ANL, Argonne, USA
The recently successful x-ray SASE (self-amplified-spontaneous emission) is a high-gain free-electron laser (FEL) that amplifies, in a single pass, the initial spontaneous emission to intense, quasi-coherent x-rays. Another type of FEL for the hard x-ray radiation is an x-ray FEL oscillator (XFELO), employing a high-Q x-ray cavity in which x-ray pulses are trapped and repeatedly amplified. The characteristics of an XFELO are complementary to an SASE; full coherence with a few meV bandwidth, high average brightness, and a peak spectral brightness comparable to an SASE. These characteristics could revolutionize the spectroscopic techniques such as inelastic scattering, nuclear resonant scattering, and hard x-ray photoemission, as well as atomic resolution imaging. A multi-GeV ERL is well-suited for driving an XFELO due to its excellent beam qualities and high repetition rate. Diamond crystal is most suitable for x-ray cavity mirror due to its excellent thermo-mechanical properties. In this talk we will review various issues for realizing an XFELO and the R&D efforts in progress at several laboratories.
