Attosecond X-ray pulses from free-electron lasers

We compare several proposals for a generation of solitary attosecond pulses using two types of free-electron lasers, which are envisioned as future light sources for studies of ultrafast dynamics using soft and hard X-rays. All of these proposals can be realized with relatively modest additions to a primary facility. Except for the first proposal, all of the proposals employ interaction of electrons with a few-cycle laser pulse with a carrier-envelope phase stabilization in a short wiggler magnet consisting of just one or two periods. This interaction plays a key role in the eventual generation of attosecond X-ray pulses with free-electron lasers. It also links the attosecond X-ray pulse to the laser pulse, thus providing an opportunity for accurate synchronization between the laser pump pulse and the X-ray probe pulse for pump-probe experiments.