Reverse taper enhanced harmonic lasing for seeding an X-ray free-electron laser

The recent development of high repetition-rate free-electron laser (FEL) facility has brought new challenges for the self-seeding scheme, mainly the problems of heat load and radiation damage on the monochromator. In this paper, a simple technique is proposed to mitigate the effect of heat-load on the monochromator of a self-seeding FEL. We combine the reverse undulator taper with the harmonic lasing technique to enhance the spectral brightness of the radiation pulse. In this way, the required radiation power for the first stage of self-seeding can be significantly reduced. Theoretical analysis and numerical simulation at 2.86 nm are performed and the results demonstrate that the proposed method can decrease the heat-load on the monochromator by nearly one order of magnitude. At the same time, the peak power of final radiation can be enhanced and the total length of undulator can be reduced comparing with the conventional self-seeding. The proposed technique can be implemented at current existing FEL facilities with variable-gap undulators.