VELOCITY BUNCHING EXPERIMENT AT SPARC

The optimization of the beam brightness is one of the main goals of the research and development efforts in rf photo-injectors devoted to short wavelength FELs. The velocity bunching experiment at SPARC has recently demonstrated the possibility of increasing the beam current via rf compression at low energies, while compensating the space charge induced emittance degradation by means of continuous magnetic focusing, increasing the final beam brightness. Stable compression ratios up to a factor 12 and 17 have been observed respectively for the high charge and low charge cases. In this paper measurements of longitudinal beam characteristics, projected and slice transverse emittances in the velocity bunching regime are presented, for different Gun-solenoid field values. Comparisons with simulations are also reported.