On the path to the next generation of light sources

At the Fourth Generation Light Source Workshop in January 1996, storage-ring-based FELs and linac-based FELs were considered for VUV/soft X-ray and hard X-ray (λ ∼ 1 Å) photon production, respectively. The general requirements of a diffraction-limited source-bunch lengths less than 1 ps (100 fs in case of the linac-based source), coherence, and a brightness at several orders of magnitude higher than the third-generation sources-were among the features discussed. Examples of progress in demonstrations, measurements, or calculations that support research towards these areas are cited. These range from the 50 fs bunch length measurement at low current in a linac to few-μrad divergence measurements on a 7 GeV storage ring. The possible extension of transition radiation-beam characterization techniques to nonintercepting diffraction radiation is briefly addressed as well.