A free electron laser (FEL) in Trieste, Italy, began producing 40- to 150-fs pulses at soft x-ray and extreme-UV wavelengths last year, and commissioning began this fall on a second FEL that will reach shorter wavelengths. Based on a conventional linac operating at 10 Hz and 1.2 GeV, the €160 million ($210 million) FERMI@Elettra source is the first user facility to implement seeding. The result: high-gain light pulses that are coherent; are stable in intensity, photon energy, and bandwidth; and have tunable energy and polarization that can be switched from linear to circular.

In seeding, an external laser imprints its coherent field and narrow bandwidth on a relativistic electron beam as the electrons enter a series of undulators. The technique overcomes the drawbacks of relying, as most FELs do, on spontaneous electron organization. The Trieste beam shows a “spectacular degree of both transverse and longitudinal coherence and will allow experiments...

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