it greatly reduces the risk of fundamental mode leakage at low temperature and ensures the safe operation of the module.

[0013] Practical high-order mode suppressors need to suppress multiple high-order modes. In addition to determining which high-order modes are dangerous and may cause beam instability, it is also necessary to determine the impedance of these high-order modes. The impedance of each high-order mode is determined by R / Q and Q at that mode frequency.

[0014] This invention proposes a design method for high-order mode couplers based on a transmission line model dual-suppression structure, achieving deep suppression of the fundamental mode (external quality factor Qe > 10).11 Furthermore, no pre-tuning of the fundamental mode is required. Starting from transmission line theory, a corresponding transmission line model is established. By adjusting various parameters of the transmission line model, the optimal transmission curve is obtained. Each part of the transmission line model is transformed into an actual three-dimensional model. After determining the preliminary model, the model is further optimized based on the transmission results, thus obtaining a high-order mode coupler design model that meets the suppression requirements. In addition, high-order mode coupler design involves solving multiple physical quantity problems related to high frequency, thermal analysis, mechanical design, and cavity-beam interaction. If these problems are not considered during the design, it will lead to the failure of the superconducting cavity during operation. This invention aims to provide a set of high-order mode coupler design methods with a dual-suppression structure, which can be used for superconducting cavity design in different frequency bands. This invention can significantly reduce the integration cost of superconducting modules, improve fundamental mode suppression, and greatly reduce the operational risks of superconducting modules.

[0015] The specific technical solution is as follows:

[0016] A design method for a high-order mode coupler based on a transmission line model dual-suppression structure includes the following steps:

[0017] Step 1: Determine the fundamental mode frequency, higher-order mode bandwidth, and external quality factor Qe requirements of the superconducting cavity. Clarify the fundamental mode frequency of the superconducting cavity, the external quality factor requirements of the higher-order mode coupler for fundamental mode suppression, the bandwidth range for extracting higher-order modes by the higher-order mode coupler, and the external quality factor requirements for suppressing higher-order modes.

[0018] Step 2: Establish the transmission line model, which includes a current source, a first parallel resonant fundamental mode suppression structure composed of inductor L1n and capacitor C1n connected in series, equivalent inductance l1, equivalent inductance M, a transmission line with equivalent inductance l2, capacitor C2t, equivalent inductance l3, a second parallel resonant fundamental mode suppression structure composed of inductor L2n and capacitor C2n connected in series, equivalent inductance l4, and a terminating load composed of capacitor Ct and matching load Z connected in parallel.

[0019] Step 3: Transmission curve analysis and transmission line model optimization. By adjusting various parameters of the transmission line model, the circuit transmission curve S21 is calculated based on the normalized scattering matrix.

[0020] Step 4: 3D modeling and transmission curve analysis and optimization. Based on the parameters of the optimized transmission line model, it is transformed into the corresponding 3D structure. A 3D model of the high-order mode coupler is established. The transmission characteristics of the high-order mode coupler are simulated and calculated using a frequency domain solver. The local parameters of the high-order mode coupler are optimized based on the transmission curve simulation results until a transmission curve that meets the requirements is obtained.

[0021] Step 5: Modeling the 3D model of the higher-order mode coupler and the superconducting cavity. The higher-order mode coupler and the superconducting cavity are modeled together. The eigenmode solver is used to perform overall simulation calculation to obtain the external quality factor Qe of the higher-order mode coupler for suppressing the fundamental mode and higher-order modes. Based on the calculation results, the local parameters of the higher-order mode coupler are adjusted until the external quality factor Qe for suppressing the fundamental mode and the external quality factor Qe for suppressing the unipolar and dipole modes in the higher-order mode are satisfied, thus completing the high-frequency parameter design of the higher-order mode coupler.

[0022] Step 6: Simulation analysis of secondary electron multiplication effect. The secondary electron multiplication simulation is performed using a particle tracking solver and a simplified simulation calculation model. Material parameters and particle source are set, and the electric and magnetic field distributions of the superconducting cavity fundamental mode calculated by the intrinsic mode solver are imported. Multi-particle secondary electron multiplication analysis is performed, and the growth rate of secondary electron multiplication is calculated until the growth rate is less than 0.

[0023] Step 7: Finite element software thermal load simulation analysis, which includes dynamic heat leakage calculation and static heat leakage calculation. The dynamic heat leakage calculation adopts an iterative process: initialize and set the static initial temperature distribution, calculate the surface resistance of each part of the device based on the temperature distribution of the current iteration, calculate the surface microwave loss of each part of the device as the surface heat source based on the surface resistance, recalculate the temperature distribution of the device, update the thermal conductivity of the device material based on the current temperature distribution, calculate the difference between the current temperature distribution and the temperature distribution of the previous iteration and compare it with the preset convergence threshold. If the difference is less than the threshold, the iteration terminates and the final steady-state temperature distribution is output. Otherwise, it returns to continue the iteration. Based on the steady-state temperature distribution result, it is determined whether a liquid helium active cooling scheme is needed.

[0024] Step 8: Mechanical Design. Based on the simulation analysis results above, the mechanical structure of the high-order mode coupler is designed. A detachable design is adopted. The main body includes a coupling hook made of high-purity niobium, a niobium outer cylinder, a niobium-titanium flange, a titanium helium tank, as well as a ceramic window, an oxygen-free copper T-shaped inner conductor, and a stainless steel flange. The components are connected by electron beam welding and brazing processes to form a complete high-order mode coupler structure.

