Diviacco, B.
| Paper | Title | Page |
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| WEOA001 | Feedback Control Of Dynamical Instabilities In Classical Lasers And Fels | 391 |
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Dynamical instabilities lead to unwanted full-scale power oscillations in many classical lasers and FEL oscillators. For a long time, applications requiring stable operation were typically performed by working outside the problematic parameter regions. A breakthrough occurred in the nineties [1], when emphasis was made on the practical importance of unstable states (stationary or periodic) that coexist with unwanted oscillatory states. Indeed, although not observable in usual experiments, unstable states can be stabilized, using a feedback control involving arbitrarily small perturbations of a parameter. This observation stimulated a set of works leading to successful suppression of dynamical instabilities (initially chaos) in lasers, sometimes with surprisingly simple feedback devices [2]. We will review a set of key results, including in particular the recent works on the stabilization of mode-locked lasers, and of the super-ACO, ELETTRA and UVSOR FELs [3]. [1] Ott et al. Phys. Rev. Lett., 64, 1196 (1990). [2] Bielawski et al. Phys. Rev. A 47, 327 (1993). [3] Bielawski et al. Phys. Rev. E. 69, 045502 (2004), De Ninno & Fanelli, Phys. Rev. Lett. 92, 094801 (2004), Bruni et al., proc. EPAC 2004. |
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| THPP001 | SRFEL Linewidth Narrowing in the Ultraviolet | 447 |
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The ELETTRA Storage Ring FEL succeded in operating in the Ultraviolet range, around 350 nm, with an etalon Fabry Perot inserted in the optical cavity. The high vacuum vessel, integrating a totally motorized control system for the principal degrees of freedom of the silica plate, allowed to obtain the laser oscillation, showing a reduction of the spectral linewidth by more than an order of magnitude. Temporal analysis by a double sweep streak camera showed also a broadening of the temporal pulse width. These major results are here exposed and compared with a numerical analysis and the Storage Ring FEL dynamics theory. |
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| THPP007 | Stabilization of the Elettra Storage-Ring Free-Electron Laser using a Derivative Feedback | |
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The possibility of establishing and maintaining a stable operation mode of a storage-ring free electron laser finally resides in a deep understanding of the strongly coupled laser-electron dynamics. Such a dynamics may be affected by electron-beam instabilities whose origin can be traced back either to electromagnetic wake fields or to "external" perturbations (e.g. line-induced modulations, mechanical vibrations, etc.). This is the case of the Elettra storage-ring free-electron laser which is significantly affected by a 50-Hz perturbation of the electron beam density. We have developed a simple theoretical model which has been proved to be able to provide insight into the evolution of the laser intensity. In this framework, we have also proposed the possibility of utilizing a derivative closed-loop feedback to create or enlarge the region of stable signal. A feedback of this type has been implemented on the Elettra storage-ring free-electron laser. The obtained results, which fully confirm our predictions, are discussed in this paper. |
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| THPP013 | Operation of the European FEL at ELETTRA Below 190 nm: A Tunable Laser Source for VUV Spectroscopy | 473 |
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Thanks to an intensive technological effort in the framework of the EEC Contract HPRI CT-2001-50025 (EUFELE), the European FEL at ELETTRA was able to break the previous record for the shortest wavelength of an FEL oscillator. Novel solutions were adopted for multilayer mirrors to allow FEL operation in the wavelength region between 160 and 190 nm, which is one of the main targets of the project. The characteristics of the FEL pulses measured at 176 nm (spectral profiles, high intensity, meV bandpass, MHz repetition rate) make it a competitive light source for spectroscopy, in particular for fluorescence studies in the VUV spectral range. Proof of principle experiments have been performed on different types of silica glasses, yielding information on the mechanisms of light absorption in this material. |
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| THPP068 | Electron Beam Diagnostic Based on a Short Seeded FEL | |
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The optical properties of an FEL amplifier are sensitively dependent on the electron beam current profile, energy spread, and transverse emittance. In this paper we consider using a short FEL amplifier operating on a low harmonic of a visible-IR input seed as a mildly destructive electron beam diagnostic able to measure these properties for sub-ps time slices. The optical methods are described as well as a planned implementation of the device for the FERMI@Elettra XUV FEL under construction at Sincrotrone Trieste, including its fiber-based seed laser closely coupled with the facility timing system, undulator parameters, and requirements on the electron and FEL pulses. This diagnostic is conveniently integrated with a "laser heater" designed to increase the very low electron beam energy spread produced by a photoinjector in order to avoid space charge and coherent synchrotron radiation instabilities. |
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| FROA003 | FERMI @ Elettra: A Seeded Harmonic Cascade FEL for EUV and Soft X-Rays | 682 |
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We describe the machine layout and major performance parameters for the FERMI FEL project funded for construction at Sincrotrone Trieste, Italy. The project will be the first user facility based on seeded harmonic cascade FELs, providing controlled, high peak-power pulses. With a high-brightness rf photocathode gun, and using the existing 1.2 GeV S-band linac, the facility will provide tunable output over a range from ~100 nm to ~10 nm, with pulse duration from 40 fs to ~ 1ps, and with fully variable output polarization. Initially, two FEL cascades are planned; a single-stage harmonic generation to operate > 40 nm, and a two-stage cascade operating from ~40 nm to ~10 nm or shorter wavelength. The output is spatially and temporally coherent, with peak power in the GW range. Lasers provide modulation to the electron beam, as well as driving the photocathode and other systems, and the facility will integrate laser systems with the accelerator infrastructure, including a state-of-the-art optical timing system providing synchronization of rf signals, lasers, and x-ray pulses. Major systems and overall facility layout are described, and key performance parameters summarized. |
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