Proposal to generate an isolated monocycle x-ray pulse by counteracting the slippage effect in free-electron lasers.

A novel scheme is proposed to generate an isolated monocycle x-ray pulse in free-electron lasers, which is based on coherent emission from a chirped microbunch passing through a strongly tapered undulator. In this scheme, the pulse lengthening by optical slippage, being intrinsic to the lasing process of free-electron lasers, can be effectively suppressed through destructive interference of electromagnetic waves emitted at individual undulator periods. Calculations show that an isolated monocycle x-ray pulse with a wavelength of 8.6 nm and a peak power of 1.2 GW can be generated if this scheme is applied to a 2-GeV and 2-kA electron beam.