Technological challenges to X-ray FELs

There is strong interest in the development of X-ray Free Electron Lasers (X-ray FELs). The interest is driven by the scientific opportunities provided by intense, coherent X-rays. An X-ray FEL has all the characteristics of a fourth-generation source: brightness several orders of magnitude greater than presently achieved in third-generation sources, full transverse coherence, and sub-picosecond long pulses. The SLAC and DESY laboratories have presented detailed design studies for X-ray FEL user-facilities around the 0.1 nm wavelength regime (LCLS at SLAC, TESLA X-ray FEL at DESY). Both laboratories are engaged in proof-of-principle experiments at longer wavelengths (TTF FEL Phase I at 71 nm, VISA at 600–800 nm) with results expected in 1999. The technologies needed to achieve the proposed performances are those of bright electron sources, of acceleration systems capable of preserving the brightness of the source, and of undulators capable of meeting the magnetic and mechanical tolerances that are required for operation in the SASE mode. This paper discusses the technological challenges presented by the X-ray FEL projects.