Overview of near vs. far field radiation characteristics of the linac coherent light source (LCLS)

The unusually long insertion devices being prepared for Ångstrom-wavelength Free Electron Lasers (FELs) will generate spectral-angular distributions in the proposed experimental areas that are substantially different from those conventionally calculated for the far field. In this paper we provide a general overview of near-field effects and report on initial computational simulations of near vs. far field distributions for the SLAC linac Coherent Light Source (LCLS) undulator, an insertion device approximately 140 meters long. The properties of the coherent radiation as a limiting case of the near-field emission, for the special condition of a microbunched beam radiating along the undulator axis, are reviewed.