The seed laser system of the FERMI free-electron laser: design, performance and near future upgrades

Abstract An important trend in extreme ultraviolet and soft X-ray free-electron laser (FEL) development in recent years has been the use of seeding by an external laser, aimed to improve the coherence and stability of the generated pulses. The high-gain harmonic generation seeding technique was first implemented at FERMI and provided FEL radiation with high coherence as well as intensity and wavelength stability comparable to table-top ultrafast lasers. At FERMI, the seed laser has another very important function: it is the source of external laser pulses used in pump–probe experiments allowing one to achieve a record-low timing jitter. This paper describes the design, performance and operational modes of the FERMI seed laser in both single- and double-cascade schemes. In addition, the planned upgrade of the system to meet the challenges of the upgrade to echo-enabled harmonic generation mode is presented

Towards jitter-free pump-probe measurements at seeded free electron laser facilities

X-ray free electron lasers (FEL) coupled with optical lasers have opened unprecedented opportunities for studying ultrafast dynamics in matter. The major challenge in pump-probe experiments using FEL and optical lasers is synchronizing the arrival time of the two pulses. Here we report a technique that benefits from the seeded-FEL scheme and uses the optical seed laser for nearly jitter-free pump-probe experiments. Timing jitter as small as 6 fs has been achieved and confirmed by measurements of FEL-induced transient reflectivity changes of Si3N4 using both collinear and non-collinear geometries. Planned improvements of the experimental set-up are expected to further reduce the timing jitter between the two pulses down to fs level

Two-bunch operation with ns temporal separation at the FERMI FEL facility

n/

Tunability experiments at the FERMI@Elettra free-electron laser

FERMI@Elettra is a free electron-laser (FEL)-based user facility that, after two years of commissioning, started preliminary users' dedicated runs in 2011. At variance with other FEL user facilities, FERMI@Elettra has been designed to deliver improved spectral stability and longitudinal coherence. The adopted scheme, which uses an external laser to initiate the FEL process, has been demonstrated to be capable of generating FEL pulses close to the Fourier transform limit. We report on the first instance of FEL wavelength tuning, both in a narrow and in a large spectral range (fine- and coarse-tuning). We also report on two different experiments that have been performed exploiting such FEL tuning. We used fine-tuning to scan across the 1s–4p resonance in He atoms, at ≈23.74 eV (52.2 nm), detecting both UV–visible fluorescence (4p–2s, 400 nm) and EUV fluorescence (4p–1s, 52.2 nm). We used coarse-tuning to scan the M4,5 absorption edge of Ge (∼29.5 eV) in the wavelength region 30–60 nm, measured in transmission geometry with a thermopile positioned on the rear side of a Ge thin foil

Le strutture e lo scarico di olle del Puntone Nuovo di Scarlino (GR), e i siti costieri specializzati della protostoria medio tirrenica

Il riesame complessivo di una serie di siti costieri del Bronzo finale-prima età del ferro (XI-VIII sec. a.C.) dell'Italia mediotirrenica caratterizzati da strutture di combustione, bacini per l'evaporazione dell'acqua marina, scarichi di frammenti di olle di colore rossiccio/arancio, evidenzia una similarità funzionale con i siti europei c.d. a briquetage, permettendo dunque di ipotizzarne una funzione legata alla produzione del sale. L'articolo comprende una sintesi generale sui complessi archeologici di questo tipo presenti nell'area mediotirrenica, che ne evidenzia la correlazione con l'avvio dei processi di urbanizzazione, e l'edizione dei risultati di uno scavo praticato a Scarlino (Toscana, GR) in un sito costiero che ha restituito strutture di combustione e scarichi di frammenti di olle, di cui viene presentato lo studio analitico dal punto di vista archeologico e minero-petrografico

Optical beam transport to a remote location for low jitter pump-probe experiments with a free electron laser

In this paper we propose a scheme that allows a strong reduction of the timing jitter between the pulses of a free electron laser (FEL) and external laser pulses delivered simultaneously at the FEL experimental stations for pump-probe–type experiments. The technique, applicable to all seeding-based FEL schemes, relies on the free-space optical transport of a portion of the seed laser pulse from its optical table to the experimental stations. The results presented here demonstrate that a carefully designed laser beam transport, incorporating also a transverse beam position stabilization, allows one to keep the timing fluctuations, added by as much as 150 m of free space propagation and a number of beam folding mirrors, to less than 4 femtoseconds rms. By its nature our scheme removes the major common timing jitter sources, so the overall jitter in pump-probe measurements done in this way will be below 10 fs (with a margin to be lowered to below 5 fs), much better than the best results reported previously in the literature amounting to 33 fs rms

Linear optics control of sideband instability for improved free-electron laser spectral brightness

Extension of stable longitudinal coherence from vacuum ultraviolet to x rays is highly sought after in the free-electron laser (FEL) community, but it is often prevented by bandwidth broadening originated in the electron beam microbunching instability. We demonstrate that a proper tuning of the linear optics before the beam enters the undulator mitigates the microbunching-induced sideband instability. The experiment was conducted at the Fermi FEL operated in echo-enabled harmonic generation mode, where the spectral brightness at 7 nm wavelength was doubled. The FEL performance is compared to nonoptimized optics solutions and characterized in terms of peak intensity and spectral bandwidth shot-to-shot stability. The technique has straightforward implementation, because it uses quadrupole magnets routinely adopted for beam transport, and it applies to any FEL architecture, so paving the way to the production of high-intensity Fourier-transform limited x-ray pulses in existing and planned FEL facilities