Exact one-particle theory of a free electron laser

From a semiclassical discussion of the one-particle quantized model we demonstrate that the free electron dynamics exhibits a phase transition in correspondence to a threshold value of the wiggler magnetic field. Below threshold the FEL can work as a small gain amplifier and can be described by the well-known pendulum analogy and gain-spread expressions. Above threshold the analogy is broken and the FEL works as a large gain amplifier which goes from spontaneous to stimulated emission regime.