Simulation of radiation build-up in an FEL oscillator: effect of electron energy spread and micro pulse width

The characteristics of radiation build-up in an FEL oscillator and the effect of electron beam energy spread on the radiation build-up including its effect on electron micropulse width, radiation wavelength and slippage distance is investigated. By the numerical simulation, it is clarified that the effect of electron beam energy spread on radiation build-up for the case using the coherent start up oscillator scheme is smaller than that for the case of the normal FEL (long electron micropulse case), while the saturation power of radiation for the normal FEL is larger than that for the coherent start up FEL. So in order to make the best use of these characteristics, at first the short electron micropulses are driven to the wiggler to attain the fast radiation build-up in the oscillator, then long electron micropulse is driven causing bunching after a few interactions, then reaching saturation. This oscillation scheme may be effective in the millimeter to sub-millimeter regime of radiation wavelength.