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Ultra-short electron bunch generation with a rectilinear compressor

We describe a new method for reducing the electron bunch length of high brightness beams in linacs, as those required to drive X-ray SASE FEL's, which is of interest also for advanced accelerator applications like plasma accelerators, where low charge femtosecond bunches are requested. The method is based on a rectilinear compressor scheme utilizing the bunching properties of RF slow waves. It is shown that whenever a beam, slower than the synchronous velocity, is injected into a RF wave at zero acceleration phase and slips back in phase up to the peak acceleration phase, it can be longitudinally compressed even when already relativistic (tens of MeV). This occurs if the beam is extracted from the wave at the synchronous velocity: in this case the bunch undergoes a quarter of synchrotron oscillation that induces a phase compression up to a factor of 20 when proper care is taken to preserve the longitudinal emittance. Compression is actually not limited by space charge effects but by RF non-linearities in the longitudinal phase space, that produce a longitudinal emittance growth. A few examples are presented to demonstrate the potentialities of this method, by which multi-kA beams with very low transverse emittance can be generated at moderate energies (about 100 MeV).