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 Self-Amplified Spontaneous Emissi on (SASE) FEL is proposed, The scheme consists of two undulators and a n X-ray monochromator located between them, The first stage of the FEL amplifier operates in the SASE linear regime, After the exit of the f irst undulator the electron bunch is guided through a non-isochronous bypass and the X-ray beam enters the monochromator, The main function of the bypass is to suppress the modulation of the electron beam induc ed in the first undulator. This is possible because of the finite valu e of the natural energy spread in the beam, At the entrance to the sec ond undulator the radiation power from the monochromator dominates sig nificantly 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 when the input signal bandwidth is small with respect to that of the FEL amplifier, Integral losses of the radiation power in t he monochromator are relatively small because grazing incidence optics can be used. The proposed scheme is illustrated for the example of th e 6 nm option SASE FEL at the TESLA Test Facility under construction a t DESY. As shown in this paper the spectral bandwidth of such a two-st age SASE FEL (Delta lambda/lambda similar or equal to 5 X 10(-5)) is c lose to the limit defined by the finite duration of the radiation puls e. The average brilliance is equal to 7 X 10(24) photons/(s X mrad(2) X mm(2) X 0.1% bandw.) which is by two orders of magnitude higher than the value which could be reached by the conventional SASE FEL, The mo nochromatization of the radiation is performed at a low level of radia tion power (about 500 times less than the saturation level) which allo ws one to use conventional X-ray optical elements (grazing incidence g rating and mirrors) for the monochromator design. (C) 1997 Elsevier Sc ience B.V.