Section 8.1.5.5. X-ray free electron laser (XFEL)

In terms of the evolution of X-ray sources, mention should be made of the X-ray free electron laser (XFEL); it now seems feasible that this will yield wavelength output well below the visible region of the electromagnetic spectrum. At DESY in Hamburg (Brinkmann et al., 1997) and at SLAC (Winick, 1995), such considerations and developments are being pursued. Compared to SR, one would obtain a transversely fully coherent beam, a larger average brilliance and, in particular, pulse lengths of ~200 fs full width at half-maximum with eight to ten orders of magnitude larger peak brilliance. Such a machine is based on a linear accelerator (linac)-driven XFEL utilizing a linear collider installation (e.g., for a high-energy physics centre-of-mass energy capability of 500 GeV). For this machine there is a `switchyard' distributing the electrons in a beam to different undulators from which the X-rays are generated in the range 0.1 to ~12 keV. The anticipated r.m.s. opening angle would be 1 mrad and the source diameter would be 20 µm. This source of X-rays would then compete in time resolution with laser-pulse-generated X-ray beams [see Helliwell & Rentzepis (1997) for a survey of that work and a comparison with synchrotron radiation] and would also have higher brilliance.