Phys. Rev. X 2,031019 (2012) Phys. Rev. Lett. (in the press); preprint via http://go.nature.com/Udw5Zi (2012)

Free-electron lasers generate high-intensity beams of coherent X-rays by manipulating the trajectory of relativistic electrons. But such sources are large and expensive. Two teams of researchers now present a blueprint for a compact source of coherent, long-wavelength radiation using a laser–plasma accelerator.

Laser-induced waves in a plasma can accelerate charged particles up to velocities near the speed of light. However, a beam generated in such a laser–plasma accelerator has not yet successfully seeded a free-electron laser, mainly due to the broad energy distribution of the accelerated electrons.

Andreas Maier and his colleagues have designed and modelled a two-metre long undulator — an arrangement of magnets that controls the direction of electron motion — that corrects for the energy spread by longitudinally decompressing the bunch of electrons. Similarly, Zhirong Huang and co-workers considered an undulator that tackled the same problem, but acted instead to produce an optimized transverse field. Both undulator designs could work on electron beams with the energy-spread characteristics of state-of-the-art laser–plasma accelerators.