• S. Bielawski, C. Bruni, C. Szwaj
  PhLAM/CERCLA, Villeneuve d'Ascq Cedex
• M.-E. Couprie, D. Garzella
  CEA/DSM/DRECAM/SPAM, Gif-sur-Yvette
• G. De Ninno, B. Diviacco, M. Trovo
  ELETTRA, Basovizza, Trieste
• D. Fanelli
  Universita di Firenze, Florence
• M. Hosaka, M. Katoh, A. Mochihashi
  UVSOR, Okazaki
• G.L. Orlandi
  ENEA C.R. Frascati, Frascati (Roma)
• Y. Takashima
  Nagoya University Graduate School of Engineering, Nagoya

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.

• D. Garzella
  CEA/Saclay, Gif-sur-Yvette
• C. Bruni
  PhLAM/CERCLA, Villeneuve d'Ascq Cedex
• M.-E. Couprie
  CEA/DSM/DRECAM/SPAM, Gif-sur-Yvette
• G. De Ninno, B. Diviacco, M. Marsi, M. Trovo
  ELETTRA, Basovizza, Trieste

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.