Generation of intense XVUV pulses with an optical klystron enhanced self-amplified spontaneous emission free electron laser

Fermi is a seeded FEL operating in high-gain harmonic generation mode. The FEL layout is constituted by a modulator and six radiators separated by a dispersive section. The modulator and the radiators can be tuned to the same resonant frequency to set up an asymmetric optical klystron configuration where self-amplified spontaneous emission can be generated and studied. This paper presents the experiment consisting in the analysis of the enhancement of the self-amplified spontaneous emission (SASE) radiation by the dispersion in the optical klystron. The FEL pulses produced with the optical klystron configuration are several orders of magnitude more intense than in pure SASE mode with the dispersion set to zero. The experimental observations are in good agreement with simulation results and theoretical expectations. A comparison with the typical high-gain harmonic generation seeded FEL operation is also provided.