X-ray free-electron lasers (XFELs) generate X-ray by electrons flying through a periodic magnetic field.-XFELs are the leading X-ray sources with ultra-high brightness and ultra-short duration.-XFELs can be launched from either the shot noise of the electron beam or the seed.-XFEL-laser collision is proposed to learn the nature of vacuum at SHINE.-XFELs are being combined with intense lasers and synchrotron radiation light sources.

Light with orbital angular momentum (OAM) provides new insights into a wide range of physical phenomena and has engendered advanced applications in various fields. Additionally, interest in X-ray OAM has been rapidly rising. We present a straightforward method to generate intense OAM beams from an X-ray free-electron laser oscillator (XFELO). The method leverages Bragg mirrors and longitudinal-transverse mode coupling to enable mode selection in a conventional XFELO configuration, thereby natively producing the fully coherent hard X-ray beams carrying OAM. Furthermore, simulation results demonstrate that fully coherent hard X-ray OAM beams at 1 MHz with a pulse energy of about 120 µJ can be generated without the optical mode converters. This simple approach could significantly advance the creation of X-ray OAM while stimulating the development of experimental methods.