An upgrade of the existing S-band Linac of the SPARC Test Facility at LNF-Frascati with high gradient C-band accelerating structures is proposed, in order to reach 250 MeV at the end of the structure. The upgrade will be done with state-of-the art technology, setting up a facility unique in Europe made of S-band and C-band Linacs (a new "hybrid" configuration, never implemented up to now). This work will be carried out by the INFN in collaboration with the PSI-Villigen.

The SPARC test facility at LNF is an advanced photo-injector producing at present a 150 MeV electron beam to generate high brilliance FEL radiation in the visible region at the fundamental wavelength and at VUV wavelengths with the harmonics.

The upgrade design proposed here is based on a S-Band photo-injector operating in a RF compression mode followed by a C-band Linac. This scheme seems very promising from the beam dynamics point of view in terms of preservation of the low emittance of the electron beam and reachable photon beam brilliance. It is also much more compact with respect to a full S-band Linac, since with relatively short (1.5 m) C-band accelerating sections it will be possible to reach an accelerating gradient of the order of 35 MV/m. Finally, this compact design will allow for performing the design and realization of the accelerating structures at LNF with the present facilities.

After the upgrade, SPARC will facilitate R&D on low emittance beams, photo-emission processes with novel cathodes, generation of polarized electrons, acceleration and synchronization of a bi-frequency Linac, efficiency and reliability of the system.  In particular, a velocity bunching R&D test, providing both a substantial pulse compression with a reduction of the compressed beam energy spread, due to the higher SPARC beam energy, will allow for opening a number of possible future FEL experiments.

The SPARC upgrade will be crucial for at least two future FEL projects in Europe (SwissFEL and SPARX), for which the use of C-band accelerators for the main Linac is being considered, since they can greatly profit from the high accelerating gradients and the compactness of the system and lower energy consumption in comparison to S-band linacs.

Finally, the SuperB collider, - a very high luminosity asymmetric (4 and 7 GeV) e⁺e^- collider at the B meson cm energy, - will also profit from the SPARC upgrade. In fact a critical SuperB subsystem is the injector, which needs to deliver very intense bunches of positrons and polarized electrons at very high rates, due to the low beam lifetimes in the ring caused by the SuperB large luminosity. For these reasons, an important topic which will need significant R&D for SuperB is the study of a high performance C-band Linac which could deliver the beams with the required properties in a very cost effective manner. It is clear then that the implementation of this technology at SPARC will be crucial for the SuperB design.

 How the SuperB collider would be built at INFN's Frascati lab

Image credit: INFN