Chaotic particle dynamics in free-electron lasers with coaxial wiggler
  The motion of a relativistic test electron in a free electron laser can be altered significantly by an ideal coaxial wiggler field and uniform axial guide field. I have investigated the group I, group II and group III (the new group that, it is found in coaxial wiggler) orbits and finally have found that those groups become chaotic at sufficiently high beam density and at sufficiently high amplitude of wiggler field. The threshold value of the wiggler amplitude for the onset of chaos is estimated analytically and confirmed by computer simulation for spatial case where self-field effects are negligibly small. It is shown that the electron dynamic is nonintegrable. There is evidence for chaos from numerical calculations of Poincare maps and nonzero Lyaponov exponents using different approaches of Benetin's method which are described and compared. Moreover, it is shown that the particle motion becomes chaotic on a time scale comparable with the beam transit time through a few wiggler periods. 
B. Farokhi, S. Mobarakabadi