How the optical timing system, the longitudinal diagnostics and the associated feedback systems provide femtosecond stable operation at the FERMI free electron laser

Abstract

FERMI, the seeded free electron laser (FEL) in operation in Italy, is providing the User Community with unique fully coherent radiation, in the wavelength range 100-4 nm. FERMI is the first FEL fully synchronized by means of optical fibers. The optical timing system ensures an ultra-stable phase reference to its distributed clients. Several femtosecond longitudinal diagnostics verify the achieved performance; the bunch length monitor (BLM) and the bunch arrival monitor (BAM) will be presented in this paper. Feedback systems play a crucial role to guarantee the needed longterm electron beam stability. A real-time infrastructure allows shot-to-shot communication between front-end computers and the servers. Orbit feedbacks are useful in machine tuning, whereas longitudinal feedbacks control electron energy, compression and arrival time. A flexible software framework allows a rapid implementation of heterogeneous multiinput-multi-output (MIMO) longitudinal loops simply by selecting the appropriate sensors and actuators.

Optical setup for two-colour experiments at the low density matter beamline of FERMI

“…The FERMI seed pulse starts from a femtosecond Ti:Sapphire oscillator (Vitara, Coherent) which is locked to the reference timing signal which determines repetition rate, and is distributed over the facility [20]. Originally, the seed laser system had a single Ti:Sapphire based amplifier consisting of a chirped-pulse regenerative amplifier and a single pass amplifier delivering pulses with an energy of up to 6 mJ and a duration of about 100 fs with a central wavelength around 785 nm.…”

Passive Linearization of the Magnetic Bunch Compression Using Self-Induced Fields

“…The chicane angles were maintained constant. The reading of a pyrodetector monitoring the long wavelength coherent emission from the bunch, placed after BC2, has been used as the target in a feedback loop to measure and keep constant the bunch duration [37]. The electron bunch temporal profile has been measured in several 25 configurations and the results are plotted in Fig.…”