Formation of multiple subpulses in a free-electron laser operating in the limit-cycle mode

The evolution of the longitudinal pulse shape and the spectrum of the short-pulse, far-infrared free-electron laser FELIX are investigated. Depending on the amount of cavity desynchronization applied, the laser is found to operate in the stable-focus mode or in the limit-cycle mode. In the latter case, auto-correlation measurements that are made with a setup that is based on second-harmonic generation in CdTe show the formation of a train of up to four subpulses. These are separated by the synchrotron length, and the corresponding sidebands in the spectrum are separated by the synchrotron frequency, in excellent agreement with theory. The measurements are made at a wavelength of 24.5 mu m.