Simulation studies of external seeding schemes for Shanghai soft X-ray free electron laser
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摘要: 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。Abstract: Shanghai Soft X-ray Free Electron Laser User Facility (SXFEL-UF) is the first X-ray FEL in China that can produce coherent radiation in the water-window regime. The main working modes of SXFEL are self-amplified spontaneous emission and external seeding schemes. This paper mainly focuses on the start to end simulations of the external seeding schemes including echo-enabled harmonic generation-high gain harmonic generation cascade (EEHG-HGHG cascade) and single stage echo-enabled harmonic generation (EEHG). 3D simulations indicates that these external seeding schemes can generate coherent X-ray radiation at soft X-ray regime directly from a conventional UV seed laser.
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图 4 级联EEHG-HGHG模式第一个辐射段出口的FEL辐射
Figure 4. Cascade EEHG-HGHG mode FEL radiation at the exit of the first radiator
图 5 级联EEHG-HGHG模式第二个辐射段出口的FEL辐射
Figure 5. Cascade EEHG-HGHG mode FEL radiation at the exit of the second radiator
图 6 单级EEHG模式波荡器出口的FEL辐射
Figure 6. Single stage EEHG FEL radiation at the exit of undulator
图 7 级联EEHG-HGHG模式中两束种子激光的时间与能量抖动对辐射的影响
Figure 7. Timing jitter and power jitter of the seed lasers in the cascade EEHG-HGHG mode
图 8 单级EEHG模式中两束种子激光的时间与能量抖动对辐射的影响
Figure 8. Timing jitter and power jitter of the seed lasers in the single stage EEHG mode
表 1 模拟所使用的主要参数
Table 1. Main parameters in the simulations
energy/MeV peak current/A sliced energy spread/keV normarlized emittance/(mm mrad) 1400 800 100 1.5 bunch length/fs power of seed lasers/MW mudulator period numbers mudulator period length/m 500(FWHM) 200/800 20/20/100 0.08/0.08/0.03 
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