Li F, Dalichaouch T N, Pierce J R, Xu X, Tsung F S, Lu W, Joshi C, Mori W B
Department of Electrical Engineering, University of California Los Angeles, Los Angeles, California 90095, USA.
Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, California 90095, USA.
Phys Rev Lett. 2022 Apr 29;128(17):174803. doi: 10.1103/PhysRevLett.128.174803.
We propose a new method for self-injection of high-quality electron bunches in the plasma wakefield structure in the blowout regime utilizing a "flying focus" produced by a drive beam with an energy chirp. In a flying focus the speed of the density centroid of the drive bunch can be superluminal or subluminal by utilizing the chromatic dependence of the focusing optics. We first derive the focal velocity and the characteristic length of the focal spot in terms of the focal length and an energy chirp. We then demonstrate using multidimensional particle-in-cell simulations that a wake driven by a superluminally propagating flying focus of an electron beam can generate GeV-level electron bunches with ultralow normalized slice emittance (∼30 nm rad), high current (∼17 kA), low slice energy spread (∼0.1%), and therefore high normalized brightness (>10^{19} A/m^{2}/rad^{2}) in a plasma of density ∼10^{19} cm^{-3}. The injection process is highly controllable and tunable by changing the focal velocity and shaping the drive beam current. Near-term experiments at FACET II where the capabilities to generate tens of kA, <10 fs drivers are planned, could potentially produce beams with brightness near 10^{20} A/m^{2}/rad^{2}.
我们提出了一种新方法，用于在等离子体尾波场结构的爆轰 regime 中自注入高质量电子束团，该方法利用具有能量啁啾的驱动束产生的“飞行焦点”。在飞行焦点中，通过利用聚焦光学的色散依赖性，驱动束团密度质心的速度可以是超光速或亚光速的。我们首先根据焦距和能量啁啾推导出焦点速度和焦斑的特征长度。然后，我们使用多维粒子模拟证明，由电子束超光速传播的飞行焦点驱动的尾波可以在密度约为 10^{19} cm^{-3} 的等离子体中产生具有超低归一化切片发射度（约 30 nm rad）、高电流（约 17 kA）、低切片能量 spread（约 0.1%），因此具有高归一化亮度（>10^{19} A/m^{2}/rad^{2}）的 GeV 级电子束团。通过改变焦点速度和塑造驱动束电流，注入过程具有高度可控性和可调性。在 FACET II 计划进行的近期实验中，具备产生数十 kA、<10 fs 驱动源的能力，有可能产生亮度接近 10^{20} A/m^{2}/rad^{2} 的束流。
