Multi-physical field coupling of C-band photocathode electron gun
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摘要: C波段光阴极微波电子枪是南方先进光源自由电子激光加速器的关键设备。针对电子枪在高功率运行下，因腔体内表面微波电磁损耗引起温升，进而导致腔体结构受热形变和谐振频率漂移的问题，通过多物理场耦合分析方法探究其内在机理，基于COMSOL Multiphysics®仿真平台构建电磁-热-结构耦合模型，首先通过高频电磁场仿真，得到真空腔体5.712 GHz的设计谐振频率；继而通过计算腔壁的电磁损耗功率密度建立等效边界热源模型，结合电子枪外部机械结构及冷却管路模型，采用流-固耦合方法得到真空腔体表面不均匀的温度分布；最终通过固体力学接口计算腔体几何形变分布，并使用此形变分布作为二次高频仿真的初始条件得到频率漂移结果。实现了电磁场、温度场与结构场的多物理场耦合建模，完整揭示了微波功率加载导致腔体谐振频率漂移的传递路径。该方法有效克服了传统单物理场分析在耦合效应表征方面的不足，为高精度微波腔体热-力耦合设计提供了有效的数值分析框架。Abstract:
Background The C-band photocathode electron gun is a key front-end device of the accelerator for the Southern Light Source Free-Electron Laser, whose resonant frequency stability is crucial for beam quality and long-term operation. During high-power microwave excitation, electromagnetic power loss on the inner surfaces of the resonant cavity produces non-uniform thermal loading, leading to structural deformation and subsequent resonant frequency drift, which cannot be accurately characterized by traditional single-physical-field analyses.Purpose To clarify the intrinsic mechanism of this phenomenon, a comprehensive electromagnetic–thermal–structural multi-physical field coupling model is developed based on the COMSOL Multiphysics® simulation platform.Methods First, high-frequency electromagnetic simulations are carried out to obtain the designed resonant frequency of the vacuum cavity at 5.712 GHz and to calculate the surface electromagnetic loss power density. Based on these results, an equivalent boundary heat source model is established. Combined with the external mechanical structure and cooling pipline model of the electron gun, the non-uniform temperature distribution of the cavity under realistic cooling conditions is obtained by employing a fluid–solid coupling method. Subsequently, the solid mechanics interface is used to compute the thermally induced deformation of the cavity geometry, and the deformed structure is introduced into a secondary high-frequency simulation to evaluate the resulting resonant frequency drift.Results The results reveal a clear transmission path from microwave power loading to temperature rise, structural deformation, and frequency shift, quantitatively demonstrating the strong coupling among electromagnetic, thermal, and mechanical fields.Conclusions This study realizes a complete multi-physical field coupling analysis of the C-band photocathode electron gun and provides an effective numerical framework for predicting resonant frequency drift, offering important guidance for the thermal–mechanical coupling design and frequency stability optimization of high-precision microwave cavities. -
图 2 C波段光阴极电子枪腔体冷却耦合传热模型
Figure 2. Cooling coupled heat transfer model of C-band photocathode electron gun
图 4 C波段光阴极电子枪腔体内表面温度分布和形变分布
Figure 4. Surface temperature distribution and displacement distribution within RF gun cavity
图 5 水温对腔体温度、形变及频率的影响
Figure 5. Effects of water temperature on cavity temperature, displacement and frequency
图 6 水流速对腔体温度、形变及频率的影响
Figure 6. Effects of flow rate on cavity temperature, displacement and frequency
图 7 占空比对腔体温度、形变及频率的影响
Figure 7. Effects of duty cycle on cavity temperature, displacement and frequency
表 1 材料热物性参数
Table 1. Thermophysical properties of materials
thermal expansion
coefficient/(W·m−1·K−1)specific heat capacity at constant
pressure/(J·kg−1·K−1)Density/
(kg·m−3)thermal conductivity/
(W·m−1·K−1)1.7×10−5 385 8960 400 
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