Nonlinear Formulation and Efficiency Enhancement of Free-Electron Lasers

We present a general, self-consistent, nonlinear theory of the free-electron laser (FEL) process. The formulation of the temporal steady-state problem results in a set of coupled nonlinear FEL equations governing the spatial evolution of the amplitudes and wavelengths of the fields. We show that intrinsic FEL efficiencies can be greatly enhanced by spatially contouring the magnetic pump-field parameters. In the optical regime, the single-pass efficiencies are found to exceed 20%.