Development of a Femtosecond Soft X-ray SASE FEL at DESY

In this paper we describe the extension of the soft X-ray SASE FEL at the TESLA Test Facility (TTF) at DESY for generation of femtosecond pulses. The idea is based on a technique of \manipulating" the energy spread along the electron bunch. Since the radiation ampli cation process is extremely sensitive to the value of the energy spread, only that part of the electron bunch radiates which has low energy spread. A novel technique is proposed for preparation of electron beam with required shaping of the energy spread allowing to overcome the problem of electron pulse jitter (about 1 ps). The system consists of two undulators separated by electron bypass and optical pulse shaping system. Undulators are tuned for ampli cation of the radiation with 523 nm wavelength. A seed radiation pulse (10 ps pulse duration, 0.3 J pulse energy, 30 kW peak power) is generated by present TTF laser facility. The rst undulator operates in the linear regime and produces 300 fs radiation pulse naturally synchronized with the electron bunch. After the exit of the rst undulator the electron bunch is guided through a bypass, and the light enters the pulse shaping system. The shaping system produces zero area light pulse (i.e. the pulse with zero value of optical eld in the central area of the pulse) which is ampli ed up to the saturation level in the second undulator. Large energy spread is induced in the signi cant fraction of the electron beam due to the FEL interaction process, and only a small part of the electron bunch (near the center of zero area light pulse) is capable to produce radiation in the 6 nm SASE FEL. The SASE FEL described in this paper will provide soft X-ray pulses with 30 fs (FWHM) duration. On the basis of the TTF parameters it should be possible to achieve an average brilliance of 1022 photons s 1mrad 2mm 2 per 0.1% BW. The average number of photons can exceed 1012 photon/pulse. Present conceptual design is compatible with the layout of the soft X-ray SASE FEL being under construction at DESY and can be realized with minimal additional e orts. Preprint submitted to 4 January 2095