An X-ray Free Electron Laser Driven by an Ultimate Storage Ring

There is a worldwide interest in developing so-called ultimate storage ring (USR) light sources having electron emittances near the X-ray diffraction limit that would provide spectral brightness one or two orders of magnitude higher than present-day, third-generation sources and very large coherent flux in the multi-keV photon energy range [1 Hettel, R. and Qin, Q. 2013. Report on the Workshop on Accelerator R&D for Ultimate Storage Rings, Oct. 30–Nov. 1, 2012, Beijing, China, SLAC-PUB-15379, February [Google Scholar]]. At the same time, there is a growing scientific interest in X-ray FEL sources that can provide a continuous train of evenly spaced, low peak power, coherent photon pulses at repetition rates of above 1 kHz, unlike the bursts of much higher frequency pulses that can be provided by linac-based FELs pulsed with repetition rates of order 100 Hz or less. These CW sources would enable dynamic imaging of materials undergoing transitions in millisecond or less time scales and would open up the development of new non-line spectroscopic techniques that could lead to a better understanding of electronic and nuclear dynamics in materials.