Numerical simulations of the UCLA experiments on a high gain SASE FEL

In this paper we present theoretical analysis of a recent SASE FEL experiment performed by 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 to apply available analytical techniques for quantitative description of the obtained results. The analysis presented in this paper is based on the results produced by a three-dimensional, time-dependent FEL simulation code FAST. It is shown that within the limit of accuracy of the experiment obtained data fully agree with the results of numerical simulations.