Traveling-wave Thomson scattering for scaling optical undulators towards the FEL regime

We present a novel concept for optical undulators that avoids the restrictions by the Rayleigh limit and thus allows to define interaction length and diameter independent of each other. With an ultrashort, high-power laser pulse in an oblique angle scattering geometry using tilted pulse fronts, electrons and laser remain overlapped while both beams travel over distances much longer than the Rayleigh length. For small scattering angles (<10°), where dispersive effects become negligible and interaction lengths scale up to the meter range, we discuss with the help of 1D-calculations the possibility of entering the SASE-FEL regime using optical undulators driven by existing lasers.