We propose the use of an ultrarelativistic electron beam interacting with a few-cycle, intense laser pulse and an intense pulse of the coherent x rays to produce a multi-MW intensity, x-ray pulses $\approx 100$ attoseconds in duration. Because of a naturally occurring frequency chirp, these pulses can be further temporally compressed.

• Received 15 December 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.224801