Self-field-induced chaotic electron motion in free electron lasers

Abstract

It is shown that the inclusion of the equilibrium self fields of the electron beam results in chaotic electron motion in the field configuration consisting of a constant-amplitude helical wiggler magnetic field and a uniform axial guide field. The threshold value of the self-field parameter e = w/4Q' is determined for the onset of chaoticity in the Group-I and Group-II orbits for the parameter regime corresponding to moderately high beam current (and density). The time scale for self-field-induced changes in the particle orbits is found to be of the order of beam-transit time through one wiggler period.