Takashima, Y.
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| WEOA002 | Storage Ring Free-Electron Laser Saturation for Chromatic and Achromatic Optics | 399 |
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In a Storage Ring Free Electron Laser (FEL), the saturation mechanism results from the so-called "bunch heating” phenomenon. The periodic interaction between the electron bunch and the laser pulse within the cavity is responsible of the enhancement of the energy spread of the bunch correlated with a bunch lengthening. Recently, new electron beam optics aiming at achieving low emittance with distributed dispersive function revealed a particular interest for the FEL. In the undulator straight section, these optics lead also to an increase of the transverse sizes of the beam, and to a significant change of the Touschek lifetime. Experimental results obtained on the Super-ACO and UVSORII FELs illustrating the change of saturation process according to the chromatic or achromatic optics will be given. They will be compared with simulations performed with the LAS model, which has been modified to represent this new saturation process. |
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| THPP002 | Detuning Curve Analysis on the UVSOR2 Free-Electron Laser | 451 |
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Storage Ring Free-Electron Laser dynamics and behaviour can be explored versus the detuning, i.e. a small difference between the frequencies of revolution of the electron bunches, and of the optical pulse circulating into the optical cavity. In fact, it provides situations ranging from the maximum initial gain over losses conditions to threshold ones. Systematic measurements of the UVSOR2 detuning curves have been performed. A complete detuning curve gives the intensity of the FEL versus the detuning. On such a plot, one can distinguish five distinct zones: three corresponding to continuous modes of emission for the FEL, and two pulsed modes. Each zone can then be described with its width and period for the pulsed modes. Streak camera also provides a full characterisation of the FEL versus detuning: position of the centre of mass of the laser, bunch lengthening. The energy spread is deduced from the electron beam transverse sizes. The analysis of the FEL behaviour versus detuning is compared with simulations performed with LAS. The detuning behaviour is then illustrated under different cases (current, control of the pulsed zone, chromatic or achromatic electron-beam optics). |
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| THPP004 | Remaining Dynamics in SR-FELs Stabilized by Feedback Control: Modeling of the Field Evolution and Experiments at UVSOR | |
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FEL oscillator operation is known to be strongly affected by instability issues. Recently, it has been demonstrated experimentally and theoretically that the "macropulse instabilities" which affects the laser pulse train enveloppe can be suppressed using feedback control techniques. From a detailed experimental and numerical study, we show here that "hidden" dynamical evolutions remain in controlled FELs. Though not visible in the evolution of the pulse enveloppe, this affects the internal pulse shape evolution, and thus the FEL spectrum and coherence properties. The experiments are performed on the UVSOR FEL. The modeling is made using the field master equation [1] based on the Dattoli/Elleaume approach. The shochastic nature of spontaneous emission is taken into account, and appears as an essential ingredient. [1] S. Bielawski, C. Bruni, D. Garzella, G.-L. Orlandi and M.E. Couprie, Phys. Rev. Lett. to appear. |
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| THPP009 | Recent Research Activities of the UVSOR-FEL | |
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On the UVSOR-II storage ring upgraded in 2003, improved performance of the electron beam allowed a high power FEL lasing in the deep UV region: an out-coupled power of the FEL reached to 0.3 W in the wavelength around 250 nm. User application of the FEL is in progress now. This spring, we installed a new RF cavity whose accelerating voltage is three times higher than that of previous one. The commissioning was successfully carried out and the voltage reached the designed value. The bunch length of the electron beam becomes shorter and thus further improved performance of the FEL is expected. In the presentation, latest performance of the UVSOR-FEL will be reported. Meanwhile we are going to perform an experiment on higher harmonic generation using an external short pulse laser (Ti:Sa). The laser system will be soon installed and the experimental setup will be reported in the presentation. |
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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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