X-ray free-electron laser (XFEL) facilities have been excellent light sources to explore ultrafast phenomena in the fields of physics, chemistry, biology and material science by supplying intense femtosecond X-ray pulses with a few tens of gigawatt power. However, shorter pulse lengths have been desired to trace the electron dynamics in atomic and molecular systems, theoretical and experimental studies also have been devoted to how to generate attosecond X-ray pulses from XFEL facilities. In this talk, I will discuss the following issues to generate isolated terawatt attosecond pulses from a free-electron laser with simulation results: (1) the generation of the current spike in the electron beam with optimal width and peak current; (2) the amplification of a photon pulse to the terawatt-level in the attosecond time domain with variable-spaced current spikes; (3) the enhancement of the signal-to-noise ratio of the attosecond pulse with optimized undulator layout. Also, the ongoing project to demonstrate the generation of attosecond pulse at Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) will be briefly introduced.
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