Deep Saturated Free Electron Laser Oscillators and Frozen Spikes
  We analyze the behavior of Free Electron Laser (FEL) oscillators operating in the deep saturated regime and point out the formation of sub-peaks of the optical pulse. They are very stable configurations, having a width corresponding to a coherence length. We speculate on the physical mechanisms underlying their growth and attempt an identification with FEL mode locked structures associated with Super Modes. Their impact on the intra-cavity nonlinear harmonic generation is also discussed along with the possibility of exploiting them as cavity out-coupler. 
P. L. Ottaviani, S. Pagnutti, G. Dattoli, E. Sabia, V. Petrillo, P.   Van Der Slot, S. Biedron, S. Milton
physics.acc-ph