• M. Labat, M.-E. Couprie
  CEA/DSM/DRECAM/SPAM, Gif-sur-Yvette
• M. Hosaka, M. Katoh, A. Mochihashi
  UVSOR, Okazaki
• Y. Takashima
  Nagoya University Graduate School of Engineering, Nagoya

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).

• M. Hosaka, M. Katoh, A. Mochihashi
  UVSOR, Okazaki
• M.-E. Couprie, M. Labat
  CEA/Saclay, Gif-sur-Yvette
• Y. Takashima
  Nagoya University Graduate School of Engineering, Nagoya

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.

• S. Bielawski, C. Szwaj
  PhLAM/CERCLA, Villeneuve d'Ascq Cedex
• M.-E. Couprie, M. Labat, G. Lambert
  CEA/Saclay, Gif-sur-Yvette
• M. Hosaka, M. Katoh, A. Mochihashi
  UVSOR, Okazaki
• Y. Takashima
  Nagoya University Graduate School of Engineering, Nagoya

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.