Collective free-electron laser in resonant pump conditions: Experiment and theory

A theory of Raman backscattering for the conditions of magneto-resonance is presented. The theoretical results predict that the Raman spectrum in this regime should be broad-band. Experiments for a free-electron laser are also presented for conditions close to magneto-resonance. Microwave emission is observed for a very weak helical pump field (20 Gauss on axis) when the resonance is approached. The emitted spectrum is wide band (10 kW/GHz, from 50 GHz to 350 GHz), in agreement with the prediction of the theoretical model.