Undulator interruption in high-gain free-electron lasers

The effect of interrupting an undulator on the performance of high-gain free-electron lasers (FELs) is evaluated by analyzing 1-D Maxwell–Vlasov equations. It is found that the effect is small for a reasonable length of the interruptions for FEL parameters envisaged for short-wavelength self-amplified spontaneous emission (SASE). Since the interruptions provide valuable space for quadrupoles and diagnostics, and at the same time permit a greater flexibility in mechanical design, the result of this paper is encouraging for construction of long undulator magnets required for SASE.