The current, so-called third generation, synchrotron radiation facilities are based on x-rays from periodic magnets, known as undulators, in straight sections of optimized storage rings. The bright x-rays from these modern facilities, of which Argonne’s Advanced Photon Source is a prime example, have become indispensable tools in physical and biological sciences and engineering. After several decades of R&D in accelerator/FEL beam dynamics as well as in linac and undulator technologies, the art of producing bright x-rays is currently undergoing another revolutionary development. Free electron laser (FEL) was finally realized recently for x-ray wavelengths. In an FEL, the interaction between the radiation and the beam in an undulator leads to a significant gain in radiation intensity. The LCLS was the first high-gain x-ray FEL amplifier built at SLAC in which the gain is so high that the initial spontaneous emission evolves into an intense, quasi-coherent radiation in a long undulator. Several other high-gain x-ray FEL facilities are either in operation or under construction around the world and efforts to improve their performance are pursued vigorously. An x-ray FEL oscillator (XFELO) is also possible by employing an x-ray resonator using diamond crystals as high-reflectivity mirrors. Not only producing fully coherent, spectrally pure x-ray beams, an XFELO can also be mode-locked to produce spectral combs by referencing to a narrow nuclear resonance line, allowing x-ray quantum optical techniques for testing fundamental physics and improving the length/frequency standards. Meanwhile, the technology of storage ring-based sources is also advancing towards an “ultimate storage ring” for the production of “diffraction-limited” radiation in the x-ray wavelength region.

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