POSSIBLE APPLICATION OF X-RAY OPTICAL-ELEMENTS FOR REDUCING THE SPECTRAL BANDWIDTH OF AN X-RAY SASE FEL

A new design for a single pass X-ray SASE FEL is proposed. The scheme consists of two undulators and an X-ray monochromator located between them. The first stage of the FEL amplifier operates in the SASE linear regime. After the exit of the first undulator the electron bunch is g uided through a non-isochronous bypass and the X-ray beam enters the m onochromator. The main function of the bypass is to suppress the modul ation of the electron beam induced in the first undulator. This is pos sible because of the finite value of the natural energy spread in the beam. At the entrance to the second undulator the radiation power from the monochromator dominates significantly over the shot noise and the residual electron bunching. As a result, the second stage of the FEL amplifier operates in the steady-state regime. The proposed scheme is illustrated for the example of the 6nm option SASE FEL at the TESLA Te st Facility under construction at DESY. The spectral bandwidth of such a two-stage SASE FEL (Delta lambda/lambda similar or equal to 5 x 10( -5)) is close to the limit defined by the finite duration of the radia tion pulse. The average spectral brilliance is equal to 2 x 10(24) pho tons/(sec x mrad(2) x mm(2) x 0.1% bandwidth) which is by two orders o f magnitude higher than the value which could be reached by the conven tional SASE FEL.