Simulation studies of a XUV/soft X-ray harmonic-cascade FEL for the proposed LBNL recirculating linac

Presently there is significant interest at LBNL in designing and building a facility for ultrafast (i.e. femtosecond time scale) X-ray science based upon a superconducting, recirculating RF linac. In addition to producing synchrotron radiation pulses in the 1-15 keV energy range, we are also considering adding one or more free-electron laser (FEL) beamlines using a harmonic cascade approach to produce coherent XUV & soft X-ray emission beginning with a strong input seed at ∼200 nm wavelength obtained from a "conventional" laser. Each cascade is composed of a radiator together with a modulator section, separated by a magnetic chicane. The chicane temporally delays the electron beam pulse in order that a "virgin" pulse region (with undegraded energy spread) be brought into synchronism with the radiation pulse, which together then undergo FEL action in the modulator. We present various results obtained with the GINGER simulation code examining final output sensitivity to initial electron beam parameters. We also discuss the effects of external laser noise and shot noise upon this particular cascade approach which can limit the final output coherence.