HIGH-GAIN HARMONIC-GENERATION OF SOFT X-RAYS WITH THE FRESH BUNCH TECHNIQUE

We report numerical simulations (using the TDA code) and analytical ve rification of the generation of 64 Angstrom high-power soft X-rays fro m an exponential regime single-pass seeded FEL. The seed is generated in the FEL using the High-Gain Harmonic Generation (HGHG) technique co mbined with the ''fresh bunch'' technique. A seed pulse at 2944 Angstr om is generated by conventional laser techniques. The seed pulse produ ces an intense energy modulation of the rear part of a 1 GeV, 1245 A e lectron beam in a ''modulator'' wiggler. In the ''radiator'' wiggler ( resonant at 64 Angstrom), the energy modulation creates beam density m odulation followed by radiation of the 46th harmonic of the seed. We u se a magnetic delay to position the 64 Angstrom radiation at the undis turbed front of the bunch to serve as a seed for a single pass, expone ntial growth FEL. After a 9 m long exponential section followed by a 7 m long tapered section the radiation power reaches 3.3 GW.