  With wavelength tunability, free-electron lasers (FELs) are well-suited for
generating orbital angular momentum (OAM) beams in a wide photon energy range.
We report the first experimental demonstration of OAM beam generation using an
oscillator FEL. Lasing around 458 nm, we have produced the four lowest orders
of coherently mixed OAM beams with good beam quality, excellent stability, and
substantial intracavity power. We have also developed a pulsed mode operation
of the OAM beam with a highly reproducible temporal structure for a range of
modulation frequencies from 1 to 30 Hz. This development can be extended to
short wavelengths, for example to x-rays using a future x-ray FEL oscillator.
The operation of such an OAM FEL also paves the way for the generation of OAM
gamma-ray beams via Compton scattering.
