Application of coaxial wire method in impedance measurement of small aperture vacuum component
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摘要: 粒子加速器中真空元件的阻抗是引起束流不稳定的重要原因。基于储存环的新一代同步辐射光源的设计发射度更小，相应地要求更小的真空盒孔径，进而带来阻抗的显著增加，这就要求在设计阶段对真空元件的阻抗进行准确的评估和优化。阻抗测量是验证阻抗模型准确性的重要手段，而同轴线法是常用的实验室测量方法。对小孔径真空元件同轴线法纵向阻抗测量进行了研究，针对窄带阻抗元件，使用pillbox腔开展了相关的阻抗测量，研究了不同的内导体尺寸对于测量结果的影响，同时基于尾场模拟、散射参数模拟以及本征模模拟对测量结果进行了验证，模拟和测量结果符合很好，并证明原有的内导体导致谐振峰频移的理论分析应用于小孔径元件时存在偏差。此外针对非窄带阻抗元件，对条带冲击磁铁结构进行了同轴线法阻抗测量，研究了内导体对结果的影响，验证了同轴线法测量的有效性。Abstract: In particle accelerators, the impedance of vacuum components is an important cause of beam instabilities. Smaller emittance is required in the new generation storage-ring-based light source, where smaller vacuum chamber is needed and the impedances are increased correspondingly.Therefore, the impedance of the vacuum components should be evaluated accurately and well optimized during the design stage. Impedance measurement is an important way to check the impedance model. The coaxial wire method is commonly used for laboratory bending measurements. For small-aperture vacuum components, the measurement error caused by the inner conductor is studied in this paper. For narrow-band impedances, pillbox cavity is used to study the influence of different inner conductor radius on the measurement results. As benchmarks, different numerical simulations, such as wake field, scattering parameters and eigenmodes, are carried out. The results show that the simulations are in good agreement with the measurement. It is also found that there exist deviations between the measurement and the existing theories on the dependence of the resonant frequency shift on the inner conductor radius. For non-narrowband impedances, we measured the impedance of a strip line kicker and studied the influence of the inner conductor, which verified the validity of the coaxial wire method.
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图 1 Pillbox腔同轴线法阻抗测量装置
Figure 1. Coaxial wire measurement setup for the pillbox cavity
图 2 采用不同内导体直径测量得到的pillbox腔纵向阻抗实部和虚部
Figure 2. Real part and imaginary part of the longitudinal impedance of the pillbox cavity measured with different inner conductor diameter
图 3 CST尾场求解器模拟计算的pillbox腔纵向阻抗实部和虚部
Figure 3. Real part and imaginary part of the longitudinal impedance of pillbox cavity simulated by CST Wakefield Solver
图 4 散射参数模拟中采用的三维模型，其中束管两端设置为波导端口
Figure 4. 3D model used for the simulation of scattering parameters, where the waveguide ports have been set at both ends of the beam pipe
图 5 模拟得到不同内导体尺寸下散射参数
Figure 5. Simulated scattering parameters with different inner conductor diameter
图 7 条带冲击磁铁同轴线法阻抗测量装置
Figure 7. Coaxial wire measurement setup for the stripline kicker
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