Enhanced emission of high frequency radiation from an electron beam in an undulator by backward wave resonance

Abstract

For an electron moving in an undulating magnetic field, there are two frequencies of radiation that can produce a resonance. There is a forward wave which corresponds to the free electron laser (FEL) resonance and there is also a backward wave. In the electron frame of reference these two resonances are identical except for the direction of the Poynting vector of the radiation. In the lab frame, however, the frequency of the forward wave appears upshifted and the frequency of the backward wave seems downshifted. Because of the correspondence between the two resonances, the backward wave can be used to stimulate incoherent emission of radiation at the frequency of the forward wave. A scheme is proposed for the utilization of the correspondence of these resonances. A backward wave at near twice the undulator wavelength can be used to produce incoherent radiation at the frequency of the forward resonance, which for sufficiently energetic electron beams could be in the XUV spectral region. The process could be used to enhance the radiation power over self-amplified spontaneous emission in single-pass XUV FELs. Some computer simulations are performed to estimate the amount of enhancement possible.