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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
Figure 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
Figure 2. Field enhancements of 90° Rogowski, Bruce, and elliptical electrode: (a) The central axis; (b) along the curved edge of cathode.
图 3 新电极截面轮廓
Figure 3. Geometry of the new high voltage electrode.
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
Figure 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
Figure 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
Figure 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
Figure 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 8 阳极小孔设计
Figure 8. Design of anode pinholes.
图 9 间距可调型超快电子衍射仪示意图
Figure 9. Schematic diagram of anode movable ultrafast electron diffractometer.
图 10 时间为0时电子源的空间分布
Figure 10. Spatial distribution of electron source at time = 0.
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
Figure 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
Figure 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
Figure 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
Figure 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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