Microbunching Instability Suppression for Improved FEL Performance

自由电子激光器（FEL）是第四代光源，能够产生宽波长范围的辐射，同时产生飞秒（10-15秒）脉冲。这使得从材料科学到生物学等多个领域的研究人员能够以高分辨率探测原子尺度的结构和动力学过程。随着对更亮的自由电子激光（FEL）X射线的需求增加，对高质量电子束和更高峰值电流的需求也在增加，然而，各种各样的效应使得实现这些要求变得困难。微聚束不稳定性（MBI）和相干同步辐射（CSR）反冲效应是导致发射度增加和FEL性能下降的主要原因。这个项目将通过几个目标来研究微聚束不稳定性和扭曲的CSR kicksthrough，这将在下面总结。CSR是一种发生在偶极子中的现象，其中相对论粒子，如电子，相干辐射。当这种辐射被束中的其他粒子吸收时，能量沿着束分布。这导致束团的纵向切片在横向坐标中偏移，因此增加了投影发射度。目前有一些CSR消除技术用于最小化这种影响，但如果CSR踢被非色散部分中的偶极子扭曲，这些技术可能不太有效。微聚束不稳定性是由CSR引起的小的初始能量调制引起的，导致线性加速器（linac）色散区的电流密度调制（或微聚束）。一种被称为焦散线的奇异性可以用来数学地描述微聚束动力学。该项目旨在通过设计一个实验来测量位于瑞典隆德的MAX-IV的焦散线诱导电流尖峰的演变，从而获得直线加速器中粒子轨迹焦散线的第一个实验测量结果。还可以研究阈值R56对微聚束增益的作用，因为两者之间的关系还没有完全弄清楚。更好地理解CSR和微聚束不稳定性，将使我们能够抑制不良影响，保持光束质量，并最终允许更明亮的自由电子激光源。

Free Electron Lasers (FELs) are a fourth-generation light source capable of producing radiationover a broad range of wavelengths, while producing femtosecond (10-15 second) pulses. This allowsresearchers across several fields, from materials science to biology, to probe atomic scale structuresand dynamical processes with a high resolution. Development of brighter FELs with shorter pulsedurations allows these researchers to make much needed advancements in medicine, biology andmaterials science.As demand for brighter Free Electron Laser (FEL) X-rays increases, so does the need for highquality electron beams and higher peak currents, however there are various effects which makeachieving these requirements difficult. Microbunching instability (MBI) and Coherent SynchrotronRadiation (CSR) kicks are the effects which lead to increased emittance and decreased FELperformance. This project will investigate microbunching instability and distorted CSR kicksthrough a few aims, which will be summarised below.CSR is a phenomena that takes place in dipoles where relativistic particles, such as electrons,radiate coherently. When this radiation is absorbed by other particles in the bunch, there is aredistribution of energy along the bunch. This leads to longitudinal slices of the bunch being offsetin the transverse coordinates, and therefore increased projected emittance. Currently there are someCSR cancellation techniques used to minimise this effect, but these may be less effective if the CSRkicks are distorted by dipoles in non-dispersive sections. The conditions which lead to distortedCSR kicks will be investigated and the effectiveness of CSR cancellation techniques on distortedkicks will be studied.Microbunching instability arises from small initial energy modulations induced by CSR leading tocurrent density modulations (or microbunching) in the dispersive regions of a linear accelerator(linac). A type of singularity known as caustics can be used to mathematically describemicrobunching dynamics. This project aims to obtain the first experimental measurements ofparticle trajectory caustics in a linac, by designing an experiment to measure the evolution ofcaustic-induced current spikes at MAX-IV, in Lund, Sweden. An investigation into the role ofthreshold R56 on microbunching gain can also be made, as the relationship between the two is notcompletely understood. Better understanding of CSR and microbunching instability, will allow us tosuppress the undesirable effects and preserve beam quality and ultimately allow for brighter FELsources.