Simulation of radiation build-up in an FEL oscillator: the effect of the electron beam micropulse shape

Using a one-dimensional multi-frequency simulation code, we have investigated how the radiation build-up time in an FEL oscillator depends upon the electron beam micropulse width and micropulse shape. From the simulation, it is found that in the Gaussian electron beam case when the electron beam micropulse width is shorter than the radiation wavelength, the radiation in the cavity builds up faster than in longer cases (because of the coherent spontaneous emission). But in the square electron beam pulse case, the spontaneous emission at the edge is much stronger than in the case of a Gaussian electron beam pulse, so the effect of coherent spontaneous emission is reduced. The radiation spectral intensity and temporal profiles have been evaluated for super Gaussian pulse shapes of a bunched electron beam.