Optical Physics of an XUV Free-Electron Laser*

Abstract

The optical physics of an XUV free-electron laser oscillator configuration driven by an RF linac is investigated. Some of the topics discussed are optical beam quality, methods of output coupling, ring resonator configurations, and the expected optical spectrum. The tool used to investigate these physics issues is the computer code, FELEX. FELEX simulates a three-dimensional FEL oscillator configuration. The electron beam is modelled by following individual electron trajectories including deceleration by the light wave, betatron motion, and external focussing due to quadrupole coils. The electron beam parameters used in the simulations are those believed to be scalable from present RF linac experiments. The optical beam is modelled by solving the paraxial wave equation on a square grid. After interaction with the electron beam in the wiggler region, the optical beam is propagated through the resonator cavity to the wiggler entrance where the interaction with a new pulse of electrons is initiated. Initial 3-D simulations at 82nm indicate strong distortions of the optical beam from the idealized Gaussian. Further, multiple pass calculations appear necessary to accurately determine the small signal gain. Finally, the optical spectrum is investigated using a one-dimensional periodic code.