Generation of high-quality electron beams based on tightly focused super-Gaussian laser
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摘要: 在激光尾场加速中，光学注入是一种有效的可控电子注入机制。然而，低电量、大发散度的电子束特性无法满足实际应用的需要。为获得大电量、高品质电子束提出采用紧聚焦的超高斯激光作为注入脉冲的新型注入方案。研究发现，相比于普通高斯激光，紧聚焦的超高斯激光不仅能够将电子束发散度降低近一个数量级，而且能够保持电子束电荷量不变。通过哈密顿理论模型证实，离轴电子是发散度的主要来源，而紧聚焦的超高斯激光极大地限制了离轴电子的注入，因此有效地降低了电子束的发散度。Abstract: Electron optical injection is an efficient all-optical injection scheme in laser wakefield accelerations. However, low-charge and large emittance electron beam is still not suitable for many practical applications. This paper presents a novel injection scheme by colliding a tightly focused super-Gaussian laser with a Gaussian pump laser. It is found that the emittance of electron beam becomes almost an order of magnitude lower than that of all-Gaussian case, while the charge of electron bunch is conserved. It is also found that the electron emittance is mainly attributed to off-axis injected electrons by a Hamiltonian model. This unique ability will pave the way towards the generation of high-quality electron beams and extend the applications of laser-plasma accelerators.
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图 1 (a) 高斯型注入脉冲下的注入电子横向相空间分布。不同颜色代表注入电子初始横向位置与主激光光轴的距离Δr。(b)相应的电子束发散度和电荷量与离轴距离Δr的关系。作为对比，采用紧聚焦超高斯激光作为注入脉冲时捕获电子的相空间分布及电子束发散度随离轴距离Δr的变化如(c)和(d)所示
Figure 1. (a)transverse phase-space distribution of trapped electrons in Gaussian case. Noted that colors represent Δr. (b)the beam emittance and total charge versus Δr. As a reference, the results by adopting a tightly focused super-Gaussian laser as the injection pulse are shown in (c) and (d)
图 2 纵向相空间分布和纵向加速电场(a) 高斯型注入脉冲(b)紧聚焦超高斯注入脉冲。(c)不同激光模式下的电子束能谱分布
Figure 2. Longitudinal phase-space distribution and the corresponding acceleration electric field Ex on the laser axis for (a) Gaussian case and (b) tightly focused super-Gaussian case. (c)the energy spectra for two cases
图 3 不同初始横向位置的电子横向相空间动力学特性。(a)和(b)初始横向位置分别为y0=19 μm和y0=5 μm的电子的横向相空间轨迹。其中不同颜色代表不同的电子能量。(c)和(d)描述不同初始位置的电子的betatron振幅及发散角随时间的演化
Figure 3. (Color online). Transverse phase-space dynamics for electrons with different initial transverse position. (a) and (b) show transverse phase-space trajectories for two electrons with y0=19 μm and y0=5 μm respectively. The color represents the electron energy and the inset is the partial enlarged drawing. (c) and (d) show the evolution of the betatron amplitude y and the divergence py/px
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