First lasing of an echo-enabled harmonic generation free-electron laser

Free-electron lasers have been successfully operated with ultrahigh brightness and excellent transverse coherence at X-ray wavelengths(1-4). One of the next goals for further improvements is full coherence(5,6). An obvious approach is to seed the free-electron laser interaction using a conventional source that has good temporal coherence(7-12). Here, we show the first lasing of a free-electron laser with an echo-enabled harmonic generation scheme(11), which shows great promise for producing coherent lasing at short wavelengths, even in the X-ray regime. The experiment was conducted at a test facility(13,14) that combines a 135.4 MeV electron accelerator with an amplifier consisting of a series of undulator magnets. Lasing was achieved at the third harmonic of the seed with a gain of similar to 100,000 over spontaneous radiation. The measurements show typical exponential growth and excellent spectral characteristics, as well as good intensity stability.