CSNS-Ⅱ低β超导椭球腔的电磁设计

Electromagnetic design of low β superconducting elliptical cavity for CSNS-Ⅱ

为中国散裂中子源二期工程(Chinese Spallation Neutron Source-Ⅱ,CSNS-Ⅱ)设计了一种低β超导椭球腔,利用2D程序SUPERFISH对该腔体的单元尺寸和高频等设计参数进行优化,获得的工作频率为648 MHz,几何β(βg)为0.47,单元个数为5,峰值电场比Epk/Eacc、峰值磁场比Bpk/Eacc和轴向电场平整度分别为3.35、6.1 mT·(MV/m)^-1和98.42%。通过2D软件MultiPac和3D软件CST粒子工作室(Computer Simulation Technology Particle Studio,CST PS)分析了优化后腔型的二次电子倍增情况,2D结果显示在峰值场0~80 MV·m^-1区间,二次电子发射系数小于1,表明整个腔体不会发生Multipacting(MP)效应。与2D结果不同,3D结果表明在椭球赤道位置存在MP效应,但在赤道处引入一个小凸起,并进行适当的后处理和高功率老练,则可以显著抑制MP效应。与国际上同类型椭球腔进行对比,主要高频性能参数基本相当。

[Background] A low β superconducting elliptical cavity was designed for the Chinese spallation neutron source-Ⅱ(CSNS-Ⅱ) project. [Purpose] This study aims to optimize the electromagnetic design of this cavity to its parameters and evaluate its secondary electron multiplication. [Methods] The 2D program SUPERFISH was used to optimize the cell size and high-frequency parameters of the cavity. The operating frequency of the cavity was 648 MHz, the geometry β(βg) was 0.47, and the number of cells was 5. Then, 2D software "Multipac" software and 3D software "computer simulation technology particle studio"(CST PS) were employed to analyze the secondary electron multiplication of the optimized cavity. [Results] Optimization design results show that the peak electric field ratio Epk/Eacc, the peak magnetic field ratio Bpk/Eaccand the axial electric field flatness are 3.35, 6.1 mT·(MV/m)^-1 and98.42%, respectively. The results 2D software show that in the peak field range from 0 to 80 MV·m^-1, the secondary electron emission coefficient is less than 1, indicating that Multipacting(MP) effect does not occur. However, the results of 3D software illustrate that the MP does exist at the equator of the ellipse. A small convexity induced at the equator, as well as proper post-processing and high-power conditioning, are shown significant suppression of MP.[Conclusions] Compared with the same type of low β elliptical cavity in the world, the main high-frequency performance parameters of designed low β superconducting elliptical cavity for CSNS-Ⅱ are basically equivalent.