Low-loss stable storage of 1.2 Å X-ray pulses in a 14 m Bragg cavity

We present an experimental demonstration of a stable, low-loss, large X-ray cavity operating at 1.2 Å wavelength. The cavity consists of four separate high-reflectivity single crystal diamond Bragg mirrors arranged in a rectangular configuration with a round-trip distance of 14.2 m. Femtosecond X-ray pulses from an X-ray free-electron laser were coupled into the cavity via a transmission phase grating. We show that a stable mode can be maintained with the introduction of cavity focusing and measure round-trip efficiencies approaching 88%, or >96% when excluding grating and lens losses, close to the Bragg mirror theoretical performance limit. 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.