Single-shot transverse coherence measurements with Young's double pinholes at FLASH2

We measured the transverse coherence at FLASH2, a variable gap undulator line at the FLASH free-electron laser user facility at DESY in Hamburg. We demonstrate, theoretically and experimentally, a revised version of Young's double pinhole approach to perform single-shot, repeatable and non-invasive transverse coherence measurements. At beamline FL24 of FLASH2, the transverse coherence of pulses was systematically characterized at wavelengths of 8, 13.5 and 18 nm for different FEL source settings. We determine degrees of coherence of 57% to 87% in the vertical and horizontal direction, respectively. These measurements can facilitate the planning of novel, coherence-based experiments at the FLASH facility.