STEP-TAPERED OPERATION OF THE FEL - EFFICIENCY ENHANCEMENT AND 2-COLOR OPERATION

We present measurements of the temporal and spectral properties of rad iation produced from the step-tapered undulator infrared free-electron lasers (FELs), CLIO in France and FELIX in the Netherlands. Using a t wo section undulator with independently adjustable deflection paramete rs, K, the FEL will operate either with an enhanced efficiency and imp roved spectral properties (with a small positive Delta K step) or simu ltaneously at two frequencies (for large Delta K). Using a dispersion- free hole output coupler the maximum wavelength difference delta lambd a/lambda, has now been extended to more than delta lambda/lambda appro ximate to 0.6. We also present measurements that show that the FEL FEL IX will produce significant power simultaneously at two wavelengths by coherent spontaneous emission when the wavelength is long and the ele ctron bunch is short. The efficiency, spectral and temporal properties have been measured. We show that at the maximum efficiency, Delta K a pproximate to 0.02, the optical pulses generated are smooth and close to Fourier transform limited. By adjusting Delta K the optical pulse d uration can be varied by a factor of 2 or more and sidebands due to sy nchrotron oscillations can be suppressed.