Effects of undulator interruptions on the performance of high-gain FEL amplifiers

The construction as well as the operation of a long undulator required for short wavelength generation with high-gain free-electron lasers will become easier if the undulator could be interrupted with drift sections every few gain lengths. To evaluate the effect of such interruption on the FEL performance, we study the following three effects; (i) the diffraction loss, (ii) the free space slippage and (iii) the phase de-coherence. The effect (i) is found to be small, the effect (ii) gives a condition on the length of the drift section and the effect (iii) is small, but not negligible.