Brilliant and efficient optical free-electron lasers with Traveling-Wave Thomson-Scattering

We demonstrate that optical free-electron lasers (OFELs) realized with Traveling-Wave Thomson-Scattering (TWTS) are by orders of magnitude more efficient and brilliant than corresponding OFELs utilizing head-on Thomson scattering geometries. In addition, we emphasize that TWTS OFELs as opposed to standard head-on Thomson scattering geometries scale favorably with regard to space-charge effects degrading emittance, energy losses through photon recoil and transverse coherence of the resulting FEL beam. The presented TWTS OFELs scenarios are assumed to be driven either by conventional, rf-accelerated electrons or by currently available laser wakefield-accelerated electrons featuring energy spreads at the one percent level.