Specific features of operation of free-electron lasers with a short bunch

The physical specific features of a free-electron laser in the short-wave region in which the bunch length in the proper frame is smaller than the undulator length in the same reference frame are discussed. The regimes of amplification of the external harmonic signal and the induced amplification of proper noise with forming the quasi-coherent radiation, i.e. the self-amplified spontaneous emission (SASE) regime, are considered.