The spectroscopic methods for the ultrafast electronic and structural dynamics of materials require fully coherent extreme ultraviolet and soft X-ray radiation with high-average brightness. Seeded free-electron lasers (FELs) are ideal sources for delivering fully coherent soft X-ray pulses. However, due to state-of-the-art laser system limitations, it is challenging to meet the ultraviolet seed laser’s requirements of sufficient energy modulation and high repetition rates simultaneously. The self-modulation scheme has been proposed and recently demonstrated in a seeded FEL to relax the seed laser requirements. Using numerical simulations, we show that the required seed laser intensity in the self-modulation is ~3 orders of magnitude lower than that in the standard high-gain harmonic generation (HGHG). The harmonic self-modulation can launch a single-stage HGHG FEL lasing at the 30th harmonic of the seed laser. Moreover, the proof-of-principle experimental results confirm that the harmonic self-modulation can still amplify the laser-induced energy modulation. These achievements reveal that the self-modulation can not only remarkably reduce the requirements of the seed laser but also improve the harmonic upconversion efficiency, which paves the way for realizing high-repetition-rate and fully coherent soft X-ray FELs.

上海软X射线自由电子激光用户装置（SXFEL）是我国首台可以运行在水窗波段的自由电子激光装置，未来可以为5个实验线站供光，其主要运行模式为自放大自发辐射模式以及外种子模式。本文就SXFEL的外种子模式进行了从头到尾的模拟研究，主要包括EEHG-HGHG混合级联与单级EEHG两种方案。模拟结果表明，虽然EEHG-HGHG混合级联模式较为复杂，但能够产生更高功率的高次谐波辐射。除此之外，我们还研究了各种三维效应对EEHG的影响。模拟和分析结果表明，通过上述两种方案，采用紫外波段的种子激光，用户可以得到全相干、窄带宽、短脉冲的水窗波段自由电子激光。