Realizable Optical Free-Electron Lasers with Traveling-Wave Thomson-Scattering

In Traveling-Wave Thomson-Scattering (TWTS) a high-power laser pulse is scattered off a relativistic electron pulse to realize optical free-electron lasers (OFELs) with a wavelength range from ultraviolet to Angstrom. TWTS employs a side-scattering geometry where laser and electron beam propagation direction enclose an interaction angle to become independent of the Rayleigh length limit for the maximum interaction distance inherent to standard head-on Thomson scattering geometries. For optimum spatial overlap between electrons and laser pulse in TWTS geometries the laser pulse features a pulse-front tilt. In this way, the electrons interact with all parts of the laser pulse and the brilliance of a TWTS light source become by orders of magnitude larger than in standard head-on geometries where spatial overlap between electrons and laser pulse is lost due to defocusing of the laser pulse. OFELs can be operated with TWTS using multi-hundred to petawatt class laser systems with beam diameters in the centimeter range since the interaction distance in TWTS can be controlled with the laser beam diameter in the interaction plane.
We show that interaction distances achieved in TWTS are long enough for microbunching of the electron beam and coherent amplification of the radiation from our 1.5D FEL theory for the interaction of electrons with laser fields in side-scattering geometries.
We give the scaling laws for the design of TWTS OFELs derived from this 1.5D theory and present possible experimental setups for TWTS OFELs using electrons from conventional and laser wakefield accelerators. We put emphasize on how the ultra-low emittance of a laser wakefield accelerator can be exploited to compensate for the one percent level energy spread and how laser pulse dispersion introduced with the pulse-front tilt in TWTS setups can be compensated with an additional pair of gratings in the laser pulse path before the interaction.