RF design of C-band photocathode electron gun for Southern Advanced Photon Source
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摘要: 作为南方先进光源直线段注入器重要设备,开展了C波段光阴极电子枪研究,包括驻波腔微波设计和耦合器设计。其中驻波腔采用3.6腔结构,π模加速模式,工作频率为5.712 GHz;耦合器采用同轴耦合方式。利用Superfish及CST完成了腔体微波结构设计,优化盘片的形状,降低腔体表面最大电场,从而有利于提高腔体加速场强;利用COMSOL开展了腔体水冷系统的分析,优化设计水路,减少腔体由于功率负载所造成的频率偏移, 控制腔体温度的上升,保持腔体最大温升小于20 ℃。在18.15 MW的入腔功率下,阴极面最高场强为180 MV/m,腔体表面最大场强与阴极面场强比值约为
0.9346 ,腔体Q值大于10 000。通过对耦合器的设计,抑制二极模和四极模的传输,S11参数小于−40 dB。Abstract: The C-band photocathode electron gun is designed. As one of important equipments for the linear injector of the Southern Advanced Photon Source. This paper discusses its RF and coupler design. The C-band electron gun has a working frequency of 5.712 GHz, a 3.6-cell structure, a π-mode acceleration mode, and it adopts a coaxial coupling method. The CST and Superfish codes are used to optimize the cavity microwave structure, reducing the surface electric field of the cavity to enhance the accelerating field strength and suppressing the multimode transmission. Furthermore, COMSOL Multiphysics is applied to analyze the cavity cooling system, reducing frequency drift caused by the cavity heating. Additionally, a water cooling design for the cavity ensures that the maximum temperature rise of the cavity is less than 20 ℃. Under an input power of 18.15 MW, the acceleration gradient of 180 MV/m on the cathode surface is achieved, the ratio of the acceleration gradient to the cathode surface electric field is approximately 0.93, and the cavity quality factor is greater than10000 . The design of the coupler suppresses the transmission of dipole and quadrupole modes, with the S11 parameter being less than −40 dB.-
Key words:
- Southern Advanced Photon Source /
- photocathode electron gun /
- RF structure /
- coupler /
- C-band
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表 1 电子枪仿真微波参数
Table 1. RF parameters of electron gun
a/b a/mm b/mm mode separation/MHz Esurf/Ecath effective shunt impedance/(MΩ/m) Q value power/MW 0.50 5.0 10.0 24.81 0.9470 48.685 11575.1 17.58 0.55 5.5 10.0 21.837 0.9346 46.775 11252.3 18.15 0.60 6.0 10.0 21.937 0.9353 44.467 10901.5 18.83 0.70 6.0 8.57 17.829 0.9614 43.687 10691.0 19.00 表 2 腔体结构各尺寸敏感度分析
Table 2. Cavity dimension and sensitivity
size/mm sensitivity/(MHz·mm−1) cavity diameter 45.053 140.800 beam tube diameter 18.000 50.400 single cell length 26.242 42.400 semi short axis 5.500 14.100 semi long axis 10.000 22.100 表 3 不同角度下X、Y方向束流的发射度
Table 3. X and Y direction beam emittance at different angle
angle of pickup/(°) emittance of X direction/(mm·mrad) emittance of Y direction/(mm·mrad) no pickup 0.198 0.187 0 0.210 0.228 30 0.223 0.203 60 0.207 0.199 90 0.219 0.195 表 4 腔体入水温度的变化对腔体结构及微波参数的影响
Table 4. Water temperature and RF parameters
Twater/℃ ΔTcavity/℃ ΔDcavity/μm Δf/MHz flatness of E-field/% 25.0 14 7 −1.3 1 22.5 12 6 −1.1 2 20.0 10 5 −0.9 3 -
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