[0025] A high-order mode coupler based on a transmission line model dual-suppression structure includes:

[0026] The hook-shaped inner conductor assembly consists of a first inner conductor, a second inner conductor, a third inner conductor, a fourth inner conductor, and a fifth inner conductor. The first, second, third, fourth, and fifth inner conductors are arranged in a hook shape, with the fifth inner conductor serving as the handle. The upper end of the fifth inner conductor is a stepped plate. The fourth inner conductor is connected to the middle of the fifth inner conductor and extends horizontally in the opposite direction to the first inner conductor. The hook-shaped inner conductor assembly is used to couple the electromagnetic field within the superconducting cavity.

[0027] The T-shaped inner conductor assembly consists of a 6th inner conductor, a 7th inner conductor, an 8th inner conductor, a 9th inner conductor, and a ceramic window. The 6th and 7th inner conductors are integrally formed into straight cylinders. The lower end of the 6th inner conductor is a stepped plate, corresponding to the upper end of the 5th inner conductor at a certain distance. The 8th inner conductor extends vertically outward from the portion formed by the 6th and 7th inner conductors. The 9th inner conductor is located above the 7th inner conductor and has a larger diameter than the 7th inner conductor. The ceramic window is located outside the 9th inner conductor. The 8th and 7th inner conductors are connected by welding at the middle position. The ceramic window is welded to the 9th inner conductor. The T-shaped inner conductor assembly is used to extract coupled electromagnetic field energy.

[0028] The outer conductor portion of the hook-shaped inner conductor assembly consists of a first flange, an outer cylinder, a liquid helium outlet pipe, a liquid helium inlet pipe, a liquid helium tank, and a second flange. The first and third flanges are welded to the outer cylinder, the liquid helium tank is welded to the first and second flanges, the liquid helium inlet pipe is welded to the liquid helium tank 2-5, the liquid helium outlet pipe is welded to the liquid helium tank, and the hook-shaped inner conductor assembly is welded to the outer cylinder 2-2 through a fourth inner conductor to form an integral coupling assembly.

[0029] The outer conductor portion of the T-shaped inner conductor assembly consists of a third flange, an outer conductor cone, and a standard N-type connector. The third flange is welded to the outer conductor cone and the standard N-type connector to form the outer conductor portion of the T-shaped inner conductor assembly. The outer conductor portion and the T-shaped inner conductor assembly are welded together through a ceramic window to form an integral extraction assembly.

[0030] The hook-shaped inner conductor assembly and the T-shaped inner conductor assembly achieve electromagnetic field coupling transmission through the connection of the fourth inner conductor and the sixth inner conductor; the overall coupling assembly and the overall extraction assembly are assembled through the second flange and the third flange to form a complete high-order mode coupler; the first inner conductor and the second inner conductor are connected in series to form the first parallel resonant fundamental mode suppression structure, and the eighth inner conductor contains the second parallel resonant fundamental mode suppression structure, thus forming a dual suppression structure.

[0031] The present invention has the following beneficial effects:

[0032] This invention proposes a design method for a high-order mode coupler based on a transmission line model and a dual-suppression structure. Starting from a theoretical transmission line model, this method optimizes the transmission line model parameters to obtain the transmission curve of the dual-suppression structure. The optimized transmission line model is then transformed into a corresponding three-dimensional structure, and the RF parameters of the high-order mode coupler with the dual-suppression structure are determined through simulation optimization. Multiphysics simulation analysis and thermal load analysis are performed on the high-order mode coupler and the superconducting cavity as a whole, and the final mechanical structure is determined through iterative optimization.

[0033] This method is applicable to the design of high-order mode couplers in superconducting cavities across any frequency band, significantly improving the design efficiency and reliability of high-order mode couplers. The proposed dual-suppression structure design greatly enhances the bandwidth for fundamental mode suppression, eliminating the need for fundamental mode tuning in practical applications. This significantly improves the integration efficiency of superconducting modules, reduces system complexity and difficulty, and lowers costs. In actual beam operation, it greatly reduces the risk of fundamental mode leakage and significantly improves the reliability of online operation of superconducting modules. Attached Figure Description

[0034] Figure 1 Schematic diagrams of two coupling methods for a coaxial high-order mode coupler;

[0035] Figure 2 The equivalent circuit diagram of a resonant higher-order mode suppressor with a fundamental mode suppression structure is shown.

[0036] Figure 3 The transmission characteristic curve of the LEPII high-order mode coupler;

[0037] Figure 4 A flowchart illustrating the design method for a high-order mode coupler with a dual-suppression structure based on a transmission line model;

[0038] Figure 5 This is a schematic diagram of the equivalent transmission line model of the double suppression structure;

[0039] Figure 6 For different circuit units and their corresponding scattering matrix expressions;

[0040] Figure 7 For the optimized transmission curve picture;

[0041] Figure 8 A 3D model diagram of a high-order mode coupler;

[0042] Figure 9 A simulation model of a high-order mode coupler and a 650MHz superconducting cavity;

[0043] Figure 10 This is a simulation model diagram of secondary electron multiplication.

[0044] Figure 11 A flowchart illustrating the iterative process for calculating the temperature distribution and dynamic heat leakage of a high-order mode coupler.

[0045] Figure 12 A schematic diagram of the boundary conditions for the temperature distribution simulation calculation of a high-order mode coupler;

[0046] Figure 13 Mechanical design drawings for high-order mode couplers. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other. To achieve the above objectives, this invention adopts the following technical solution.

