Experimental study on the tuning of a very-high-frequency photocathode electron gun
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摘要: 甚高频光阴极微波电子枪是一种工作于连续波模式的电子源，用于产生MHz级高重频、高品质的电子束，是高重频X射线自由电子激光、高重频超快电子衍射等科学装置的关键核心部件。在运行过程中，电子枪腔体谐振频率随着馈入功率及冷却水温度变化而变化，因此需要对电子枪频率进行实时测量并通过调谐器对腔体频率进行调整以保障腔体内微波场幅度和相位的稳定。基于LCR振荡电路模型，分析了腔体取样微波与入射微波的相位差与电子枪谐振频率的关系，采用了频率扫描、频率跟踪和主动调谐的电子枪腔体三步调谐方法，成功应用于清华大学甚高频电子枪调谐和微波幅度相位控制。通过该调谐方法，可以使电子枪在高功率运行时始终保持谐振状态。电子枪在满功率运行下腔体谐振频率偏差控制在94.2 Hz （RMS），腔体微波采样口的幅值稳定性达到
0.0046 % （RMS），锁相精度达到0.0023 ° （RMS），实现电子枪在满功率下长时间稳定运行。Abstract:Background The very-high-frequency (VHF) photocathode electron gun operates in continuous-wave mode and serves as a critical electron source for generating high-repetition-rate, high-quality electron beams. It is widely used in advanced scientific facilities such as X-ray free-electron lasers and ultrafast electron diffraction systems. However, during operation, resonant frequency shifts caused by variations in feed power and cooling water temperature can destabilize the radio-frequency (RF) field inside the cavity.Purpose This study aims to achieve stable amplitude and phase control of the RF field in a VHF electron gun under high-power continuous-wave operation by accurately tracking and tuning the resonant frequency of the cavity in real time.Methods Based on an LCR oscillator circuit model, the phase difference between the cavity-sampled microwave and the incident wave was analyzed to determine the cavity's resonant frequency. A three-step tuning strategy—comprising frequency scanning, frequency tracking, and active tuning—was implemented and applied to a VHF electron gun at Tsinghua University.Results Using the proposed tuning method, the electron gun maintained resonance during high-power operation, with a resonant frequency deviation controlled at an RMS value of 94.2 Hz under full power. The amplitude stability at the microwave sampling port reached an RMS value of0.0046 %, and the phase-locking accuracy achieved an RMS value of0.0023 °. These results enabled long-term, stable full-power operation of the electron gun.Conclusions The developed three-step active tuning method effectively ensures high amplitude and phase stability for the VHF photocathode electron gun under continuous-wave operation, providing a reliable tuning solution for high-repetition-rate accelerator-based light sources and scientific instruments.-
Key words:
- very high frequency /
- electron gun /
- low-level radio frequency /
- tuning
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图 1 甚高频光阴极电子枪的LCR等效电路图
Figure 1. LCR Equivalent Circuit Diagram of a Very High Frequency Photocathode Electron Gun
图 5 电子枪幅值相位特性扫频曲线，其中蓝色实线为LLRF测量值，红色虚线为理论值
Figure 5. Sweep-frequency curve of the VHF gun’s amplitude-phase characteristics, where the blue solid line represents the LLRF measured values, and the red dashed line represents the theoretical values
图 7 电子枪主动调谐后调谐器受力和微波取样探针与入射波相位差
$ {\Delta \phi } $ 随时间的变化Figure 7. The variation of the force on the tuner and the phase difference
$ {\Delta \phi } $ between the RF probe and the incident wave over time after active tuning of the VHF gun
图 8 电子枪主动调谐后24 h微波幅值稳定性测试
Figure 8. Microwave amplitude stability test in 24 h after active tuning
图 9 电子枪主动调谐后24 h微波相位稳定性测试
Figure 9. Microwave phase stability test in 24 h after active tuning
图 10 电子枪主动调谐后24 h谐振频率稳定性测试
Figure 10. Gun resonant frequency stability test in 24 h after active tuning
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