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

We present measurements that demonstrate the existence of spontaneous coher ence between independently generated laser pulses in the FELIX free-electro n laser, The experiments show that the interpulse coherence is caused by a high level of coherently enhanced spontaneous emission. We have been able t o manipulate the level of interpulse coherence by changing the settings of the electron accelerating system. The shape of the electron bunches critica lly determines the amount of coherent emission. Results of simulations of t he acceleration process are used to discuss the influence of the accelerato r parameters on the electron bunch shape. The experimentally determined cor relation function of the laser pulses is reproduced by a simple pulse-evolu tion model. Thereby, an estimate of the timing jitter of the RF acceleratin g system is obtained.