We report on the first demonstration of generating high-power and short FEL pulses using the fresh-slice multi-stage amplification scheme at Athos, the soft X-ray beamline of SwissFEL. We use a transversely tilted electron beam traveling through the unique Athos layout with magnetic chicanes between every two undulator modules. Our first results show the production of FEL radiation with pulse energies of few hundreds microjoules and pulse durations at the femtosecond level. This operation mode will allow us to advance the scientific opportunities of single-particle imaging experiments.

High-resolution measurements of the uncorrelated energy spread at SwissFEL indicate energy spread levels much larger than predicted by state-of-the-art particle tracking. This contribution presents measurements of the energy spread at the SwissFEL injector as a function of the electron bunch charge, the optics and the longitudinal dispersion of the lattice. The results indicate that both intra-beam scattering and micro-bunching, not covered in the conventional modeling of injectors, cause a blow-up of the energy spread. The work underlines the importance of considering the energy spread in the optimization and design of high-brightness electron beam sources and the need to develop new models to adequately understand and simulate the observed physics effects.