Gain-guided X-ray free-electron laser oscillator

The X-ray free-electron laser oscillator (XFELO) has recently been reconsidered a promising candidate for producing high-brightness, fully coherent pulses in the hard X-ray regime. This letter proposes a gain-guided XFELO scheme, avoiding the external focusing elements required for conventional configuration. Self-consistent theoretical analysis and three-dimensional numerical simulation results verify that the X-ray transverse mode in such an "unstable" cavity is stable and robust due to electron beam gain-guiding. This scheme is capable of generating 14.3 keV photons of FEL radiation with peak power on the order of a gigawatt using Shanghai coherent light source parameters. The output pulse energy and transverse and longitudinal coherence are comparable with those from conventional XFELOs. This promising scheme is expected to contribute significantly to the construction and operation of a real XFELO.