• A. Falone, H. Fitze, R. Ischebeck, Y. Kim, M. Pedrozzi, V. Schlott, B. Steffen, L. Stingelin
  PSI, Villigen
• D. Alesini, L. Ficcadenti, L. Palumbo
  INFN/LNF, Frascati (Roma)

RF deflectors are crucial diagnostic tools for bunch length and slice emittance measurements with sub-picosecond resolution. Their use is essential in commissioning and operation of VUV and X-ray FELs. The 250MeV FEL injector, under construction at PSI, will use two of them. The first one will be installed after the gun at low energy (~7MeV), the second one at the end of the Linac at high energy (250MeV). The first RF deflector consists of a single cell standing wave cavity working on the TM110 deflecting mode, and tuned at 2997.912 MHz (frequency of the linac structures). In this note we report the motivation of this measurement, beam dynamics and beam diagnostics considerations and the RF design and simulations of this cavity.