New J. Phys. 13, 063012 (2011)

Credit: © 2011 IOP

Researchers have now improved a recent proposal for mode-locking free-electron lasers. In the original concept, placing magnetic chicanes between magnetic undulator sections imposes an axial-mode structure on the generated radiation, and the adjacent modes are locked in phase by appropriately modulating the electron beam energy. Now, Eugene Kur and colleagues from the USA and the UK have proposed that the lasers could be mode-locked by modulating the beam current instead of the beam energy. They say that this should result in a cleaner output and increased spectral brightness to beyond what is possible with current X-ray source schemes of comparable bandwidth, such as high-order harmonic generation and storage-ring-based synchrotrons. Numerical simulations predict that a spectral brightness of 30–180 μJ eV−1 should be possible at a central wavelength of 6.2 nm. The researchers also show that mode-locking might allow the output bandwidth to be controlled by varying the undulator section length.