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The chaotic nature of a storage-ring free electron laser (FEL) is investigated. The derivation of a low embedding dimension for the dynamics allows the low-dimensionality of this complex system to be observed, whereas its unpredictability is demonstrated, in some ranges of parameters, by a positive Lyapounov exponent. The route to chaos is then explored by tuning a single control parameter, and a period-doubling cascade is evidenced, as well as intermittence. [ABSTRACT FROM AUTHOR]

To take complete benefit of the Free Electron Laser (FEL) features, we have developed a feedback control scheme designed to suppress the dynamical instabilities affecting the FEL output intensity. The principle is to stabilize an unstable stationary state that coexists with the unwanted limit cycle. This paper gives a theoretical and experimental description of the stabilized stationary regime of a storage ring FEL in presence of the feedback control. We present a numerical model of the FEL with control, that describes the longitudinal dynamics of the laser pulse in condition of detuning between the FEL and the electron beam. These theoretical results are compared with experiments performed on the Super-ACO FEL. The established feedback control provides stable operation of the FEL. [ABSTRACT FROM AUTHOR]

In a Storage Ring Free Electron Laser (SRFEL) there is a strong interdependence between the laser beam and the electron beam from which the laser is generated. The Super ACO storage ring has a second Radio Frequency (RF) cavity at the 5th harmonic of the main RF cavity. It is used to shorten the bunch length, thereby enhancing the laser gain. Employing this RF harmonic cavity instabilities are observed with a strong effect on both the laser radiation properties and the electron beam behaviour. In this paper, we first present beam characteristics of Super-ACO as influenced by the harmonic cavity, and the instabilities of the beam due to this RF cavity. Then we discuss the FEL properties in presence of the harmonic RF cavity. In general the harmonic cavity functions as intended, and it is observed that the laser suppresses the instabilities caused by the harmonic cavity in the absence of the FEL. [ABSTRACT FROM AUTHOR]

In this paper we illustrate the main features of a new scheme which aims at obtaining coherent radiation at very short wavelengths (down to 1.5 nm) by using the Harmonics of an intense laser, generated in a gas, as a seed to inject in a long undulator, either in the pure SASE or in the HGHG configuration. The unique spatial and temporal coherence properties of the High-Order Harmonics are then conserved, and the High Gain FEL amplifier system saturates up to the GW level. An experiment is here proposed, to be performed on the new compact LINAC-based source SCSS, currently under development in SPring-8. [Copyright &y& Elsevier]

The response of the Free Electron Laser (FEL) to an external perturbation can be studied by applying a laser gain modulation. When its frequency is close to the laser resonance one, a non-linear gain variation of the Super-ACO storage ring FEL can lead to a chaotic dynamic behaviour of the laser macro-temporal structure. At a fixed frequency of the modulation, and increasing its amplitude, the systematic analysis of the macro-temporal structure of the laser, shows that chaos appears between two periodic regimes. The attractors, i.e. the phase-space representation of the macro-temporal structure, characterize the regime as periodic, quasi-periodic, or chaotic. The bifurcation scheme evolves with the frequency of the modulation. The experimental macro-temporal sequences can be reproduced by LAS the numerical empirical code developed in the Super-ACO FEL group. Here the comparison between the experimental and numerical results obtained on the non-linear dynamics of the Super-ACO FEL is presented. [Copyright &y& Elsevier]

After having reached the status of user facilities in the infrared, Free Electron Laser (FEL) oscillators are becoming interesting light sources for scientific research also in the UV/VUV wavelength region. Thanks to continuous advances in accelerator and mirror technology, it has been possible to reach with tunability energies which are hardly accessible with conventional lasers (the European FEL project at Elettra recently lased below 190 nm), with the realistic perspective of reaching even shorter wavelengths in the near future. Thanks to their unique properties, such as tunability, full coherence, high intensity, spectral and temporal stability, they offer new opportunities for many kinds of spectroscopy: in particular, storage ring FELs are ideal sources for pump-probe experiments in conjunction with synchrotron radiation, as demonstrated by a series of studies of the nonequilibrium space charge distribution at photoexcited Si surfaces and interfaces performed at SuperACO. Besides, especially when operated at shorter wavelengths, FEL oscillators are very attractive also for one-photon experiments requiring high power and full tunability. We describe here the properties of the sources, and provide an overview of the experiments that have been performed and that are planned to exploit the new opportunities they offer. [ABSTRACT FROM AUTHOR]

Significant improvements in the performances of the Super-ACO storage ring free-electron laser (FEL) at 800 MeV have been obtained recently: enhancement of the output power in the ultraviolet, laser duration of 10 h for the same injection of positrons, long-term stability with a micropulse of 60 ps FWHM. A first series of experiments using this FEL has then been successfully performed. Taking advantage of.the time structure, the polarization and the high power of the source at 350 nm, the polarized fluorescence decays of the reduced nicotinamide adenine dinucleotide coenzyme (NADH) were studied in aqueous solution, using the single-photon counting (SPC) technique. The experimental setup is described, including the Super-ACO FEL characteristics and diagnostics. The FEL working point has been first optimized by monitoring the SPC apparatus function. A complete fluorescence experiment required about 30 min of data acquisition, during which the best integrated instrumental response had a FWHM of 110 ps. Measurements performed in such a way lead to the unambiguous separation of two close lifetime components of 0.28 and 0.62 ns in the fluorescence decays of NADH at 20 °C, in good agreement with previous works. The thermodynamic parameters obtained from temperature studies show that the NADH fluorescence heterogeneity is consistent with the ground-state folding equilibrium of the coenzyme, as characterized by many other spectroscopic techniques. From the fluorescence anisotropy decays, an apparent hydrodynamic radius of about 6 Å is determined, while on the other hand, a large initial depolarization of the fluorescence indicates a fast independent motion of the nicotinamide ring. The quality of the collected data fully meets the requirements for the study of more complex systems such as fluorescent compounds bound to proteins or membranes. Thus, the feasibility of use of a storage ring UV FEL for this type of time-resolved experiments on the subnanosecond... [ABSTRACT FROM AUTHOR]