[0048] This invention proposes a design method for high-order mode couplers based on a transmission line model with a dual-suppression structure. Building upon the traditional single-suppression structure for high-order mode couplers, this method proposes a dual-suppression structure based on the transmission line model. This design method is applicable to high-order mode couplers for superconducting cavities across all frequency bands. Designers can start from the theoretical transmission line model and, based on the suppression requirements for both the fundamental and higher-order modes, iteratively design various parameters of the transmission line model and corresponding three-dimensional structural parameters to optimize transmission performance, ultimately completing a high-order mode coupler that meets the suppression requirements for both the fundamental and higher-order modes. The dual-suppression structure significantly increases the fundamental mode suppression bandwidth, eliminates the need for fundamental mode tuning in practical applications, saves module assembly time, reduces system complexity, decreases economic expenditure, and greatly improves the safety of the superconducting module in actual operation.

[0049] The flowchart of the design method for high-order mode couplers based on the transmission line model and the dual suppression structure is as follows: Figure 4 As shown.

[0050] Taking a superconducting cavity with an operating frequency of 650MHz required for a large electron-positron collider as an example, the following details the specific process of the design method of this invention.

[0051] (1) Determine the fundamental mode frequency, higher-order mode bands, and external quality factor Qe requirements of the superconducting cavity:

[0052] The fundamental mode frequency of the superconducting cavity is 650MHz. The external quality factor requirement for the high-order mode coupler to suppress the fundamental mode is: Qe > 1×10⁻⁶. 11 The high-order mode coupler extracts the high-order mode bandwidth from 800 to 1400 MHz. The external quality factor requirement for high-order mode suppression is that the single-pole mode Qe is 1 × 10⁻⁶. 5 Dipole mode Qe in 1×10 4 .

[0053] (2) Establish the transmission line model:

[0054] Establish as Figure 5The equivalent transmission line model diagram shown has I0 on the far left representing a current source. Inductor L1n and capacitor C1n are connected in series to form the first parallel resonant fundamental mode suppression structure. This parallel resonant suppression structure is connected at the beginning of the main transmission line and is connected in parallel with subsequent transmission lines. After the first suppression structure, the main transmission line is connected in series with the equivalent inductance l1, and then in parallel with the equivalent inductance M. The other end of the equivalent inductance M is connected to the next transmission line with an equivalent inductance of l2, followed by a capacitor C2t and an equivalent inductance l3 connected in series. Inductor L2n and capacitor C2n are connected in series to form the second parallel resonant fundamental mode suppression structure, the size of which is given by the formula... The second suppression structure has equivalent inductors l3 and l4 connected in series before and after it, respectively. On the far right of the equivalent transmission line model diagram, capacitor Ct and matching load Z are connected in parallel, serving as the terminal load of the entire circuit. The equivalent inductors l1, l2, l3, and l4 correspond to the length of each transmission line segment. The equivalent inductance M is the cylindrical support inductance in the inner conductor of the higher-order mode coupler. C2t is the equivalent gap capacitance between transmission lines l2 and l3, and Ct is the equivalent capacitance at the extraction end of the inner conductor of the higher-order mode coupler.

[0055] (3) Transmission curve analysis and transmission line model optimization:

[0056] Using the Circuit & Systems module in the multiphysics simulation software CST STUDIO SUITE, a system was built. Figure 5 The equivalent transmission line model shown is used to adjust the circuit parameters and calculate the circuit transmission curve. . The scattering matrices of different circuit units can be obtained from the normalized scattering matrix theory, such as... Figure 6 As shown.

[0057] 1) Parallel admittance unit:

[0058] Circuit symbol: Parallel admittance express;

[0059] The corresponding normalized transfer matrix: ;

[0060] Parameter definition: It is the normalized admittance value of the parallel unit, which is equal to the actual admittance. With characteristic admittance The ratio, i.e. This matrix describes the relationship between the parallel admittance and the amplitude and phase of the transmitted signal.

[0061] 2) Series impedance unit:

[0062] Circuit symbol: series impedance express;

[0063] The corresponding normalized transfer matrix: ;

[0064] Parameter definition: It is the normalized impedance value of the series unit, which is equal to the actual impedance. With characteristic impedance The ratio, i.e. This matrix describes the relationship between the series impedance and the amplitude and phase of the transmitted signal.

[0065] 3) Transmission line segment unit:

[0066] Circuit symbol: Electrical length is Characteristic impedance Transmission lines;

[0067] The corresponding normalized transfer matrix: ;

[0068] Parameter definition: This is the normalized characteristic impedance value of the transmission line, set to 1 in the diagram, indicating that the characteristic impedance of the transmission line is equal to the system reference impedance. The electrical length of the transmission line is... The calculation formula is: ,in The phase constant, denoted as the physical length of the transmission line. This matrix describes the delay and phase shift effects of the transmission line on the signal.

[0069] 4) Cascading of circuit units:

[0070] Circuit symbol: , , ;

[0071] Parameter definition: , , These are the normalized scattering matrices of each sub-circuit unit, representing the input and output scattering characteristics of the corresponding unit.

[0072] The normalized scattering matrices (S matrix) are concatenated as follows: If the scattering matrices of each element are respectively , , By connecting different circuit units in series, the total transmission matrix after circuit cascading is obtained. It is a 2×2 matrix. This matrix is ​​obtained by multiplying the transfer matrices of each sub-unit in the circuit in sequence, and it completely describes the input-output relationship of the entire circuit. The normalized scattering matrix is ​​included. With scattering matrix The matrix element transformation relationships are shown below:

[0073] ;

[0074] in, It is a normalized scattering matrix used to characterize the scattering characteristics of the circuit port, and its elements are dimensionless amplitude ratios. It is the transmission matrix of the circuit, used to describe the voltage and current relationship between the input and output ports of the circuit. It is the characteristic impedance of the circuit input port (port 1), measured in ohms (Ω), which characterizes the impedance matching characteristics of the input port; The characteristic impedance of the circuit output port (port 2), measured in ohms (Ω), characterizes the impedance matching properties of the output port. The square root operation and impedance ratio in the formula are used to achieve the normalization transformation from the transmission matrix to the scattering matrix, ensuring the consistency of matrix elements under different port impedances.

