1-D SIMULATIONS OF FREE ELECTRON LASER OSCILLATORS

Evolutions of electron beam and optical field in an oscillator are simulated numerically with MFEL 1 and FEL 0 codes, based on 1 -D free electron laser theory. The code MFEL 1 is applied for short pulse case of, for example, rf linac FELs, FELO is for single mode simulation which can describe the motion in phase space of electrons with energy spread and give some interesting results in short computer running time. In the short pulse case, the calculations show that the power of the laser does not reach saturation in macro-pulse duration because the current of beam is too small. Frequency spectra and pulse profile of optical field are strongly dependent on the beam current, but rather insensitive to the energy spread in the region under consideration. The duration of optical pulse is only one tenth of that of the electron beam. Small signal's gain-spectrum calculations almost reproduce the Madey's theorem with the optimum detune at △λ =0.098μm, while △λ=0.088μm from Madey's theorem. When strength of optical field is approaching saturation, the gain-spectrum is completely different from that of small signal and the non-zero gains appear in the region far from the resonant frequency and optimum detune. This means that the instability of sidebands occur which has been observed by the Los Alamas oscillator experiments.