Abstract: In free electron laser with a long, linear wiggler, the magnetic field at the entrance of wiggler and the external focusing required to deep the electron beam from dispersing can determine the character of the electron orbits and seriously influence the PEL gain. To obtain good results in PEL experiments, it is necessary to investigate the linearly polarized wiggler with good magnetic field at the entrance and self-focusing.
In this paper, it is researched in details that the entrance magnetic field and self-focusing of the linearly polarized wiggler produced by doubje bifilar helix lines. (1) The integral formula of the magnetic field on Z axis of the wiggler is derived, the prediction by the formula is good with the experiment, the results show that the field symmetry is better than the bifilar helix wiggler's. (2) According to Lorentz equation, we have derived the electron trajactory equations with the stable motions, and the formula about the threshold transport e-beam current and the wiggler peak field, simulated the orbits of the electron at edge in the wiggler, those give us a result that the wiggler has a ability of self-focusing, and can be applied in the linearly polarized PEL experiment.