The effects of the guide field and tapered wiggler in free-electron laser

The interaction between electromagnetic field and electron beam was investigated numerically in a free-electron laser, using a new simulation code which consist of the three-dimensional electron motion, amplitude and phase of emitted radiation. The maintenance of the quality and stability of electron beam and the resonance condition through the wiggler is important to obtain the high gain and the high efficiency. But the electron orbits diverge and become unstable, and the resonance condition is not preserved because of the three-dimensional effects on the off-axis electron and the conversion of electron energy to electromagnetic field. To prevent those defects and improve the efficiency compared with the conventional case, we used the new methods such as the axial-guide field and tapered wiggler.