Evolution of Spontaneous and Coherent Radiation in the Free Electron Laser Oscillator

摘要

An analysis of the free electron laser (FEL) oscillator start-up problem in the linear regime is presented. The model is spatially one dimensional, though many important three dimensional effects are included heuristically. The electron beam consists of pulses of arbitrary shape separated by approximately twice the radiation transit time. The small gain/pass approximation is employed in deriving an energy rate equation, which described the evolution of the radiation pulses within the resonator. The wiggler field is assumed to occupy a portion of the finite Q resonator. In the energy rate equation, the spontaneous (incoherent) radiation term is represented by a source matrix, while the stimulated (coherent) radiation term is represented by a gain matrix. The effect of small variations in the mirror separation are investigated in the context of laser lethargy. Our analysis suggests possible methods which would substantially shorten the start-up times in FEL oscillators. Finally, our results are compared with the FEL oscillator experiments performed at Stanford University. (author)