title: High-gain harmonic-generation free-electron laser seeded by harmonics generated in gas.
abstract: The injection of a seed in a free-electron laser (FEL) amplifier reduces the saturation length and improves the longitudinal coherence. A cascaded FEL, operating in the high-gain harmonic-generation regime, allows us to extend the beneficial effects of the seed to shorter wavelengths. We report on the first operation of a high-gain harmonic-generation free-electron laser, seeded with harmonics generated in gas. The third harmonics of a Ti:sapphire laser, generated in a gas cell, has been amplified and up-converted to its second harmonic (λ(rad)=133 nm) in a FEL cascaded configuration based on a variable number of modulators and radiators. We studied the transition between coherent harmonic generation and superradiant regime, optimizing the laser performances with respect to the number of modulators and radiators.
authors.affiliation: ENEA C.R. Frascati, Via E. Fermi, 45 00044 Frascati, Roma, Italy.
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authors.lastname: Labat
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authors.lastname: Bellaveglia
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authors.lastname: Bougeard
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authors.lastname: Carré
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authors.lastname: Ciocci
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authors.lastname: Garzella
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authors.lastname: Gatti
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authors.lastname: Giannessi
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authors.lastname: Lambert
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authors.lastname: Mostacci
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authors.lastname: Petralia
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authors.lastname: Petrillo
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authors.lastname: Spassovsky
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authors.lastname: Surrenti
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authors.lastname: Vaccarezza
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authors.lastname: Vicario