Ji Shaozheng, Piazza Luca, Cao Gaolong, Park Sang Tae, Reed Bryan W, Masiel Daniel J, Weissenrieder Jonas
KTH Royal Institute of Technology, Material Physics, Electrum 229, SE-16440 Kista, Sweden.
Integrated Dynamic Electron Solutions, Inc. (IDES), Pleasanton, California 94588, USA.
Struct Dyn. 2017 Jul 17;4(5):054303. doi: 10.1063/1.4994004. eCollection 2017 Sep.
Efforts to understand matter at ever-increasing spatial and temporal resolutions have led to the development of instruments such as the ultrafast transmission electron microscope (UEM) that can capture transient processes with combined nanometer and picosecond resolutions. However, analysis by UEM is often associated with extended acquisition times, mainly due to the limitations of the electron gun. Improvements are hampered by tradeoffs in realizing combinations of the conflicting objectives for source size, emittance, and energy and temporal dispersion. Fundamentally, the performance of the gun is a function of the cathode material, the gun and cathode geometry, and the local fields. Especially shank emission from a truncated tip cathode results in severe broadening effects and therefore such electrons must be filtered by applying a Wehnelt bias. Here we study the influence of the cathode geometry and the Wehnelt bias on the performance of a photoelectron gun in a thermionic configuration. We combine experimental analysis with finite element simulations tracing the paths of individual photoelectrons in the relevant 3D geometry. Specifically, we compare the performance of guard ring cathodes with no shank emission to conventional truncated tip geometries. We find that a guard ring cathode allows operation at minimum Wehnelt bias and improve the temporal resolution under realistic operation conditions in an UEM. At low bias, the Wehnelt exhibits stronger focus for guard ring than truncated tip cathodes. The increase in temporal spread with bias is mainly a result from a decrease in the accelerating field near the cathode surface. Furthermore, simulations reveal that the temporal dispersion is also influenced by the intrinsic angular distribution in the photoemission process and the initial energy spread. However, a smaller emission spot on the cathode is not a dominant driver for enhancing time resolution. Space charge induced temporal broadening shows a close to linear relation with the number of electrons up to at least 10 000 electrons per pulse. The Wehnelt bias will affect the energy distribution by changing the Rayleigh length, and thus the interaction time, at the crossover.
人们致力于以不断提高的空间和时间分辨率来理解物质，这推动了诸如超快透射电子显微镜（UEM）等仪器的发展，该显微镜能够以纳米和皮秒的组合分辨率捕捉瞬态过程。然而，UEM分析通常与较长的采集时间相关，这主要是由于电子枪的局限性所致。在实现源尺寸、发射度、能量和时间色散等相互冲突目标的组合时，权衡取舍阻碍了改进。从根本上说，电子枪的性能是阴极材料、电子枪和阴极几何形状以及局部场的函数。特别是截顶尖端阴极的柄部发射会导致严重的展宽效应，因此必须通过施加韦内尔特偏压来过滤此类电子。在此，我们研究阴极几何形状和韦内尔特偏压对热电子配置中光电子枪性能的影响。我们将实验分析与有限元模拟相结合，追踪相关三维几何结构中单个光电子的路径。具体而言，我们将无柄部发射的保护环阴极的性能与传统截顶尖端几何形状进行比较。我们发现，保护环阴极能够在最小韦内尔特偏压下运行，并在UEM的实际运行条件下提高时间分辨率。在低偏压下，韦内尔特对保护环的聚焦作用比对截顶尖端阴极更强。随着偏压增加，时间展宽的增加主要是由于阴极表面附近加速场的减小。此外，模拟表明，时间色散还受光发射过程中的固有角分布和初始能量展宽的影响。然而，阴极上较小的发射光斑并非提高时间分辨率的主要驱动因素。空间电荷引起的时间展宽与每个脉冲至少10000个电子的电子数量呈现近乎线性的关系。韦内尔特偏压将通过改变交叉点处的瑞利长度从而改变相互作用时间来影响能量分布。
