Quantitative study of optical guiding by three-dimensional simulation

In order to further broaden the fields of FEL application, it is necessary to generate short wavelength FELs as well as to make the bandwidth much narrower. These call for, why self-amplified spontaneous emission, and/or long undulators are conceived. However, such methods require the precise evaluation of the optical guiding effects. For this reason, we have developed a three-dimensional numerical FEL code based on ELFIN, which can treat optical guiding effects through the Fourier transformation of the laser into a number of modes in the plane perpendicular to the beam axis. Numerical results show optical guiding effects for the parameters in the FEL experiments at Free Electron Laser Institute (FELI), and the effectiveness of the long undulator as well.