The Generation of Ultra-Low Emittance Beams by Downramp Injection in a Plasma Wakefield Accelerator

该项目的目标是探索一种产生高质量电子束的新方法。电子显微镜、自由电子激光器和高能对撞机等许多应用都需要高质量的电子束。电子束质量的一个衡量标准是它的发射度。这是因为小发射率光束可以聚焦到非常小的光斑尺寸，这在这些应用中是理想的。该项目将探索利用三维模拟一种新的电子加速方案来产生极低发射度的电子束，该方案利用了等离子体加速器中控制粒子密度的能力。目前，基于等离子体的加速器（PBAs）能够自产生含有大量电荷、持续时间短、归一化发射率低的超相对论电子束。这些光束量通常组合成一个称为归一化光束亮度的单一指标。基于加速器的x射线光源需要超高亮度光束。虽然PBA实验已经产生了有用的光束，但它们还没有产生具有驱动x射线自由电子激光器所需的必要亮度和能量扩散的光束，也没有产生作为未来线性对撞机注入器所需的电荷和发射度。该项目将通过理论和计算机模拟，以及在SLAC的FACET II设备上进行的实验，探索等离子体中电子在密度急剧下降时的加速度。

The goal of this project is to explore a new method for generating high quality electron beams. High quality electron beams are needed for many applications such as electron microscopy, free electron lasers and high-energy colliders. One measure of quality of an electron beam is its emittance. This is because a small emittance beam can be focused to a very small spot size, desirable in these applications. This project will explore generation of extremely low emittance electron beams using three-dimensional simulations of a new electron acceleration scheme that takes advantage of the ability to control particle density in a Plasma-Based Accelerator.It is now well established that Plasma-Based Accelerators (PBAs) are capable of self-generating ultra-relativistic electron bunches that contain a significant amount of charge, have a short duration, and a low normalized emittance. These beam quantities are often combined into a single figure of merit called the normalized beam brightness. Ultra-high brightness beams are needed in accelerator-based X-ray light sources. While PBA experiments have produced useful beams, they have not yet produced beams with the necessary brightness and energy spread needed to drive an X-ray free-electron-laser, nor the charge and emittance needed as an injector for a future linear collider. This project will explore the acceleration of electrons across a sharp density downramp in a plasma through theory and computer simulations, and through an experiment that will be carried out at the FACET II facility at SLAC.