Zhao Zhouyu, Li Heting, Li Weiwei, Jia Qika, Jiang Shimin, Wang Lin
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, People's Republic of China.
J Synchrotron Radiat. 2019 Sep 1;26(Pt 5):1473-1480. doi: 10.1107/S1600577519009317. Epub 2019 Aug 12.
A multi-color light source is a significant tool for nonlinear optics experiments, pump-dump/repump-probe experiments and in other fields. Here, a novel method is proposed to create three-color pulses based on a high-gain harmonic-generation (HGHG) free-electron laser with a tilted electron bunch. In this method, the initial bunch tilt is created by transverse wakefields after the bunch passes through a corrugated structure with an off-axis orbit, and is further enlarged in a following drift section. Then the tilted bunch experiences the off-axis field of a quadrupole magnet to cool down the large transverse velocity induced before. After that, it enters an HGHG configuration adopting a transverse gradient undulator (TGU) as the radiator, where only three separated fractions of the tilted bunch will resonate at three adjacent harmonics of the seed wavelength and are enabled to emit three-color pulses simultaneously. In addition, the use of the natural transverse gradient of a normal planar undulator instead of the TGU radiator to emit three-color pulses is also studied in detail. Numerical simulations including the generation of the tilted bunch and the free-electron laser radiation confirm the validity and feasibility of this scheme both for the TGU radiator and the natural gradient in the extreme-ultraviolet waveband.
多色光源是用于非线性光学实验、泵浦-卸载/再泵浦-探测实验及其他领域的重要工具。在此，提出了一种基于具有倾斜电子束团的高增益谐波产生（HGHG）自由电子激光器来产生三色脉冲的新方法。在该方法中，初始束团倾斜是在束团穿过具有离轴轨道的波纹结构后由横向尾场产生的，并在随后的漂移段中进一步增大。然后，倾斜的束团经历四极磁铁的离轴场，以冷却之前诱导产生的大横向速度。之后，它进入采用横向渐变波荡器（TGU）作为辐射器的HGHG结构，在此结构中，倾斜束团中只有三个分离的部分会在种子波长的三个相邻谐波处发生共振，并能够同时发射三色脉冲。此外，还详细研究了使用普通平面波荡器的自然横向梯度而非TGU辐射器来发射三色脉冲的情况。包括倾斜束团的产生和自由电子激光辐射的数值模拟证实了该方案对于TGU辐射器和极紫外波段的自然梯度而言的有效性和可行性。
