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由于空间电荷效应的限制, 产生百飞秒的极短电子脉冲是超快电子衍射技术的一大难点. 同时, 电子的穿透深度随着电子能量的增加而增加, 而电子的散射几率却具有相反的规律. 因而, 除了时间分辨的提升, 还需要可宽范围调节的电子能量以优化不同厚度样品对其的需求. 基于此, 提出并设计了一种新型超紧凑电子枪, 结合均匀场阴极和可移动阳极的配置, 可在10—125 kV加速电压范围内实现100 fs量级时间分辨率. 通过优化设计高压电极轮廓, 使得其轴上和整个阴极面的场增强因子在不同阴阳极间距下均小于约4%, 从而保证了不同加速电压下最大轴上场强均可达10 MV/m量级, 有效地抑制了电子脉冲的展宽效应; 进一步将阳极小孔设计成可放置致密电镜载网的阶梯孔, 一方面可将载网支撑的样品紧贴小孔后方放置, 最大程度上缩短了电子从阴极到样品的时间弥散, 同时也可以有效地减弱阳极孔对电子束的散焦效应, 提升电子束的横向聚焦性能.One of the grand challenges in ultrafast science is real-time visualization of the microscopic structural evolution on atomic time and length scales. A promising pump-probe technique using a femtosecond laser pulse to initiate the ultrafast dynamics and another ultrashort electron pulse to probe the resulting changes has been developed and widely used to study ultrafast structural dynamics in chemical reactions, phase transitions, charge density waves, and even biological functions. In the past three decades, a number of different ultrafast electron guns have been developed to generate ultashort electron sources, mainly including hybrid electron gun with radio-frequency (RF) cavities for compressing the pulse broadening, relativistic electron gun for suppressing the coulomb interaction, single-electron pulses without space charge effect and compact direct current (DC) electron gun for minimizing the electron propagation distance. At present, these developments with different final electron energy and available total charge have improved the time response of ultrafast electron diffraction (UED) setups to a new frontier approaching to 100 fs regime. Although enormous efforts have been made, the superior capabilities and potentials of ultrafast electron diffraction (UED) are still hindered by space-charge induced pulse broadening. Besides, the penetration depth of electrons increases with the electron energy, while the scattering probability of electrons has the opposite consequence. Thus, in addition to the temporal resolution enhancement, it is also important that the electron energy should be tunable in a wide range to meet the requirements for samples with different thickness. Here in this work, we design a novel ultra-compact electron gun which combines a well-designed cathode profile, thereby providing a uniform field and a movable anode configuration to achieve a temporal resolution on the order of 100 fs over an accelerating voltage range from 10 kV to 125 kV. By optimizing the design of the high-voltage electrode profile, the field enhancement factor on the axis and along the cathode surface are both less than ~4% at different cathode-anode spacings, and thus the maximum on-axis field strength of ~10 MV/m is achieved under various accelerating voltages. This effectively suppresses the space charge broadening effect of the electron pulse. Furthermore, the anode aperture is designed as a stepped hole in which the dense sample grid can be placed, and the sample under study is directly supported by the grid and located at the anode, which reduces the cathode-to-sample distance, thus minimizing the electron pulse broadening from the cathode to sample. Moreover, the defocusing effect caused by the anode hole on the electron beam can be effectively reduced, therefore improving the lateral focusing performance of the electron beam.
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Keywords:
- ultrafast process /
- molecular movie /
- uniform field electrode /
- ultrashort electron sources
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图 1 常见UED用电极截面 (a) 平面型; (b) Rogowski型; (c) Bruce型; (d) 椭球型
Fig. 1. Definition of the (a) Plane; (b) Rogowski; (c) Bruce; (d) elliptical electrode profiles for UED.
图 2 90° Rogowski, Bruce, 椭球型和新电极四类电极的场增强效应 (a)中心轴区域场增强因子; (b)沿阴极表面场增强因子
Fig. 2. Field enhancements of 90° Rogowski, Bruce, and elliptical electrode: (a) The central axis; (b) along the curved edge of cathode.
图 4 总半径固定时中心平面区域尺寸变化对场增强的影响
Fig. 4. Effect of dimensional change of center plane area on field enhancement when the overall radius is constant.
图 5 不同阴阳极间距下的场增强效应 (a) 沿阴极面的场增强; (b)轴上场增强
Fig. 5. Field enhancement effect at different cathode-anode spacings: (a) Field enhancement on the axis; (b) field enhancement along the cathode surface.
图 6 电极配置对场增强的影响 (a) 非对称配置; (b) 对称配置
Fig. 6. Influence of the electrode configuration on field enhancement: (a) Asymmetric electrode configuration; (b) symmetric electrode configuration.
图 7 小孔处的电场分布 (a) 无载网; (b) 50目载网; (c) 不同目数载网的对比
Fig. 7. Electric field distribution at the anode pinholes: (a) Without TEM grid; (b) 50 mesh TEM grid; (c) comparison of different meshes of TEM grid.
图 9 间距可调型超快电子衍射仪示意图
Fig. 9. Schematic diagram of anode movable ultrafast electron diffractometer.
图 11 加速电压、初始能量弥散以及电子数目对电子脉宽的影响 (a) V = 10 kV, z = 0−5 mm; (b) V = 125 kV, z = 0−20 mm; (c) V = 10 kV, z = 0−20 mm; (d) V = 125 kV, z = 0−100 mm
Fig. 11. Effect of accelerating voltage, initial electron dispersion and number of electrons on the length of the electron pulse: (a) V = 10 kV, z = 0−5 mm; (b) V = 125 kV, z = 0−20 mm; (c) V = 10 kV, z = 0−20 mm; (d) V = 125 kV, z = 0−100 mm.
