Self-amplified spontaneous emission for a single pass free-electron laser

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