超快X射线光源的相干性

With the development of the 4th generation light source (i.e. free electron laser), it comes into reality to generate the high brilliance ultrafast X-ray pulse with broad tunability. The free electron laser includes two major categories: SASE (Self-Amplified Spontaneous Emission) or Seeded (e.g. HGHG-High Gain Harmonic Generation) FEL. In a SASE-FEL, the electron bunch is interacting with and modulated by its own radiation, forming micro-bunches to give off coherent radiation, which origins from the random phase noise. SASE-FEL has large shot-by-shot fluctuation, and its longitudinal radiation profile includes many individual coherent spikes which are mutually uncorrelated and incoherent, corresponding to a partial coherent light-source. HGHG-FEL adopts a seed optical laser to modulate the electron bunch both in energy and in density, after dispersion compensation, the "micro-bunched" electron pulse irradiates at the harmonics of the seed laser. Compared to SASE-FEL, HGHG-FEL has higher peak power and brilliance, with a narrow Gaussian spectrum, corresponding to a stable and fully coherent light source. The major mission of this proposal is to develop an advance X-ray instrument, to measure the longitudinal coherence and the pulse duration of both SASE and HGHG pulses. It is supposed to provide the technical support for the commissioning and optimization of the future X-ray free electron laser sources , then to investigate the new phenomena and physics in the X-ray FEL. We also plan to use this device to explore the unknown phenomena in the atto-second science, especially it would be expected to directly measure the atto-second pulse duration in the time domain as the first time.

前沿科学实验技术的发展对高亮度短波长的X射线光源充满了期待，而近年来自由电子激光技术的发展使产生高亮度可调谐的超快X射线光源成为了可能。根据其发光机理主要分为自发放大模式(即SASE)和激光诱导的高增益模式(如HGHG)。本课题以在建中的X射线自由电子激光大科学装置为例，具体阐述了两种模式的特性。SASE辐射脉冲有随机和不稳定性，它的辐射功率时域分布包含多个独立相干的毛刺，是部分相干光源。而HGHG脉冲是一种具有全相干特性的高稳定光源，有更高的峰值功率和光源亮度。本项目旨在研发高精度的相干性测量装置，对X射线自由电子激光的这两种模式的时间相干长度和脉冲长度进行测量，可为X射线自由电子激光装置的调试和优化提供技术支持，同时系统研究其辐射增益中新的物理机制和现象。本项目也计划将此测量装置用于阿秒脉冲的测量，将首次在时域中直接测量阿秒脉冲的长度，并探索许多阿秒科学中的未知问题。

本项目立足于测量短波长自由电子激光超快辐射脉冲长度及相干性这一国际难题，为我国兴建的第四代光源，即大连极紫外相干光源(DCLS)和上海X射线自由电子激光装置(SXFEL)提供辐射脉冲诊断测量，为验证其相干辐射特性提供最直接的依据。本项目成功研制了中国首台极紫外至水窗波段的超快X射线相干测量装置，并率先在大连极紫外光源DCLS开展实验，成功测得了DCLS的波前相干性和时间相干性，后续还将开展对上海X射线自由电子激光装置(SXFEL)的相干测量。本研究通过细致严谨的工作，首次测得并验证了DCLS光源具有全相干特性，提供了第一手的光源时域特征数据，为后续开展前沿科学研究(如相干衍射光学，原子分子相干动力学过程等)奠定了基础，具有重要参考价值。本项目的实施，为今后研制硬X射线自由电子激光相干测量技术奠定了基础，并积累了宝贵的经验。