Optical afterburner for a SASE FEL: First results from FLASH

Radiation pulse from a Self-Amplified Spontaneous Emission Free Electron Laser (SASE FEL) consists out of spikes (wavepackets). Energy loss in the electron beam (averaged over radiation wavelength) also exhibit spiky behavior on a typical scale of coherence length, and follows the radiation pulse envelope. These modulations of the electron beam energy are converted into large density (current) modulations on the same temporal scale with the help of a dispersion section, installed behind the x-ray undulator.
Powerful optical radiation is then generated with the help of a dedicated radiator (afterburner). We have recently demonstrated this principle at the Free Electron Laser in Hamburg (FLASH), and this paper present the first experimental results.