  A feasible method is proposed to generate isolated attosecond terawatt x-ray
radiation pulses in high-gain free-electron lasers. In the proposed scheme, a
frequency chirped laser pulse is employed to generate a gradually-varied
spacing current enhancement of the electron beam and a series of spatiotemporal
shifters are applied between the undulator sections to amplify a chosen
ultra-short radiation pulse from self-amplified spontaneous emission.
Three-dimensional start-to-end simulations have been carried out and the
calculation results demonstrated that 0.15 nm x-ray pulses with peak power over
1TW and duration of several tens of attoseconds could be achieved by using the
proposed technique.
