Generation and measurement of ultralow-emittance electron beams at SwissFEL

The emittance is a fundamental parameter of particle distributions accounting for the average spread of the particles’ positions and momenta. We have generated and characterized intense ultralow-emittance electron beams, setting new standards for electron linear accelerators. The measurements have been carried out at the SwissFEL accelerator of PSI. SwissFEL is one of the few X-ray free-electron lasers (FELs) worldwide, which are cutting-edge research instruments to investigate matter with resolutions at the level of atomic processes. We attribute our low emittance values to the outstanding quality of our hardware, a sophisticated and comprehensive optimization procedure, and the opportunity to measure very small emittances with precise, robust and high-resolution measurement methods. Our low-emittance electron beams give rise to a significant improvement of the FEL performance. In particular, our beams allow generating X-ray FEL radiation with much lower electron beam energies than in standard FEL facilities. This paves the way for compact and affordable facilities such as SwissFEL and beyond, potentially expanding FEL research to a much larger community of scientists.