[0075] The transmission parameters of the circuit are calculated using the following formula. :

[0076] ;

[0077] in, The forward transmission coefficient (insertion loss) of the circuit is expressed in decibels (dB), which characterizes the amplitude transmission characteristics of the signal from the input port (port 1) to the output port (port 2). The total transmission matrix of the radio frequency circuit is obtained by multiplying the transmission matrices of each sub-unit in the circuit in sequence, and is used to describe the input-output relationship of the entire circuit. , , , The total transmission matrix The elements in the first row and first column, the first row and second column, the second row and first column, and the second row and second column; in the formula, log is the common logarithm with base 10, the coefficient 20 is used to convert the linear amplitude ratio to decibels (dB), and the numerator 2 is a constant under the port impedance normalization condition.

[0078] By optimizing various circuit parameters, the optimized transmission curve was finally obtained. like Figure 7 As shown. At the dual-inhibition structure <-80dB, within the frequency range of 800-1400MHz >-10dB, optimized to meet requirements, proceed to the next step.

[0079] (4) Three-dimensional modeling and transmission curve analysis and optimization:

[0080] Depend on Figure 5 The transmission line model is obtained Figure 7 After optimizing the transmission curve, the parameters of the optimized transmission line model in CST STUDIO SUITE can be used to convert it into a corresponding three-dimensional structure, such as... Figure 8 The high-order mode coupler shown mainly includes: inner conductor 1-1, corresponding to... Figure 5 The first suppression structure C1n in the middle; inner conductors 1-2, corresponding to Figure 5 The first suppression structure L1n in the middle; inner conductors 1-3, corresponding to Figure 5 Transmission line l1; inner conductors 1-4, corresponding to Figure 5 Inductor M; inner conductors 1-5, corresponding to Figure 5 Transmission line l2; the aforementioned inner conductors 1-1 to 1-5 together form a hook-shaped inner conductor assembly for coupling the electromagnetic field within the superconducting cavity; inner conductors 1-6 and 1-7 correspond to... Figure 5 Transmission line l3; inner conductors 1-8, corresponding to Figure 5 The second suppression structures L2n and C2n; inner conductors 1-9, corresponding to Figure 5 Transmission line l4; the inner conductor of the ring is ceramic window 1-10, corresponding to Figure 5 Ct; Inner conductors 1-6, 1-7, and 1-9 form a complete inner conductor. Inner conductor 1-8 and inner conductor 1-7 are welded together at the middle position of 1-7. Ceramic window 1-10 is welded to inner conductor 1-9. Inner conductors 1-6 to 1-10 and ceramic windows together form a T-shaped inner conductor assembly for extracting coupled electromagnetic field energy. Flanges 2-1 and 2-7 are welded to outer cylinder 2-2. The hook-shaped inner conductor assembly is welded to outer cylinder 2-2 via inner conductor 1-4. Liquid helium tank 2-5 is welded to flanges 2-1 and 2-6. Liquid helium inlet pipe 2- 4 is welded to the liquid helium tank 2-5, and the liquid helium outlet pipe 2-3 is welded to the liquid helium tank 2-5; 2-1~2-6 together form the outer conductor part of the hook-shaped inner conductor assembly, and the inner conductor assembly is welded to the outer conductor part through the inner conductor 1-4 to form an integral coupling assembly; flange 2-7 is welded to the outer conductor cone 2-8 and the standard N-type connector 2-9 to form the outer conductor part of the T-shaped inner conductor assembly, and the outer conductor part and the T-shaped inner conductor are welded to the ceramic window 1-10 to form an integral extraction assembly; the integral coupler assembly and the cube extraction assembly are assembled through flange 2-6 and flange 2-7 to form a complete high-order mode coupler.

[0081] After the high-order mode coupler model is initially determined, the frequency domain solver of CST STUDIO SUITE is used to simulate and calculate the transmission characteristics of the high-order mode coupler. The local parameters of the high-order mode coupler are optimized based on the simulation results of the transmission curve until a transmission curve that meets the requirements is obtained.

[0082] (5) Three-dimensional model of high-order mode coupler and superconducting cavity modeling:

[0083] After optimizing the transmission characteristics of the higher-order mode coupler, CST STUDIO SUITE was used to model the higher-order mode coupler and a 650MHz superconducting cavity together. Then, an eigenmode solver was used for overall simulation calculations to obtain the external quality factor Qe of the higher-order mode coupler's suppression of both the fundamental and higher-order modes. Since the suppression effect of the higher-order mode coupler is only related to the inner conductor structure and the inner diameter of the outer conductor, the modeling process... Figure 8 The model shown is appropriately simplified, and the simplified high-order mode coupler and superconducting cavity model is as follows: Figure 9 As shown, the simulation model includes a 650MHz superconducting cavity, higher-order mode coupler 1#, and higher-order mode coupler 2#. Based on the calculation results, the local parameters of the higher-order mode couplers were adjusted, and the final optimized result satisfies the condition Qe > 1×10 for fundamental mode suppression. 11 The external quality factor Qe for suppression of unipolar modes in higher-order modes is 1×10 5 The magnitude, dipole mode Qe in 1×10 4 At this level, the high-frequency parameter design of the high-order mode coupler can be completed. The external quality factor Qe of the optimized 650MHz high-order mode coupler for the suppression of the fundamental mode and the high-order mode is shown in Table 1.

