BUNCH ARRIVAL MONITOR AT FERMI@ELETTRA

The bunch arrival monitor (BAM) for the IV generation synchrotron light source FERMI@Elettra is presented. It is based on an original idea developed at FLASH/DESY, specifically designed and built in-house for FERMI@Elettra. Each BAM station consists of a frontend module, located in the machine tunnel, and of a backend unit located in the service area. It makes use of the pulsed optical phase reference along with the stabilized fiber link. The front end converts the bunch arrival times into amplitude variations of the optical phase reference pulses distributed over the link. The analogue signal is generated at the e-beam's passage in a broadband pick-up and is sent to the modulation input of an electro-optical modulator (EOM). The back end acquires, synchronously, the amplitude modulated pulses, using a broadband photodiode and a fast analog-to-digital converter. The digitized data is sent to the machine control system for further processing. The dedicated analog-to-digital conversion, processing and communication board, part of the monitor back end, is also briefly described. INTRODUCTION New 4 Generation Light Sources (4GLS) currently in operation, construction or design at several National Laboratories are posing demanding requirements on the associated Timing and Synchronization (T&S) systems, identified as femto-second (fsec) T&S systems. 4GLS are typically Free Electron Lasers (FEL) driven by single pass Linear Accelerators (LINAC). Such demanding requirements on jitter (<10fsecRMS), and drift, originate from the adopted scheme for the generation of the electron beam and the FEL radiation. The typical bunch length ( B<50fsecFWHM), achievable in single pass accelerators thanks to the beam longitudinal manipulation techniques, the required beam quality (6-D emittance) and the radiation generation (seeding) and exploitation (time resolved / pump-probe experiments) schemes call for an ultimate jitter of <10fsecRMS. This ultra-low jitter value is typically required either between the electron bunch and the seed laser pulse or between the FEL pulse and the User laser pulse. To achieve this goal, the whole accelerator components need to share a Phase Reference with <10fsecRMS jitter and drift. BUNCH ARRIVAL MONITOR In 4GLS, the time position of the bunch is identified by the combination of the timing pulse (as a reference) and the slope of the electrical signal produced by the bunch passage through the pick up mounted on the beam line. This scheme was first proposed by the Desy group [1]. The pick up signal is fed to an Electro-Optical Modulator (Mach-Zehnder modulator MZM) and modulates the optical pulses of the optical master oscillator (OMO) as a reference. The timing jitter of the bunch shifts the temporal position of the slope and thus amplitude modulates the optical pulses. Therefore the amplitude variations correlate to the timing jitter of bunches. The BAM system consists of two main parts, the front end installed in the tunnel close to the beam pick up and the back end installed in the service area for the readout of the modulated pulses and further processing.