Nonlinear saturation and thermal effects on the free electron laser using an electromagnetic pump

Abstract

The production of visible laser radiation by a low energy relativistic electron beam (MeV) is most readily accomplished through the use of a high frequency (far infrared) electromagnetic pump (λr=λ0/4γ02); such radiation can be generated by reflecting the radiation the relativistic electron beam generates when it interacts with a rippled static magnetic field. It is important to investigate what limits the efficiency in this situation. It is found that with a weak pump field, the pump energy can be amplified by a factor of 4γ20 and the pump intensity by 16γ40 with the instability being terminated by pump depletion. With a strong pump field, particle trapping in the longitudial potential wave stops the growth of the instability and the initial momentum spread (temperature) limits the saturation amplitude.