Narrow-band GeV photons generated from an x-ray free-electron laser oscillator

We propose a scheme to generate narrow-band GeV photons, γ-rays, via Compton scattering of hard x-ray photons in an x-ray free-electron laser oscillator. Generated γ-rays show a narrow-band spectrum with a sharp peak, ~0.1% (FWHM), due to large momentum transfer from electrons to photons. The γ-ray beam has a spectral density of ~10² ph/(MeV s) with a typical set of parameters based on a 7-GeV electron beam operated at 3-MHz repetition, Such γ-rays will be a unique probe for studying hadron physics. Features of the γ-ray source, flux, spectrum, polarization, tunability and energy resolution are discussed.