Design of a femtosecond electron diffractometer with adjustable gaps

Abstract

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.

100 fs time-resolved streak tube design based on anisotropy and post-acceleration technology

“…2.1 时间分辨影响因素分析 条纹管时间分辨率可由公式(1)给出 [9] ： 电场, 因此电极设计的重点是既能获得高强度的电场, 又能使光电管获得较大的 光子效率 [5] . 飞秒条纹管对光阴极附近电场的要求为 E≥14 kV/mm, 即需要在最 小的加速电极间距内产生尽可能高的均匀电场.…”

“…目前, 最小化光电子渡越时间弥散(降低物理时间弥散)及最大化扫描速度(降 低技术时间弥散)是提升条纹管时间分辨率的主要途径, 主要取决于阴极附近的 加速电场及扫描偏转灵敏度 [5,6] . 为了增强加速电场, Gallant 等人在单发模式下采 用 25 kV/mm 的牵引场和 0.56 c 的扫描偏转场实现了 350 fs 的时间分辨率, 且在 狭缝方向实现了 40 μm 的空间分辨率 [7] .…”

Development of High-Voltage Electrodes for Neutron Scattering Sample Environment Devices

“…The edging effect, on the other hand, will significantly limit the achievable field strength and therefore should be strictly avoided or minimized. Previous researchers have developed various electrode profiles to avoid the edging effect, ranging from simple planar to more complex elliptical [23], Rogowski [24], and Bruce [25] profiles, but the applicability of those profiles, particularly the compromise between field uniformity and strength and the spatial confinement for neutron scattering experiments, has not yet been examined.…”