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We report measurements demonstrating the concept of the free-electron laser (FEL) superradiant cascade. Radiation (λrad = 200 nm) at the second harmonic of a short, intense seed laser pulse (λseed = 400 nm) was generated by the cascaded FEL scheme at the transition between the modulator and radiator undulator sections. The superradiance of the ultrashort pulse is confirmed by detailed measurements of the resulting spectral structure, the intensity level of the produced harmonics, and the trend of the energy growth along the undulator. These results are compared to numerical particle simulations using the FEL code GENESIS 1.3 and show a satisfactory agreement. [ABSTRACT FROM AUTHOR]

Higher order harmonic generation in a free-electron laser amplifier operating in the superradiant regime [R.H. Dicke, Phys. Rev. 93, 99 (1954).] has been observed. Superradiance has been induced by seeding a single-pass amplifier with the second harmonic of a Ti:sapphire laser, generated in a /3-Barium borate crystal, at seed intensities comparable to the free-electron laser saturation intensity. Pulse energy and spectral distributions of the harmonics up to the 11th order have been measured and compared with simulations. [ABSTRACT FROM AUTHOR]

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 (λrad = 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. [ABSTRACT FROM AUTHOR]

The single-shot spatial characteristics of the vacuum ultraviolet self-amplified spontaneous emission of a free electron laser (FEL) is measured at different stages of amplification up to saturation with a Hartmann wavefront sensor. We show that the fundamental radiation at 61.5 nm tends towards a single-mode behavior as getting closer to saturation. The measurements are found in good agreement with simulations and theory. A near diffraction limited wavefront was measured. The analysis of Fresnel diffraction through the Hartmann wavefront sensor hole array also provides some further insight for the evaluation of the FEL transverse coherence, of high importance for various applications.

We report the first experimental implementation of a method based on simultaneous use of an energy chirp in the electron beam and a tapered undulator, for the generation of ultrashort pulses in a self-amplified spontaneous emission mode free-electron laser (SASE FEL). The experiment, performed at the SPARC FEL test facility, demonstrates the possibility of compensating the nominally detrimental effect of the chirp by a proper taper of the undulator gaps. An increase of more than 1 order of magnitude in the pulse energy is observed in comparison to the untapered case, accompanied by FEL spectra where the typical SASE spiking is suppressed.

We investigate a fundamental limitation occurring in vacuum ultraviolet and extreme ultraviolet seeded free electron lasers (FELs). For a given electron beam and undulator configuration, an increase of the FEL output energy at saturation can be obtained via an increase of the seed pulse duration. We put in evidence a complex spatiotemporal deformation of the amplified pulse, leading ultimately to a pulse splitting effect. Numerical studies of the Colson-Bonifacio FEL equations reveal that slippage length and seed laser pulse wings are core ingredients of the dynamics.

In spatiotemporal systems with advection, suppression of noise-sustained structures involves questions that are outside of the framework of deterministic dynamical systems control (such as Ott-Grebogi-Yorke-type methods). Here we propose and test an alternate strategy where a nonlocal additive feedback is applied, with the objective to create a new deterministic solution that becomes robust to noise. As a remarkable fact-though the needed parameter perturbations required have essentially a finite size-they turn out to be extraordinarily small in principle: 10;{-8} in the free-electron laser experiment presented here.

Seeded free-electron lasers (FELs) are among the future fourth-generation light sources in the vacuum ultraviolet and x-ray spectral regions. We analyze the seed temporal coherence preservation in the case of coherent harmonic generation FELs, including spectral narrowing and structure degradation. Indeed, the electron synchrotron motion driven by the seeding laser can cause sideband growth in the FEL spectrum.

Seeded single pass free-electron lasers are promising coherent, short-duration, and intense light sources, from the visible to x rays. Operated with adjustable undulators, they are also a unique device for providing fully variable polarized radiation. We report here the first seeding of helical undulators with a variable polarized source. We demonstrate that the adjustment of the seed polarization and focusing allows the free-electron laser radiation to be optimized in terms of intensity and quality.

We evidence numerically and experimentally that advection can induce spectrotemporal defects in a system presenting a localized structure. Those defects in the spectrum are associated with the breakings induced by the drift of the localized solution. The results are based on simulations and experiments performed on the super-ACO free-electron laser. However, we show that this instability can be generalized using a real Ginzburg-Landau equation with (i) advection and (ii) a finite-size supercritical region.

We show that the pulsed regimes observed in free-electron lasers (FELs) can be suppressed using feedback control. By applying tiny parameter perturbations, the feedback allows to keep the systems onto a stationary state that is naturally existing in phase space, but is usually inaccessible because of its unstable nature. We test this method numerically on a master equation derived from the classical iterative model. Then we present the experimental results obtained on the super-ACO FEL. This method is in principle directly applicable to the other free-electron lasers, whose instabilities have a dynamical (deterministic) origin.

We present a phenomenological treatment of free-electron laser storage ring saturation dynamics. The model includes longitudinal instabilities, Touschek intrabeam scattering, and nonzero off-energy-function contributions. The model predictions are compared with Super ACO experimental results and the agreement is shown to be satisfactory.

This paper gives an analytical description of the stationary regime of a storage-ring free-electron laser in the presence of the maximum detuning (compatible with the laser onset) between the laser pulse and the electron beam when they pass and interact in the optical cavity. In this condition, the conservation of the first moments of the laser intensity distribution allows one to express the peak gain of the light amplification process and the maximum detuning as a function of system parameters that are directly measurable. These theoretical results are compared with experiments performed on the Super-ACO free-electron laser.

The longitudinal distribution of a free-electron laser (FEL) may present a complex internal structure. This phenomenon has been already observed in the case of LINAC based oscillators and self-amplified spontaneous emission devices (for which the presence of "spikes" in the temporal distribution is systematically observed). We investigate here the physical process responsible for the growth of complex substructures inside the micropulse of a storage-ring free-electron laser. This "hole-burning-like" process results from the localized character of the interaction between the ultrarelativistic electron beam circulating in the storage ring and the laser pulse. Experimental results concerning the case of the super-ACO FEL are presented and interpreted by means of a pass-to-pass tracking code containing all the relevant features of the system dynamics.

The stabilization of nonlinear excitations by noise is a topic of fundamental importance in many physical problems. We discuss a genuine example within the context of storage ring-free electron laser physics, by presenting a model which allows the characterization of the system evolution and the determination of the conditions leading to the suppression of instabilities of sawtooth type. The conclusions of the model are confirmed by a comparison with experimental results on the Super Aco Storage Ring-Free Electron Laser.

In this paper a theoretical model is presented, which is based on a pass to pass analysis of the localized interaction between a short laser pulse with a wider electron distribution. It can be applied to a large class of physical phenomena and, in particular, to the case of a storage-ring free-electron laser (FEL). Numerical results are confirmed by experimental measurements done on the ACO and Super-ACO FELs.