[0084] Table 1. Simulation results of the external quality factor Qe for suppression of fundamental and higher-order modes by the higher-order mode coupler.

[0085]

[0086] (6) Simulation and analysis of secondary electron multiplication effect:

[0087] Using CST STUDIO SUITE's particle tracking solver and Figure 10 The simplified simulation model shown is used for secondary electron multiplication simulation. To simulate the secondary electron multiplication effect, the materials must first be set. The materials of each part of the model are as follows: Figure 10 As shown in the left figure, the 650MHz superconducting cavity is made of high-purity niobium, and the outer cylinder of the high-order mode coupler is also made of high-purity niobium. Figure 8 The hook-shaped inner conductor components shown in 1-1 to 1-5 are made of high-purity niobium. Figure 8The T-shaped inner conductor components shown in Figures 1-6 to 1-9 are made of oxygen-free copper. During the simulation, the built-in material library in the CST software was used to import materials from the software's material list. These included niobium that had been baked at 300℃, niobium cleaned by argon (Ar) discharge, and copper materials using Ferman secondary emission probability models. For niobium, material parameters from two different post-processing methods were selected for simulation. When using the CST particle tracking solver for multi-particle secondary electron multiplication analysis, multiple suspected secondary electron multiplication sites are usually present. Therefore, a shell structure needs to be created around the higher-order mode coupler and an independent initial particle source needs to be configured. This allows for independent evaluation of the secondary electron multiplication effect on different surfaces. Figure 10 The right figure shows the four different particle sources selected for the secondary electron multiplication simulation.

[0088] The simulation solver was configured with an initial energy of 2 eV, energy dissipation of 200%, and an emission angle of 89° at the surface of each particle source region. The spatial position and energy changes of the particles under the influence of the electromagnetic field within the cavity were then tracked. The electric and magnetic field distributions of the superconducting cavity's fundamental mode, calculated using the eigenmode solver, were imported. The accelerating electric field gradient was set to scan from 2 MV / m to 20 MV / m, and the initial phase of the electric field under each gradient was scanned at 30° intervals, from 30° to 330°. Finally, the fastest growth rate of the particle curve in each electric field gradient was selected as the growth rate of the secondary electron multiplication. When secondary electron multiplication occurs, the change in the number of particles can be approximated by an exponential function over time, where the growth rate is the product of the exponential coefficient and time. The number of particles after the initiation of secondary electron multiplication can be expressed as: ,in It is the exponential growth rate coefficient (usually expressed in nanoseconds). -1 (in units) express The number of particles at time t. If A value greater than 0 indicates that secondary electron multiplication has occurred. In this case, local adjustments need to be made to the location where secondary electron multiplication occurred until... Until it is less than 0.

[0089] (7) Thermal load analysis using finite element software:

[0090] Thermal load analysis can be performed using finite element analysis software ANSYS or multiphysics simulation software CST STUDIOSUITE. The high-order mode coupler mainly consists of three parts: a niobium hook-shaped inner conductor coupling assembly, a copper-stainless steel T-shaped inner conductor extraction assembly, and a ceramic window. The thermal load analysis of the high-order mode coupler is divided into two parts: dynamic heat leakage calculation and static heat leakage calculation. Dynamic heat leakage calculation is an iterative process, employing... Figure 11The iterative process shown analyzes the temperature distribution and dynamic heat leakage calculation of the high-order mode coupler. The specific implementation steps are as follows:

[0091] 1) Initialization: Set the static initial temperature distribution of the device. , which serves as the initial value for temperature iteration.

[0092] 2) Calculate surface resistance based on the temperature distribution of the current iteration. Calculate the surface resistance of various parts of the device, and the surface resistance of non-superconducting materials such as copper and stainless steel. The calculation formula is:

[0093] ;

[0094] in, It is surface resistance, measured in ohms (Ω), which characterizes the AC resistance of the metal surface of a microwave device under a high-frequency electromagnetic field. Pi is a mathematical constant with a value of approximately 3.14159. It is the microwave operating frequency, measured in Hertz (Hz), which refers to the frequency of the electromagnetic signal when the device is actually working. It is the vacuum permeability, a physical constant, with a value of 4π × 10⁻⁶. -7 H / m (henry / meter); It is the relative permeability of a material, dimensionless, and is the ratio of the permeability of the metal material to the permeability of vacuum. It is the electrical conductivity of metallic materials, measured in Siemens per meter (S / m), which characterizes the material's ability to conduct electric current, and its value varies with temperature.

[0095] Niobium-based superconducting materials at temperature Surface resistance under certain conditions The calculation formula is:

[0096] ;

[0097] in, It is the surface resistance of BCS, measured in ohms (Ω). It is the surface resistance of a superconductor in the superconducting state (when the temperature is below the critical temperature) and is used to characterize the loss characteristics of a superconductor in a high-frequency microwave field. It is the microwave operating frequency, measured in Hertz (Hz), which refers to the frequency of the electromagnetic signal when the device is actually working. It is the actual operating temperature of the superconductor, and the unit is Kelvin (K). Kelvin (K) is the critical temperature of a superconductor, measured in Kelvin (K). It is the temperature at which a superconductor transitions from its normal state to its superconducting state. This formula is only applicable when... (i.e., superconducting state) is applicable; 17.67 is a fitting constant related to the intrinsic properties of superconducting materials. Its value is obtained by experimental testing and theoretical fitting, reflecting the dependence of the superconductor bandgap on temperature.

