Short-Wavelength Free-Electron Lasers with Periodic Plasma Structures

Concepts of compact free-electron lasers that are based on beam-plasma interactions and that operate in the vacuum ultraviolet and x-ray wavelength ranges are discussed. Coherent radiation can not only be produced by periodic transverse motions of an electron beam, but also by its longitudinal motions. In this latter case, coherent transition radiation is generated when an electron beam passes through a structure with a deep periodic modulation of the plasma electron density. A number of structures are considered as short-wavelength radiators: standing Langmuir solitons or collapsing caverns, fast nonlinear longitudinal plasma waves, artificial periodical structures that can be converted into the plasma state by a powerful current generator or by a laser pulse, and periodic z-pinches produced by a thin wire.