Elettra – a seeded harmonic cascade fel for euv and soft x-rays

We describe the machine layout and major performance parameters for the FERMI FEL project funded for construction at Sincrotrone Trieste, Italy, within the next five years. The project will be the first user facility based on seeded harmonic cascade FEL’s, providing controlled, high peak-power pulses. With a high-brightness rf photocathode gun, and using the existing 1.2 GeV S-band linac, the facility will provide tunable output over a range from ~100 nm to ~10 nm, with pulse duration from 40 fs to ~ 1ps, peak power ~GW, and with fully variable output polarization. Initially, two FEL cascades are planned; a single-stage harmonic generation to operate > 40 nm, and a two-stage cascade operating from ~40 nm to ~10 nm or shorter wavelength. The output is spatially and temporally coherent, with peak power in the GW range. Lasers provide modulation to the electron beam, as well as driving the photocathode and other systems, and the facility will integrate laser systems with the accelerator infrastructure, including a state-ofthe-art optical timing system providing synchronization of rf signals, lasers, and x-ray pulses. Major systems and overall facility layout are described, and key performance parameters summarized. OVERVIEW OF THE FACILITY The FERMI @ Elettra facility will make use of the existing GeV linac at Sincrotrone Elettra, which will become available for dedicated FEL applications following the completion of construction of a new injector booster complex for the storage ring. With a new rf photocathode injector, and some additional accelerating sections, this linac will be capable of providing high brightness bunches at 1.2 GeV and up to 50 Hz repetition rates. Figure 1 shows a preliminary CAD drawing of the proposed facility, with the linac-based FEL facility adjacent to the Elettra storage ring building. To accommodate the new rf photocathode gun, the tunnel which houses the existing linac and thermionic gun will be extended upstream. This will be a relatively minor investment in additional excavation and will take advantage of the present roadway cutting, already at the level of the linac and extending backward several meters. An S-band rf photocathode gun, with spatial and temporal control of the photocathode laser system, will provide high brightness electron bunches at up to 50 Hz rate. Flexibility in bunch parameters will be incorporated into the systems design. Accelerating sections raise the beam energy to ~100 MeV at the exit of the injector. A laser heater system following the injector system will provide control of the uncorrelated energy spread in the beam and minimize potential impact of the microbunching instability. The laser heater also allows opportunity for implementing useful diagnostics systems. Two magnetic bunch compressors are planned, inserted at 230 MeV and at 650 MeV. The final energy of the beam, 1.2 GeV, is determined by the accelerator section maximum gradient, available RF power including overhead and de-rating for reliable operations, and offcrest operation for control of energy chirp. The linac is installed in a tunnel about 5 m below ground level, and the new facility will include a transport line to take the beam up to an undulator hall at or near the surface. This vertical ramp allows inclusion of useful diagnostics, and is carefully designed to minimize perturbations to the beam quality. In the undulator hall, the electron beam may be directed to the longer-wavelength FEL (FEL-I) by a transport line which introduces a horizontal offset to the beam, or to the short-wavelength FEL (FEL-II) in a direct line to avoid perturbation to beam quality due to CSR in bend magnets. A timing system based on transmission of optical signals over a highly stabilized fiber optic system will distribute timing signals throughout the facility. This provides synchronization of the photocathode laser to the RF gun phase, stabilized drive signals to RF systems in the facility, and synchronization of the FEL seed laser with the arrival time of the electron beam. The seeded FEL process occurs in one stage of harmonic generation for FEL-I, and in a two-stage cascade for FEL-II. For FEL-II, both a fresh-bunch approach and a whole-bunch seeding technique are being developed. The x-ray pulse duration is determined by the seed laser, and both short-pulse (~40-100 fs) and long pulse (~0.5-1.0 ps) schemes are under development. The photon beams from the FELs are transported in beamlines to hutches an adjoining downstream experimental hall. The electron beams are dumped following the final radiating undulator. Laser systems in the experimental area are synchronized to the FEL output using the stabilized optical timing system distributed around the facility. SLAC-PUB-11488