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

We propose a scheme to generate narrow-band GeV photons, gamma-rays, via Compton scattering of hard x-ray photons in an x-ray free-electron laser oscillator. Generated gamma-rays show a narrow-band spectrum with a sharp peak, similar to 0.1% (FWHM), due to large momentum transfer from electrons to photons. The gamma-ray beam has a spectral density of similar to 10(2) ph/(MeV s) with a typical set of parameters based on a 7-Ge V electron beam operated at 3-MHz repetition, Such gamma-rays will be a unique probe for studying hadron physics. Features of the.-ray source, flux, spectrum, polarization, tunability and energy resolution are discussed.