<title>Considerations in short-wavelength (extreme-UV and x-ray) free-electron laser using quantum interference</title>

Abstract

Ordinary Free-Electron Lasers (FELs) can be found in successful operation in the spectral range from millimeters to ultraviolet wavelengths. However the operation of the common FELs in the extreme ultraviolet and X-ray wavelength regimes faces certain adverse effects. Some of the main obstacles in the way of the realization of X-ray FEL are electron momentum spread and angular divergence. Another point to keep in mind is that ordinary FELs work on the principle of 'momentum population inversion.' By this one means that electrons with momenta larger than the resonant value contribute to the gain whereas electrons with momenta smaller than the resonant value contribute to the loss. Thus to ensure a net gain we need more electrons with momenta lying in the upper momentum domain than in the lower one i.e. a 'momentum population inversion.' Keeping these points in mind Scully and co-workers have proposed using the ideas of Lasing Without Inversion (LWI) to achieve the successful operation of short-wavelength (extreme UV and X-ray) FELs. The purpose of this work is to, as a first step, critically analyze the theoretical and the practical feasibility of the proposals by Scully and co-workers. In particular we take a look at the following issues: Can the LWI FEL's operate efficiently even with a strongly inhomogeneous, broad electron momentum distribution? How practically feasible is the two-section Cherenkov Transition Radiation (TR) FEL? Does this Cherenkov TR FEL allow a complete absorption cancellation and LWI operation even in the case of a very broad electron momentum distribution compared to the homogeneous width? Is the gain of LWI FEL greater than the gain of the usual FEL by a factor of 100 (i.e. by two orders of magnitude)? How realistic is the proposed 'classical selective interference?' Moreover is the 'classical selective interference' free of the angular spread limitation? How valid is the small-signal gain calculation? The saturation would invalidate the small-signal analysis: when precisely does the saturation set in? The saturation is expected to affect the phase coherence required for the interference: what is the form of dependence of phase coherence on the saturation?© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.