Generation of stable and coherent hard x-rays utilizing a superrandiance-enhanced self-seeding scheme

Abstract Self-seeding free-electron laser (FEL) enables the generation of a quasi-fully coherent hard x-ray pulse, and yet the stability of the pulse energy remains to be enhanced. Combining self-seeding with the superradiance, we propose a scheme for generating hard x-ray pulses characterized by stable intensity and narrow bandwidth. In this scheme, a large energy chirp is induced at the bunch head using a few-cycle laser. Powerful FEL radiation is generated in the linear chirp region and further evolves into an intensity-stable, single-spike spectral structure in the nonlinear chirp region. This evolution is driven by the superradiant effect, which utilizes the interaction between a nonlinear undulator taper and the nonlinear chirp of the bunch head. A diamond monochromator filters a narrow-bandwidth spectrum from the stable pulse, which is subsequently amplified to FEL saturation through interaction with the flat–top of the bunch tail. Numerical simulations and statistical analysis indicate that the stability of the hard x-ray FEL pulse is improved to 1.5%, providing a feasible pathway to generate stable and coherent hard x-ray pulses, enhancing the application potential of hard x-ray FEL facilities.