INFLUENCE OF A STEP-TAPERED UNDULATOR FIELD ON THE OPTICAL PULSE-SHAPE OF A FAR-INFRARED FREE-ELECTRON LASER

The optical output of the free-electron laser for infrared experiments (FELIX), which operates in the regime of strong slippage, consists of picosecond pulses, Depending on the amount of cavity desynchronizatio n, the optical pulse can develop substantial structure in the form of multiple subpulses, We present second-order autocorrelation measuremen ts of the subpulses at several far-infrared wavelengths while applying a step-taper in the undulator field. The operation with a step-tapere d undulator prevents the electrons from reabsorbing the optical field energy, leading to a smooth optical pulse, For different settings of t he undulator the measured pulse shape and corresponding power spectrum are discussed, It is possible without decreasing the small-signal gai n to produce a smooth high-power optical pulse during the whole satura ted part of the machine pulse in an FEL oscillator with a reverse-step tapered undulator.