[0098] 3) Calculate the surface microwave loss based on the surface resistance obtained in step 2). and Surface microwave loss of various parts of the computing device The power dissipated by the metal part is:

[0099] ;

[0100] In the above formula It is the microwave dissipation power of the metal component, measured in watts (W). It is the surface resistance of the metal; Indicates the location on the metal surface Location, along the normal direction The tangential component of the magnetic field intensity, expressed in amperes per meter (A / m), represents the magnitude of the tangential magnetic field induced by the microwave field on the metal surface. Indicates surface position The square of the modulus of the tangential component of the magnetic field strength reflects the magnitude of the magnetic field energy density. The area vector of a metal surface, measured in square meters (m²), is used to integrate the area of ​​the entire metal surface.

[0101] The power dissipation of the ceramic part is:

[0102] ;

[0103] In the above formula It is the microwave dissipation power of the ceramic dielectric component, measured in watts (W). It is the loss tangent of ceramic materials, dimensionless, and is the core parameter characterizing the microwave loss characteristics of dielectric materials. Its value varies with temperature, frequency and material composition. It is the vacuum permittivity, a physical constant, with a value of 8.854 × 10⁻⁶. -12 F / m (fa / meter); It is the relative permittivity of the ceramic material, dimensionless, and is the ratio of the permittivity of the ceramic material to the permittivity of vacuum. It is the square of the modulus of the electric field strength, with units of volts² / meter² (V² / m²), reflecting the magnitude of the electric field energy density; It is a volume element of the ceramic medium, with the unit being cubic meters (m³), used to divide the entire ceramic medium region by volume.

[0104] 4) Apply a surface heat source to reduce the surface microwave loss obtained in step 3). As a surface heat source.

[0105] 5) Update the temperature distribution: Based on the surface heat source applied in step 4), recalculate the temperature distribution of the device. .

[0106] 6) Thermal conductivity iteration: Update the thermal conductivity of the device material according to the current temperature distribution to reflect the change of thermal conductivity with temperature.

[0107] 7) Convergence check: Calculate the difference between the current temperature distribution and the temperature distribution of the previous iteration. ,like Less than the preset convergence factor If the condition is met, the iteration terminates and the final steady-state temperature distribution is output; otherwise, the iteration terminates and the final steady-state temperature distribution is output. As a new iterative temperature distribution Return to step 2) and continue iterating until a steady-state temperature distribution is obtained. This temperature distribution result is used to determine whether an active liquid helium cooling scheme should be considered, either inside or outside the higher-order mode coupler.

[0108] The model profile used for thermal load simulation calculations of high-order mode couplers is as follows: Figure 12 As shown, this model is a dedicated model for thermal load simulation calculations, containing complete structural features and boundary condition settings. It can be used to accurately solve the temperature distribution of devices under low-temperature operating conditions. The main structural material composition required for the simulation calculation, from the microwave extraction port to the low-temperature cavity, is as follows from top to bottom:

[0109] A transducer section with an oxygen-free copper (RRR50) inner conductor, used to optimize microwave transmission characteristics;

[0110] B. Stainless steel (316L) outer cylinder provides mechanical support and vacuum isolation;

[0111] C. Ceramic window ring, achieving vacuum sealing and transmitting microwave signals;

[0112] DT type oxygen-free copper (RRR50) inner conductor, used for microwave transmission;

[0113] E. Oxygen-free copper (RRR50) inner conductor, used for microwave transmission;

[0114] F aluminum sealing rings are used for vacuum sealing of cavity flanges and high-order mold coupler flanges;

[0115] G-Niobium (RRR300) hook-shaped inner conductor, used for coupling electromagnetic field energy within the cavity;

[0116] H-type niobium (RRR300) outer cylinder provides mechanical support and vacuum isolation;

[0117] I. Stainless steel (316L) outer cylinder provides mechanical support and vacuum isolation;

[0118] The electrical and thermal conductivity parameters of different materials used in the simulation calculations at low temperatures are shown in Table 2. The thermal conductivity, electrical conductivity, and superconducting properties of each material in the model are consistent with those of the actual device to ensure the accuracy of the thermal load simulation calculations. The boundary conditions in this simulation are divided into two categories: surface heat sources and fixed temperatures, as shown in Table 3. Based on the above settings, multiphysics software is used for simulation analysis, according to... Figure 11 The calculation process shown can ultimately yield the heat loss and steady-state temperature distribution of each part of the high-order mode coupler.

[0119] Table 2. Material parameters of the high-order mode coupler used in thermal load simulation analysis

[0120]

[0121] Table 3 Boundary Conditions for Thermal Load Simulation Analysis Model

[0122]

[0123] (8) Mechanical Design:

[0124] The high-order mode coupler features a detachable design, primarily consisting of two parts: a high-purity niobium coupling hook, a niobium outer cylinder, a niobium-titanium flange, and a titanium helium tank; and a ceramic window, a copper-free T-shaped inner conductor, and a stainless steel flange. This design offers advantages such as lower manufacturing complexity, easier installation, and a compact structure. The niobium coupling hook and niobium outer cylinder, as well as the niobium outer cylinder and niobium-titanium flange, are connected using electron beam welding. The ceramic and oxygen-free copper, and ceramic and stainless steel interfaces are welded using brazing. The final mechanical design drawing is shown below. Figure 13 As shown, the key dimensions are as follows:

[0125] The total length from the bottom of the coupling end to the top of the extraction end of the high-order mode coupler is 295.53 mm;

[0126] The outer diameter of the stainless steel outer cylinder at the extraction end is 44.82 mm;

[0127] Stainless steel and niobium-titanium flange radius 69.5 mm;

[0128] The inner diameter of the niobium outer cylinder is 80 mm;

[0129] The distance from the top of the T-shaped oxygen-free copper inner conductor to the point where the inner conductor diameter changes is 56.66 mm;

[0130] The distance from the inner edge of the inner conductor of the niobium hook to the inner wall of the niobium outer cylinder is 14 mm;

[0131] The distance from the top to the bottom center of the inner conductor of the niobium hook is 116 mm.

