Short pulse and three-dimensional effects in a FEL oscillator

Using the FELOS code developed by us, short pulse and three-dimensional effects are investigated. The spiking phenomenon is observed. Though there is the ''lethargy'' effect during the early stage of FEL oscillation, the speed of spikes is always larger than the speed of light in vacuum after the length of the synchrotron oscillation period equals the wiggler length. Comparing with 1D calculations, we point out that 3D effects destroy the bunching of the electron beam. This results in washing out the periodic structure of the optical spikes appearing in a 1D simulation and makes the optical beam nearly a single spike of sub-picosecond length. It is stressed that the sub-picosecond pulse will open new possibilities of applications of FELs. (orig.)