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.
Highlights
Free-electron lasers (FELs) are state-of-the-art tools to generate high-brightness x-ray radiation pulses by exploiting a resonant instability that develops while a high-quality electron beam propagates through a properly tuned undulator
If the FEL operates in self-amplified spontaneous emission (SASE) mode, the longitudinal phase space of the electron beam is imprinted into the time-frequency representation of the emitted radiation according to the resonance condition ω
We propose to extend the original autocorrelation method to account for the linear component of the electron beam energy chirp, so that in addition to more accurate estimations of the radiation pulse duration it provides both sign and value of the linear chirp in the electron beam and radiation pulse
Summary
Free-electron lasers (FELs) are state-of-the-art tools to generate high-brightness x-ray radiation pulses by exploiting a resonant instability that develops while a high-quality electron beam propagates through a properly tuned undulator. The chicane delay acts as a phase-shifter on the delay scale of the radiation wavelength [9], it exponentially smears out microbunching and delays the radiation with respect to the radiation field on the scale of the SASE pulse duration The latter effect was studied in [10] where it was proposed to measure the autocorrelation of the intensity envelope of SASE radiation pulses by delaying the electron beam with respect to emitted radiation in the middle of the exponential growth. This method, while being simple and effective [11] does not account for chirps in the electron beam and, as result, may yield an underestimated pulse duration. We propose to extend the original autocorrelation method to account for the linear component of the electron beam energy chirp, so that in addition to more accurate estimations of the radiation pulse duration it provides both sign and value of the linear chirp in the electron beam and radiation pulse
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