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减小空间电荷效应及扫描偏转系统边缘场效应引起的时间弥散是实现百飞秒级时间分辨条纹管的关键.本文提出并设计了一种新型飞秒条纹管,结合超高加速电场、高扫描速度和后加速电场的设计,可在光电阴极4 mm×10 μm的范围内实现100 fs量级的时间分辨率.通过优化设计加速电极结构,使光电阴极有效探测范围内的电子均可在15 kV/mm量级的强电场中加速运行,有效地抑制了电子脉冲的物理时间弥散;在阳极入口处放置窄狭缝以减小大角度光电子引起的时空弥散对性能的影响;最后在荧光屏处设置+5000 V的高电位,以缩短光电子在等位区的渡越时间,进一步减小空间电荷效应引起的时间弥散.最终,此设计方法能够将条纹管的时间分辨率提高至百飞秒量级.Reducing the space charge effect and the time dispersion caused by the edge field effect of the scanning deflection system is the key to realize the 100-femtosecond streak tube. In this paper, a novel femtosecond streak tube is proposed and designed. The factors affecting the temporal resolution are analyzed theoretically and the specifications are given. Parameters including the electric field distribution and electron transmittance on the two common acceleration system structures (planar cathode -mesh accelerating electrode and planar cathode – slit accelerating electrode) are compared and analyzed theoretically. The results show that although the electric field distribution formed by the planar cathode – mesh accelerating electrode could form uniform electric field, the electron transmittance is very low; planar cathode-slit accelerating structure would defocus the photoelectron beam along the scanning direction, but the electron transmittance in the effective detection range of the cathode is as high as 100%. The defocusing of the photoelectron beam can be removed by setting a narrow slit in front of the anode. The focusing electrode adopts two groups of plate-like structures which are vertically placed front and back, forming one-dimensional focusing electric fields along the scanning and the slit direction, respectively. The spatial focusing electrode is placed close to the phosphor screen, which is beneficial to push back the cross-point of the electron beam along the spatial direction. Thus, the electron transit time dispersion in the condition of large electron density would decrease. At the same time, the anode can provide a post-accelerating voltage of +5000 V, which is beneficial to shorten the transit time and dispersion of the photoelectrons, thereby improving the temporal resolution. Based on the above theoretical analysis, a novel femtosecond streak tube is designed by using planar cathode-slit accelerating electrode, anisotropic focusing system and post-accelerating method. The influence of the anode slit width on the spatial and temporal resolution is simulated. The results show that the temporal resolution deteriorates with the increase of the anode slot width (10 μm ~ 50 μm), due to the increase of the anode slit width will lead to the gradual increase of the size of the electron spot along the scanning direction, which would lead to the increase of the technical time dispersion. In addition, this study gives the simulation results of the femtosecond streak tube when the anode slit width is in the range of 10~50 μm. The results show that the static spatial resolution is higher than 100 lp/mm @ MTF=10%, dynamic spatial resolution is higher than 29 lp/mm @ MTF = 10%, the temporal resolution is better than 122 fs in the range of 4 mm cathode effective detection length. When the effective detection length of the cathode is increased to 8 mm, the dynamic spatial resolution of the streak tube tube is higher than 22 lp/mm @ MTF=10%, and the temporal resolution is better than 191 fs.
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Keywords:
- Anisotropy focusing technology /
- Post-accelerating technology /
- Temporal resolution /
- femtosecond streak tube
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