The production of ultrashort electron bunches represents a fundamental process for the realization of short wavelength free electron lasers (FEL). Due to space charge effects at low energy, a short electron bunch with a high charge will degrade its emittance and will be lengthened within a few centimeters downstream the cathode. As a conse- quence, bunch compression is necessary to reach the required length for high peak currents. A common device used so far for this goal is the magnetic com- pressor, in which a bunch with a time-energy corre- lation (or chirp) is driven along an energy-dependent path by a dispersive, non-isochronous beam trans- port section, consisting of four dipoles placed in a chicane configuration. The magnetic compression may often degrade the beam quality, however, due to the so called micro-bunching instability caused by coherent synchrotron radiation effects in bends, that limits the performances of high intensities electron accelerators. In recent years, a new method, named velocity bunching, has been developed and it is able to com- press the bunch using rectilinear trajectories at rel- atively low energy. In this process the electrons on the tail of the bunch are faster than electrons in the bunch head, thus producing, at the exit of a trav- elling rf structure, a compressed beam. The possi- bility to achieve RF compression without emittance degradation has been recently demonstrated at the SPARC accelerator, that is the prototype of the in- jector of the SPARX FEL facility in which a hybrid (RF plus magnetic chicane) compression scheme is going to be employed. In this work we present an analytical model for studying the evolution of the longitudinal phase space modulation of a particle beam through an RF compressor and we compare the results with numer- ical simulations based on a macro-particle code. Collective effects have been considered all along the schematic case of a drift plus an RF compressor, downstream the photocathode emission.
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