Lasing of a cavity-based X-ray source

Cavity-based X-ray Free Electron Lasers (CBXFELs) promise to transform thefield of hard X-ray science by delivering laser-like radiation with exceptional bril-liance, stability, and full three-dimensional coherence. At the European XFEL,we have constructed a proof-of-concept CBXFEL featuring a 133 m round-triphard X-ray cavity using diamond Bragg crystals. Here, we report the first las-ing with multi-pass gain in an X-ray cavity, marking a major milestone for thefield. Our results establish the feasibility of CBXFELs and open the door tonext-generation X-ray sources with unprecedented spectral purity and stability.