The FERMI FEL2 electro optical sampling design and operational experience

This paper reports on the design and operational experience of an electro optical sampling (EOS) diagnostics based on the same femtosecond laser used for the seeding process of the FERMI FEL2. This design allows operating in parallel with the FEL2, minimizing time jitter with respect to the seed laser which is the main time marker of the FEL process by removing the time jitter contributions related to the use of an external laser. In our setup a fraction of the energy from the seed laser amplifier is extracted and delivered via a dedicated low vacuum transport system to the EOS setup in the tunnel with beam pointing stabilization actively obtained by piezo actuators. The laser beam size, pulse compression management, polarization cleaning and selection are performed in the tunnel prior to injecting the laser beam in the vacuum chamber. The spatial encoding scheme is employed, with a 100-micron thick GaP crystal and a detection performed in cross polarizers setup using a CMOS camera. The EOS has been characterized in terms of temporal resolution, jitter and drift. Finally, its capability to operate as longitudinal feedback system is also reported.