We investigate finite pulse effects in self-amplified spontaneous emission (SASE), especially the role of coherent spontaneous emission (CSE) in the start and the evolution of the free-electron laser (FEL) process. When the FEL interaction is negligible, we solve the one-dimensional Maxwell equation exactly and clarify the meaning of the slowly varying envelope approximation (SVEA). In the exponential gain regime, we solve the coupled Vlasov-Maxwell equations and extend the linear theory to a bunched beam with energy spread. A time-dependent, nonlinear simulation algorithm is employed to study the CSE effect for a general beam distribution.
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