Constraints on Photon Pulse Duration from Longitudinal Electron Beam Diagnostics at a Soft X-ray Free-Electron Laser

The successful operation of X-ray free-electron lasers (FELs), like the Linac Coherent Light Sourceor the Free-Electron Laser in Hamburg (FLASH), makes unprecedented research on matter at atomiclength and ultrafast time scales possible. However, in order to take advantage of these unique lightsources and to meet the strict requirements of many experiments in photon science, FEL photonpulse durations need to be known and tunable. This can be achieved by controlling the FEL drivingelectron beams, and high-resolution longitudinal electron beam diagnostics can be utilized to provideconstraints on the expected FEL photon pulse durations. In this paper, we present comparativemeasurements of soft X-ray pulse durations and electron bunch lengths at FLASH. The soft X-raypulse durations were measured by FEL radiation pulse energy statistics and compared to electronbunch lengths determined by frequency-domain spectroscopy of coherent transition radiation inthe terahertz range and time-domain longitudinal phase space measurements. The experimentalresults, theoretical considerations, and simulations show that high-resolution longitudinal electronbeam diagnostics provide reasonable constraints on the expected FEL photon pulse durations. Inaddition, we demonstrated the generation of soft X-ray pulses with durations below 50 fs (FWHM)after the implementation of the new uniform electron bunch compression scheme used at FLASH