Numerical simulations of the UCLA/LANL/RRCKI/SLAC experiment on a high-gain SASE FEL

In this paper we present a theoretical analysis of a recent SASE FEL experiment performed by a UCLA/LANL/RRCKI/SLAC team reporting on a high power gain of about 105 at the wavelength of 12 μm. The region of physical parameters of this experiment (as well as of future X-ray FELs) does not allow one to apply available analytical techniques for a quantitative description of the results obtained. The analysis presented in this paper is based on the results produced by the three-dimensional, time-dependent FEL simulation code FAST. It is shown that, within the limit of accuracy of the experiment, the data obtained agree fully with the results of numerical simulations.