Generating Fourier transform limited FEL pulses using hard X-ray split-and-delay

In our study we have characterized the spatial and temporal coherence properties of the hard FEL pulses after the split-and-delay line. The experiment has been performed at the XCS instrument of the LCLS at SLAC National Accelerator Laboratory. A dried colloidal sample was used to generate static speckle patterns.
The analysis of speckle contrast from small-angle scattering measurements from the sample shows well-preserved transverse coherence of the FEL beam. By measuring fluctuations of intensity from successive FEL pulses we show that only single or double temporal modes remain in the spectrum of the transmitted beam, corresponding to nearly Fourier transform limited FEL pulses. This capability is an important step towards routine split-pulse XPCS experiments aims to probe ultrafast phenomena in condensed matter systems at FEL sources.

Scientific Reports 10, 5054 (2020)