High Bragg reflectivity of diamond crystals exposed to multi-kW mm⁻² X-ray beams

X-ray free-electron lasers in the oscillator configuration (XFELO) are future fully coherent hard X-rays sources with ultrahigh spectral purity. X-ray beams circulate in an XFELO optical cavity comprising diamond single crystals. They function as high-reflectance (close to 100%), narrowband (∼10 meV) Bragg backscattering mirrors. The average power density of the X-ray beams in the XFELO cavity is predicted to be as high as ∼10 kW mm⁻². Therefore, XFELO feasibility relies on the ability of diamond crystals to withstand such a high radiation load and preserve their high reflectivity. Here the endurance of diamond crystals to irradiation with multi-kW mm⁻² power density X-ray beams is studied. It is shown that the high Bragg reflectivity of the diamond crystals is preserved after the irradiation, provided it is performed at ∼1 × 10⁻⁸ Torr high-vacuum conditions. Irradiation under 4 × 10⁻⁶ Torr results in a ∼1 meV shift of the Bragg peak, which corresponds to a relative lattice distortion of 4 × 10⁻⁸, while the high Bragg reflectivity stays intact.