The Free Electron Laser is a powerful research tool, potentially allowing scientists to ‘see’ the processes of atoms at their own spatial and temporal scales. Current operational x-ray FEL’s, rely on the amplification of noise (aka Self Amplified Spontaneous Emission - ‘SASE’) which limits the temporal coherence of their output. The Echo-Enabled Harmonic Generation FEL is a method that has been proposed to allow the transfer of the good temporal coherence properties of relatively long wavelength conventional seed lasers into the x-ray regime, where seeds lasers are currently unavailable.
A new method that combines EEHG with mode-coupling from conventional laser techniques, has now been proposed in a paper published in Europhysics Letters by James Henderson and Brian McNeil . The EEHG method creates a beam of electrons that possesses a phase correlated modal structure of their microbunching in frequency space. Using analysis and simulations, the authors show that this can be exploited by injecting the beam into a Mode-Locked FEL radiating structure , to generate a train of short-pulses (~106 attoseconds FWHM duration) at x-ray wavelengths – see Figure.
The importance of short FEL pulses could be seen at the LCLS facility in 2010-11 when, for the first time, structural information of tiny protein nano-crystals was captured and the first single shot images of intact viruses were recorded using the longer duration SASE output. With a coherent train of much shorter pulses, in effect an x-ray stroboscope, it may be possible to go further and create clear ‘molecular movies’ of other important chemical and biological processes.
 J. R. Henderson & B. W. J. McNeil, Echo enabled harmonic generation free electron laser in a mode-locked configuration, Europhysics Letters, 100, 64001 (2012)
 N.R. Thompson & B.W.J. McNeil, Phys. Rev. Lett., 100, 203901 (2008)