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The microbunching instability has been a long-standing issue for high-brightness free-electron lasers (FELs), and is a significant show-stopper to achieving full longitudinal coherence in the x-ray regime. This paper reports the first experimental demonstration of microbunching instability mitigation through transverse Landau damping, based on linear optics control in a dispersive region. Analytical predictions for the microbunching content are supported by numerical calculations of the instability gain and confirmed through the experimental characterization of the spectral brightness of the FERMI FEL under different transverse optics configurations of the transfer line between the linear accelerator and the FEL.

Diagnostics of the beam transverse profile with ever more demanding spatial resolution is required by the progress on novel particle accelerators - such as laser and plasma driven accelerators - and by the stringent beam specifications of the new generation of X-ray facilities. In a linac driven Free-Electron-Laser (FEL), the spatial resolution constraint joins with the further requirement for the diagnostics to be minimally invasive in order to protect radiation sensitive components - such as the undulators - and to preserve the lasing mechanism. As for high resolution measurements of the beam transverse profile in a FEL, wire-scanners (WS) are the top-ranked diagnostics. Nevertheless, conventional WS consisting of a metallic wire (beam-probe) stretched onto a frame (fork) can provide at best a rms spatial resolution at the micrometer scale along with an equivalent surface of impact on the electron beam. In order to improve the spatial resolution of a WS beyond the micrometer scale along with the transparency to the lasing, PSI and FERMI are independently pursuing the technique of the nano-lithography to fabricate a free-standing and sub-micrometer wide WS beam-probe fully integrated into a fork. Free-standing WS with a geometrical resolution of about 250 nm have been successfully tested at SwissFEL where low charge electron beams with a vertical size of 400-500 nm have been characterized. Further experimental tests carried out at SwissFEL at the nominal beam charge of 200 pC confirmed the resilience to the heat-loading of the nano-fabricated WS. In this work, details on the nano-fabrication of free-standing WS as well as results of the electron-beam characterization are presented.

The hitherto unexplored two-photon doubly-excited states [Ne$^{*}$($2p^{-1}3s$)]$_{2}$ were experimentally identified using the seeded, fully coherent, intense extreme ultraviolet free-electron laser FERMI. These states undergo ultrafast interatomic Coulombic decay (ICD) which predominantly produces singly-ionized dimers. In order to obtain the rate of ICD, the resulting yield of Ne$_{2}^{+}$ ions was recorded as a function of delay between the XUV pump and UV probe laser pulses. The extracted lifetimes of the long-lived doubly-excited states, 390 (-130 / +450} fs, and of the short-lived ones, less than 150~fs, are in good agreement with \emph{ab initio} quantum mechanical calculations.

Single photon laser enabled Auger decay (spLEAD) has been redicted theoretically [Phys. Rev. Lett. 111, 083004 (2013)] and here we report its first experimental observation in neon. Using coherent, bichromatic free-electron laser pulses, we have detected the process and coherently controlled the angular distribution of the emitted electrons by varying the phase difference between the two laser fields. Since spLEAD is highly sensitive to electron correlation, this is a promising method for probing both correlation and ultrafast hole migration in more complex systems., Comment: 5 pages, 3 figures

XUV and X-ray Free Electron Lasers (FELs) produce short wavelength pulses with high intensity, ultrashort duration, well-defined polarization and transverse coherence, and have been utilised for many experiments previously possible at long wavelengths only: multiphoton ionization, pumping an atomic laser, and four-wave mixing spectroscopy. However one important optical technique, coherent control, has not yet been demonstrated, because Self- Amplified Spontaneous Emission FELs have limited longitudinal coherence. Single-colour pulses from the FERMI seeded FEL are longitudinally coherent, and two-colour emission is predicted to be coherent. Here we demonstrate the phase correlation of two colours, and manipulate it to control an experiment. Light of wavelengths 63.0 and 31.5 nm ionized neon, and the asymmetry of the photoelectron angular distribution was controlled by adjusting the phase, with temporal resolution 3 attoseconds. This opens the door to new shortwavelength coherent control experiments with ultrahigh time resolution and chemical sensitivity.

We present the experimental demonstration of a method for generating two spectrally and temporally separated pulses by an externally seeded, single-pass free-electron laser operating in the extreme-ultraviolet spectral range. Our results, collected on the FERMI@Elettra facility and confirmed by numerical simulations, demonstrate the possibility of controlling both the spectral and temporal features of the generated pulses. A free-electron laser operated in this mode becomes a suitable light source for jitter-free, two-colour pump-probe experiments.

Tunable polarization over a wide spectral range is a required feature of light sources employed to investigate the properties of local symmetry in both condensed and low-density matter. Among new-generation sources, free-electron lasers possess a unique combination of very attractive features, as they allow to generate powerful and coherent ultra-short optical pulses in the VUV and X-ray spectral range. However, the question remains open about the possibility to freely vary the light polarization of a free-electron laser, when the latter is operated in the so-called nonlinear harmonic-generation regime. In such configuration, one collects the harmonics of the free-electron laser fundamental emission, gaining access to the shortest possible wavelengths the device can generate. In this letter we provide the first experimental characterization of the polarization of the harmonic light produced by a free-electron laser and we demonstrate a method to obtain tunable polarization in the VUV and X-ray spectral range. Experimental results are successfully compared to those obtained using a theoretical model based on the paraxial solution of Maxwell's equations. Our findings can be expected to have a deep impact on the design and realization of experiments requiring full control of light polarization to explore the symmetry properties of matter samples.

General relativity can be cast as a gauge theory by introducing a tetrad field and a spin-connection. This formalism was extended by replacing the tetrad field with a mixed tensor field independent of the metric tensor in order to develop a mechanism of adjustment of the vacuum energy density that takes advantage of Weinberg's no-go theorem. With no anthropic considerations, it was shown that the vacuum energy density is bounded and the gravitational and cosmological constants are proportional to a tiny dimensionless parameter determined by the coupling constants of the model., Comment: 9 pages