COHERENT X-RAY PRODUCTION BY CASCADING STAGES OF HIGH GAIN HARMONIC GENERATION FREE ELECTRON LASERS SEEDED BY IR LASER DRIVEN HIGH-ORDER HARMONIC GENERATION∗

Coherent x-ray production achieved by a seeded free electron laser (FEL) with cascaded high gain harmonic generation (HGHG) is important for next generation de-velopment of synchrotron light sources. We examine the feasibility and some features of FEL emission seeded by a high order harmonic of an intense infrared conventional laser source (HHG). In addition to the intrinsic FEL chirp phenomenon, the longitudinal profile and spectral band-width of the HHG seed are modified significantly by the FEL interaction well before saturation occurs. This smears out original attosecond pulselet structure. As an example, we describe a cascaded HGHG scheme for coherent x-ray FEL generation that is seeded by the twenty-seventh har-monic of an ultrashort 800 nm laser pulse with 10 fs rms duration. By cascading two stages of HGHG, 15 GW peak power FEL emission at 0.3 nm can be produced with 90 MW peak power radiation at 0.1 nm via the non-linear har-monic generation