Coherent radiation from a relativistic electron beam is a valuable way to overcome the present limitations of conventional lasers and synchrotron radiation light sources. The typical scheme has electrons, directly from a linac, in a single-pass interaction with a laser pulse in the presence of a static undulator magnetic field. We demonstrate that a storage-ring free-electron laser can also achieve harmonic generation (down to 36.5 nm), presenting both experimental and theoretical results, and offer a reliable interpretation of the peculiar underlying physical processes involved.

• Received 18 November 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.104801