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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.

Echo-enabled harmonic generation free-electron lasers (EEHG FELs) are promising candidates to produce fully coherent soft x-ray pulses by virtue of efficient high harmonic frequency up-conversion from UV lasers. The ultimate spectral limit of EEHG, however, remains unclear, because of the broadening and distortions induced in the output spectrum by residual broadband energy modulations in the electron beam. We present a mathematical description of the impact of incoherent (broadband) energy modulations on the bunching spectrum produced by the microbunching instability through both the accelerator and the EEHG line. The model is in agreement with a systematic experimental characterization of the FERMI EEHG FEL in the photon energy range $130-210$ eV. We find that amplification of electron beam energy distortions primarily in the EEHG dispersive sections explains an observed reduction of the FEL spectral brightness that is proportional to the EEHG harmonic number. Local maxima of the FEL spectral brightness and of the spectral stability are found for a suitable balance of the dispersive sections' strength and the first seed laser pulse energy. Such characterization provides a benchmark for user experiments and future EEHG implementations designed to reach shorter wavelengths.

High-brightness electron bunches, such as those generated and accelerated in free-electron lasers (FELs), can develop small-scale structure in the longitudinal phase space. This causes variations in the slice energy spread and current profile of the bunch which then undergo amplification, in an effect known as the microbunching instability. By imposing energy spread modulations on the bunch in the low-energy section of an accelerator, using an undulator and a modulated laser pulse in the centre of a dispersive chicane, it is possible tomanipulate the bunch longitudinal phase space. This allows for the control and study of the instability in unprecedented detail. We report measurements and analysis of such modulated electron bunches in the 2Dspectro-temporal domain at the FERMI FEL, for three different bunch compression schemes. We also perform corresponding simulations of these experiments and show that the codes are indeed able to reproduce the measurements across a wide spectral range. This detailed experimental verification of the ability of codes to capture the essential beam dynamics of the microbunching instability will benefit the design and performance of future FELs., Comment: 12 pages, 14 figures

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.

An understanding of collective effects is of fundamental importance for the design and optimisation of the performance of modern accelerators. In particular, the design of an accelerator with strict requirements on the beam quality, such as a free electron laser (FEL), is highly dependent on a correspondence between simulation, theory and experiments in order to correctly account for the effect of coherent synchrotron radiation (CSR), and other collective effects. A traditional approach in accelerator simulation codes is to utilise an analytic one-dimensional approximation to the CSR force. We present an extension of the 1D CSR theory in order to correctly account for the CSR force at the entrance and exit of a bending magnet. A limited range of applicability to this solution, in particular in bunches with a large transverse spot size or offset from the nominal axis, is recognised. More recently developed codes calculate the CSR effect in dispersive regions directly from the Lienard-Wiechert potentials, albeit with approximations to improve the computational time. A new module of the General Particle Tracer (GPT) code was developed for simulating the effects of CSR, and benchmarked against other codes. We experimentally demonstrate departure from the commonly used 1D CSR theory for more extreme bunch length compression scenarios at the FERMI FEL facility. Better agreement is found between experimental data and the codes which account for the transverse extent of the bunch, particularly in more extreme compression scenarios., Comment: 33 pages, 19 figures, submitted to New J. Phys

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.

Frequency pulling is a well-known phenomenon in standard laser physics, leading to a shift of the laser frequency when the cavity and maximum gain frequencies are detuned. In this letter we present the first experimental demonstration of frequency pulling in single-pass free-electron lasers. Measurements are performed using the single-pass free-electron laser installed on the Elettra storage ring.

Using harmonic emission from circularly polarized undulator is a procedure that is normally employed on synchrotron beamlines in order to extend the covered spectral range. A similar capability is likewise beneficial for next generation free-electron lasers. In this paper, we perform a first quantitative experimental analysis of the angular distribution of free-electron laser harmonic emission from helical undulators. Experimental results are compared to those obtained by means of a theoretical model based on the paraxial solution of Maxwell's equations.

We examine the mutual synchronization of a one dimensional chain of chaotic identical objects in the presence of a stimulus applied to the first site. We first describe the characteristics of the local elements, and then the process whereby a global nontrivial behaviour emerges. A propensity criterion for networking is introduced, consisting in the coexistence within the attractor of a localized chaotic region, which displays high sensitivity to external stimuli,and an island of stability, which provides a reliable coupling signal to the neighbors in the chain. Based on this criterion we compare homoclinic chaos, recently explored in lasers and conjectured to be typical of a single neuron, with Lorenz chaos., Comment: 4 pages, 3 figures

We investigate the response of an open chain of bidirectionally coupled chaotic homoclinic systems to external periodic stimuli. When one end of the chain is driven by a periodic signal, the system propagates a phase synchronization state in a certain range of coupling strengths and external frequencies. When two simultaneous forcings are applied at different points of the array, a rich phenomenology of stable competitive states is observed, including temporal alternation and spatial coexistence synchronization domains., Comment: 4 pages, 5 figures, submitted to Phys. Rev. Lett

We study experimentally and theoretically the polarization alternation during the switch-on transient of a quasi-isotropic CO$_2$ laser emitting on the fundamental mode. The observed transient dynamics is well reproduced by means of a model which provides a quantitative discrimination between the intrinsic asymmetry due to the kinetic coupling of molecules with different angular momenta, and the extrinsic anisotropies, due to a tilted intracavity window. Furthermore, the experiment provides a numerical assignment for the decay rate of the coherence term for a CO$_2$ laser., Comment: 14 pages, 6 figures, submitted to Opt. Comm

We numerically investigate the dynamics of a closed chain of unidirectionally coupled oscillators in a regime of homoclinic chaos. The emerging synchronization regimes show analogies with the experimental behavior of a single chaotic laser subjected to a delayed feedback, Comment: 4 pages, 5 figures, submitted to PRE

In this work we describe a simple method of encoding information at real time in the inter-spike intervals of a homoclinic chaotic system. This has been experimentally tested by means of an instantaneous synchronization between the laser intensity of a CO2 laser with feedback in the regime of Sil'nikov chaos and an external pulsed signal of very low power. The information is previously encoded in the temporal intervals between consecutive pulses of the external signal. The value of the inter-pulse intervals is varied each time a new pulse is generated., Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett

The chaotic spike train of a homoclinic dynamical system is self-synchronized by re-inserting a small fraction of the delayed output. Due to the sensitive nature of the homoclinic chaos to external perturbations, stabilization of very long periodic orbits is possible. On these orbits, the dynamics appears chaotic over a finite time, but then it repeats with a recurrence time that is slightly longer than the delay time. The effect, called delayed self-synchronization (DSS), displays analogies with neurodynamic events which occur in the build-up of long term memories., Comment: Submitted to Phys. Rev. Lett., 13 pages, 7 figures

A continuous train of irregularly spaced spikes, peculiar of homoclinic chaos, transforms into clusters of regularly spaced spikes, with quiescent periods in between (bursting regime), by feeding back a low frequency portion of the dynamical output. Such autonomous bursting results to be extremely robust against noise; we provide experimental evidence of it in a CO2 laser with feedback. The phenomen here presented display qualitative analogies with bursting phenomena in neurons., Comment: Submitted to Phys. Rev. Lett., 14 pages, 5 figures