In this Letter we report the first experiments aimed at the simultaneous demonstration of the emittance compensation process and velocity bunching in a high brightness electron source, the SPARC photoinjector in INFN-LNF. While a maximum compression ratio up to a factor 14 has been observed, in a particular case of interest a compression factor of 3, yielding a slice current of 120 A with less than 2 microm slice emittance, has been measured. This technique may be crucial in achieving high brightness beams in photoinjectors aiming at optimized performance of short wavelength single-pass free electron lasers or other advanced applications in laser-plasma accelerators.

SPARC (acronym of ‘‘Sorgente Pulsata ed Amplificata di Radiazione Coerente’’, i.e. Pulsed and\ud Amplified Source of Coherent Radiation) is a single pass free-electron laser designed to obtain high gain\ud amplification at a radiation wavelength of 500 nm. Self-amplified spontaneous emission has been\ud observed driving the amplifier with the high-brightness beam of the SPARC linac. We report measurements\ud of energy, spectra, and exponential gain. Experimental results are compared with simulations from\ud several numerical codes

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Peak current represents a key demand for new generation electron beam photoinjectors. Many beam\ud applications, such as free electron laser, inverse Compton scattering, terahertz radiation generation, have\ud efficiencies strongly dependent on the bunch length and current. A method of beam longitudinal\ud compression (called velocity bunching) has been proposed some years ago, based on beam longitudinal\ud phase space rotation in a rf field potential. The control of such rotation can lead to a compression factor in\ud excess of 10, depending on the initial longitudinal emittance. Code simulations have shown the possibility\ud to fully compensate the transverse emittance growth during rf compression, and this regime has been\ud experimentally proven recently at SPARC. The key point is the control of transverse beam plasma\ud oscillations, in order to freeze the emittance at its lowest value at the end of compression. Longitudinal\ud and transverse phase space distortions have been observed during the experiments, leading to asymmetric\ud current profiles and higher final projected emittances. In this paper we discuss in detail the results obtained\ud at SPARC in the regime of velocity bunching, analyzing such nonlinearities and identifying the causes.\ud The beam degradation is discussed, both for slice and projected parameters. Analytical tools are derived to\ud experimentally quantify the effect of such distortions on the projected emittanc

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