Numerical study of effects of the beam tube on laser fields with a three-dimensional simulation code using the finite element method

In January 1997, the Beijing FEL observed large laser amplification at 8–18 μm. However, through the collaborative work, it was found from both experiments and numerical simulations that the laser loss on the beam tube wall was not negligible, and that the saturation was not seen in the relatively long wavelength range because of this loss. This calls for further investigation on the effects of the beam tube of finite size. In order to include such effects self-consistently, we have developed a new three-dimensional code that can solve equations with the boundary conditions of the beam tube by using the Finite Element Method. Results show that the beam tube effects are dominant in deriving higher laser modes in the tube, compared with the optical guiding effects, and consequently reduced gain especially in the longer wavelength range, where the beam tube effects are greatly emphasized. It is also found that TEM02 mode is the most dominant higher mode in the beam tube, and is also the main cause of gain reduction.