A two-frequency RF cavity for the PSI-XFEL: Design and beam dynamics simulations

In the frame of conceptual studies for a compact (less than 6 GeV) X-ray FEL at PSI, we report here on the design and beam dynamics simulations of a two-cell two-frequency RF gun-like cavity. The goal of this study is to demonstrate the feasibility and the RF flexibility of such a cavity for manipulating the longitudinal phase space while preserving the initial low transverse emittance. Shaping the longitudinal phase space is mandatory for efficient velocity bunching at an early stage of the accelerator. The cavity, located after the gap of a 500-kV to 1-MV diode, is dimensioned to operate at 1.5 and 4.5 GHz. design of the two-cell cavity and the coupling schemes for both frequencies are presented. Mode separation, which ensures adequate operation, is achieved by profiling the cavity walls. The flexibility of such a design to shape the longitudinal phase space is illustrated with beam dynamics simulations. The transverse emittance is shown to be nearly unchanged by combining emittance compensation and a beam matching scheme. The deleterious effects of the transverse RF fields are consequently reduced to a minimum.