Spatiospectral properties of redshifted free electron laser radiation

Free electron lasers (FELs) are advanced light sources delivering pulses with unprecedented intensity down to the hard x-ray range. Redshifted FEL represents a new scheme exploiting strongly chirped electron beams to emit tunable coherent radiation at wavelengths longer than the FEL's resonant condition. While theoretical studies and proposals for this scheme are rapidly advancing, experimental evidences are so far very limited. Here, we report on the nature of spatiospectral properties of a redshifted FEL, presenting analytical and numerical models benchmarked with experimental data from the COXINEL (coherence x-ray source inferred from electrons accelerated by laser) FEL beamline driven by a laser plasma accelerator at Helmholtz-Zentrum Dresden-Rossendorf (Germany). We reveal how those properties are governed by a peculiar, V-shape like, transverse spatial phase inherited from single-electron synchrotron radiation. Moving the observation point inside the undulator, i.e., backpropagating the radiation, it is shown how this synchrotron radiation inheritage imprints localized fringes on the spatiospectral intensity distribution, uniquely allowing to retrieve the FEL source point location. Building on these results, we anticipate a twofold impact towards development of the future high-average power cavity-based x-ray regenerative amplifier FEL and further advancement of a compact laser-plasma-based x-ray FEL.