Claims

1. A design method for a high-order mode coupler based on a transmission line model dual-suppression structure, characterized in that, Includes the following steps: Step 1: Determine the fundamental mode frequency, higher-order mode bandwidth, and external quality factor Qe requirements of the superconducting cavity. Clarify the fundamental mode frequency of the superconducting cavity, the external quality factor Qe requirements of the higher-order mode coupler for fundamental mode suppression, the bandwidth range for extracting higher-order modes by the higher-order mode coupler, and the external quality factor Qe requirements for higher-order mode suppression. Step 2: Establish the transmission line model, which includes a current source, a first parallel resonant fundamental mode suppression structure composed of inductor L1n and capacitor C1n connected in series, a transmission line with equivalent inductance l1, equivalent inductance M, a transmission line with equivalent inductance l2, capacitor C2t, a transmission line with equivalent inductance l3, a second parallel resonant fundamental mode suppression structure composed of inductor L2n and capacitor C2n connected in series, a transmission line with equivalent inductance l4, and a terminating load composed of capacitor Ct and matching load Z connected in parallel. Step 3: Transmission curve analysis and transmission line model optimization. By adjusting various parameters of the transmission line model, the circuit transmission curve S21 is calculated based on the normalized scattering matrix. Step 4: 3D modeling and transmission curve analysis and optimization. Based on the parameters of the optimized transmission line model, it is transformed into the corresponding 3D structure. A 3D model of the high-order mode coupler is established. The transmission characteristics of the high-order mode coupler are simulated and calculated using a frequency domain solver. The local parameters of the high-order mode coupler are optimized based on the transmission curve simulation results until a transmission curve that meets the requirements is obtained. Step 5: Modeling the 3D model of the higher-order mode coupler and the superconducting cavity. The higher-order mode coupler and the superconducting cavity are modeled together. The eigenmode solver is used to perform overall simulation calculation to obtain the external quality factor Qe of the higher-order mode coupler for suppressing the fundamental mode and higher-order modes. Based on the calculation results, the local parameters of the higher-order mode coupler are adjusted until the external quality factor Qe for suppressing the fundamental mode and the external quality factor Qe for suppressing the unipolar and dipole modes in the higher-order modes are satisfied, thus completing the high-frequency parameter design of the higher-order mode coupler. Step 6: Simulation analysis of secondary electron multiplication effect. The secondary electron multiplication simulation is performed using a particle tracking solver and a simplified simulation calculation model. Material parameters and particle source are set, and the electric and magnetic field distributions of the superconducting cavity fundamental mode calculated by the intrinsic mode solver are imported. Multi-particle secondary electron multiplication analysis is performed, and the growth rate of secondary electron multiplication is calculated until the growth rate is less than 0. Step 7: Finite element software thermal load simulation analysis, which includes dynamic heat leakage calculation and static heat leakage calculation. The dynamic heat leakage calculation adopts an iterative process: initialize and set the static initial temperature distribution, calculate the surface resistance of each part of the device based on the temperature distribution of the current iteration, calculate the surface microwave loss of each part of the device as the surface heat source based on the surface resistance, recalculate the temperature distribution of the device, update the thermal conductivity of the device material based on the current temperature distribution, calculate the difference between the current temperature distribution and the temperature distribution of the previous iteration and compare it with the preset convergence threshold. If the difference is less than the threshold, the iteration terminates and the final steady-state temperature distribution is output. Otherwise, it returns to continue the iteration. Based on the steady-state temperature distribution result, it is determined whether a liquid helium active cooling scheme is needed. Step 8: Mechanical Design. Based on the simulation analysis results of Steps 6 and 7, the mechanical structure of the high-order mode coupler is designed. A detachable design is adopted. The main body includes a coupling hook made of high-purity niobium, a niobium outer cylinder, a niobium-titanium flange, a titanium helium tank, as well as a ceramic window, an oxygen-free copper T-shaped inner conductor, and a stainless steel flange. The components are connected by electron beam welding and brazing processes to form a complete high-order mode coupler structure.

2. The design method for a high-order mode coupler based on a transmission line model dual-suppression structure according to claim 1, characterized in that, In the transmission line model described in step 2, the first parallel resonant fundamental mode suppression structure is connected to the front end of the main transmission line and connected in parallel with the subsequent transmission lines. The equivalent inductance M is the cylindrical support inductance in the inner conductor of the higher-order mode coupler. The capacitance C2t is the equivalent gap capacitance between l2 and l3, and the capacitance Ct is the equivalent capacitance at the extraction end of the inner conductor of the higher-order mode coupler.

3. The design method for a high-order mode coupler based on a transmission line model dual-suppression structure according to claim 1, characterized in that, The normalized scattering matrix mentioned in step 3 includes: establishing the normalized transmission matrix of parallel admittance unit, series impedance unit, and transmission line segment unit; obtaining the total transmission matrix by cascading circuit units; and then calculating the circuit transmission curve S21 by the transformation relationship between the normalized scattering matrix and the transmission matrix.

4. The design method for a high-order mode coupler based on a transmission line model dual-suppression structure according to claim 1, characterized in that, The three-dimensional structure described in step 4 includes a hook-shaped inner conductor assembly and a T-shaped inner conductor assembly. The hook-shaped inner conductor assembly is used to couple the electromagnetic field within the superconducting cavity, and the T-shaped inner conductor assembly is used to extract the coupled electromagnetic field energy. The hook-shaped inner conductor assembly is welded to the outer cylinder through the inner conductor to form an integral coupling assembly, and the T-shaped inner conductor assembly is welded to the outer conductor through a ceramic window to form an integral extraction assembly. The integral coupler assembly and the extraction assembly are assembled through a flange to form a complete high-order mode coupler.

