Efficiency enhancement in free-electron laser amplifier with one dimensional helical wiggler and ion-channel guiding

A method for efficiency enhancement in free-electron laser is studied which uses both tapered wiggler magnetic field and ion-channel density. Derivation of a set of nonlinear and coupled differential equations leads to the self-consistent description of the evolution of both an ensemble of electrons and the electromagnetic radiation. Numerical solution of these equations reveals considerable enhancements of the interaction efficiency. In order to obtain a better insight into physical basis of the problem, a modified pendulum equation for the interaction is derived and a small signal theory of the efficiency enhancement is developed.