Design of RF shielding CF flange copper ring structure
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摘要: 高能电子加速器中，束流与真空室相互作用产生的尾场会引发束流不稳定性，此现象在高重频(>105 Hz)超导加速器中尤为明显。全金属加速器真空室大量使用的CF刀口法兰，其连接处截面突变是导致产生束流耦合阻抗的主要来源之一。设计了一种RF屏蔽型法兰-密封圈连接结构，其目的是通过实现法兰-密封圈-法兰预紧密封后的平滑过渡，有效减少阻抗。首先，采用3D电磁仿真CST软件对比仿真了连接过渡段不同径向台阶和轴向间隙参数下的阻抗效应，给出了相关参数的允许范围。然后，通过ANSYS软件对屏蔽法兰-铜圈结构进行了形变仿真，初步制定了不同型号的屏蔽密封圈的内径几何参数，在屏蔽法兰-密封圈的真空密封试验中，验证了预紧力矩≥6 N·m时即可实现有效的超高真空密封，并且通过屏蔽法兰-密封铜圈过渡段径向台阶和轴向间隙测试试验，得到了最优预紧力矩和屏蔽铜圈的关键尺寸参数。最后，采用对光滑真空管段、标准法兰-密封圈过渡段和屏蔽法兰-密封圈过渡段的功率损失和阻抗进行了仿真计算，验证了所设计的RF屏蔽型法兰-密封圈连接结构可以有效地实现阻抗屏蔽。Abstract:
Background High-repetition-rate electron accelerators face beam instabilities induced by wake fields from beam-vacuum chamber interactions. Geometric discontinuities at ubiquitous con flat (CF) knife-edge flange connections are a dominant source of beam-induced impedance in all-metal vacuum chambers.Purpose To mitigate this impedance, this paper presents the design of an RF-shielded flange-gasket connection structure that achieves a smooth post-tightening transition at the interface, thereby minimizing impedance.Methods Electromagnetic simulation: 3D simulations (CST) analyzed impedance effects of radial step heights and axial gaps at the transition, establishing allowable parameter ranges. Deformation simulation: ANSYS simulations modeled the shielded flange-copper gasket assembly to preliminarily determine inner diameter specifications for various gasket models. Vacuum sealing tests: Verified ultra-high vacuum integrity under applied tightening torque. Transition geometry testing: Measured the achieved radial step and axial gap post-tightening to define optimal copper gasket dimensions and tightening torque. Comparative simulation: CST simulations compared power loss and impedance for smooth chambers, standard flange-gasket transitions, and the proposed shielded transition.Results Electromagnetic simulations were used to define critical tolerance ranges for radial step and axial gap. Deformation simulations were utilized to provide initial gasket inner diameter specifications. Vacuum tests confirmed effective sealing at a tightening torque ≥6 N·m. Transition testing established the optimal tightening torque and key copper gasket dimensions ensuring minimal geometric discontinuity. Comparative simulations demonstrated that the RF-shielded flange-gasket transition significantly reduces power loss and impedance compared to a standard CF transition, achieving performance close to that of a smooth vacuum chamber.Conclusions The designed RF-shielded flange-gasket connection structure effectively minimizes geometric discontinuity at the joint. Through combined electromagnetic, mechanical, and vacuum testing, critical parameters, including radial step, axial gap, gasket dimensions, and tightening torque, were optimized.. Electromagnetic verification confirms this design provides effective impedance shielding, offering a solution to mitigate wake-field-induced instabilities at flange connections in high-energy accelerators.-
Key words:
- electron accelerator /
- high repetition rate /
- wakefield /
- beam coupling impedance /
- RF shielding
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图 1 法兰铜圈连接过渡段示意图
Figure 1. Schematic diagram of the transition section of the flange copper ring connection
图 2 法兰铜圈连接过渡段射频阻抗仿真
Figure 2. RF impedance simulation of vacuum flange transition with copper gasket
图 5 不同型号的屏蔽铜圈塑性变形仿真结果
Figure 5. Simulation results of plastic deformation of different types of shielded copper ring
图 9 不同预紧力矩下装置漏率结果
Figure 9. Leakage rate of the device under different fastening torques
图 10 高温烘烤后装置漏率检测结果
Figure 10. Leakage rate of the device after high temperature baking
表 1 不同型号屏蔽铜圈内外径加工尺寸
Table 1. Different types of shielded copper inner diameter and outer diameter processing size
types internal diameter/mm external diameter/mm DN35 $35_{0.02}^{0.04} $ $48.3_{-0.011}^{0} $ DN50 $50_{0.04}^{0.06} $ $61.6_{-0.013}^{0} $ DN63 $63_{0.06}^{0.08} $ $82.5_{-0.015}^{0} $ DN100 $100_{0.08}^{0.16} $ $120.6_{-0.018}^{0} $ 
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表 2 屏蔽法兰-密封圈连接过渡段径向台阶高度
Table 2. The height of the radial steps of the copper ring flange connection transition section
moment/(N·m) ΔD/μm sample 1 sample 2 sample 3 sample 4 sample 5 10 −58 −50 −55 −56 −58 11 −40 −43 −37 −40 −39 12 −30 −38 −29 −30 30 13 −15 −25 −10 −15 −10 14 −5 −10 −0 −5 −0 15 10 5 5 10 9 16 20 0 10 20 15
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表 3 铜圈法兰连接过渡段轴向间隙
Table 3. The copper ring flange connects the axial clearance of the transition section
ΔL/mm position/m ΔD=0.4 mm ΔD=0.45 mm ΔD=0.5 mm screw side nut side screw side nut side screw side nut side 1 0.2 0.17 0.15 0.16 0.1 0.084 2 0.2 0.18 0.14 0.17 0.09 0.088 3 0.2 0.19 0.13 0.16 0.062 0.088 4 0.18 0.2 0.13 0.19 0.058 0.118 5 0.18 0.18 0.13 0.18 0.068 0.118 6 0.18 0.18 0.14 0.18 0.068 0.118 7 0.2 0.18 0.16 0.18 0.096 0.11 8 0.2 0.17 0.16 0.18 0.1 0.09
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