Towards zeptosecond-scale pulses from x-ray free electron lasers

The short wavelength and high peak power of the presentgeneration of free-electron lasers (FELs) opens the possibilityof ultra-short pulses even surpassing the present(tens to hundreds of attoseconds) capabilities of other lightsources - but only if x-ray FELs can be made to generatepulses consisting of just a few optical cycles. For hard xrayoperation (<~0.1nm), this corresponds to durations ofapproximately a single attosecond, and below into the zeptosecondscale. This talk will describe a novel method [1] to generate trains of few-cycle pulses, at GW peak powers,from existing x-ray FEL facilities by using a relativelyshort 'afterburner'. Such pulses would enhance researchopportunity in atomic dynamics and push capability towardsthe investigation of electronic-nuclear and nucleardynamics. The corresponding multi-colour spectral output, with a bandwidth envelope increased by up to two orders of magnitudes over SASE, also has potential applications