Characterisation of microbunching instability with 2D Fourier analysis.

The optimal performance of high-brightness free-electron lasers (FELs) is limited by the microbunching instability, which can cause variations in both the slice energy spread and longitudinal profile of electron beams. In this paper, we perform 2D Fourier analysis of the full bunch longitudinal phase space, such that modulations in both planes can be studied simultaneously. Unlike the standard 1D analysis, this method is able to reveal modulations in a folded phase space, which would otherwise remain uncovered. Additionally, the plasma oscillation between energy and density modulations is also revealed by this method. The damping of the microbunching instability, through the use of a laser heater, is also analysed with this technique. We confirm a mitigation of the amplitude of modulation and a red-shift of the microbunching frequency as the energy spread added increases. As an outcome of this work, a systematic experimental comparison of the development of the instability in the presence of different compression schemes is here presented for the first time.

Nanoscale transient gratings excited and probed by extreme ultraviolet femtosecond pulses.

Advances in developing ultrafast coherent sources operating at extreme ultraviolet (EUV) and x-ray wavelengths allow the extension of nonlinear optical techniques to shorter wavelengths. Here, we describe EUV transient grating spectroscopy, in which two crossed femtosecond EUV pulses produce spatially periodic nanoscale excitations in the sample and their dynamics is probed via diffraction of a third time-delayed EUV pulse. The use of radiation with wavelengths down to 13.3 nm allowed us to produce transient gratings with periods as short as 28 nm and observe thermal and coherent phonon dynamics in crystalline silicon and amorphous silicon nitride. This approach allows measurements of thermal transport on the ~10-nm scale, where the two samples show different heat transport regimes, and can be applied to study other phenomena showing nontrivial behaviors at the nanoscale, such as structural relaxations in complex liquids and ultrafast magnetic dynamics.

Coherent THz Emission Enhanced by Coherent Synchrotron Radiation Wakefield.

We demonstrate that emission of coherent transition radiation by a ∼1 GeV energy-electron beam passing through an Al foil is enhanced in intensity and extended in frequency spectral range, by the energy correlation established along the beam by coherent synchrotron radiation wakefield, in the presence of a proper electron optics in the beam delivery system. Analytical and numerical models, based on experimental electron beam parameters collected at the FERMI free electron laser (FEL), predict transition radiation with two intensity peaks at ∼0.3 THz and ∼1.5 THz, and extending up to 8.5 THz with intensity above 20 dB w.r.t. the main peak. Up to 80-µJ pulse energy integrated over the full bandwidth is expected at the source, and in agreement with experimental pulse energy measurements. By virtue of its implementation in an FEL beam dump line, this work promises dissemination of user-oriented multi-THz beamlines parasitic and self-synchronized to EUV and x-ray FELs.

Extreme ultraviolet resonant inelastic X-ray scattering (RIXS) at a seeded free-electron laser.

In the past few years, we have been witnessing an increased interest for studying materials properties under non-equilibrium conditions. Several well established spectroscopies for experiments in the energy domain have been successfully adapted to the time domain with sub-picosecond time resolution. Here we show the realization of high resolution resonant inelastic X-ray scattering (RIXS) with a stable ultrashort X-ray source such as an externally seeded free electron laser (FEL). We have designed and constructed a RIXS experimental endstation that allowed us to successfully measure the d-d excitations in KCoF3 single crystals at the cobalt M2,3-edge at FERMI FEL (Elettra-Sincrotrone Trieste, Italy). The FEL-RIXS spectra show an excellent agreement with the ones obtained from the same samples at the MERIXS endstation of the MERLIN beamline at the Advanced Light Source storage ring (Berkeley, USA). We established experimental protocols for performing time resolved RIXS experiments at a FEL source to avoid X ray-induced sample damage, while retaining comparable acquisition time to the synchrotron based measurements. Finally, we measured and modelled the influence of the FEL mixed electromagnetic modes, also present in externally seeded FELs, and the beam transport with ~120 meV experimental resolution achieved in the presented RIXS setup.

The FERMI free-electron lasers.

FERMI is a seeded free-electron laser (FEL) facility located at the Elettra laboratory in Trieste, Italy, and is now in user operation with its first FEL line, FEL-1, covering the wavelength range between 100 and 20 nm. The second FEL line, FEL-2, a high-gain harmonic generation double-stage cascade covering the wavelength range 20-4 nm, has also completed commissioning and the first user call has been recently opened. An overview of the typical operating modes of the facility is presented.

Stenting of the arterial duct in newborns with duct-dependent pulmonary circulation.

OBJECTIVE: To evaluate the feasibility and results of stenting of the arterial duct in newborns with duct-dependent pulmonary circulation using low-profile, high-flexibility premounted coronary stents. DESIGN: Prospective interventional and clinical follow-up study. SETTING: Tertiary referral centre. PATIENT POPULATION: Between April 2003 and December 2006, 26 neonates (mean (SD) age 15.2 (19.9) days, mean (SD) weight 3.3 (0.8) kg) underwent attempts at stenting of the arterial duct. MAIN OUTCOME MEASURES: Procedural success and complication rates. Early and mid-term follow-up results. RESULTS: The procedure was successfully completed in 24/26 (92.3%) cases. Minor complications occurred in 2/26 (7.7%) cases. No mortality occurred. After stenting, the ductal diameter increased from 1.2 (1.0) mm to 3.1 (0.4) mm (p<0.001) and the percutaneous O(2) saturation increased from 70 (14)% to 86 (10)% (p<0.001), respectively. Over a mid-term follow-up, 2/24 patients (8.3%) needed a systemic-to-pulmonary artery shunt because of inadequate ductal flow and 4/24 patients (16.7%) underwent stent redilatation after 6.0 (4.4) months, but before corrective surgery. Cardiac catheterisation before corrective surgery in 9 patients showed an increase of the Nakata index from 112 (49) mm/mm(2) to 226 (108) mm/mm(2) (p<0.001), without any left-to-right imbalance of the pulmonary artery size. In the subset of 11 patients who improved without needing an additional source of pulmonary blood supply, the stented arterial duct closed uneventfully in 45.5% of cases after 4.0 (2.2) months. CONCLUSIONS: Stenting of the arterial duct is a feasible, safe and effective palliation in newborns with duct-dependent pulmonary circulation, supporting the spontaneous improvement process or promoting significant and balanced pulmonary artery growth for subsequent corrective surgery.