We describe a technique for the generation of a solitary attosecond x-ray pulse in a free-electron laser (FEL), via a process of self-amplified spontaneous emission. In this method, electrons experience an energy modulation upon interacting with laser pulses having a duration of a few cycles within single-period wiggler magnets. Two consecutive modulation sections, followed by compression in a dispersive section, are used to obtain a single, subfemtosecond spike in the electron peak current. This region of the electron beam experiences an enhanced growth rate for FEL amplification. After propagation through a long undulator, this current spike emits a $\sim 250\mathrm{\text{attosecond}}$ x-ray pulse whose intensity dominates the x-ray emission from the rest of the electron bunch.

• Received 13 January 2005

DOI:https://doi.org/10.1103/PhysRevSTAB.8.050704

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