Progress Toward an Ultimate Storage Ring Light Source

Developments such as the low emittance NSLS-II storage ring, followed by the even lower emittance MAX IV ring, indicate that the technology of storage ring light sources has not reached full maturity. Indeed, these new sources are paving the way toward realizing diffraction-limited angstrom-wavelength storage ring light sources in the not-too-distant future. In this paper, we survey ongoing work around the world to develop concepts and designs for so-called "ultimate" storage ring light sources. Several of these designs target horizontal emittances that are two or more orders of magnitude less than present machines, and thus brightness and coherent fraction that is several orders of magnitude better than existing storage ring light sources.