Partially coherent X-rays from modern storage rings

Several routes to the development of ideal soft X-ray sources are now being actively pursued. These include atomic lasers, free electron lasers (FELs), and storage ring (synchrotron) undulators. For short wavelength (XUV) applications, FELs and undulators are very closely related, each being dependent on the development of well controlled (low emittance) electron storage rings, with energies of order 0.5 to 1.5 GeV beam energy, and many period magnetic structures, referred to as undulators. The major thrust of this article is that undulators provide the only sure route to coherent soft X-rays in the near term, that they are tuneable throughout the region of interest, and that they will serve a multitude of users, in disparate fields of science and technology, albeit at a large central research facility.