meV-resolution single-shot hard x-ray spectrograph for cavity-based x-ray free-electron lasers

Cavity-based X-ray free-electron lasers (CBXFEL) is a possible realization of fully coherent hard X-ray sources of high spectral brilliance with a narrow spectral bandwidth of $\simeq 1 - 50 meV, a high repetition pulse rate of ~1 MHz, and stability. A diagnostic tool is required to measure CBXFEL spectra with a meV resolution and high luminosity on a shot-to-shot basis. A high-luminosity single-shot hard X-ray spectrograph is designed to image 9.831-keV x-rays in a ~200-meV spectral window with a spectral resolution of a few meV. Operational principle of the spectrograph is based on angular dispersion in Bragg diffraction from crystals. The spectrograph operates close to design specification exhibiting a linear dispersion rate of 1.4~$\mu$m/meV and a 200-meV window of high-fidelity spectral imaging. The demonstrated in the experiment spectral resolution of 20 meV is limited by high sensitivity to crystal angular instabilities at the bending magnet beamline 1BM at the Advanced Photon Source, where it was tested.