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, $\sim 0.1\%$ (FWHM), due to large momentum transfer from electrons to photons. The $\gamma$-ray beam has a spectral density of $\sim {10}^2\mathrm{ph}/(\mathrm{MeV}\mathrm{s})$ with a typical set of parameters based on a 7-GeV electron beam operated at 3-MHz repetition, Such $\gamma$-rays will be a unique probe for studying hadron physics. Features of the $\gamma$-ray source, flux, spectrum, polarization, tunability and energy resolution are discussed.

Figure 1

Schematic views of x-ray FEL oscillators. (a) An oscillator with two normal-incidence sapphire crystals and compound refractive lenses (CRL). (b) An oscillator consisting of two diamond crystals with nonzero Bragg angle, ${\mathrm{\Theta }}_B\ne 0$ and two focusing mirrors with a small grazing angle ${\varphi }_G<1\mathrm{mrad}$.

Figure 2

A schematic view of laser Compton scattering.

Figure 3

Energy-differential cross section of Compton scattering for 7 GeV electrons and laser photons at three different energies of 1.2 eV, 120 eV and 12 keV.

Figure 4

$\gamma$-ray spectra of the two-mirror 7-GeV XFELO-$\gamma$ calculated by an analytical formula without electron energy spread (green) and with energy spread (red), and numerical simulation by cain (blue).

Figure 5

Correlation of $\gamma$-ray energy and scattering angle in the two-mirror 7-GeV XFELO-$\gamma$ calculated by an analytical formula without electron energy spread (red line) and numerical simulation by cain (blue dots).

Figure 6

$\gamma$-ray spectra of the four-mirror 7-GeV XFELO-$\gamma$ calculated by an analytical formula without electron energy spread (green) and with energy spread (red), and numerical simulation by cain (blue).

Figure 7

Results of the cain simulation for $\gamma$-ray photons generated via Compton scattering with a spin polarized electron beam in the two-mirror XFELO-$\gamma$. (a) Degree of polarization, (b) The Stokes parameter for circular polarization.