SHORT-PULSE EFFECTS IN A FREE-ELECTRON LASER

The Free-Electron Laser for Infrared eXperiments (FELIX) offers a unqi ue combination of short electron bunches and long wavelengths, i.e., a slippage parameter A, ranging up to 10. As a consequence, pronounced short-pulse effects can be observed. In this paper the experimental ob servation of two of these effects is discussed, namely the occurrence of limit-cycle oscillations and the feasibility of tuning of the micro pulse duration. The stable limit-cycle oscillation of the macropulse p ower is due to a modulation of the optical micropulse shape. This is a consequence of a combination of high optical power and short pulses. The former causes synchrotron oscillations of the electrons and the ef fect is, therefore, closely related to spiking phenomena. The short-pu lse nature of FELIX ensures that the oscillations do not evolve into t he chaotic behavior normally associated with spiking and the sideband instability. Experimental results are compared with numerical simulati ons.