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Having seen proof that FEL principles work for hard x-rays for lCLS, it is time to seriously consider more advanced hard x-ray sources. An x-ray FEL oscillator (XFELO) promises to provide an extremely narrow bandwidth of a few meV (corresponding to 10-7 in relative bandwidth) with an average brightness higher by several orders of magnitude than, and peak brightness comparable to, the SASE XFEL. These capabilities will provide breakthrough opportunities in areas complementary to SASE XFELs as well as drastically enhance the capabilities of techniques developed at third-generation synchrotron radiation facilities-particularly those requiring high coherence and high spectral purity. The main technological challenges for XFELO are: an injector providing a continuous sequence of ultralow emittance, low intensity electron bunches at a few MHz repetition rate; high quality diamond crystals serving as the main mirrors for x-ray cavity; high-reflectivity grazing incidence, curved mirrors for controlling the transverse mode profile; tight tolerances for angular stability of the optical elements. We will present progresses in conceptual and experimental efforts to advance the state-of-the-arts in these topics.
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