Fig. 1. Longitudinal phase space of the electron beam with normal laser modulation (left), frequency beating with the unchirped laser (middle) and with the chirped laser (right).

Fig. 2. Schematic layout of the proposed scheme.

Fig. 3. Longitudinal phase space of the electron beam at the end of the linac (left) and at the exit of the ESASE section (right). The bunch head is to the right.

Fig. 4. Electron beam energy modulation from the longitudinal space charge field from the 39 m undulator.

Fig. 5. The output radiation evolution and the spectrum along the undulator line at the end of the first, fourth and ninth undulator segments.

Fig. 6. FEL gain curves for the proposed scheme and the normal SASE.