Method for reducing the radiation bandwidth of an x-ray FEL

Abstract

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 guided through a non-isochronous bypass and the X-ray beam enters the monchromator. The mail function of the bypass is to suppress the modulation of the electron beam induced in the first undulator. This is possible because of the finite value of the natural energy spread in the bem. 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 TEst Facility under construction at DESY. The spectral bandwidth of such a two-stage SASE FEL (Δλ/λ≈ 5 x 10^-5) is close to the limit defined by the finite duration of the radiation pulse. The average spectral brilliance is equal to 7 x 10²⁴ photons/(secxmrad2 x mm2 x 0.1 % bandw.) which is by two orders of magnitude higher than the value which could be reached by the conventional SASE FEL.