SPECTRAL CHARACTERISTICS OF A FREE-ELECTRON LASER WITH TIME-DEPENDENT BEAM ENERGY

The spectral characteristics of a continuous beam free electron laser are studied both numerically and analytically. The average spectral width and the probability distribution of mode amplitudes determined from nonlinear numerical simulations are found to agree with a simple theory based on the linear growth of each mode in the presence of a white noise source. Included in the numerical simulation model is the time dependence of the beam energy and its effect on the spontaneous emission rate. Based on this model predictions of the frequency of observing a single dominant mode in the recent University of California at Santa Barbara experiments [FEL '89, 11th International Conference on Free Electron Lasers, Conference Digest (IEEE, New York, 1989), Paper P2.19] are made. The predicted frequency is somewhat lower than observed. However, other features of the simulations do agree with experimental results. © 1990 American Institute of Physics.