Scientists and engineers at DESY have so far made remarkable progress. In autumn 2000 they finished building a free-electron laser that produces vacuum-ultraviolet light down to wavelengths of 80 nm. The laser has already produced its first scientific results in a study of how clusters of xenon atoms interact with intense femtosecond pulses of ultraviolet light. It is now being upgraded into a full user facility that will, by late 2004, produce light with a wavelength of just 6 nm.
By the start of the next decade, the lab hopes to have completed a fully functioning X-ray free-electron laser that can produce light with a wavelength of 0.1 nm. The laser will form part of the ambitious -3.5 bn TeV Energy Superconducting Linear Accelerator (TESLA) facility that will also be used as an electron-positron collider. Although the original plan was for the laser and collider to share the same accelerator, they will now be run as separate projects.
Designing X-ray free-electron lasers requires three groups of scientists to join forces and focus on a common goal. Specializing in accelerators, synchrotron radiation and lasers, each group views these new facilities as a natural development in its respective field.
Synchrotron scientists, for instance, see X-ray free-electron lasers as a way of continuing the almost exponential growth in the intensity of radiation obtained from synchrotron sources over the past 50 years. The new lasers will deliver a peak "brilliance" that will be some eight orders of magnitude higher than the current synchrotron sources.
Laser physicists, meanwhile, have long sought to extend optical laser technology from the visible into the X-ray region. Although the radiation from a free-electron laser is similar to that obtained from conventional optical lasers - featuring high power, narrow bandwidth, femtosecond pulses and diffraction-limited beam propagation - there is one big difference. Free-electron lasers can be continuously tuned over a wide range of wavelengths, rather than having to operate at a fixed frequency or over a very narrow range.
Finally, X-ray free-electron lasers present a new challenge for accelerator scientists, who have to create electron beams of exceptionally high quality. The systems are also more difficult to design than existing free-electron lasers operating in the infrared, visible and ultraviolet regime, as these are based on short linear accelerators or synchrotron-radiation storage rings.
In the July issue of Physics World Elke Plönjes, Josef Feldhaus and Thomas Möller from the Hamburger Synchrotronstrahlungslabor (HASYLAB) in Germany describe the huge scientific potential of free-electron lasers that operate at X-ray wavelengths.