We present recent results obtained by the European Free Electron Laser (FEL) project at the ELETTRA storage ring. Coherent and tunable light was produced in the ultraviolet down to below 190 nm, the shortest lasing wavelength obtained so far with a FEL oscillator. The performance of the FEL is described, together with a series of technical solutions that have been adopted on it. These solutions increase its validity as a source for applications, and demonstrate the potential of FEL oscillators to become sources with features in the vacuum ultraviolet. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The Super-ACO storage ring Free Electron Laser (FEL) has been operated using an alternative operating point characterized by a non-zero dispersion function in the undulator straight section, linking the transverse motion of the electrons to the energy spread, and by a low momentum compaction factor. The longitudinal to transverse coupled electron beam motion induces a modified dynamics of the Super-ACO FEL. The FEL characteristics, such as the FEL power, pulse duration and spectral width are presented and analyzed in terms of storage ring FEL dynamics. [Copyright &y& Elsevier]

A new optics providing an emittance close to the minimum theoretical value has been proposed for the Super-ACO storage ring with a reduced value of the momentum compaction factor and non-zero dispersive function in the Free-Electron Laser (FEL) section. It has been adapted to the FEL operation mode, with two RF cavities at 100 and 500 MHz. The obtained results concerning this new optics and the FEL oscillation are presented. [Copyright &y& Elsevier]

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]

At the initiative of the CEA (Commissariat à l’Énergie Atomique), discussions were conducted in France on fourth generation light sources. A new independent accelerator based radiation facility ARC-EN-CIEL (Accelerator Radiation Complex for ENhanced Coherent Intense Extended Light) is proposed, aiming at providing coherent femtosecond light pulses in the UV- to X-ray range for scientific applications. The project is based on a 700 MeV superconducting LINAC, providing low emittance 200 fs RMS electron bunches. They can be injected in undulators used in the Self Amplified Spontaneous Emission mode (SASE), or in the High Gain Harmonic Generation (HGHG), seeded with high harmonics in gases at 20 nm.The SASE source covers the 100–5 nm spectral range, the HGHG goes down to 0.8 nm.Two optional loops, for Energy Recovery or energy enhancement (1.4 GeV), will accommodate fs synchrotron radiation sources in the IR-, VUV- and X-ray ranges, together with a FEL oscillator providing radiation down to 10 nm, taking advantage of the optical development for lithography. The facility also proposes to test plasma acceleration and to provide a Thomson radiation source. Characteristics of the light source will be described. [Copyright &y& Elsevier]

A Storage Ring Free Electron Laser presents different regimes versus the synchronization between the optical pulses bouncing in the optical cavity and the electron bunches stored in the ring. Both experiments and simulations show that, on Super-ACO, the so-called detuning curve presents five zones: a “CW” regime at the ms time scale in the central and lateral ones, and a pulsed behavior at intermediate detunings. Apart from a longitudinal feedback allowing the FEL to be maintained in the central “cw” region for ensuring a high level of stability for the users, a recent feedback allowed the pulsed regions to be stabilized. The new detuning curves are presented. [Copyright &y& Elsevier]

The operation of the Super-ACO storage ring FEL using a harmonic RF cavity exhibits vertical instabilities, which are generally compensated with an increase of the chromaticity of the operating point. It has been previously reported that the FEL itself can compete, and even damps various types of electron beam instabilities. Here, we show that, when the FEL is operating, the threshold of appearance of the vertical instability is modified and that the FEL enlarges the zone of stable regime of the beam operation. Experiments showing the evolution of the chromaticity allowing the beam to be stable versus current are compared, in the presence of the FEL or for the natural beam operation. [Copyright &y& Elsevier]

We explore the self-induced harmonic generation in Storage Ring Optical Klystron (SR OK) FEL devices. We use a numerical code which accounts for the SR OK FEL longitudinal dynamics, includes the laser pulse propagation, and gives an accurate image of the phenomenology of instabilities associated with the laser–electron beam interplay. The coherent harmonic generation evolution is developed using the bunching parameter theory. The analysis we develop yields the turn-by-turn evolution of the harmonics and provides an accurate physical insight into their dependence on the intracavity laser power. The results obtained with this code are considered with mainly a Q-switched configuration evaluated for an Elettra-like facility. [Copyright &y& Elsevier]

The interaction between laser pulse and electron beam of a storage ring free electron laser (SRFEL) can be described in the longitudinal phase space as a function of the longitudinal detuning, which we define as the delay per pass between the laser pulse and the electron bunch. SRFEL dynamics versus detuning is studied by comparison of measurements performed at Super ACO with a numerical code. Experimental phenomena regarding detuning have been properly predicted by simulations. Quantitative agreement for pulse duration, pulse repetition frequency, laser power, etc. has been found. [Copyright &y& Elsevier]

The advanced understanding of the complete degradation phenomena is crucial in order to develop robust optics for FEL. Under very harsh Synchrotron Radiation conditions, results show that multiscale wavelength damages could be observed, inducing local crystalline structure modifications of the high optical index material with a severe increase of the surface roughness. [Copyright &y& Elsevier]

Recent improvements in the performance of the European storage ring FEL at ELETTRA including shorter wavelength operation (190 nm), wider tunability and increased power are described. We report briefly also on initial use for experimental studies. [Copyright &y& Elsevier]