Low-loss Stable Storage of X-ray Free Electron Laser Pulses in a 14 m Rectangular Bragg Cavity

Abstract We present an experimental demonstration of a stable, low-loss, large x-ray cavity operating at 1.2 Angstrom wavelength. The cavity consists of 4 high reflectivity single crystal diamond Bragg mirrors arranged in a rectangular configuration with a design round trip distance of 14.2 m. Femtosecond x-ray laser pulses from an x-ray free electron laser were coupled into the cavity via a transmission phase grating. The cavity round trip efficiency and lifetime were characterized quantitatively, indicating close to the theoretical optical performance. By introducing focusing optics, we show that a stable cavity mode can be trapped within. The direct observation of sustained stable x-ray circulation provides the most direct evidence to date that cavity-based x-ray free electron lasers and other cavity-based hard x-ray systems are feasible.