Toward TW hard x-ray free-electron lasers

The number of x-ray free-electron lasers is increasing around the world to service a fast-growing user community. The unique properties, i.e., wavelengths well into the hard x-ray region, a few femtosecond optical pulse duration and high peak brilliance, enable novel experimental studies. However, for some of these experimental techniques to reached their full potential a higher number of photons per pulse, by two to three orders of magnitude beyond the state-of-the-art, is required, i.e., x-ray pulses with beyond TW peak power. We found that a possible route towards TW powers is extreme transverse compression of the electron beam in a strong FODO lattice in conjunction with a tapered undulator. We show that when the current density reaches extremely high levels, the characteristic growth length in the tapered undulator becomes shorter than the Rayleigh range and gives rise to optical guiding over the whole undulator length, leading to hard x-ray pulses with beyond TW peak power.