Two-bunch self-seeding for narrow-bandwidth hard x-ray free-electron lasers∗

It is well known that seeding can be used to produce narrow-bandwidth and fully coherent x-ray freeelectron lasers (FELs). Self-seeding, which uses an extra undulator to generate the seed pulse, is perhaps one of the most promising methods to accomplish this. In the hard x-ray regime with high-energy electrons, this method requires a large magnetic chicane to match the path-length delay of the x-ray monochromator that selects a narrow bandwidth of radiation. Such a chicane not only takes a large footprint to build, but also may degrade the electron-beam qualities through incoherent and coherent synchrotron radiation. In this paper, we present an alternative two-bunch self-seeding scheme. The two bunches are precisely separated to match the x-ray delay of the monochromator and eliminate the need for a long, complex magnetic chicane. The spectrally filtered self-amplified spontaneous emission x-ray pulse produced by the first bunch is combined with the second electron bunch at the entrance of the second undulator and then amplified to the saturation level. We present start-to-end simulation results based on the linac coherent light source hard x-ray FEL and show that this method can produce a nearly fully coherent x-ray pulse at a few GW power level.