Abstract: The invention envisages control devices acting on the laser to vary in time the electron/photon interaction, means proceeding to a State I in which they alter the optimal conditions for laser amplification; and a State II in which that reestablish said optimal conditions, which control devices permit the improvement of the peak amplitude and stability of the laser pulses and the rate and stability of the pulse period. According to another aspect of this invention, the two states are alternated, preferably periodically or almost periodically. The duration of State I should be selected by comparison with the characteristics of the electron packet and, in particular of the relaxation time observed for said packet. If the duration of State II is set to a value to be comparable to or longer than the electron relaxation time, the peak amplitude of the laser pulses is increased. The duration of State II should be selected by comparison with the natural macropulse width.