5. The design method for a high-order mode coupler based on a transmission line model dual-suppression structure according to claim 1, characterized in that, In the secondary electron multiplication simulation described in step 6, a shell structure is created around the high-order mode coupler and an independent initial particle source is configured. The initial energy, energy dissipation, and emission angle are set for particle emission. The electric and magnetic field distributions of the superconducting cavity fundamental mode are imported. The accelerating electric field gradient is set to scan from 2MV / m to 20MV / m. Under each electric field gradient, the initial phase of the electric field is scanned from 30° to 330° every 30° interval. The fastest growth rate of the particle curve in each electric field gradient is selected as the growth rate of secondary electron multiplication.

6. The design method for a high-order mode coupler based on a transmission line model dual-suppression structure according to claim 1, characterized in that, The surface resistance calculation in step 7 includes: for non-superconducting materials such as copper and stainless steel, the surface resistance is calculated using the relationship between frequency, magnetic permeability, and electrical conductivity; for niobium-type superconducting materials, the BCS surface resistance is calculated using the relationship between frequency, temperature, and critical temperature when the temperature is below the critical temperature.

7. The design method for a high-order mode coupler based on a transmission line model dual-suppression structure according to claim 1, characterized in that, The calculation of surface microwave loss in step 7 includes: for the metal part, the area integral is calculated based on the square of the modulus of the surface resistance and the tangential component of the magnetic field strength; for the ceramic part, the volume integral is calculated based on the loss tangent, the vacuum permittivity, the relative permittivity, and the square of the modulus of the electric field strength.

8. A high-order mode coupler based on a transmission line model dual-suppression structure, characterized in that, include: The hook-shaped inner conductor assembly consists of a first inner conductor, a second inner conductor, a third inner conductor, a fourth inner conductor, and a fifth inner conductor. The first, second, third, fourth, and fifth inner conductors are arranged in a hook shape, with the fifth inner conductor serving as the handle. The upper end of the fifth inner conductor is a stepped plate. The fourth inner conductor is connected to the middle of the fifth inner conductor and extends horizontally in the opposite direction to the first inner conductor. The hook-shaped inner conductor assembly is used to couple the electromagnetic field within the superconducting cavity. The T-shaped inner conductor assembly consists of a 6th inner conductor, a 7th inner conductor, an 8th inner conductor, a 9th inner conductor, and a ceramic window. The 6th and 7th inner conductors are integrally formed into straight cylinders. The lower end of the 6th inner conductor is a stepped plate, corresponding to the upper end of the 5th inner conductor at a certain distance. The 8th inner conductor extends vertically outward from the portion formed by the 6th and 7th inner conductors. The 9th inner conductor is located above the 7th inner conductor and has a larger diameter than the 7th inner conductor. The ceramic window is located outside the 9th inner conductor. The 8th and 7th inner conductors are connected by welding at the middle position. The ceramic window is welded to the 9th inner conductor. The T-shaped inner conductor assembly is used to extract coupled electromagnetic field energy. The outer conductor portion of the hook-shaped inner conductor assembly consists of a first flange, an outer cylinder, a liquid helium outlet pipe, a liquid helium inlet pipe, a liquid helium tank, and a second flange. The first and third flanges are welded to the outer cylinder, the liquid helium tank is welded to the first and second flanges, the liquid helium inlet pipe is welded to the liquid helium tank, the liquid helium outlet pipe is welded to the liquid helium tank, and the hook-shaped inner conductor assembly is welded to the outer cylinder through a fourth inner conductor to form an integral coupling assembly. The outer conductor portion of the T-shaped inner conductor assembly consists of a third flange, an outer conductor cone, and a standard N-type connector. The third flange is welded to the outer conductor cone and the standard N-type connector to form the outer conductor portion of the T-shaped inner conductor assembly. The outer conductor portion and the T-shaped inner conductor assembly are welded together through a ceramic window to form an integral extraction assembly. The hook-shaped inner conductor assembly and the T-shaped inner conductor assembly achieve electromagnetic field coupling transmission through the equivalent capacitance formed between the 5th inner conductor and the 6th inner conductor plate; the overall coupling assembly and the overall extraction assembly are assembled through the 2nd flange and the 3rd flange to form a complete high-order mode coupler; the 1st inner conductor and the 2nd inner conductor are connected in series to form the first parallel resonant fundamental mode suppression structure, and the 8th inner conductor contains the second parallel resonant fundamental mode suppression structure, forming a dual suppression structure.

9. The high-order mode coupler according to claim 8, characterized in that, The hook-shaped inner conductor assembly is made of high-purity niobium. The sixth to ninth inner conductors in the T-shaped inner conductor assembly are made of oxygen-free copper. The outer cylinder is made of high-purity niobium. The liquid helium tank in the outer conductor part of the hook-shaped inner conductor assembly is made of titanium, and the first and second flanges are made of niobium-titanium. The outer conductor part of the T-shaped inner conductor assembly is made of stainless steel.

10. The high-order mode coupler according to claim 8, characterized in that, The connections between the first and second inner conductors, the second and third inner conductors, the third and fourth inner conductors, and the fourth and fifth inner conductors in the hook-shaped inner conductor assembly are all made using electron beam welding. The connections between the hook-shaped inner conductor assembly and the T-shaped inner conductor assembly, as well as the connections between the ceramic window and the ninth inner conductor and the outer conductor cone, are all made using brazing.

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