Efficiency enhancement and saturation length reduction of harmonically coupled, two-beam free electron laser

In this paper, efficiency enhancement and saturation length reduction of three-dimensional free-electron laser (FEL) operating with harmonically coupled two cold electron beams with different energies was studied. To enhance the efficiency of the facility, method of wiggler tapering was used and prebunching of the lower energy electron beam was considered in order to reduce the saturation length of the system. A wiggler with planar geometry was considered to produce higher harmonics of radiation. Energies of the electron beams were chosen so that the fundamental resonance of the fast beam to be at the third harmonic of the slow beam. A set of nonlinear equations based on the scheme introduced in MEDUSA code, were solved numerically. To consider the effect of wiggler tapering, the magnetic field of the wiggler decreased linearly when saturation of the third harmonic radiation field started. The optimum slope and starting point of the wiggler field were found by successive runs of the simulation code to obtain the maximum radiation power. It was found that after tapering of the wiggler field, the power of up-converted third harmonic radiation enhances dramatically and prebunching of the lower energy beam has significant effect on saturation length reduction of the facility. The introduced schemes can be used in production of high power, short wavelength FELs.