Coherent spontaneous emission and spontaneous phase locking in a free-electron laser

We present measurements that demonstrate the existence of spontaneous coherence between independently generated laser pulses in the FELIX free-electron laser. The experiments show that the interpulse coherence is caused by a high level of coherently enhanced spontaneous emission. We have been able to manipulate the level of interpulse coherence by changing the settings of the electron accelerating system. The shape of the electron bunches critically determines the amount of coherent emission. Results of simulations of the acceleration process are used to discuss the influence of the accelerator parameters on the electron bunch shape. The experimentally determined correlation function of the laser pulses is reproduced by a simple pulse-evolution model. Thereby, an estimate of the timing jitter of the RF accelerating system is obtained.