Simple and robust free-electron laser doubler

We present the design of a free-electron laser (FEL) doubler suitable for the simultaneous operation of two FEL lines, in either self-amplified spontaneous emission or externally seeded configuration. The doubler relies on the physical selection of two longitudinal portions (beamlets) of a single electron bunch at a low energy and on their spatial separation at high energy. Since the two beamlets are naturally synchronized, FEL pump–FEL probe experiments are enabled when the two photon pulses are sent to the same experimental station. The proposed solution offers improved flexibility of operation compared with existing or designed two-pulse, two-color FEL schemes, as it allows independent control and continuous tunability of the color, timing, intensity, and angle of incidence of the radiation pulses at the user end station. Detailed numerical simulations and experimental results demonstrate its feasibility at the Free Electron laser Radiation for Multidisciplinary Investigations (FERMI) FEL facility.