高亮度超快电子衍射光电子束线中的点到点库仑效应
Point-to-Point Coulomb Effects in High Brightness Photoelectron Beamlines for Ultrafast Electron Diffraction
摘要：
为了提高超快电子衍射和显微镜的空间和时间分辨率，人们积极寻求超高亮度光电阴极来改善电子束质量。束流动力学代码通常近似库仑与平均场空间电荷的相互作用，这在传统束流中是一个很好的近似。然而，点对点库仑效应，如无序诱导加热和Boersch效应，不能忽略冷，密集的光束由这样的光阴极产生。在本文中，我们介绍了两种新的数值方法来计算的光阴极图像电荷的重要影响时，使用点对点的相互作用模型。利用精确的像电荷模型，计算了点到点相互作用对两条高亮度光电子谱线超快衍射的影响。第一束线使用200 keV电子枪，而第二束线使用5 MeV电子枪，每个电子枪都以$10^5$电子/脉冲的单发衍射方式工作。假设一个零的光电发射温度，它示出，包括随机库仑效应增加了这些光束线的最终发射度超过2的一个因素，并降低了峰值横向相空间密度超过3的一个因素相比，平均场模拟。然后，我们介绍了一种方法来计算的无序诱导加热释放的能量使用对关联函数。这种无序引起的加热能量被发现缩放非常接近静止超冷等离子体的理论结果，它占了一半以上的平均场模拟的发射度增长。
英文摘要：
In an effort to increase spatial and temporal resolution of ultrafast electron diffraction and microscopy, ultra-high brightness photocathodes are actively sought to improve electron beam quality. Beam dynamics codes often approximate the Coulomb interaction with mean-field space charge, which is a good approximation in traditional beams. However, point-to-point Coulomb effects, such as disorder induced heating and the Boersch effect, can not be neglected in cold, dense beams produced by such photocathodes. In this paper, we introduce two new numerical methods to calculate the important effects of the photocathode image charge when using a point-to-point interaction model. Equipped with an accurate model of the image charge, we calculate the effects of point-to-point interactions on two high brightness photoemission beamlines for ultrafast diffraction. The first beamline uses a 200 keV gun, whereas the second uses a 5 MeV gun, each operating in the single-shot diffraction regime with $10^5$ electrons/pulse. Assuming a zero photoemission temperature, it is shown that including stochastic Coulomb effects increases the final emittance of these beamlines by over a factor of 2 and decreases the peak transverse phase space density by over a factor of 3 as compared to mean-field simulations. We then introduce a method to compute the energy released by disorder induced heating using the pair correlation function. This disorder induced heating energy was found to scale very near the theoretical result for stationary ultracold plasmas, and it accounts for over half of the emittance growth above mean-field simulations.
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