Femtosecond X-ray pulses from a spatially chirped electron bunch in a SASE FEL

We propose a simple method to produce short X-ray pulses using a spatially chirped electron bunch in a SASE FEL. The spatial chirp is generated using an RF deflector which produces a transverse offset (in y and/or y′) correlated with the longitudinal bunch position. Since the FEL gain is very sensitive to an initial offset in the transverse phase space at the entrance of the undulator, only a small portion of the electron bunch with relatively small transverse offset will interact significantly with the radiation, resulting in an X-ray pulse length much shorter than the electron bunch length. The X-ray pulse is also naturally phase locked to the RF deflector and so allows high precision timing synchronization. We discuss the generation and transport of such a spatially chirped electron beam and show that tens of femotsecond long pulse can be generated for the linac coherent light source.