图 12 加速电压、初始能量弥散以及电子数目对电子束斑尺寸的影响 (a) V = 10 kV, z = 0−0.02 m; (b) V = 125 kV, z = 0−0.02 m
Fig. 12. Effect of accelerating voltage, initial electron dispersion, and number of electrons on beam spot size: (a) V = 10 kV, z = 0−0.02 m; (b) V = 125 kV, z = 0−0.02 m.
图 13 磁透镜对电子脉冲长度的影响 (a) n = 1000; (b) n = 10000
Fig. 13. Effect of magnetic lens on electronic pulse length: (a) n = 1000; (b) n = 10000.
图 14 磁透镜对电子束斑尺寸的影响 (a) n = 1000; (b) n = 10000
Fig. 14. Effect of magnetic lens on beam spot size: (a) n = 1000; (b) n = 10000.
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[1] Williamson J, Zewail A H 1991 Proc. Natl. Acad. Sci. USA 88 5021
Google Scholar
[2] Ihee H, Lobastov V A, Gomez U M, Goodson B M, Srinivasan R, Ruan C Y, Zewail A H 2001 Science 291 458
Google Scholar
[3] Siwick B J, Dwyer J R, Jordan R E, Miller R D 2003 Science 302 1382
Google Scholar
[4] Morrison V R, Chatelain R P, Tiwari K L, Hendaoui A, Bruhács A, Chaker M, Siwick B J 2014 Science 346 445
Google Scholar
[5] Sie E J, Nyby C M, Pemmaraju C D, Park S J, Shen X, Yang J, Hoffmann M C, Ofori-Okai B K, Li R K, Reid A H, Weathersby S 2019 Nature 565 61
Google Scholar
[6] Wolf T J, Sanchez D M, Yang J, Parrish R M, Nunes J P F, Centurion M, Coffee R, Cryan J P, Gühr M, Hegazy K, Kirrander A 2019 Nat. Chem. 11 504
Google Scholar
[7] Mo M, Murphy S, Chen Z, Fossati P, Li R K, Wang Y, Wang X J, Glenzer S 2019 Sci. Adv. 5 eaaw0392
Google Scholar
[8] 裴敏洁, 齐大龙, 齐迎朋, 贾天卿, 张诗按, 孙真荣 2015 64 034101
Google Scholar
Pei M J, Qi D L, Qi Y P, Jia T Q, Zhang S A, Sun Z R 2015 Acta Phys. Sin. 64 034101
Google Scholar
[9] 罗端, 惠丹丹, 温文龙, 刘蓉, 王兴, 田进寿 2017 66 152901
Google Scholar
Luo D, Hui D D, Wen W L, Liu R, Wang X, Tian J S 2017 Acta Phys. Sin. 66 152901
Google Scholar
[10] Gulde M, Schweda S, Storeck G, Maiti M, Yu H K, Wodtke A M, Schäfer S, Ropers C 2014 Science 345 200
Google Scholar
[11] Gao M, Lu C, Jean-Ruel H, Liu L C, Marx A, Onda K, Koshihara S, Nakano Y, Shao X F, Hiramatsu T, Saito G, Yamochi H, Cooney R R, Moriena G, Sciani G, Miller R J D 2013 Nature 496 343
Google Scholar
[12] 刘运全, 张杰, 田进寿, 赵宝升, 吴建军, 赵卫 2006 55 3368
Google Scholar
Liu Y Q, Zhang J, Tian J S, Zhao B S, Wu J J, Zhao W 2006 Acta Phys. Sin. 55 3368
Google Scholar
[13] Harb M, Ernstorfer R, Hebeisen C T, Sciaini G, Peng W, Dartigalongue T, Eriksson M A, Lagally M G, Kruglik S G, Miller R J D 2008 Phys. Rev. Lett. 100 155504
Google Scholar
[14] Gerbig C, Senftleben A, Morgenstern S, Sarpe C, Baumert T 2015 New J. Phys. 17 043050
Google Scholar
[15] Waldecker L, Bertoni R, Ernstorfer R 2015 J. Appl. Phys. 117 044903
Google Scholar
[16] Sciaini G, Miller R J D 2011 Rep. Prog. Phys. 74 096101
Google Scholar
[17] 刘运全, 张杰, 田进寿, 赵宝升, 吴建军, 赵卫, 侯洵 2007 56 123
Google Scholar
Liu Y Q, Zhang J, Tian J S, Zhao B S, Wu J J, Zhao W, Hou X 2007 Acta Phys. Sin. 56 123
Google Scholar
[18] Kassier G H, Haupt K, Erasmus N, Rohwer E G, Schwoerer H 2009 J. Appl. Phys. 105 113111
Google Scholar
[19] Rogowski W 1923 Die Elektrische Festigkeit am Rande des Plattenkondensators (Berlin: Springer-Verlag) pp1–15
[20] Badali D S, Gengler R Y, Miller R J D 2016 Structural Dynamics-US 3 034302
Google Scholar
[21] Bruce F 1947 J. Inst.Electr. Eng.-Part II; Power Eng. 94 138
[22] van der Geer S http://www.pulsar.nl/gpt/ [2019.11.23]
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