Scientific Opportunities with an X-ray Free-Electron Laser Oscillator

An X-ray free-electron laseroscillator (XFELO) is a new type of hard X-raysource that would produce fully coherent pulses with meV bandwidth and stable intensity. The XFELO complements existing sources based on self-amplified spontaneous emission(SASE) from high-gain X-ray free-electron lasers(XFEL) that produce ultra-short pulses with broad-band chaotic spectra. This report is based on discussions of scientific opportunities enabled by an XFELO during a workshop held at SLAC on June 29 - July 1, 2016