Few-cycle vortices from superradiant nonlinear Thomson scattering by a relativistic chirped mirror

We propose a scheme to generate few-cycle vortices based on nonlinear Thomson scattering by microbunched electrons from a circularly polarized laser pulse with chirped frequency. At sufficiently high intensities, the generation of harmonics that carry orbital angular momentum occurs. At the same time, the electrons collectively act as a relativistic chirped mirror, which superradiantly reflects a chosen harmonic into a single localized beat. Calculations show that a few-cycle soft x-ray vortex with gigawatt peak power can be generated if this scheme is applied to a pC electron bunch with a few MeV kinetic beam energy.