非相干能量调制下软X射线回波谐波产生自由电子激光脉冲的特性
Characterization of soft X-ray echo-enabled harmonic generation free-electron laser pulses in the presence of incoherent energy modulations
摘要：
回波谐波产生自由电子激光器（EEHG FEL）利用紫外激光的高次谐波频率上转换产生完全相干的软X射线脉冲。然而，EEHG的最终光谱极限仍然不清楚，因为在输出光谱中由电子束中的残余宽带能量调制引起的加宽和失真。我们提出了一个数学描述的非相干（宽带）的能量调制的聚束谱产生的微聚束不稳定性通过加速器和EEHG线的影响。该模型是在协议的费米EEHG自由电子激光的光子能量范围$130-210$ eV的系统实验表征。我们发现，放大的电子束能量畸变主要在EEHG色散部分解释了观察到的自由电子激光光谱亮度的减少，是成比例的EEHG谐波数。在色散区强度和第一种子激光脉冲能量的适当平衡下，发现了自由电子激光光谱亮度和光谱稳定性的局部极大值。这种特性为用户实验和未来的EEHG实现提供了一个基准，旨在达到更短的波长。
英文摘要：
Echo-enabled harmonic generation free-electron lasers (EEHG FELs) are promising candidates to produce fully coherent soft x-ray pulses by virtue of efficient high harmonic frequency up-conversion from UV lasers. The ultimate spectral limit of EEHG, however, remains unclear, because of the broadening and distortions induced in the output spectrum by residual broadband energy modulations in the electron beam. We present a mathematical description of the impact of incoherent (broadband) energy modulations on the bunching spectrum produced by the microbunching instability through both the accelerator and the EEHG line. The model is in agreement with a systematic experimental characterization of the FERMI EEHG FEL in the photon energy range $130-210$ eV. We find that amplification of electron beam energy distortions primarily in the EEHG dispersive sections explains an observed reduction of the FEL spectral brightness that is proportional to the EEHG harmonic number. Local maxima of the FEL spectral brightness and of the spectral stability are found for a suitable balance of the dispersive sections' strength and the first seed laser pulse energy. Such characterization provides a benchmark for user experiments and future EEHG implementations designed to reach shorter wavelengths.
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