Nonlinear interactions of lasers with channeled beams as sources of coherent X-radiation

One of the main trends in the area of stimulated X-ray emission has been the development of sources based on beams of relativistic or sub-relativistic emitters. The emission from such sources is continuously tunable, owing to the Doppler shift towards progressively shorter wavelengths as the beam velocity increases. The better known sources in this category are various versions of free-electron lasers (FELs), encompassing structures wherein a spatially periodic perturbation acts either on the electron beam or on the emitted radiation. This article is intended to draw attention to recent work concerning a different type of high-velocity emitters, namely, relativistic electrons or positrons channeled in crystals or in artificial periodic structures. The appeal of such emitters is the possibility of using them as a source of fully coherent, tunable, stimulated X-radiation. (author)