We review the synchrotron type radiation sources with focus on undulator and free-electron laser (FEL) schemes, aimed on working in X-ray range and ultra-short time interval. Main FEL schemes, useful for generation of high frequency radiation, extending to X-rays, are presented. High harmonic generation is explored. The advantages and disadvantages of single pass and of multipass designs are discussed. The viable ways to reduce the duration of the pulse, with the goal to generate femtosecond pulses, are indicated. Future developments of X-ray FELs (X-FELs) and the ways to improve the quality of the FEL radiation in this context are discussed.
Part of the book: High Energy and Short Pulse Lasers