Narrow-bandwidth operation of a free-electron laser enforced by seeding

Narrow-bandwidth operation of a quasicontinuous free-electron laser by seed injection has been demonstrated. The pulse-to-pulse frequency bandwidth has been reduced from 1.6 to 0.16 GHz. An order-of-magnitude reduction of the sideband-mode beating in a single pulse and a 30% reduction in laser start-up time has been obtained. Numerical simulations of the multimode free-electron laser show that the sideband-mode suppression and the buildup of coherence are rather slow, underlining the importance of seeding to obtain single-mode operation for the available pulse length.