Delay-detune scan for longitudinal electron beam characterization at free-electron lasers

Abstract

We present first experimental results of a novel method to study the energy chirp of the electron beam lasing window at a free-electron laser operating in self-amplified spontaneous emission mode. The method requires a single magnetic chicane splitting the FEL undulator into two parts of several gain lengths each. Undulator segments should have variable gaps. By scanning both the delay in the chicane and detuning in a part of the undulators, one can retrieve the linear component of both the radiation frequency chirp and the electron energy chirp. In addition, such scan improves the accuracy of the previously proposed autocorrelation-based pulse duration measurement technique. The proposed method targets facilities that lack direct diagnostics of the electron beam longitudinal phase space distribution. It also can be extended to diagnose transverse tilts of the beam.