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首页> 外文会议>IEEE International Conference on Plasma Science >A Model of Photoemission Delay Mechanisms and Its Application to Beam Optics Codes
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A Model of Photoemission Delay Mechanisms and Its Application to Beam Optics Codes

机译：光发射延迟机制的模型及其在光束光学代码中的应用

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High performance Free Electron Lasers (FEL) and x-ray FELs (xFEL) seek high quantum efficiency (QE) photocathodes (typically semiconductors). An important requirement for the shorter wavelength FELs is the response time of the photocathode as it affects pulse shaping [1]: delayed emission effects, although correlated with enhanced QE [2], contribute to emittance (longitudinal and transverse) and halo formation [3], particularly for short pulses where the tails are not emitted at optimal times. The delayed contribution is a consequence of laser penetration depth effects but also (multiple) scattering mechanisms [4]-[6] that give rise to a diffusive rather than ballistic component to the photocurrent. The incorporation of such effects is mostly absent in beam optics codes that are critical to xFEL simulation and design. In this work, we describe efforts to a delayed emission model developed for the Particle-in-Cell code MICHELLE [7] that includes not only unscattered electrons excited deeper into the bulk photocathode material, but also partially thermalized electron contributions that emerge at later times. The method is based on a string of excitation sites and the monitoring of their contribution to the photocurrent using a Moments-based model (comparable to the Spicer three-step model) as they lose energy to scattering. The significant challenges of doing so shall be discussed regarding the integration of the models into MICHELLE, particularly how the most recent model improves upon the simpler models of Ref. [8]. The methods will be applied to showing the effects of delayed emission on pulse duration and laser jitter as a function of the number of scattering events the electrons typically experience for both metals and semiconductors. The extension to semiconductor photocathodes is critical as their QE is higher given that the scattering events are not nearly as fatal to emission.
机译：高性能自由电子激光器（FEL）和X射线FEL（xFEL）寻找高量子效率（QE）光电阴极（通常是半导体）。波长较短的FEL的一个重要要求是光阴极的响应时间，因为它影响脉冲整形[1]：尽管延迟发射效应与增强的QE相关[2]，但会影响发射率（纵向和横向）和光晕形成[3]。 ，特别是对于短脉冲，在最佳时间不发射尾巴。延迟的贡献是激光穿透深度效应的结果，也是（多种）散射机制[4]-[6]的结果，这些散射机制对光电流产生了扩散而不是弹道分量。在对xFEL仿真和设计至关重要的光束光学代码中，大多数情况下都没有这种效果的结合。在这项工作中，我们描述了为细胞内粒子代码MICHELLE [7]开发的延迟发射模型的工作，该模型不仅包括更深地激发到整体光电阴极材料中的未散射电子，而且还包括后来出现的部分热化电子贡献。 。该方法基于一串激发位点，并使用基于矩的模型（可与Spicer三步模型相比）监测它们对光电流的贡献，因为它们会损失能量以进行散射。关于将模型集成到MICHELLE中的问题，将讨论这样做的重大挑战，特别是最新模型如何在Ref。的较简单模型上进行改进。 [8]。该方法将用于显示延迟发射对脉冲持续时间和激光抖动的影响，该影响是电子通常在金属和半导体中经历的散射事件数量的函数。鉴于散射事件对发射的致命性不高，半导体光电阴极的扩展至关重要，因为它们的QE更高。

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