Remaining noise-induced spatiotemporal dynamics in a free electron laser stabilized by feedback control

We investigate the efficiency of the pulsed regime stabilization by feedback control in a free electron laser (FEL) in oscillator configuration. We show that a hidden dynamics remains in the controlled FEL. Though not visible in the evolution of the pulse envelope, this dynamics affects the internal pulse shape evolution, and thus the FEL spectrum and coherence properties. The experimental results obtained on the UVSOR FEL are interpreted in the framework of a field master equation model. The stochastic nature of spontaneous emission is taken into account, and appears as an essential ingredient since the dynamics of the controlled structures is governed by noise like the other stable regimes of the FEL.