DESIGN OF THE CAVITY BPM FOR FERMI@ELETTRA

High resolution Beam Position Monitors (BPM) are fundamental diagnostics for a seeded FEL, like FERMI@ELETTRA, as they allow to measure the electron beam trajectory non destructively and on a shot-by shot basis. Cavity BPMs provide the required sub-micrometer resolution relying on the excitation of the TM 110 dipole mode when the beam passes through the cavity off axis. For the seeded FEL FERMI, we adopted a pair of cavity BPMs located upstream the modulating undulator to measure the electron beam trajectory at the sub-micrometer level. In this paper we first discuss the requirements for such a cavity BPM including those for the beam based alignment. The scaling from an X-band design to the final C-band design is presented. The resolution to stay below one micrometer has been cross-checked, both analytically and numerically. The losses of the common mode TM 010 have been also checked, leading to the final dimensions which keep the losses at the level of the X-band cavity BPM.