High-gain harmonic generation of soft X-rays with the "fresh bunch" technique

We report numerical simulations (using the TDA code) and analytical verification of the generation of 64 Å high-power soft X-rays from an exponential regime single-pass seeded FEL. The seed is generated in the FEL using the High-Gain Harmonic Generation (HGHG) technique combined with the "fresh bunch" technique. A seed pulse at 2944 Å is generated by conventional laser techniques. The seed pulse produces an intense energy modulation of the rear part of a 1 GeV, 1245 A electron beam in a "modulator" wiggler. In the "radiator" wiggler (resonant at 64 Å), the energy modulation creates beam density modulation followed by radiation of the 46th harmonic of the seed. We use a magnetic delay to position the 64 Å radiation at the undisturbed front of the bunch to serve as a seed for a single pass, exponential growth FEL. After a 9 m long exponential section followed by a 7 m long tapered section the radiation power reaches 3.3 GW.