Beam diagnostics of a C-band photocathode RF gun
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摘要: C波段光阴极微波电子枪凭借超高加速梯度（>150 MV/m），成为第四代光源获取高亮度电子束的核心技术路线。然而，其输出束流具有ps级超窄脉冲、大动态范围电荷量（50～
2500 pC）及极低横向发射度0.18 mm·mrad@100 pC，现有基于L/S波段的测量手段难以满足其测量精度与带宽要求。为此，本文依托针南方先进光源（SAPS）测试平台，研制了一套适配C波段电子枪特性的高精度束流测量系统。针对窄脉冲电荷测量难题，自主研制了法兰式有源积分型电荷探测器（Active-ICT），提出基于商用高灵敏度探头的交叉标定方法，实现了优于±1% FS的测量线性度；针对极小发射度测量中空间电荷力影响显著的问题，通过Astra模拟优化了双缝发射度仪的狭缝参数与漂移距离，在0.15～0.25 mm·mrad范围内将系统误差控制在10%以内；为解决能散测量中的噪底干扰，设计了双缝准直结合扇形二极铁的能散测量光路。利用该系统开展了初步束流实验，结果表明：光电流与暗电流测量结果与法拉第筒吻合良好，不同加速梯度下的束流能量测量曲线与动力学模拟高度一致，验证了测量系统的可靠性与测量精度。本研究工作解决了国内C波段光阴极微波电子枪调试中的关键束诊技术难题，为同类高梯度注入器的工程研制提供了核心技术支撑。Abstract:Background The C-band photocathode radio frequency (RF) electron gun, with an ultra-high accelerating gradient exceeding 150 MV/m, is a key technology for generating high-brightness electron beams in fourth-generation light sources. However, its output beam features picosecond-scale ultrashort pulses, a wide charge dynamic range of 50 pC to2500 pC, and an ultra-low transverse emittance of 0.18 mm·mrad@100 pC. Existing measurement methods developed for L/S-band systems can hardly meet the stringent requirements on measurement accuracy and bandwidth for such beams.Purpose This study aims to develop a high-precision beam measurement system adapted to the characteristics of the C-band electron gun, based on the test platform of the South Advanced Light Source (SAPS).Methods Firstly, a flange-mounted active integrating charge transformer (Active-ICT) was independently developed to address the challenge of narrow-pulse charge measurement, and a cross-calibration method based on a set of commercial high-sensitivity ICT and terminator was proposed, achieving a measurement linearity better than ±1% full scale (FS). Secondly, to mitigate the significant influence of space charge force in ultra-low emittance measurement, the slit parameters and drift length of the double-slit emittance meter were optimized via Astra simulation, confining the systematic error within 10% in the emittance range of 0.15～0.25 mm·mrad. Thirdly, an optical path combining double-slit collimation and a sector dipole magnet was designed to suppress noise floor interference in energy spread measurement.Results Preliminary beam experiments were conducted with the established system. The results show that the measured photocurrent and dark current are in good agreement with Faraday cup measurements, and the beam energy curves obtained under different accelerating gradients are highly consistent with beam dynamics simulation results, verifying the reliability and measurement accuracy of the system.Conclusions This work solves the key beam diagnostics technical bottlenecks in the commissioning of domestic C-band photocathode RF electron guns, and provides core technical support for the engineering development of similar high-gradient injectors. -
图 1 C 波段光阴极微波电子枪束流诊断光束线束测设备布局图
Figure 1. Beam diagnostics layout of the C-band RF electron gun with a photocathode
图 5 Bergoz ICT探头与自研两款ICT探头的标定曲线和拟合公式
Figure 5. Calibration of Bergoz ICT sensor and two sets of self-designed ICT sensors
图 9 Monte-Carlo模拟得到的能散测量系统电子通量二维分布
Figure 9. Monte-carlo simulation of the lectron flux distribution in the energy spread monitor
图 10 第二缝出射的杂散电子能谱
Figure 10. Simulation of the stray electron energy spectrum at the second slit exit
图 11 模拟的荧光靶成像及其剖面拟合结果
Figure 11. Simulation of the beam spot and profile at the YAG screen of double-slit beam energy spread monitor
图 12 电子束法拉第筒结构示意图
Figure 12. Structure of the faraday cup in C-band photocathode electron gun test platform
图 13 不同材料（铜、石墨、铝）收集体对应的背散射电子能谱
Figure 13. Back scattered electron spectrum in different materials (copper, graphite and aluminum)
图 15 C波段光阴极微波电子枪测试平台首套束流测量系统安装完成
Figure 15. Installation of the first beam instruments of C-band photocathode electron gun test platform
图 16 利用法拉第筒（a）和带电子学法拉第筒（b）分别对电子枪暗电流的测量
Figure 16. Dark current measured by faraday cup without(a) / with (b) electronics
图 17 利用ICT和FCT同时对光电流和暗电流进行测量的波形
Figure 17. Beam current waveforms of faraday cup and ICT with electronics
图 18 不同加速相位下，利用ICT和FCT测量光电流电荷量的结果对比
Figure 18. Normalized photoelectron bunch charge measured by faraday cup and ICT under phase scanning
图 19 不同加速梯度下，扫描加速相位对应的束流能量测量结果与仿真的对比
Figure 19. Beam energy measurements versus scanned acceleration phase under different accelerating gradients
表 1 C波段光阴极微波电子枪束流参数[7]
Table 1. Beam parameters of the C-band RF electron gun with a photocathode
Parameter Unit Value Parameter Unit Value RF frequency GHz 5.712 Bunch Charge (min.) pC 100 Accelerating gradient MV/m 150 Transverse emittance mm-mrad 0.175 Repetition rate Hz 1-100 Bunch length ps 5 Beam energy at the gun exit MeV 7.3 Beam rms size μm 42.5 
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表 2 不同光电子束团电荷量对应的束流物理参数
Table 2. Beam parameters for different electron bunch charge
bunch charge/pC normalized Emittance/(mm·mrad) RMS beam size/mm RMS beam spread/mrad energy/MeV 100 0.175 0.0425 0.369 7.26 300 0.323 0.0793 0.565 7.25 500 0.463 0.112 0.792 7.24 1000 0.845 0.215 1.27 7.22 1500 1.16 0.297 1.64 7.19 2000 1.36 0.379 1.88 7.11
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表 3 ICT探头等效电路对应的参数值
Table 3. Beam parameters of the C-band RF electron gun with a photocathode
symbol meaning value symbol meaning value IB beam current 20～400 A L3、L4 secondary leakage inductance negligible C1 capacitance in loop 2 20*180 pF R1 third leakage inductance negligible CL capacitance in loop 3 360 pF R2 equivalent resistance of toroid 1 1.75 Ω@100 kHz L1 inductance of toroid 1 4.86 μH RL equivalent resistance of toroid 2 33 Ω@100 kHz L2 inductance of toroid 2 121.5 μH N turns of toroid 2 5
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表 4 不同电荷量对应的光电流束团参数
Table 4. Beam parameters of the C-band RF electron gun with a photocathode
charge/pC beam spot size/mm repetition rate/Hz average power/mW peak power/mW average intensity/nA peak intensity/A 100 0.6 100 70 144.033 10 20.576 1500 2.5 10 105 2160.494 15 308.642 2500 3.5 10 175 3600.823 25 514.403
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表 5 不同材料（铜、石墨、铝）收集体背散射电子的逃逸概率
Table 5. Escape probability of the backscattered electrons for different materials (copper, graphite, aluminum)
material escape possibility/% total possibility/% copper 0.863 ～1.0 graphite 0.068 0.12 aluminum 0.161 0.25
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