X-ray free-electron laser in the quantum regime

808-817 (April 1994) The action of free electron lasers (FELs) operating in a fundamentally new regime is examined. In this regime the size of an emitted photon exceeds the radiation line width. It is shown that such an FEL consists of a two-level quantum generator with a completely inverted active medium. Analysis of the nonlinear dynamics of an FEL in this regime shows, in analogy with the process by which a .rr-pulse is formed in a coherently amplifying medium, that the inversion can be completely removed in a single pass. The nature of the physical processes in a "two-level" FEL is essentially quantum-mechanical and cannot be described using classical electrodynamics. Lasing in this quantum regime can be achieved in an x-ray FEL with an optical undulator.